Transformers-wikipedia.pdf by harish1991

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   Short circuit test                          1
   Transformer                                 2
   Amorphous metal transformer                22
   Austin transformer                         23
   Autotransformer                            23
   Balun                                      27
   Buchholz relay                             31
   Buck–boost transformer                     32
   Capacitor voltage transformer              35
   Center tap                                 36
   Compensation winding                       37
   Copper loss                                37
   Current transformer                        38
   Delta-wye transformer                      42
   Dissolved gas analysis                     43
   Distribution transformer                   45
   Enameled wire                              48
   Energy efficient transformer               49
   Flyback transformer                        49
   Growler (electrical device)                52
   Hybrid coil                                53
   Induction coil                             55
   Iron loss                                  60
   Isolation transformer                      67
   Leakage inductance                         69
   Linear variable differential transformer   71
   Magnifying transmitter                     72
   Metadyne                                   78
   Multiple Gas Extractor                     79
   Neon sign transformer                      79
   Oil sample tube                            80
   Oudin coil                                 81
   Padmount transformer                       82
   Paraformer                                 85
   Polyphase coil                              85
   Prolec GE                                   86
   Quadrature booster                          86
   Repeating coil                              89
   Resolver (electrical)                       89
   Rotary transformer                          91
   Rotary variable differential transformer    92
   Scott-T transformer                         94
   Synchro                                     96
   Tap (transformer)                           99
   Tesla coil                                 103
   Toroidal inductors and transformers        118
   Transformer oil                            124
   Transformer oil testing                    127
   Transformer types                          129
   Trigger transformer                        138
   Vector group                               139
   Wet Transformer                            140
   Zigzag transformer                         140

   Article Sources and Contributors           141
   Image Sources, Licenses and Contributors   144

Article Licenses
   License                                    147
Short circuit test                                                                                                            1

     Short circuit test
     The purpose of Short circuit test is to determine the series branch parameters of the equivalent circuit. As the name
     suggests, in this test primary applied voltage, the current and power input are measured keeping the secondary
     terminals short circuited. Let these values be Vsc, Isc and Wsc respectively. The supply voltage required to circulate
     rated current through the transformer is usually very small and is of the order of a few percent of the nominal
     voltage. The excitation current which is only 1 percent or less even at rated voltage becomes negligibly small during
     this test and hence is neglected. The shunt branch is thus assumed to be absent. Wsc is the sum of the copper losses
     in primary and secondary put together. The reactive power consumed is that absorbed by the leakage reactance of the
     two windings.

     For carrying Short Circuit Test on Power Transformer Do the following:
     • Isolate the Power Transformer from service.
     • Remove HV/LV Jumps and Disconnect Neutral from Earth/Ground.
     • Short LV Phases and connect these short circuited terminals to Neutral
     • Energise HV side by LV supply.
     • Measure Current in Neutral, LV line voltages, HV Voltage and HV Line Currents.
     and this is wrong
     See also "Open circuit test".

     If Neutral current is near to zero transformer windings are OK
     If Neutral current is higher or equal to Line current between LV Phase one of the winding is Open.
     Source : Saurav Chaudhary,ece,ssgpurc 100rav
Transformer                                                                                                                          2

    A transformer is a static device that transfers electrical energy from
    one circuit to another through inductively coupled conductors—the
    transformer's coils. A varying current in the first or primary winding
    creates a varying magnetic flux in the transformer's core and thus a
    varying magnetic field through the secondary winding. This varying
    magnetic field induces a varying electromotive force (EMF) or
    "voltage" in the secondary winding. This effect is called mutual

    If a load is connected to the secondary, an electric current will flow in
    the secondary winding and electrical energy will be transferred from
    the primary circuit through the transformer to the load. In an ideal
    transformer, the induced voltage in the secondary winding (Vs) is in
    proportion to the primary voltage (Vp), and is given by the ratio of the
    number of turns in the secondary (Ns) to the number of turns in the
    primary (Np) as follows:

                                                                                  Pole-mounted power distribution transformer
                                                                                with center-tapped secondary winding (note use
                                                                                 of grounded conductor, right, as one leg of the
                                                                                primary feeder). It transforms the high voltage of
                                                                                   the overhead distribution wires to the lower
                                                                                          voltage used in house wiring.

    By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be
    "stepped up" by making Ns greater than Np, or "stepped down" by making Ns less than Np.
    In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers
    being a notable exception.
    Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge
    units weighing hundreds of tons used to interconnect portions of power grids. All operate with the same basic
    principles, although the range of designs is wide. While new technologies have eliminated the need for transformers
    in some electronic circuits, transformers are still found in nearly all electronic devices designed for household
    ("mains") voltage. Transformers are essential for high-voltage electric power transmission, which makes
    long-distance transmission economically practical.
Transformer                                                                                                                        3


    The phenomenon of electromagnetic induction was
    discovered independently by Michael Faraday and
    Joseph Henry in 1831. However, Faraday was the
    first to publish the results of his experiments and
    thus receive credit for the discovery.[2] The
    relationship between electromotive force (EMF) or
    "voltage" and magnetic flux was formalized in an
    equation now referred to as "Faraday's law of

                                                                 Faraday's experiment with induction between coils of wire


    where      is the magnitude of the EMF in volts and ΦB is the magnetic flux through the circuit (in webers).[3]
    Faraday performed the first experiments on induction between coils of wire, including winding a pair of coils around
    an iron ring, thus creating the first toroidal closed-core transformer.[4]

    Induction coils
                                                 The first type of transformer to see wide use was the induction coil,
                                                 invented by Rev. Nicholas Callan of Maynooth College, Ireland in
                                                 1836. He was one of the first researchers to realize that the more turns
                                                 the secondary winding has in relation to the primary winding, the
                                                 larger is the increase in EMF. Induction coils evolved from scientists'
                                                 and inventors' efforts to get higher voltages from batteries. Since
                                                 batteries produce direct current (DC) rather than alternating current
                                                 (AC), induction coils relied upon vibrating electrical contacts that
                                                 regularly interrupted the current in the primary to create the flux
                                                 changes necessary for induction. Between the 1830s and the 1870s,
                                                 efforts to build better induction coils, mostly by trial and error, slowly
              Faraday's ring transformer
                                                 revealed the basic principles of transformers.

                                                In 1876, Russian engineer Pavel Yablochkov invented a lighting
    system based on a set of induction coils where the primary windings were connected to a source of alternating
    current and the secondary windings could be connected to several "electric candles" (arc lamps) of his own design.[5]
        The coils Yablochkov employed functioned essentially as transformers.[5]
    In 1878, the Ganz Company in Hungary began manufacturing equipment for electric lighting and, by 1883, had
    installed over fifty systems in Austria-Hungary. Their systems used alternating current exclusively and included
    those comprising both arc and incandescent lamps, along with generators and other equipment.[7]
Transformer                                                                                                                         4

    Lucien Gaulard and John Dixon Gibbs first exhibited a device with an open iron core called a "secondary generator"
    in London in 1882, then sold the idea to the Westinghouse company in the United States.[8] They also exhibited the
    invention in Turin, Italy in 1884, where it was adopted for an electric lighting system.[9] However, the efficiency of
    their open-core bipolar apparatus remained very low.[10]
    Induction coils with open magnetic circuits are inefficient for transfer of power to loads. Until about 1880, the
    paradigm for AC power transmission from a high voltage supply to a low voltage load was a series circuit.
    Open-core transformers with a ratio near 1:1 were connected with their primaries in series to allow use of a high
    voltage for transmission while presenting a low voltage to the lamps. The inherent flaw in this method was that
    turning off a single lamp affected the voltage supplied to all others on the same circuit. Many adjustable transformer
    designs were introduced to compensate for this problematic characteristic of the series circuit, including those
    employing methods of adjusting the core or bypassing the magnetic flux around part of a coil.[11]
    Efficient, practical transformer designs did not appear until the 1880s, but within a decade the transformer would be
    instrumental in the "War of Currents", and in seeing AC distribution systems triumph over their DC counterparts, a
    position in which they have remained dominant ever since.[12]

    Closed-core lighting transformers
    In the autumn of 1884[14] , Ganz Company engineers Károly
    Zipernowsky, Ottó Bláthy and Miksa Déri had determined that
    open-core devices were impracticable, as they were incapable of
    reliably regulating voltage. In their joint patent application for the
    "Z.B.D." transformers, they described two designs with closed
    magnetic circuits: the "closed-core" and "shell-core" transformers. In
    the closed-core, the primary and secondary windings were wound
    around a closed iron ring; in the shell-core, the windings were passed
    through the iron core. In both designs, the magnetic flux linking the
    primary and secondary windings traveled almost entirely within the
    iron core, with no intentional path through air. The new Z.B.D.
                                                                                 Drawing of Ganz Company's 1885 prototype.
    transformers reached 98 percent efficiency, which was 3.4 times higher
                                                                                 Capacity: 1400 VA, frequency: 40 Hz, voltage
    than the open core bipolar devices of Gaulard and Gibs.[15] When they                       ratio: 120/72 V
    employed it in parallel connected electric distribution systems,
    closed-core transformers finally made it technically and economically
    feasible to provide electric power for lighting in homes, businesses and
    public spaces.[16] [17] Bláthy had suggested the use of closed-cores,
    Zipernowsky the use of shunt connections, and Déri had performed the
    experiments;[18] Bláthy also discovered the transformer formula,
    Vs/Vp = Ns/Np. The vast majority of transformers in use today rely on
    the basic principles discovered by the three engineers. They also
    reportedly popularized the word "transformer" to describe a device for
    altering the EMF of an electric current,[16] [19] although the term had
    already been in use by 1882.[20] [21] In 1886, the Ganz Company
    installed the world's first power station that used AC generators to          Prototypes of the world's first high-efficiency
                                                                                transformers. They were built by the Z.B.D. team
    power a parallel-connected common electrical network, the                                                        [13]
                                                                                          on 16th September 1884.
    steam-powered Rome-Cerchi power plant.[22]
Transformer                                                                                                                      5

                                                Although George Westinghouse had bought Gaulard and Gibbs' patents in
                                                1885, the Edison Electric Light Company held an option on the U.S. rights
                                                for the Z.B.D. transformers, requiring Westinghouse to pursue alternative
                                                designs on the same principles. He assigned to William Stanley the task of
                                                developing a device for commercial use in United States.[24] Stanley's first
                                                patented design was for induction coils with single cores of soft iron and
                                                adjustable gaps to regulate the EMF present in the secondary winding. (See
                                                drawing at left.)[23] This design was first used commercially in the U.S. in
                                                1886.[12] But Westinghouse soon had his team working on a design whose
       Stanley's 1886 design for adjustable gap
                                       [23]     core comprised a stack of thin "E-shaped" iron plates, separated
             open-core induction coils
                                                individually or in pairs by thin sheets of paper or other insulating material.
    Prewound copper coils could then be slid into place, and straight iron plates laid in to create a closed magnetic
    circuit. Westinghouse applied for a patent for the new design in December 1886; it was granted in July 1887.[18] [25]

    Other early transformers
    In 1889, Russian-born engineer Mikhail Dolivo-Dobrovolsky developed the first three-phase transformer at the
    Allgemeine Elektricitäts-Gesellschaft ("General Electricity Company") in Germany.[26]
    In 1891, Nikola Tesla invented the Tesla coil, an air-cored, dual-tuned resonant transformer for generating very high
    voltages at high frequency.[27] [28]
    Audio frequency transformers ("repeating coils") were used by early experimenters in the development of the

    Basic principles
    The transformer is based on two principles: first, that an electric current can produce a magnetic field
    (electromagnetism), and, second that a changing magnetic field within a coil of wire induces a voltage across the
    ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that
    is developed. The changing magnetic flux induces a voltage in the secondary coil.
                                                                                    An ideal transformer is shown in the
                                                                                    adjacent figure. Current passing
                                                                                    through the primary coil creates a
                                                                                    magnetic field. The primary and
                                                                                    secondary coils are wrapped around a
                                                                                    core    of    very    high     magnetic
                                                                                    permeability, such as iron, so that most
                                                                                    of the magnetic flux passes through
                                                                                    both the primary and secondary coils.

                                                                                    Induction law
                                                                                    The voltage induced across the
                                                                                    secondary coil may be calculated from
                                                                                    Faraday's law of induction, which
                                                                                    states that:
                                  An ideal transformer
Transformer                                                                                                                      6

    where Vs is the instantaneous voltage, Ns is the number of turns in the secondary coil and Φ is the magnetic flux
    through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is
    the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to
    the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the
    excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an
    ideal transformer,[29] the instantaneous voltage across the primary winding equals

    Taking the ratio of the two equations for Vs and Vp gives the basic equation[30] for stepping up or stepping down the

    Np/Ns is known as the turns ratio, and is the primary functional characteristic of any transformer. In the case of
    step-up transformers, this may sometimes be stated as the reciprocal, Ns/Np. Turns ratio is commonly expressed as
    an irreducible fraction or ratio: for example, a transformer with primary and secondary windings of, respectively,
    100 and 150 turns is said to have a turns ratio of 2:3 rather than 0.667 or 100:150.

    Ideal power equation
    If the secondary coil is attached to a load that
    allows current to flow, electrical power is
    transmitted from the primary circuit to the
    secondary circuit. Ideally, the transformer is
    perfectly efficient; all the incoming energy is
    transformed from the primary circuit to the
    magnetic field and into the secondary circuit. If
    this condition is met, the incoming electric
    power must equal the outgoing power:

                                                                          The ideal transformer as a circuit element

    giving the ideal transformer equation

    Transformers normally have high efficiency, so this formula is a reasonable approximation.
    If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is
    transformed by the square of the turns ratio.[29] For example, if an impedance Zs is attached across the terminals of
    the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns)2Zs. This relationship is
    reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np)2Zp.
Transformer                                                                                                                   7

    Detailed operation
    The simplified description above neglects several practical factors, in particular the primary current required to
    establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.
    Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero
    resistance.[31] When a voltage is applied to the primary winding, a small current flows, driving flux around the
    magnetic circuit of the core.[31] The current required to create the flux is termed the magnetizing current; since the
    ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still
    required to create the magnetic field.
    The changing magnetic field induces an electromotive force (EMF) across each winding.[32] Since the ideal windings
    have no impedance, they have no associated voltage drop, and so the voltages VP and VS measured at the terminals
    of the transformer, are equal to the corresponding EMFs. The primary EMF, acting as it does in opposition to the
    primary voltage, is sometimes termed the "back EMF".[33] This is due to Lenz's law which states that the induction
    of EMF would always be such that it will oppose development of any such change in magnetic field.

    Practical considerations

    Leakage flux
                                                                            The ideal transformer model assumes that
                                                                            all flux generated by the primary winding
                                                                            links all the turns of every winding,
                                                                            including itself. In practice, some flux
                                                                            traverses paths that take it outside the
                                                                            windings.[34] Such flux is termed leakage
                                                                            flux, and results in leakage inductance in
                                                                            series with the mutually coupled transformer
                                                                            windings.[33] Leakage results in energy
                                                                            being alternately stored in and discharged
                                                                            from the magnetic fields with each cycle of
                                                                            the power supply. It is not directly a power
                                                                            loss (see "Stray losses" below), but results in
                                                                            inferior voltage regulation, causing the
                            Leakage flux of a transformer                   secondary voltage to fail to be directly
                                                                            proportional to the primary, particularly
    under heavy load.      Transformers are therefore normally designed to have very low leakage inductance.

    However, in some applications, leakage can be a desirable property, and long magnetic paths, air gaps, or magnetic
    bypass shunts may be deliberately introduced to a transformer's design to limit the short-circuit current it will
    supply.[33] Leaky transformers may be used to supply loads that exhibit negative resistance, such as electric arcs,
    mercury vapor lamps, and neon signs; or for safely handling loads that become periodically short-circuited such as
    electric arc welders.[35]
    Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in circuits that
    have a direct current flowing through the windings.
    Leakage inductance is also helpful when transformers are operated in parallel. It can be shown that if the "per-unit"
    inductance of two transformers is the same (a typical value is 5%), they will automatically split power "correctly"
    (e.g. 500 kVA unit in parallel with 1,000 kVA unit, the larger one will carry twice the current).
Transformer                                                                                                                    8

    Effect of frequency
    Transformer universal EMF equation
    If the flux in the core is purely sinusoidal, the relationship for either winding between its rms voltage Erms of the
    winding , and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density
    B is given by the universal EMF equation:[31]

    If the flux does not contain even harmonics the following equation can be used for half-cycle average voltage Eavg
    of any waveshape:

    The time-derivative term in Faraday's Law shows that the flux in the core is the integral with respect to time of the
    applied voltage.[36] Hypothetically an ideal transformer would work with direct-current excitation, with the core flux
    increasing linearly with time.[37] In practice, the flux would rise to the point where magnetic saturation of the core
    occurs, causing a huge increase in the magnetizing current and overheating the transformer. All practical
    transformers must therefore operate with alternating (or pulsed) current.[37]
    The EMF of a transformer at a given flux density increases with frequency.[31] By operating at higher frequencies,
    transformers can be physically more compact because a given core is able to transfer more power without reaching
    saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and
    conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies
    which reduce core and winding weight.[38] Conversely, frequencies used for some railway electrification systems
    were much lower (e.g. 16.7 Hz and 25 Hz) than normal utility frequencies (50 – 60 Hz) for historical reasons
    concerned mainly with the limitations of early electric traction motors. As such, the transformers used to step down
    the high over-head line voltages (e.g. 15 kV) are much heavier for the same power rating than those designed only
    for the higher frequencies.
    Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced
    magnetizing current; at lower frequency, the magnetizing current will increase. Operation of a transformer at other
    than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is
    practical. For example, transformers may need to be equipped with "volts per hertz" over-excitation relays to protect
    the transformer from overvoltage at higher than rated frequency.
    One example of state-of-the-art design is those transformers used for electric multiple unit high speed trains,
    particularly those required to operate across the borders of countries using different standards of electrification. The
    position of such transformers is restricted to being hung below the passenger compartment. They have to function at
    different frequencies (down to 16.7 Hz) and voltages (up to 25 kV) whilst handling the enhanced power
    requirements needed for operating the trains at high speed.
    Knowledge of natural frequencies of transformer windings is of importance for the determination of the transient
    response of the windings to impulse and switching surge voltages.
Transformer                                                                                                                         9

    Energy losses
    An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers energy is
    dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and
    those rated for electricity distribution usually perform better than 98%.[39]
    Experimental transformers using superconducting windings achieve efficiencies of 99.85%.[40] The increase in
    efficiency can save considerable energy, and hence money, in a large heavily-loaded transformer; the trade-off is in
    the additional initial and running cost of the superconducting design.
    Losses in transformers (excluding associated circuitry) vary with load current, and may be expressed as "no-load" or
    "full-load" loss. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to
    over 99% of the no-load loss. The no-load loss can be significant, so that even an idle transformer constitutes a drain
    on the electrical supply and a running cost; designing transformers for lower loss requires a larger core, good-quality
    silicon steel, or even amorphous steel, for the core, and thicker wire, increasing initial cost, so that there is a trade-off
    between initial cost and running cost. (Also see energy efficient transformer).[41]
    Transformer losses are divided into losses in the windings, termed copper loss, and those in the magnetic circuit,
    termed iron loss. Losses in the transformer arise from:
    Winding resistance
          Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin
          effect and proximity effect create additional winding resistance and losses.
    Hysteresis losses
          Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For
          a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to
          which it is subjected.[41]
    Eddy currents
          Ferromagnetic materials are also good conductors, and a core made from such a material also constitutes a
          single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a
          plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is
          a complex function of the square of supply frequency and inverse square of the material thickness.[41] Eddy
          current losses can be reduced by making the core of a stack of plates electrically insulated from each other,
          rather than a solid block; all transformers operating at low frequencies use laminated or similar cores.
          Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly
          with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound
          commonly associated with transformers,[30] and can cause losses due to frictional heating.
    Mechanical losses
          In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the primary
          and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise, and
          consuming a small amount of power.[42]
    Stray losses
          Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned to the
          supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as
          the transformer's support structure will give rise to eddy currents and be converted to heat.[43] There are also
          radiative losses due to the oscillating magnetic field, but these are usually small.
Transformer                                                                                                                      10

    Dot convention
    It is common in transformer schematic symbols for there to be a dot at the end of each coil within a transformer,
    particularly for transformers with multiple primary and secondary windings. The dots indicate the direction of each
    winding relative to the others. Voltages at the dot end of each winding are in phase; current flowing into the dot end
    of a primary coil will result in current flowing out of the dot end of a secondary coil.

    Equivalent circuit
          Refer to the diagram below
    The physical limitations of the practical transformer may be brought together as an equivalent circuit model (shown
    below) built around an ideal lossless transformer.[44] Power loss in the windings is current-dependent and is
    represented as in-series resistances Rp and Rs. Flux leakage results in a fraction of the applied voltage dropped
    without contributing to the mutual coupling, and thus can be modeled as reactances of each leakage inductance Xp
    and Xs in series with the perfectly coupled region.
    Iron losses are caused mostly by hysteresis and eddy current effects in the core, and are proportional to the square of
    the core flux for operation at a given frequency.[45] Since the core flux is proportional to the applied voltage, the iron
    loss can be represented by a resistance RC in parallel with the ideal transformer.
    A core with finite permeability requires a magnetizing current Im to maintain the mutual flux in the core. The
    magnetizing current is in phase with the flux; saturation effects cause the relationship between the two to be
    non-linear, but for simplicity this effect tends to be ignored in most circuit equivalents.[45] With a sinusoidal supply,
    the core flux lags the induced EMF by 90° and this effect can be modeled as a magnetizing reactance (reactance of
    an effective inductance) Xm in parallel with the core loss component. Rc and Xm are sometimes together termed the
    magnetizing branch of the model. If the secondary winding is made open-circuit, the current I0 taken by the
    magnetizing branch represents the transformer's no-load current.[44]
    The secondary impedance Rs and Xs is frequently moved (or "referred") to the primary side after multiplying the
    components by the impedance scaling factor (Np/Ns)2.

    Transformer equivalent circuit, with secondary impedances referred to the primary side
    The resulting model is sometimes termed the "exact equivalent circuit", though it retains a number of
    approximations, such as an assumption of linearity.[44] Analysis may be simplified by moving the magnetizing
    branch to the left of the primary impedance, an implicit assumption that the magnetizing current is low, and then
    summing primary and referred secondary impedances, resulting in so-called equivalent impedance.
    The parameters of equivalent circuit of a transformer can be calculated from the results of two transformer tests:
    open-circuit test and short-circuit test.
Transformer                                                                                                                      11

    A wide variety of transformer designs are used for different applications, though they share several common
    features. Important common transformer types include:

                                                  In an autotransformer portions of the same winding act as both the
                                                  primary and secondary. The winding has at least three taps where
                                                  electrical connections are made. An autotransformer can be smaller,
                                                  lighter and cheaper than a standard dual-winding transformer however
                                                  the autotransformer does not provide electrical isolation.
                                                  Autotransformers are often used to step up or down between voltages
                                                  in the 110-117-120 volt range and voltages in the 220-230-240 volt
                                                  range, e.g., to output either 110 or 120V (with taps) from 230V input,
                                                  allowing equipment from a 100 or 120V region to be used in a 230V

              A variable autotransformer            A variable autotransformer is made by exposing part of the winding
                                                    coils and making the secondary connection through a sliding brush,
    giving a variable turns ratio.[46] Such a device is often referred to by the trademark name variac.

    Polyphase transformers
    For three-phase supplies, a bank of three individual
    single-phase transformers can be used, or all three
    phases can be incorporated as a single three-phase
    transformer. In this case, the magnetic circuits are
    connected together, the core thus containing a
    three-phase flow of flux.[47] A number of winding
    configurations are possible, giving rise to different
    attributes and phase shifts.[48] One particular polyphase
    configuration is the zigzag transformer, used for
    grounding and in the suppression of harmonic

                                                                 Three-phase step-down transformer mounted between two utility
Transformer                                                                                                                         12

       Screenshot of a FEM simulation of the magnetic
       flux inside a three-phase power transformer. Full
                   animation is available too.

    Leakage transformers
                                                           A leakage transformer, also called a stray-field transformer, has a
                                                           significantly higher leakage inductance than other transformers,
                                                           sometimes increased by a magnetic bypass or shunt in its core between
                                                           primary and secondary, which is sometimes adjustable with a set
                                                           screw. This provides a transformer with an inherent current limitation
                                                           due to the loose coupling between its primary and the secondary
                                                           windings. The output and input currents are low enough to prevent
                                                           thermal overload under all load conditions—even if the secondary is

                                                           Leakage transformers are used for arc welding and high voltage
                                                           discharge lamps (neon lights and cold cathode fluorescent lamps,
                                                           which are series-connected up to 7.5 kV AC). It acts then both as a
                                                           voltage transformer and as a magnetic ballast.

                   Leakage transformer                     Other applications are short-circuit-proof         extra-low   voltage
                                                           transformers for toys or doorbell installations.

    Resonant transformers
    A resonant transformer is a kind of leakage transformer. It uses the leakage inductance of its secondary windings in
    combination with external capacitors, to create one or more resonant circuits. Resonant transformers such as the
    Tesla coil can generate very high voltages, and are able to provide much higher current than electrostatic
    high-voltage generation machines such as the Van de Graaff generator.[50] One of the applications of the resonant
    transformer is for the CCFL inverter. Another application of the resonant transformer is to couple between stages of
    a superheterodyne receiver, where the selectivity of the receiver is provided by tuned transformers in the
    intermediate-frequency amplifiers.[51]

    Audio transformers
    Audio transformers are those specifically designed for use in audio circuits. They can be used to block radio
    frequency interference or the DC component of an audio signal, to split or combine audio signals, or to provide
    impedance matching between high and low impedance circuits, such as between a high impedance tube (valve)
    amplifier output and a low impedance loudspeaker, or between a high impedance instrument output and the low
    impedance input of a mixing console.
    Such transformers were originally designed to connect different telephone systems to one another while keeping
    their respective power supplies isolated, and are still commonly used to interconnect professional audio systems or
Transformer                                                                                                                         13

    system components.
    Being magnetic devices, audio transformers are susceptible to external magnetic fields such as those generated by
    AC current-carrying conductors. "Hum" is a term commonly used to describe unwanted signals originating from the
    "mains" power supply (typically 50 or 60 Hz). Audio transformers used for low-level signals, such as those from
    microphones, often include shielding to protect against extraneous magnetically coupled signals.

    Instrument transformers
    Instrument transformers are used for measuring voltage and current in electrical power systems, and for power
    system protection and control. Where a voltage or current is too large to be conveniently used by an instrument, it
    can be scaled down to a standardized, low value. Instrument transformers isolate measurement, protection and
    control circuitry from the high currents or voltages present on the circuits being measured or controlled.
    A current transformer is a transformer designed to provide a current in
    its secondary coil proportional to the current flowing in its primary
    Voltage transformers (VTs), also referred to as "potential transformers"
    (PTs), are designed to have an accurately known transformation ratio
    in both magnitude and phase, over a range of measuring circuit
    impedances. A voltage transformer is intended to present a negligible
    load to the supply being measured. The low secondary voltage allows         Current transformers, designed for placing around
    protective relay equipment and measuring instruments to be operated at                         conductors
    a lower voltages.[53]

    Both current and voltage instrument transformers are designed to have predictable characteristics on overloads.
    Proper operation of over-current protective relays requires that current transformers provide a predictable
    transformation ratio even during a short-circuit.

    Transformers can be classified in many different ways; an incomplete list is:
    • By power capacity: from a fraction of a volt-ampere (VA) to over a thousand MVA;
    • By frequency range: power-, audio-, or radio frequency;
    • By voltage class: from a few volts to hundreds of kilovolts;
    • By cooling type: air-cooled, oil-filled, fan-cooled, or water-cooled;
    • By application: such as power supply, impedance matching, output voltage and current stabilizer, or circuit
    • By purpose: distribution, rectifier, arc furnace, amplifier output, etc.;
    • By winding turns ratio: step-up, step-down, isolating with equal or near-equal ratio, variable, multiple windings.
Transformer                                                                                                                        14



    Laminated steel cores

    Transformers for use at power or audio frequencies typically
    have cores made of high permeability silicon steel.[54] The
    steel has a permeability many times that of free space, and the
    core thus serves to greatly reduce the magnetizing current, and
    confine the flux to a path which closely couples the
    windings.[55] Early transformer developers soon realized that
    cores constructed from solid iron resulted in prohibitive
    eddy-current losses, and their designs mitigated this effect
    with cores consisting of bundles of insulated iron wires.[8]
                                                                       Laminated core transformer showing edge of laminations at
    Later designs constructed the core by stacking layers of thin
                                                                                              top of photo
    steel laminations, a principle that has remained in use. Each
    lamination is insulated from its neighbors by a thin
    non-conducting layer of insulation.[47] The universal transformer equation indicates a minimum cross-sectional area
    for the core to avoid saturation.

    The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and so reduce
    their magnitude. Thinner laminations reduce losses,[54] but are more laborious and expensive to construct.[56] Thin
    laminations are generally used on high frequency transformers, with some types of very thin steel laminations able to
    operate up to 10 kHz.
                                                             One common design of laminated core is made from
                                                             interleaved stacks of E-shaped steel sheets capped with
                                                             I-shaped pieces, leading to its name of "E-I transformer".[56]
                                                             Such a design tends to exhibit more losses, but is very
                                                             economical to manufacture. The cut-core or C-core type is
                                                             made by winding a steel strip around a rectangular form and
                                                             then bonding the layers together. It is then cut in two, forming
                                                             two C shapes, and the core assembled by binding the two C
                                                             halves together with a steel strap.[56] They have the advantage
                                                             that the flux is always oriented parallel to the metal grains,
                                                             reducing reluctance.

                                                                A steel core's remanence means that it retains a static
        Laminating the core greatly reduces eddy-current losses magnetic field when power is removed. When power is then
                                                                reapplied, the residual field will cause a high inrush current
    until the effect of the remaining magnetism is reduced, usually after a few cycles of the applied alternating
    current.[57] Overcurrent protection devices such as fuses must be selected to allow this harmless inrush to pass. On
    transformers connected to long, overhead power transmission lines, induced currents due to geomagnetic
    disturbances during solar storms can cause saturation of the core and operation of transformer protection devices.[58]

    Distribution transformers can achieve low no-load losses by using cores made with low-loss high-permeability
    silicon steel or amorphous (non-crystalline) metal alloy. The higher initial cost of the core material is offset over the
    life of the transformer by its lower losses at light load.[59]
Transformer                                                                                                                       15

    Solid cores
    Powdered iron cores are used in circuits (such as switch-mode power supplies) that operate above main frequencies
    and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulk electrical
    resistivity. For frequencies extending beyond the VHF band, cores made from non-conductive magnetic ceramic
    materials called ferrites are common.[56] Some radio-frequency transformers also have movable cores (sometimes
    called 'slugs') which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

    Toroidal cores

    Toroidal transformers are built around a ring-shaped core, which,
    depending on operating frequency, is made from a long strip of silicon
    steel or permalloy wound into a coil, powdered iron, or ferrite.[60] A
    strip construction ensures that the grain boundaries are optimally
    aligned, improving the transformer's efficiency by reducing the core's
    reluctance. The closed ring shape eliminates air gaps inherent in the
    construction of an E-I core.[35] The cross-section of the ring is usually
    square or rectangular, but more expensive cores with circular
    cross-sections are also available. The primary and secondary coils are
    often wound concentrically to cover the entire surface of the core. This
    minimizes the length of wire needed, and also provides screening to                    Small toroidal core transformer

    minimize the core's magnetic field from generating electromagnetic

    Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other
    advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum
    (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses
    (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. The main
    disadvantages are higher cost and limited power capacity (see "Classification" above). Because of the lack of a
    residual gap in the magnetic path, toroidal transformers also tend to exhibit higher inrush current, compared to
    laminated E-I types.
    Ferrite toroidal cores are used at higher frequencies, typically between a few tens of kilohertz to hundreds of
    megahertz, to reduce losses, physical size, and weight of switch-mode power supplies. A drawback of toroidal
    transformer construction is the higher labor cost of winding. This is because it is necessary to pass the entire length
    of a coil winding through the core aperture each time a single turn is added to the coil. As a consequence, toroidal
    transformers are uncommon above ratings of a few kVA. Small distribution transformers may achieve some of the
    benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and
    secondary windings.

    Air cores
    A physical core is not an absolute requisite and a functioning transformer can be produced simply by placing the
    windings near each other, an arrangement termed an "air-core" transformer. The air which comprises the magnetic
    circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysteresis in the core material.[33]
    The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for
    use in power distribution.[33] They have however very high bandwidth, and are frequently employed in
    radio-frequency applications,[61] for which a satisfactory coupling coefficient is maintained by carefully overlapping
    the primary and secondary windings. They're also used for resonant transformers such as Tesla coils where they can
    achieve reasonably low loss in spite of the high leakage inductance.
Transformer                                                                                                                               16

                                                                             The conducting material used for the windings depends
                                                                             upon the application, but in all cases the individual
                                                                             turns must be electrically insulated from each other to
                                                                             ensure that the current travels throughout every turn.[36]
                                                                             For small power and signal transformers, in which
                                                                             currents are low and the potential difference between
                                                                             adjacent turns is small, the coils are often wound from
                                                                             enamelled magnet wire, such as Formvar wire. Larger
                                                                             power transformers operating at high voltages may be
                                                                             wound with copper rectangular strip conductors
                                                                             insulated by oil-impregnated paper and blocks of

                                                                             High-frequency transformers operating in the tens to
                                                                             hundreds of kilohertz often have windings made of
        Windings are usually arranged concentrically to minimize flux
                                                                             braided Litz wire to minimize the skin-effect and
                                                                             proximity effect losses.[36] Large power transformers
                                                                             use multiple-stranded conductors as well, since even at
                                                                             low power frequencies non-uniform distribution of
                                                                             current would otherwise exist in high-current
                                                                             windings.[62] Each strand is individually insulated, and
                                                                             the strands are arranged so that at certain points in the
                                                                             winding, or throughout the whole winding, each
                                                                             portion occupies different relative positions in the
                                                                             complete conductor. The transposition equalizes the
                                                                             current flowing in each strand of the conductor, and
                                                                             reduces eddy current losses in the winding itself. The
                                                                             stranded conductor is also more flexible than a solid
                                                                             conductor of similar size, aiding manufacture.[62]

                                                                             For signal transformers, the windings may be arranged
                                                                             in a way to minimize leakage inductance and stray
                                                                             capacitance to improve high-frequency response. This
         Cut view through transformer windings. White: insulator. Green
      spiral: Grain oriented silicon steel. Black: Primary winding made of   can be done by splitting up each coil into sections, and
         oxygen-free copper. Red: Secondary winding. Top left: Toroidal      those sections placed in layers between the sections of
       transformer. Right: C-core, but E-core would be similar. The black    the other winding. This is known as a stacked type or
     windings are made of film. Top: Equally low capacitance between all
                                                                             interleaved winding.
         ends of both windings. Since most cores are at least moderately
                                                                  Both the primary and secondary windings on power
     conductive they also need insulation. Bottom: Lowest capacitance for
             one end of the secondary winding needed for low-powertransformers may have external connections, called
          high-voltage transformers. Bottom left: Reduction of leakage
                                                                  taps, to intermediate points on the winding to allow
                inductance would lead to increase of capacitance.
                                                                  selection of the voltage ratio. In distribution
                                                                  transformers the taps may be connected to an automatic
    on-load tap changer for voltage regulation of distribution circuits. Audio-frequency transformers, used for the
    distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A
    center-tapped transformer is often used in the output stage of an audio power amplifier in a push-pull circuit.
    Modulation transformers in AM transmitters are very similar.
Transformer                                                                                                                                17

    Certain transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under
    a vacuum, one can replace air spaces within the windings with epoxy, thus sealing the windings and helping to
    prevent the possible formation of corona and absorption of dirt or water. This produces transformers more suited to
    damp or dirty environments, but at increased manufacturing cost.[63]

    High temperatures will damage the winding insulation.[64] Small
    transformers do not generate significant heat and are cooled by air
    circulation and radiation of heat. Power transformers rated up to
    several hundred kVA can be adequately cooled by natural convective
    air-cooling, sometimes assisted by fans.[65] In larger transformers, part
    of the design problem is removal of heat. Some power transformers are
    immersed in transformer oil that both cools and insulates the
    windings.[66] The oil is a highly refined mineral oil that remains stable
    at transformer operating temperature. Indoor liquid-filled transformers
    are required by building regulations in many jurisdictions to use a
    non-flammable liquid, or to be located in fire-resistant rooms.[67]
    Air-cooled dry transformers are preferred for indoor applications even
    at capacity ratings where oil-cooled construction would be more
    economical, because their cost is offset by the reduced building
    construction cost.

    The oil-filled tank often has radiators through which the oil circulates
    by natural convection; some large transformers employ forced
    circulation of the oil by electric pumps, aided by external fans or
    water-cooled heat exchangers.[66] Oil-filled transformers undergo
                                                                                        Cut-away view of three-phase oil-cooled
    prolonged drying processes to ensure that the transformer is completely        transformer. The oil reservoir is visible at the top.
    free of water vapor before the cooling oil is introduced. This helps                Radiative fins aid the dissipation of heat.
    prevent electrical breakdown under load. Oil-filled transformers may
    be equipped with Buchholz relays, which detect gas evolved during internal arcing and rapidly de-energize the
    transformer to avert catastrophic failure.[57] Oil-filled transformers may fail, rupture, and burn, causing power
    outages and losses. Installations of oil-filled transformers usually includes fire protection measures such as walls, oil
    containment, and fire-suppression sprinkler systems.

    Polychlorinated biphenyls have properties that once favored their use as a coolant, though concerns over their
    environmental persistence led to a widespread ban on their use.[68] Today, non-toxic, stable silicone-based oils, or
    fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for
    a transformer vault.[64] [67] Before 1977, even transformers that were nominally filled only with mineral oils may
    also have been contaminated with polychlorinated biphenyls at 10-20 ppm. Since mineral oil and PCB fluid mix,
    maintenance equipment used for both PCB and oil-filled transformers could carry over small amounts of PCB,
    contaminating oil-filled transformers.[69]
    Some "dry" transformers (containing no liquid) are enclosed in sealed, pressurized tanks and cooled by nitrogen or
    sulfur hexafluoride gas.[64]
    Experimental power transformers in the 2 MVA range have been built with superconducting windings which
    eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.[70]
Transformer                                                                                                                           18

    Insulation drying
    Construction of oil-filled transformers requires that the insulation covering the windings be thoroughly dried before
    the oil is introduced. There are several different methods of drying. Common for all is that they are carried out in
    vacuum environment. The vacuum makes it difficult to transfer energy (heat) to the insulation. For this there are
    several different methods. The traditional drying is done by circulating hot air over the active part and cycle this with
    periods of vacuum (hot-air vacuum drying, HAV). More common for larger transformers is to use evaporated
    solvent which condenses on the colder active part. The benefit is that the entire process can be carried out at lower
    pressure and without influence of added oxygen. This process is commonly called vapour-phase drying (VPD).
    For distribution transformers, which are smaller and have a smaller insulation weight, resistance heating can be used.
    This is a method where current is injected in the windings to heat the insulation. The benefit is that the heating can
    be controlled very well and it is energy efficient. The method is called low-frequency heating (LFH) since the
    current is injected at a much lower frequency than the nominal of the grid, which is normally 50 or 60 Hz. A lower
    frequency reduces the effect of the inductance in the transformer, so the voltage can be reduced.

    Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base
    of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage
    insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide
    careful control of the electric field gradient without letting the transformer leak oil.[71]

    A major application of transformers is to
    increase voltage before transmitting
    electrical energy over long distances
    through wires. Wires have resistance and so
    dissipate electrical energy at a rate
    proportional to the square of the current
    through the wire. By transforming electrical
    power to a high-voltage (and therefore
    low-current) form for transmission and back
    again afterward, transformers enable
    economical transmission of power over long
    distances. Consequently, transformers have
    shaped the electricity supply industry,
    permitting generation to be located remotely
    from points of demand.[72] All but a tiny              Image of an electrical substation in Melbourne, Australia showing 3 of 5
    fraction of the world's electrical power has                220kV/66kV transformers, each with a capacity of 185MVA

    passed through a series of transformers by
    the time it reaches the consumer.[43]

    Transformers are also used extensively in electronic products to step down the supply voltage to a level suitable for
    the low voltage circuits they contain. The transformer also electrically isolates the end user from contact with the
    supply voltage.
    Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and
    record players to the input of amplifiers. Audio transformers allowed telephone circuits to carry on a two-way
    conversation over a single pair of wires. A balun transformer converts a signal that is referenced to ground to a signal
Transformer                                                                                                                                              19

    that has balanced voltages to ground, such as between external cables and internal circuits.
    The principle of open-circuit (unloaded) transformer is widely used for characterisation of soft magnetic materials,
    for example in the internationally standardised Epstein frame method[73] .

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Transformer                                                                                                                                               20

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    [48] Say, M. G. (February, 1984). Alternating Current Machines, Fifth Edition. Halsted Press. p. 166. ISBN 0470274514.
    [49] Hindmarsh. Electrical Machines and their Applications. p. 173.
    [50] Abdel-Salam, M. et al.. High-Voltage Engineering: Theory and Practice. pp. 523–524. ISBN 0824741528.
    [51] Carr, Joseph. Secrets of RF Circuit Design. pp. 193–195. ISBN 0071370676.
    [52] Guile, A. and Paterson, W. (1978). Electrical Power Systems, Volume One. Oxford: Pergamon Press. pp. 330–331. ISBN 008021729X.
    [53] Institution of Electrical Engineers (1995). Power System Protection. London: Institution of Electrical Engineers. pp. 38–39.
        ISBN 0852968345.
    [54] Hindmarsh, John (1984). Electrical Machines and their Applications. Pergamon. pp. 29–31. ISBN 0080305733.
    [55] Gottlieb, Irving (1998). Practical Transformer Handbook. Newnes. p. 4. ISBN 075063992X.
    [56] McLyman, Colonel Wm. T. (2004). Transformer and Inductor Design Handbook. CRC. Chap. 3, pp. 9–14. ISBN 0824753933.
    [57] Harlow, James H.. Electric Power Transformer Engineering. Taylor & Francis. Chap. 2, pp. 20–21.
    [58] Boteler, D. H.; Pirjola, R. J.; Nevanlinna, H. (1998). "The effects of geomagnetic disturbances on electrical systems at the Earth's surface".
        Advances in Space Research 22: 17–27. doi:10.1016/S0273-1177(97)01096-X.
    [59] Hasegawa, Ryusuke (June 2, 2000). "Present status of amorphous soft magnetic alloys". Journal of Magnetism and Magnetic Materials
        215-216: 240–245. doi:10.1016/S0304-8853(00)00126-8.
    [60] McLyman. Transformer and Inductor Design Handbook. Chap. 3 p1.
    [61] Lee, Reuben. "Air-Core Transformers" (http:/ / www. vias. org/ eltransformers/ lee_electronic_transformers_07b_22. html). Electronic
        Transformers and Circuits. . Retrieved May 22, 2007.
    [62] Central Electricity Generating Board (1982). Modern Power Station Practice. Pergamon Press.
    [63] Heathcote, Martin (November 3, 1998). J & P Transformer Book. Newnes. pp. 720–723. ISBN 0750611588.
    [64] Kulkarni, S. V. and Khaparde, S. A. (May 24, 2004). Transformer Engineering: design and practice. CRC. pp. 2–3. ISBN 0824756533.
    [65] Pansini, Anthony J. (1999). Electrical Transformers and Power Equipment. Fairmont Press. p. 32. ISBN 0881733113.
    [66] Willis, H. Lee (2004). Power Distribution Planning Reference Book. CRC Press. p. 403. ISBN 0824748751.
    [67] ENERGIE (1999) (PDF). The scope for energy saving in the EU through the use of energy-efficient electricity distribution transformers
        (http:/ / www. leonardo-energy. org/ drupal/ files/ Full project report - Thermie. pdf?download). .
    [68] "ASTDR ToxFAQs for Polychlorinated Biphenyls" (http:/ / www. atsdr. cdc. gov/ tfacts17. html). 2001. . Retrieved June 10, 2007.
    [69] McDonald, C. J. and Tourangeau, R. E. (1986). PCBs: Question and Answer Guide Concerning Polychlorinated Biphenyls (http:/ / www.
        ec. gc. ca/ wmd-dgd/ default. asp?lang=En& n=AD2C1530-1& offset=3& toc=show#anchor6). Government of Canada: Environment Canada
        Department. p. 9. ISBN 066214595X. . Retrieved November 7, 2007.
Transformer                                                                                                                                     21

    [70]   Pansini, Anthony J. (1999). Electrical Transformers and Power Equipment. Fairmont Press. pp. 66–67. ISBN 0881733113.
    [71]   Ryan, Hugh M. (2001). High Voltage Engineering and Testing. Institution Electrical Engineers. pp. 416–417. ISBN 0852967756.
    [72]   Heathcote. J & P Transformer Book. p. 1.
    [73]   IEC 60404-2 (http:/ / webstore. iec. ch/ webstore/ webstore. nsf/ mysearchajax?Openform& key=60404-2& sorting=& start=1& onglet=1)

    • Central Electricity Generating Board (1982). Modern Power Station Practice. Pergamon. ISBN 0-08-016436-6.
    • Daniels, A.R. (1985). Introduction to Electrical Machines. Macmillan. ISBN 0-333-19627-9.
    • Flanagan, William (1993). Handbook of Transformer Design and Applications. McGraw-Hill.
      ISBN 0-0702-1291-0.
    • Gottlieb, Irving (1998). Practical Transformer Handbook. Elsevier. ISBN 0-7506-3992-X.
    • Hammond, John Winthrop. Men and Volts, the Story of General Electric, published 1941 by J.B.Lippincott.
      Citations: design, early types - 106-107; design, William Stanley, first built - 178; oil-immersed, began use of -
    • Harlow, James (2004). Electric Power Transformer Engineering. CRC Press. ISBN 0-8493-1704-5.
    • Heathcote, Martin (1998). J & P Transformer Book, Twelfth edition. Newnes. ISBN 0-7506-1158-8.
    • Hindmarsh, John (1977). Electrical Machines and their Applications, 4th edition. Exeter: Pergammon.
      ISBN 0-08-030573-3.
    • Kulkarni, S.V. & Khaparde, S.A. (2004). Transformer Engineering: design and practice. CRC Press.
      ISBN 0-8247-5653-3.
    • McLaren, Peter (1984). Elementary Electric Power and Machines. Ellis Horwood. ISBN 0-4702-0057-X.
    • McLyman, Colonel William (2004). Transformer and Inductor Design Handbook. CRC. ISBN 0-8247-5393-3.
    • Pansini, Anthony (1999). Electrical Transformers and Power Equipment. CRC Press. p. 23.
      ISBN 0-8817-3311-3.
    • Ryan, H.M. (2004). High Voltage Engineering and Testing. CRC Press. ISBN 0-8529-6775-6.
    • Say, M.G. (1983). Alternating Current Machines, Fifth Edition. London: Pitman. ISBN 0-273-01969-4.
    • Winders, John (2002). Power Transformer Principles and Applications. CRC. ISBN 0-8247-0766-4.
    • Gururaj, B.I. (June 1963). "Natural Frequencies of 3-Phase Transformer Windings" (
      xpl/freeabs_all.jsp?isnumber=4072786&arnumber=4072800&count=25&index=12). IEEE Transactions on
      Power Apparatus and Systems 82 (66): 318–329. doi:10.1109/TPAS.1963.291359.

    External links
    • Transformers - Interactive Java Tutorial (
      index.html) National High Magnetic Field Laboratory
    • Inside Transformers from Denver University (
    • Understanding Transformers: Characteristics and Limitations from Conformity Magazine (http://www.
    • 3 Phase Transformer Information and Construction — The 3 Phase Power Resource Site (http://www.
    • Substation and Transmission (
      Substation_and_Transmission//) at the Open Directory Project
    • J.Edwards and T.K Saha, Power flow in transformers via the Poynting vector (
      ~aupec/aupec00/edwards00.pdf)PDF (264 KB)
    • Introduction to Current Transformers (
      pdf)PDF (94.6 KB)
    • Java applet of transformer (
    • HD video tutorial on transformers (
Transformer                                                                                                                                          22

    • Three-phase transformer circuits ( from All About

    Amorphous metal transformer
    Amorphous Metal Transformer (AMT) is a type of energy efficient transformer found on electric grids.[1] The
    magnetic core of this transformer is made with amorphous metal, which is easily magnetized / demagnetized.
    Typically, core loss can be 70–80% less than its traditional counterpart. This leads to a reduction of generation
    requirement and, when using electric power generated from fossilized fuels, less CO2 emission.[2] [3] It has been
    widely adopted by large developing countries such as China[4] and India[5] where energy conservation and CO2
    emission reduction have been put on priority. These two countries can potentially save 25–30 TWh electricity
    annually, eliminate 6-8 GW generation investment, and reduce 20–30 million tons of CO2 emission by fully utilizing
    this technology.
    As one of the major programs to improve grid efficiency (also see Ultra High Voltage (UHV) Transmission in
    China), China has started to massively install amorphous metal transformers in a number of energy intensive
    provinces since 2005. Over 20,000 MVA of such transformers are installed every year.[6] [7] This movement has also
    led to the successful development and production of amorphous metal ribbon in China.[8]

    Notes and references
    [1] Kennedy, Barry (1998), Energy Efficient Transformers, McGraw-Hill
    [2] Li, Jerry (2005), Climate Change and Energy—Opportunities in China, presented in Climate Change: The Business Forecast" Conference,
        London, Oct 2005. (Available from links on Jerry Li 's page at http:/ / www. jerryli. co. nr)
    [3] Li, Jerry (2000), Use of Energy Efficient Transformers in Asia, presented in Asian Energy Conference 2000, Hong Kong. (Available from
        links on Jerry Li 's page at http:/ / www. jerryli. co. nr)
    [4] “SPC Note on T&D network loss reduction and energy saving plan” SPC Transportation and Energy Section, Document #123, 1997 (in
    [5] B.S.K. Naidu, “Amorphous Metal Transformers—New Technology Developments”, Keynote Speech, CBIP-AlliedSignal Seminar (India),
        April 1999.
    [6] Li, Jerry (2009), From Strong to Smart: the Chinese Smart Grid and its relation with the Globe, AEPN, Article No. 0018602, Asia Energy
        Platform. Available at http:/ / www. aepfm. org/ link. php
    [7] Li, Jerry (2008), Deployment of Amorphous Metal Distribution Transformer in China, China Electric Power Yearbook 2008, P.793-795,
        China Electric Power Press (In Chinese)
    [8] Chu, Christina (2009), China's AT&M launches amorphous metal ribbon in conjunction with the country's emission reduction plan (In
        Chinese). Available at http:/ / www. chinapower. com. cn/ newsarticle/ 1103/ new1103612. asp . A rough English translation can be found at
        http:/ / www. aepfm. org/ link. php

    • Amorphous Metals in Electric-Power Distribution Applications (
    • (
    • Amorphous Ribbon for Transformers (
Austin transformer                                                                                                                                   23

    Austin transformer
    An Austin transformer is a special type of an Isolation transformer used for feeding the air-traffic obstacle lamps
    and other devices on a mast radiator antenna insulated from ground. As the electrical potential difference between
    the antenna and ground is high (up to 300 kV), feeding the lamps directly is impossible. The transformer consists of
    two ring-like windings with a large air space between the winding and the magnetic core. The large spacing provides
    both isolation from high voltage and low inter-winding coupling capacitance. [1]
    The Austin transformer is named after its inventor, Arthur O. Austin , who graduated from Stanford University in
    1903 and who obtained 225 patents in his career. [2]

    [1] B. Whitfield Griffith, Radio-electronic transmission fundamentals, Sci Tech Publishing, 2000, ISBN 1884932134, page 367, preview at http:/
        / books. google. ca/ books?id=m5DIroWLw2EC& pg=PA367& lpg=PA367& dq=%22austin+ transformer%22& source=bl&
        ots=CS2a9-9lp_& sig=GYyqF168oSH5IaEJhb8NFoncVYw& hl=en& ei=XPvUSZ7_E5vlnQfotJCEDw& sa=X& oi=book_result&
        ct=result& resnum=5
    [2] [ Austin insulators history, retrieved 2010 Nov 1

    External links
    • Picture of an Austin transformer at a broadcast transmitter site, retrieved 2010 Nov 3 (http://gallery.bostonradio.

    An autotransformer (sometimes called autoformer)[1] is an electrical transformer with only one winding. The auto
    prefix refers to the single coil rather than any automatic mechanism. In an autotransformer portions of the same
    winding act as both the primary and secondary. The winding has at least three taps where electrical connections are
    made. An autotransformer can be smaller, lighter and cheaper than a standard dual-winding transformer however the
    autotransformer does not provide electrical isolation.
    Autotransformers are often used to step up or down between voltages in the 110-117-120 volt range and voltages in
    the 220-230-240 volt range, e.g., to output either 110 or 120V (with taps) from 230V input, allowing equipment from
    a 100 or 120V region to be used in a 230V region.
Autotransformer                                                                                                                        24

    An autotransformer has a single winding
    with two end terminals, and one or more
    terminals at intermediate tap points. The
    primary voltage is applied across two of the
    terminals, and the secondary voltage taken
    from two terminals, almost always having
    one terminal in common with the primary
    voltage. The primary and secondary circuits
    therefore have a number of windings turns
    in common.[2] Since the volts-per-turn is the
    same in both windings, each develops a
    voltage in proportion to its number of turns.
    In an autotransformer part of the current
    flows directly from the input to the output,
    and only part is transferred inductively,
    allowing a smaller, lighter, cheaper core to        Single-phase tapped autotransformer with output voltage range of 40%–115% of
    be used as well as requiring only a single
    winding[3] .

    One end of the winding is usually connected in common to both the voltage source and the electrical load. The other
    end of the source and load are connected to taps along the winding. Different taps on the winding correspond to
    different voltages, measured from the common end. In a step-down transformer the source is usually connected
    across the entire winding while the load is connected by a tap across only a portion of the winding. In a step-up
    transformer, conversely, the load is attached across the full winding while the source is connected to a tap across a
    portion of the winding.

    As in an ordinary transformer, the ratio of secondary to primary voltages is equal to the ratio of the number of
    turns of the winding they connect to. For example, connecting the load between the middle and bottom of the
    autotransformer will reduce the voltage by 50%. Depending on the application, that portion of the winding used
    solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire
    winding is directly connected.

    An autotransformer does not provide electrical isolation between its windings as an ordinary transfomer does. A
    failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output.
    This is an important safety consideration when deciding to use an autotransformer in a given application.
    Furthermore, if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either.
    Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically
    lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1; beyond that range, a
    two-winding transformer is usually more economical.
    In three phase power transmission applications, autotransformers have the limitations of not suppressing harmonic
    currents and as acting as another source of ground fault currents. A large three-phase autotransformer may have a
    "buried" delta winding, not connected to the outside of the tank, to absorb some harmonic currents.
    In practice, transformer losses mean that autotransformers are not perfectly reversible; one designed for stepping
    down a voltage will deliver slightly less voltage than required if used to step up. The difference is usually slight
Autotransformer                                                                                                               25

    enough to allow reversal where the actual voltage level is not critical. This is true of isolated winding transformers
    Like multiple-winding transformers, autotransformers operate on time-varying magnetic fields and so cannot be used
    directly on DC.

    Autotransformers are frequently used in power applications to interconnect systems operating at different voltage
    classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built
    (for example) for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions
    between the two common domestic mains voltage bands in the world (100-130 and 200-250). The links between the
    UK 400 kV and 275 kV 'Super Grid' networks are normally three phase autotransformers with taps at the common
    neutral end.
    On long rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted
    as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to
    the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.
    A special form of autotransformer called a zig zag is used to provide grounding (earthing) on three-phase systems
    that otherwise have no connection to ground (earth). A zig-zag transformer provides a path for current that is
    common to all three phases (so-called zero sequence current).
    In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution
    systems, and for impedance matching such as between a low-impedance microphone and a high-impedance amplifier
    In UK railway applications, it is common to power the trains at 25 kV AC. To increase the distance between
    electricity supply Grid feeder points they can be arranged to supply a 25-0-25 kV supply with the third wire
    (opposite phase) out of reach of the train's overhead collector pantograph. The 0 V point of the supply is connected
    to the rail while one 25 kV point is connected to the overhead contact wire. At frequent (about 10 km) intervals, an
    autotransformer links the contact wire to rail and to the second (antiphase) supply conductor. This system increases
    usable transmission distance, reduces induced interference into external equipment and reduces cost. A variant is
    occasionally seen where the supply conductor is at a different voltage to the contact wire with the autotransformer
    ratio modified to suit.[4]
Autotransformer                                                                                                                                            26

    Variable autotransformers
                                                                              A variable autotransformer is made by exposing part of
                                                                              the winding coils and making the secondary connection
                                                                              through a sliding brush, giving a variable turns ratio.[5]
                                                                              Such a device is often referred to by the trademark name
                                                                              As with two-winding transformers, autotransformers may
                                                                              be equipped with many taps and automatic switchgear to
                                                                              allow them to act as automatic voltage regulators, to
                                                                              maintain a steady voltage at the customers' service during
                                                                              a wide range of load conditions. They can also be used to
                                                                              simulate low line conditions for testing. Another
                                                                              application is a lighting dimmer that doesn't produce the
                                                                              EMI typical of most thyristor dimmers.

                                                                              By exposing part of the winding coils and making the
        A variable autotransformer, with a sliding-brush secondary
      connection and a toroidal core. Cover has been removed to show          secondary connection through a sliding brush, an almost
                        copper windings and brush.                            continuously variable turns ratio can be obtained,
                                                                              allowing for very smooth control of voltage. Applicable
                                                                              only for relatively low voltage designs, this device is
                                                                              known as a variable AC transformer, or commonly by the
                                                                              trade name of Variac.

                                                                              From 1934 to 2002, Variac was a U.S. trademark of
                                                                              General Radio for a variable autotransformer intended to
                                                                              conveniently vary the output voltage for a steady AC
                                                                              input voltage. In 2004, Instrument Service Equipment
                                                                              applied for and obtained the Variac trademark for the
                                                                              same type of product.
        Variable Transformer - part of Tektronix 576
                       Curve Tracer
    [1] Paul Horowitz and Winfield Hill, The Art of Electronics Second Edition, Cambridge University Press, Cambridge MA, 1989, ISBN
        0-521-37095-7, page 58
    [2] Pansini. Electrical Transformers and Power Equipment. pp. 89–91.
    [3] Commercial site explaining why autotransformers are smaller (http:/ / victoruae. com/ victor/ index. php?option=com_content& task=view&
        id=25& Itemid=49)
    [4] "Fahrleitungen electrischer Bahnen" BG Teubner-Verlag, Stuttgart, page 672. An English edition "Contact Lines for Electric Railways"
        appears to be out of print. This industry standard text describes the various European electrification principles. See the website of the UIC in
        Paris for the relevant international rail standards, in English. No comparable publications seem to exist for American railways, probably due to
        the paucity of electrified installations there.
    [5] Bakshi, M. V. and Bakshi, U. A.. Electrical Machines - I. p. 330. ISBN 8184310099.
Autotransformer                                                                                                                27

    • Terrell Croft and Wilford Summers (ed), American Electricians' Handbook, Eleventh Edition, McGraw Hill, New
      York (1987) ISBN 0-07013932-6
    • Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh
      Edition,McGraw-Hill, New York, 1978, ISBN 0-07020974-X

    A balun, pronounced /ˈbælʌn/, is a type of electrical transformer that
    can convert electrical signals that are balanced about ground
    (differential) to signals that are unbalanced (single-ended) and vice
    versa. They are also often used to connect lines of differing impedance.
    The origin of the word balun is bal(ance) + un(balance).
    Baluns can take many forms and their presence is not always obvious.
    They always use electromagnetic coupling for their operation.

                                                                               Pair of AC&E 120 Ohm twisted pair (Krone IDC)
                                                                                 to 75 Ohm coaxial cable balun transformers.
                                                                                         Actual length is about 3cm.

                                                                                       2 balun matching transformers

    Types of balun
Balun                                                                                                                           28

                                                   Autotransformer type
                                                   In an autotransformer, two coils on a ferrite rod can be used as a balun
                                                   by winding the individual strands of enameled wire comprising the coil
                                                   very tightly together. This winding can take one of two forms: either
                                                   the two windings must be wound such that the two form a single layer
                                                   where each turn is touching each of the adjacent turns of the other
                                                   winding; or the two wires are twisted together before being wound into
                                                   the coil.

                                                    The two windings are joined to become a single coil. The end of one of
       Autotransformer 4:1 wideband balun using two the windings on one side of the coil is connected to the end of the other
                 windings on a ferrite rod.         winding on the other side of the coil. This point then becomes the
                                                    ground for the unbalanced circuit. One of the remaining ends is
    connected to the ungrounded side of the unbalanced circuit, and one side of the balanced circuit. Finally, the other
    side of the balanced circuit is connected to the remaining end.

    Classical transformer type
    Isolated transformers have a real impedance at a resonance frequency
    where self-inductance and self-capacitance for each individual winding
    cancel themselves out.

    Transmission-line transformer type
    Baluns can be considered as simple forms of transmission line
    A more complex (and subtle) type results when the transformer type                     Isolated transformer

    (magnetic coupling) is combined with the transmission line type
    (electro-magnetic coupling). This is where whole transmission lines are used as windings, resulting in devices
    capable of very wideband operation. This whole class known generally as "Transmission Line Transformers" spawn
    their own huge variety. Very commonly, they use small ferrite cores in toroidal or "binocular" shapes. Something as
    simple as 10 turns of coaxial cable coiled up on a diameter about the size of a dinner plate makes an extremely
    effective choke balun for frequencies from about 10 MHz to beyond 30 MHz. The magnetic material may be "air",
    but it is a transmission line transformer.
Balun                                                                                                                                   29

                                                             The Guanella transmission line transformer is often combined with a
                                                             balun to act as an impedance matching transformer. Putting balancing
                                                             aside a 1:4 transformer of this type consists of a 75 Ohm transmission
                                                             line divided in parallel into two 150 Ohm cables, which are then
                                                             combined in series for 300 Ohm. It is implemented as a specific wiring
                                                             around the ferrite core of the balun.

                                                             Delay line type
                                                             A large class of baluns uses connected transmission lines of specific
                                                             lengths, with no obvious "transformer" part. These are usually built for
                                                             (narrow) frequency ranges where the lengths involved are some
                                                             multiple of a quarter wavelength of the intended frequency in the
         Homemade 1:1 balun using a toroidal core and        transmission line medium. A common application is in making a
         coaxial cable. This simple RF choke works as a
                                                             coaxial connection to a balanced antenna, and designs include many
          balun by preventing signals passing along the
        outside of the braid. Such a device can be used to   types involving coaxial loops and variously connected "stubs".
            cure television interference by acting as a
                                                 One easy way to make a balun is a one-half wavelength (λ/2) length of
                                                 coaxial cable. The inner core of the cable is linked at each end to one
                                                 of the balanced connections for a feeder or dipole. One of these
    terminals should be connected to the inner core of the coaxial feeder. All three braids should be connected together.
    This then forms a 4:1 balun which works at only one frequency.
    Another narrow band design is to use a λ/4 length of metal pipe. The coaxial cable is placed inside the pipe; at one
    end the braid is wired to the pipe while at the other end no connection is made to the pipe. The balanced end of this
    balun is at the end where the pipe is wired to the braid. The λ/4 conductor acts as a transformer converting the
    infinite impedance at the unconnected end into a zero impedance at the end connected to the braid. Hence any
    current entering the balun through the connection, which goes to the braid at the end with the connection to the pipe,
    will flow into the pipe. This balun design is not good for low frequencies because of the long length of pipe that will
    be needed. An easy way to make such a balun is to paint the outside of the coax with conductive paint, then to
    connect this paint to the braid.

    Balun alternatives
    An RF choke can be used in place of a balun. If a coil is made using coaxial cable near to the feed point of a
    balanced antenna then the RF current that flows on the outer surface of the coaxial cable can be attenuated. One way
    of doing this would be to wrap a lossy material, such as ferrite around the coaxial cable;

    A balun's function is generally to achieve compatibility between systems, and as such, finds extensive application in
    modern communications, particularly in realising frequency conversion mixers to make cellular phone and data
    transmission networks possible. They are also used to convert an E1 carrier signal from coaxial cable to UTP CAT-5
Balun                                                                                                                            30

    Radio and television
    In television, amateur radio, and other antenna installations and
    connections, baluns convert between 300 ohm ribbon cable or 450 ohm
    ladder line (balanced) and 75 Ω coaxial cable (unbalanced) or to
    directly connect a balanced antenna to (unbalanced) coax. To avoid
    EMC problems it is a good idea to connect a centre fed dipole antenna
                                                                                  A 75-to-300 ohm balun built into the antenna
    to coaxial cable via a balun. Match 300 Ω twin-lead cable to 75 Ω
    coaxial cable

    In electronic communications, baluns convert Twinax cables to Category 5 cables, and back, or they convert between
    coaxial cable and ladder line.
    In measuring the impedance or radiation pattern of a balanced antenna using a coaxial cable, it is important to place a
    balun between the cable and the antenna feed. Unbalanced currents that may otherwise flow on the cable will make
    the measured antenna impedance sensitive to the configuration of the feed cable, and the radiation pattern of small
    antennas may be distorted by radiation from the cable.
    Baluns are present in radars, transmitters, satellites, in every telephone network, and probably in most wireless
    network modem/routers used in homes. It can be combined with transimpedance amplifiers to compose high-voltage
    amplifiers out of low-voltage components.

    While not as high as most RF applications, baseband video still uses frequencies up to several megahertz. Since this
    bandwidth is now well within range of modern twisted-pair cables, they are now being used to send video which
    would otherwise run over coaxial cable. Many better security cameras now have both a balanced UTP output and an
    unbalanced coaxial one via an internal balun, though any camera can be used with an external balun. A balun is also
    used on the video recorder end to convert back from the 100-ohm balanced to 75-ohm unbalanced. A balun of this
    type has a BNC connector with two screw terminals. VGA/DVI baluns are baluns with electronic circuitry used to
    connect VGA/DVI sources (laptop, DVD, etc.) to VGA/DVI display devices over long runs of CAT-5/CAT-6 cable.
    Runs over 130 m (400 ft) may lose quality due to attenuation and variations in the arrival time of each signal. A
    skew control and special low skew or skew free cable is used for runs over 130 m (400 ft).

    In audio applications, baluns convert between high impedance (see
    Nominal impedance) unbalanced and low impedance balanced lines.
    Except for the connections, the three devices in the image are
    electrically identical, but only the leftmost two can be used as baluns.
    The device on the left would normally be used to connect a high
    impedance source, such as a guitar, into a balanced microphone input,
    serving as a passive DI unit. The one in the centre is for connecting a
    low impedance balanced source, such as a microphone, into a guitar
    amplifier. The one at the right is not a balun, as it provides only
    impedance matching.

    In power line communications, baluns are used in coupling signals
                                                                                       Three audio baluns (transformers).
    onto a power line.
Balun                                                                                                                     31

    • Building and Using Baluns and Ununs: Practical Designs for the Experimenter, Jerry Sevick (W2FMI), 1994.
    • Radio communication handbook, Edition five, Radio Society of Great Britain (RSGB), 1976, pages 12.41 and
    • SWDXER [1] (click ENTER) ¨The SWDXER¨ (was - with general SWL
      information and radio antenna tips (now hosted by
    • Coaxial Balun [2] Coaxial Balun sample by

    [1] http:/ / oh2ffy. 50gigs. net/ swdxer/ index. html
    [2] http:/ / yagi-uda. com/ coaxial_balun. php

    Buchholz relay
    In the field of electric power distribution and transmission, a Buchholz
    relay is a safety device mounted on some oil-filled power transformers
    and reactors, equipped with an external overhead oil reservoir called a
    conservator. The Buchholz Relay is used as a protective device
    sensitive to the effects of dielectric failure inside the equipment.
    Depending on the model, the relay has multiple methods to detect a
    failing transformer. On a slow accumulation of gas, due perhaps to
    slight overload, gas produced by decomposition of insulating oil
    accumulates in the top of the relay and forces the oil level down. A
    float switch in the relay is used to initiate an alarm signal. Depending
    on design, a second float may also serve to detect slow oil leaks.

    If an arc forms, gas accumulation is rapid, and oil flows rapidly into
    the conservator. This flow of oil operates a switch attached to a vane
    located in the path of the moving oil. This switch normally will operate
    a circuit breaker to isolate the apparatus before the fault causes
    additional damage. Buchholz relays have a test port to allow the
    accumulated gas to be withdrawn for testing. Flammable gas found in the relay indicates some internal fault such as
    overheating or arcing, whereas air found in the relay may only indicate low oil level or a leak.

    Buchholz relays have been applied to large power transformers at least since the 1940s. The relay was first
    developed by Max Buchholz (1875–1956) in 1921 [1] .
    Names like beechwood relay or beech relay are an indication of incorrectly translated German language manuals.
Buchholz relay                                                                                                                 32

    [1] http:/ / www. transformerworld. co. uk/ buchholz. htm Tutorial T5

    External links
    • Buchholz relay technical specifications from Electromotoren und Gerätebau Barleben GmbH (http://www. Prospekt Buchholzrelais ENGLISCH.pdf) Contains a detailed description (in
      English) of the Buchholz detection principles and mechanisms
    • Koncar A.k.a Rade Koncar , Buchholz relays , for power and distribution transformers. (http://www.koncar-nsp.
    • Buchholz Relay ( Buchholz.pdf)

    Buck–boost transformer
    A buck–boost transformer is a type of
    autotransformer used to make small
    adjustments to the voltage applied to
    alternating current equipment. Buck–boost
    connections are used in several places such
    as uninterruptible power supply (UPS) units
    for computers, electric power distribution,
    and in the tanning bed industry. Operating
    electrical equipment at other than its
    designed voltage may result in poor
    performance, short operating life, or
    possibly overheating and damage.

    For large adjustments in voltage (more than
    15% to 20%), usually a two-winding
                                                                    Typical multi-tap buck–boost transformer
    transformer is used with the required voltage
    ratio, for example 240VAC to 120VAC.
    These transformers are more costly than buck–boost transformers since both windings must carry the full power
    delivered to the load, whereas the buck–boost winding must only carry a fraction of the load power.

    There are two basic types, self adjusting (active) or passive designs. The active types monitor incoming voltages and
    will adjust the outgoing voltage to be within an acceptable range. This is typically between 115VAC and 225VAC
    for computer UPS systems. The system will either buck (lower) or boost (raise) the voltage if it senses a variance in
    the incoming voltage. Several taps are provided on the transformer winding which allow adjustment of the ratio. In
    an active buck–boost transformer, a control circuit selects which tap to use to maintain the output voltage within the
    desired range, over a range of input voltages. The control portion of the device that senses the voltage drop or rise is
    not technically part of the transformer, but rather a part of the larger transformer assembly.
    Passive transformers are used for larger equipment where the amount of buck or boost is fixed. For example, a fixed
    boost would be used when connecting equipment rated for 230 VAC to a 208 V power source.
    The passive transformers are rated in volt-amperes (or more rarely, amperes) and are rated for a percent of voltage
    drop or rise. For example, a buck–boost transformer rated at 10% rise at 208VAC will raise incoming voltage of
Buck–boost transformer                                                                                                                33

    210VAC to 231VAC. A rating of 5% drop at 240VAC will yield the result of 233VAC if the actual incoming
    voltage is 245VAC.

    All transformers operate only with alternating current. Transformers change only voltage, not frequency. Equipment
    that uses induction motors will operate at a different speed if operated at other than the design frequency. Some
    equipment is marked on its nameplate to run at either 50 Hz or 60 Hz, and would need only the voltage adjusted with
    a buck–boost transformer.

    Consumer and business applications
    Most passive transformers come semi-wired, where the installer
    completes the last internal connections to have the unit perform
    the amount of buck or boost needed. They have multiple taps on
    both the primary and secondary coils to achieve this flexibility.
    They are designed for hard wired installations (no plugs) and
    allow the same transformer to be used in several different
    applications. The same transformer can be rewired to raise or
    lower voltage by 5%, 10% or 15% for either 208VAC or 240VAC
    applications, depending on the final wiring done by the electrician.
    Fixed transformers with around the same cost were introduced
    primarily for the tanning market. They are prewired, and must be
    purchased with the exact amount of buck or boost needed for the
                                                                             Fixed ratio transformer with cord, plug and receptacle
    application. They have factory-installed plugs and receptacles
                                                                                for light to medium loads. 30A version shown.
    making installation very quick and easy, and reducing the need for
    hard-wiring small loads.

    A typical fixed unit will have a NEMA 6-20 plug for attachment to the prewired 240V wall receptacle, and a
    receptacle for the load equipment. This eliminates the need for professional installation if the exact incoming voltage
    can be determined. To make them easier for end-users to select, they are rated in load amps (A) rather than
    buck–boost volt-amps (kVA). These are used almost exclusively in light to moderate applications that require 40
    amps or less.
    Not all 240V equipment requires voltage correction. These transformers are used when electrical equipment has a
    voltage requirement that is slightly out of tolerance with the incoming power supply. This is most common when
    using 240V equipment in a business with 208V service or vice versa.
    Equipment is typically labeled with its voltage rating, and may advertise the amount of tolerance it will accept before
    degraded performance or damage can be expected. A unit that requires 230VAC with a tolerance of 5% will not
    require a buck–boost transformer if the branch circuit (under load) is between 219VAC and 241VAC. Measurement
    should be made while the circuit is loaded, as the voltage can drop several volts compared to the open measurement.
    The transformer must be rated to carry the full load current or it will be damaged.
Buck–boost transformer                                                                                                                    34

    Electric power distribution applications

   Video clip of very large three-phase buck–boost transformers spread across utility poles (320x240, 180 kbps, 23 sec, no audio) – see
   also at 640x480 size, 600 kbps.

    Power traveling long distances in electric power grids can experience a condition known as voltage drop due to the
    slight resistance of the metallic conductor having a cumulative effect over tens to hundreds of miles/kilometers.
    Distant customers may experience a drop of 5% to 10% of rated nominal line voltage.
    To combat these effects, large boost transformers are placed on poles in remote locations to increase system voltage
    and bring the voltage back up to acceptable levels. The boosting consumes current capacity from the unboosted main
    line, reducing the main line current carrying capacity for all customers.
    On three-phase power lines, the very large and heavy transformers are often spaced apart across three separate
    consecutive poles on the powerline, each boosting one of the three phases.

Capacitor voltage transformer                                                                                                  35

    Capacitor voltage transformer
    A capacitor voltage transformer
    (CVT), or capacitance coupled voltage
    transformer (CCVT) is a transformer
    used in power systems to step down
    extra high voltage signals and provide a
    low voltage signal, for measurement or
    to operate a protective relay. In its most
    basic form the device consists of three
    parts: two capacitors across which the
    transmission line signal is split, an
    inductive element to tune the device to
    the line frequency, and a transformer to isolate and further step down the voltage for the instrumentation or
    protective relay. The device has at least four terminals: a terminal for connection to the high voltage signal, a ground
    terminal, and two secondary terminals which connect to the instrumentation or protective relay. CVTs are typically
    single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage
    transformers would be uneconomical. In practice, capacitor C1 is often constructed as a stack of smaller capacitors
    connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2.

    The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high
    frequency communication signals from power frequency. This forms a carrier communication network throughout
    the transmission network.

    External links
    • Specifications for a commercial CVT [1]

    [1] http:/ / www. ritz-international. de/ pub/ CVT-E. pdf
Center tap                                                                                                                    36

    Center tap
    In electronics, a center tap is a connection made to a point half way along a winding of a transformer or inductor, or
    along the element of a resistor or a potentiometer. Taps are sometimes used on inductors for the coupling of signals,
    and may not necessarily be at the half-way point, but rather, closer to one end. A common application of this is in the
    Hartley oscillator. Inductors with taps also permit the transformation of the amplitude of alternating current (AC)
    voltages for the purpose of power conversion, in which case, they are referred to as autotransformers, since there is
    only one winding. An example of an autotransformer is an automobile ignition coil. Potentiometer tapping provides
    one or more connections along the device's element, along with the usual connections at each of the two ends of the
    element, and the slider connection. Potentiometer taps allow for circuit functions that would otherwise not be
    available with the usual construction of just the two end connections and one slider connection.

    Volts center tapped
    Volts center tapped (VCT) describes the voltage output of a center tapped transformer. For example: A 24 VCT
    transformer will measure 24 VAC across the outer two taps (winding as a whole), and 12VAC from each outer tap to
    the center-tap (half winding). These two 12 VAC supplies are 180 degrees out of phase with each other, thus making
    it easy to derive positive and negative 12 volt DC power supplies from them.

    Common applications of center-tapped transformers
    • In a rectifier, a center-tapped transformer and two diodes can form a full-wave rectifier that allows both
      half-cycles of the AC waveform to contribute to the direct current, making it smoother than a half-wave rectifier.
      This form of circuit saves on rectifier diodes compared to a diode bridge, but has poorer utilization of the
      transformer windings. Center-tapped two-diode rectifiers were a common feature of power supplies in vacuum
      tube equipment. Modern semiconductor diodes are low-cost and compact so usually a 4-diode bridge is used (up
      to a few hundred watts total output) which produces the same quality of DC as the center-tapped configuration
      with a more compact and cheaper power transformer. Center-tapped configurations may still be used in
      high-current applications, such as large automotive battery chargers, where the extra transformer cost is offset by
      less costly rectifiers.

                                    A full-wave rectifier using two diodes and a center tap transformer.

    • In an audio power amplifier center-tapped transformers are used to drive push-pull output stages. This allows two
      devices operating in Class B to combine their output to produce higher audio power with relatively low distortion.
      Design of such audio output transformers must tolerate a small amount of direct current that may pass through the
      Hundreds of millions of pocket-size transistor radios used this form of amplifier since the required transformers
      were very small and the design saved the extra cost and bulk of an output coupling capacitor that would be
      required for an output-transformerless design. However, since low-distortion high-power transformers are costly
      and heavy, most consumer audio products now use a transformerless output stage.
Center tap                                                                                                                    37

       The technique is nearly as old as electronic amplification and is well-documented, for example, in "The Radiotron
       Designer's Handbook, Third Edition" of 1940.
    • In analog telecommunications systems center-tapped transformers can be used to provide a DC path around an
      AC coupled amplifier for signalling purposes.
    • In electronic amplifiers, a center-tapped transformer is used as a phase splitter in coupling different stages of an
    • Power distribution, see 3 wire single phase.
    • A center-tapped rectifier is preferred to the full bridge rectifier when the output DC current is high and the output
      voltage is low.

    F. Langford Smith, The Radiotron Designer's Handbook Third Edition, (1940), The Wireless Press, Sydney,
    Australia, no ISBN, no Library of Congress card electronic circuits involving centre tapped transformers

    Compensation winding
    A compensation winding is an isolated coil wound into a transformer's primary to effectively create fractional
    numbers of turns.

    Copper loss
    Copper loss is the term often given to heat produced by electrical currents in the conductors of transformer
    windings, or other electrical devices. Copper losses are an undesirable transfer of energy, as are core losses, which
    result from induced currents in adjacent components. The term is applied regardless of whether the windings are
    made of copper or another conductor, such as aluminium. Hence the term winding loss is often preferred. A related
    term, load loss closely related but not identical, since an unloaded transformer will have some winding loss.
    Copper losses result from Joule heating and so are also referred to as "I squared R losses", in deference to Joule's
    First Law. This states that the energy lost each second, or power, increases as the square of the current through the
    windings and in proportion to the electrical resistance of the conductors.

    where I is the current flowing in the conductor and R the resistance of the conductor. With I in amperes and R in
    ohms, the calculated power loss is given in watts.
    With high-frequency currents, winding loss is affected by proximity effect and skin effect, and cannot be calculated
    as simply.
    For low-frequency applications, the power lost can be minimized by employing conductors with a large
    cross-sectional area, made from low-resistivity metals.
Copper loss                                                                                                               38

    External links
    • Reduction of copper losses [1]

    [1] http:/ / www. articleworld. org/ index. php/ Copper_loss

    Current transformer
    In electrical engineering, a current transformer (CT) is used for
    measurement of electric currents. Current transformers, together with voltage
    transformers (VT) (potential transformers (PT)), are known as
    instrument transformers. When current in a circuit is too high to directly
    apply to measuring instruments, a current transformer produces a reduced
    current accurately proportional to the current in the circuit, which can be
    conveniently connected to measuring and recording instruments. A current
    transformer also isolates the measuring instruments from what may be very
    high voltage in the monitored circuit. Current transformers are commonly
    used in metering and protective relays in the electrical power industry.

    Like any other transformer, a current transformer has a primary winding, a
    magnetic core, and a secondary winding. The alternating current flowing in
    the primary produces a magnetic field in the core, which then induces a
    current in the secondary winding circuit. A primary objective of current
    transformer design is to ensure that the primary and secondary circuits are
    efficiently coupled, so that the secondary current bears an accurate
    relationship to the primary current.

                                                                                    A CT for operation on a 110 kV grid

    The most common design of CT consists of a length of wire wrapped
    many times around a silicon steel ring passed over the circuit being
    measured. The CT's primary circuit therefore consists of a single 'turn'
    of conductor, with a secondary of many hundreds of turns. The primary
    winding may be a permanent part of the current transformer, with a
    heavy copper bar to carry current through the magnetic core.
    Window-type current transformers are also common, which can have
    circuit cables run through the middle of an opening in the core to
    provide a single-turn primary winding. When conductors passing
    through a CT are not centered in the circular (or oval) opening, slight
    inaccuracies may occur.
Current transformer                                                                                                                   39

    Shapes and sizes can vary depending on the
    end user or switchgear manufacturer.
    Typical examples of low voltage single ratio
    metering current transformers are either ring
    type or plastic moulded case. High-voltage
    current transformers are mounted on
    porcelain bushings to insulate them from
    ground. Some CT configurations slip around
    the bushing of a high-voltage transformer or
    circuit breaker, which automatically centers         Current transformers used in metering equipment for three-phase 400 ampere
                                                                                      electricity supply
    the conductor inside the CT window.
    The primary circuit is largely unaffected by
    the insertion of the CT. The rated secondary current is commonly standardized at 1 or 5 amperes. For example, a
    4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. The
    secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. The load, or
    burden, of the CT should be of low resistance. If the voltage time integral area is higher than the core's design rating,
    the core goes into saturation towards the end of each cycle, distorting the waveform and affecting accuracy.

    Current transformers are used extensively for measuring current and monitoring the operation of the power grid.
    Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building
    with three-phase service and single-phase services greater than 200 amp.
    The CT is typically described by its current ratio from primary to secondary. Often, multiple CTs are installed as a
    "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide
    isolation between metering and protection circuits, and allows current transformers with different characteristics
    (accuracy, overload performance) to be used for the different purposes.

    Safety precautions
    Care must be taken that the secondary of a current transformer is not disconnected from its load while current is
    flowing in the primary, as the transformer secondary will attempt to continue driving current across the effectively
    infinite impedance. This will produce a high voltage across the open secondary (into the range of several kilovolts in
    some cases), which may cause arcing. The high voltage produced will compromise operator and equipment safety
    and permanently affect the accuracy of the transformer.

    The accuracy of a CT is directly related to a number of factors including:
    •   Burden
    •   Burden class/saturation class
    •   Rating factor
    •   Load
    •   External electromagnetic fields
    •   Temperature and
    • Physical configuration.
    • The selected tap, for multi-ratio CTs
Current transformer                                                                                                          40

    For the IEC standard, accuracy classes for various types of measurement are set out in IEC 60044-1, Classes 0.1,
    0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class designation is an approximate measure of the CT's accuracy. The ratio
    (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5%
    or less. Errors in phase are also important especially in power measuring circuits, and each class has an allowable
    maximum phase error for a specified load impedance. Current transformers used for protective relaying also have
    accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays
    during system faults.

    The load, or burden, in a CT metering circuit is the (largely resistive) impedance presented to its secondary winding.
    Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA & 60 VA. As
    for ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with a burden
    rating of B-0.2 can tolerate up to 0.2 Ω of impedance in the metering circuit before its output current is no longer a
    fixed ratio to the primary current. Items that contribute to the burden of a current measurement circuit are
    switch-blocks, meters and intermediate conductors. The most common source of excess burden in a current
    measurement circuit is the conductor between the meter and the CT. Often, substation meters are located significant
    distances from the meter cabinets and the excessive length of small gauge conductor creates a large resistance. This
    problem can be solved by using CT with 1 ampere secondaries which will produce less voltage drop between a CT
    and its metering devices (used for remote measurement).

    Knee-point voltage
    The knee-point voltage of CT is the magnitude of the secondary voltage after which the output current ceases to
    follow the input current. This means that the one-to-one or proportional relationship between the input and output is
    lost and the output current increase abruptly even with small increment in the input, if the voltage across the
    secondary terminals exceeds the knee-point voltage. It is important to note that the knee-point voltage is not
    applicable for metering current transformers or even for potential transformers (both metering & protection), the
    concept of knee point voltage is pertinent to protection current transformers only.

    Rating factor
    Rating factor is a factor by which the nominal full load current of a CT can be multiplied to determine its absolute
    maximum measurable primary current. Conversely, the minimum primary current a CT can accurately measure is
    "light load," or 10% of the nominal current (there are, however, special CTs designed to measure accurately currents
    as small as 2% of the nominal current). The rating factor of a CT is largely dependent upon ambient temperature.
    Most CTs have rating factors for 35 degrees Celsius and 55 degrees Celsius. It is important to be mindful of ambient
    temperatures and resultant rating factors when CTs are installed inside pad-mounted transformers or poorly
    ventilated mechanical rooms. Recently, manufacturers have been moving towards lower nominal primary currents
    with greater rating factors. This is made possible by the development of more efficient ferrites and their
    corresponding hysteresis curves. This is a distinct advantage over previous CTs because it increases their range of
    accuracy, since the CTs are most accurate between their rated current and rating factor.

    Special designs
    Specially constructed wideband current transformers are also used (usually with an oscilloscope) to measure
    waveforms of high frequency or pulsed currents within pulsed power systems. One type of specially constructed
    wideband transformer provides a voltage output that is proportional to the measured current. Another type (called a
    Rogowski coil) requires an external integrator in order to provide a voltage output that is proportional to the
    measured current. Unlike CTs used for power circuitry, wideband CTs are rated in output volts per ampere of
Current transformer                                                                                                 41

    primary current.

    Depending on the ultimate clients requirement, there are two main standards to which current transformers are
    designed. IEC 60044-1 (BSEN 60044-1) & IEEE C57.13 (ANSI), although the Canadian & Australian standards are
    also recognised.

    • Guile, A.; Paterson, W. (1977). Electrical Power Systems, Volume One. Pergamon. p. 331. ISBN 0-08-021729-X.

    External links
    •     High Frequency (Wideband) Current Transformers [1]
    •     Introduction to Current Transformers [2]
    •     Introduction to Metering / Measurement Current Transformers [3]
    •     Introduction to Protection Current Transformers [4]
    •     Transformer Terminology [5]
    • Selecting the Appropriate CT for the Metering Installation [6]
    • What Does CT Burden Mean? [7]

    [1]   http:/ / www. ipec. co. uk/ asm/ solution/ pd_sensors. php?site=2& section=2& subnav=3
    [2]   http:/ / www. elkor. net/ pdfs/ AN0305-Current_Transformers. pdf
    [3]   http:/ / www. itl-uk. com/ introduction-metering-transformers. html
    [4]   http:/ / www. itl-uk. com/ introduction-protection-transformers. html
    [5]   http:/ / www. itl-uk. com/ instrument-transformer-characteristics. html
    [6]   http:/ / www. powermetrix. com/ downloads/ Selecting%20CTs. pdf
    [7]   http:/ / www. sentrancorp. com/ index. cfm?Page=WhatdoesCTburdenmean
Delta-wye transformer                                                                                                        42

    Delta-wye transformer
    A delta-wye (Δ-Y) transformer is a transformer that converts three-phase electric power without a neutral wire into
    3-phase power with a neutral wire. It can be a single three-phase transformer, or built from three independent
    single-phase units. The term Delta-Wye transformer is used in North America, and Delta-Star system in Europe.
    Delta-wye transformers are common in commercial, industrial,
    and high-density residential locations, to supply three-phase
    distribution systems.
    An example would be a distribution transformer with a delta
    primary, running on three 11kV phases with no neutral or earth
    required, and a star (or wye) secondary providing a 3-phase supply
    at 400 V, with the domestic voltage of 230 available between each
    phase and an earthed neutral point.

    Output voltages
    The line-to-neutral voltage is the voltage between any phase and
    neutral. The line-to-line voltage is the voltage between any two
    phases, and is equal to the phase to neutral voltage x   .                                       Delta Wye Transformer

                                                            Phase voltage   Line voltage (rounded)

                                                           120              208

                                                           127              220

                                                           220              380

                                                           230              400

                                                           240              415

                                                           277              480

                                                           347              600

    External links
    • Three-phase transformer circuits [1]

    [1] http:/ / www. allaboutcircuits. com/ vol_2/ chpt_10/ 6. html
Dissolved gas analysis                                                                                                         43

    Dissolved gas analysis
    Dissolved Gas Analysis or DGA is the study of dissolved gases in transformer oil.
    It is the most sensitive and reliable technique which gives an early indication of abnormal behavior of a
    transformer.DGA is advanced tool to diagnose the health of a transformer under Preventive Maintenance
    DGA consists of three steps. They are, sampling of transformer oil in an airtight glass tube, complete extraction of
    gases from the sample and subsequent analysis of the extracted gases for their quantity and combination.

    Transformer Oil
    Transformer oil is used as a coolant and insulator in a transformer. It baths every internal component and contains a
    lot of diagnostic information in the form of dissolved gases. Since these gases reveal the faults of a transformer, they
    are known as Fault Gases. They are formed in transformer oil, due to natural ageing and as a result of faults inside
    the transformer. Formation of fault gases is due to oxidation, vaporization, insulation decomposition, oil breakdown
    and electrolytic action.

    Oil Sample Tube
    Oil sample tube is used to draw, retain and transport the oil sample of transformer oil in the same condition as it is
    inside a transformer with all fault gases dissolved in it.
    It is a gas tight borosilicate glass tube of capacity 150 ml or 250 ml, having two airtight Teflon valves on both the
    ends. The outlets of these valves have been provided with a screw thread which helps in convenient connection of
    synthetic tubes while drawing sample from transformer. Also this provision is useful in transferring the oil into
    Sample oil burette of the Multiple Gas Extractor without any exposure to atmosphere, thereby retaining all its
    dissolved and evolved fault gases contents.
    It has got a septum arrangement on one side of the tube for drawing sample oil to test its moisture content.
    Thermo foam boxes are used to transport the above Oil Sample Tubes without any exposure to sunlight

    Extraction of Fault Gases using Multiple Gas Extractor
    Complete extraction of fault gases from transformer oil is achieved by Multiple Gas Extractor. This is a unique glass
    apparatus designed by Central Power Research Institute, Bangalore, India and developed by Dakshin Lab Agencies,
    Bangalore. In this apparatus, the same sample oil is exposed to high vacuum many times until there is no further
    increase in the volume of extracted fault gases. The entire extraction takes place at very high vacuum under ambient
    temperature, without any escape of fault gases in to atmosphere. A fixed volume of sample oil is directly drawn from
    sample tube into degassing vessel under high vacuum, where the gases are released. These gases are isolated using a
    mercury piston to measure its volume at atmospheric pressure (Total Gas Content) and subsequent transfer to Gas
    Chromatograph using gas tight syringe or auto sampler. The fault gases are measured, in milliliter of gases per
    milliliter of transformer oil and converted into parts per million. Moreover, in this method small traces of incipient
    fault gases are detected at very early stage.This method alone, provides the repeated accurate results for Total Gas
Dissolved gas analysis                                                                                                    44

    Fault Gases
    Atmospheric Gases: Hydrogen, Nitrogen and Oxygen Oxides of Carbon: Carbon Monoxide and Carbon dioxide
    Hydro Carbons: Acetylene, Ethylene, Methane and Ethane

    Quantitative determination and analysis of Fault gases
    The gases extracted from the sample oil are injected into Gas Chromatograph where the columns separate gases. The
    separated gases are detected by Thermal Conductivity Detector for atmospheric gases, by Flame Ionization Detector
    for hydro carbons and oxides of carbon. Methanator is used to detect oxides of carbon, when they are in very low

    Types of Faults

    Insulation Overheating
    When transformer is overloaded it generates more heat and deteriorates the cellulose insulation. In this case DGA
    results show high carbon monoxide and high carbon dioxide. In extreme cases methane and ethylene are at higher

    Insulation Liquid Overheating
    The overheating of insulation liquid results in breakdown of liquid by heat and formation of high thermal gases.
    They are methane, ethane and ethylene.

    It is a partial discharge and detected in a DGA by elevated hydrogen.

    Arcing is the most severe condition in a transformer and indicated even by low levels of acetylene.

    In a new transformer the levels of hydrocarbons in transformer oil after vacuum filtration shall be 5 ppm. After
    commissioning a new transformer DGA shall be done every month or earlier depending on the DGA results
    In a overhauled and repaired transformer, DGA is to be done a week after re-commission. Subsequently DGA is
    required every month or earlier depending the DGA results.

    In interpretation of the results obtained for a particular transformer, due regard should be given to the following
    factors before arriving at a specific conclusion:
    • Date of commissioning of the transformer
    • Loading cycle of the transformer
    • Date on which the oil was last filtered
Dissolved gas analysis                                                                                                            45

    External links

    Distribution transformer
    A distribution transformer is a transformer that provides the final
    voltage transformation in the electric power distribution system,
    stepping down the voltage used in the distribution lines to the level
    used by the customer. If mounted on a utility pole, they are called
    pole-mount transformers (or colloquially a pole pig). If the
    distribution lines are located underground, distribution transformers are
    mounted on concrete pads and locked in steel cases, thus known as
    pad-mount transformers. Because of weight restrictions transformers
    for pole mounting are only built for primary voltages under 30 kV.

    Distribution transformers[1] are classified into different categories
    based on certain factors such as
    • Type of insulation - liquid-immersed distribution transformers or
      dry-type distribution transformers
    • Number of Phases - single-phase distribution transformers or
                                                                                Single-phase distribution transformer in Canada
      three-phase distribution transformers
    • voltage class (for dry-type) – Low voltage distribution transformers
      or medium voltage distribution transformers
    • Basic impulse insulation level (BIL), for medium-voltage, dry-type.
    Liquid immersed distribution transformers can be further classified into:
    •   Padmount transformer
    •   Station transformer
    •   Substation transformer
    •   Grounding transformer

    Distribution transformers are normally located at a service drop, where wires run from a utility pole to a customer's
    premises. They are often used for the power supply of facilities outside settlements, such as isolated houses,
    farmyards or pumping stations at voltages below 30kV. Another application is the power supply of switch heatings
    from the overhead wire of railways electrified with AC. In this case single phase distribution transformers are used.
    In North American utility practice, these devices are very commonly used in areas with overhead primary
    distribution lines. Pad-mount transformers are used in urban areas and neighborhoods where the primary distribution
    lines run underground. Many large buildings have electric service provided at primary distribution voltage. These
    buildings have customer-owned transformers in the basement for step-down purposes.
    High voltage hobbyists often use these transformers in reverse (step-up) by feeding 120 or 240 volts into the
    secondary and drawing the resulting high voltage off the primary bushings, using it to power devices like Jacob's
    Ladders and Tesla coils, and many other high voltage experiments.
Distribution transformer                                                                                                          46

    Both pole-mount and pad-mount transformers convert the high 'primary' voltage of the overhead or underground
    distribution lines to the lower 'secondary' voltage of the distribution wires inside the building. The primaries use the
    three-phase system. Main distribution lines always have three wires, while smaller "laterals" (close to the customer)
    may include one or two phases, used to serve all customers with single-phase power. If three-phase service is
    desired, one must have a three-phase supply. Primaries are at one of a wide range of voltages from 4 to 33 kilovolts
    but most commonly about 7,200 or 14,400 volts.

    The high voltage primary windings are brought out to bushings on the top of
    the case.
    • Single phase transformers, generally used in the USA system, are attached
      to the overhead wires with two different types of connections:
       • If a primary neutral wire is available, a 'wye' or 'phase to neutral'
         transformer can be used. This usually has only one bushing on top,
         connected to one of the primary phases. The other end of the primary
         winding is 'grounded' to the transformer's case, which is connected to
         the neutral wire of the 3 phase system, and also earth ground. This type
         of distribution system, called 'grounded wye', is preferred because the
         transformers present unbalanced loads on the line, causing currents in
         the neutral wire. With the 'delta' connection, this can cause variations in
         the voltages on the 3 phase wires.
                                                                                        Three phase distribution transformer in
      • If no neutral wire is available, a 'delta' or 'phase to phase' transformer
         must be used. This has two bushings on top which are connected to two
         of the three primary wires, so the voltage across the primary winding is the phase-to-phase voltage. This type
         is used on long distribution lines where it is uneconomical to run a fourth neutral wire.
    • Transformers providing three-phase secondary power, which are used for residential service in the European
      system, have three secondary windings and are attached to all three primary phase wires. The windings are almost
      always connected in a 'wye' configuration, with the ends of the three windings connected together and grounded.
    The transformer is always connected to the primary distribution lines through protective fuses and disconnect
    switches. For pole-mounted transformers this usually takes the form of a 'fused cutout'. An electrical fault causes the
    fuse to melt, and the device drops open to give a visual indication of trouble. It can also be manually opened while
    the line is energized by lineworkers using insulated hot sticks.

    The low voltage secondary windings are attached to three or four terminals on the transformer's side.
    • In the USA and countries using its system, the secondary is most often the split-phase 240/120 volt system. The
      240 V secondary winding is center-tapped and the center neutral wire is grounded, making the two end
      conductors "hot" with respect to the center tap. These three wires run down the service drop to the electric meter
      and service panel inside the building. Connecting a load between either hot wire and the neutral gives 120 volts.
      Connecting between both hot wires gives 240 volts.
    • In Europe and countries using its system, the secondary is often the three phase 416Y/240 system. There are three
      240 V secondary windings, each receiving power from a primary winding attached to one of the primary phases.
      One end of the 3 secondary windings are connected together to a 'neutral' wire, which is grounded. The other end
      of the 3 secondary windings, along with the neutral, are brought down the service drop to the service panel. 240 V
Distribution transformer                                                                                                              47

       loads are connected between any of the three phase wires and the neutral.
    Higher secondary voltages, such as 480 volts, are sometimes required for commercial and industrial uses. Some
    industrial customers require three-phase power at secondary voltages. To provide this, three-phase transformers can
    be used. In the US, which uses mostly single phase transformers, three identical single phase transformers are often
    wired in a transformer bank in either a wye or delta connection, to create a three phase transformer.

    The transformers for these are made much the same way smaller
    transformers are made. Most use a "C" or "E" shaped core made from
    laminations of sheet steel stacked and either glued together with resin
    or banded together with steel straps. The low current, high voltage
    primaries are wound from enamel coated copper wire and the high
    current, low voltage secondaries are wound using a thick ribbon of
    aluminum or copper insulated with resin-impregnated paper. The entire
    assembly is baked to cure the resin then submerged in a large (usually
    gray) powder coated steel tank which is then filled with high purity
    mineral oil, which is inert and non-conductive. The mineral oil helps
    dissipate heat and protects the transformer from moisture, which will
    float on the surface of the oil. The tank is temporarily depressurized to
    remove any remaining moisture that would cause arcing and is sealed
    against the weather with a gasket at the top.

                                                                                   Oil-cooled three-phase distribution transformer,
    Reference                                                                      similar to one in above photo, with housing off,
    [1] Distribution transformers (http:/ / www. pacificcresttrans. com/                        showing construction.
        liquid-filled-distribution-transformers. html)
Enameled wire                                                                                                               48

    Enameled wire
    Enameled wire is wire (such as magnet wire) coated with a very thin insulating layer. It is used in applications such
    as winding electric motor coils, speakers and transformers. It is also used in the construction of electromagnets and
    The core material is copper or aluminum, coated with a thin layer of a polyurethane, polyamide, or polyester etc
    resin - the so-called "enamel". Aluminum is lighter than copper, but has higher resistivity.
    For ease of manufacturing inductive components like transformers and inductors, most new enameled wire has
    enamel that acts as a flux when burnt during soldering. This means that the electrical connections at the ends can be
    made without stripping off the insulation first. Older enameled copper wire is normally not like this, and requires
    sandpapering or scraping to remove the insulation before soldering.
    Enameled wires are classified by their diameter (AWG number or SWG) or area (square millimetres), temperature
    class and insulation class. Enameled wires are manufactured in both round and rectangular shapes. Rectangular wire
    is used in larger windings to make the most efficient use of available winding space. Also aluminium wire is coated
    with enamel
    Breakdown voltage depends on the thickness of the covering, which can be of 3 types: Grade 1, Grade 2 and Grade
    3. Higher grades have thicker insulation and thus higher breakdown voltages.
    The temperature class indicates the temperature of the wire where it has a 20,000 hour service life. At lower
    temperatures the service life of the wire is longer (about a factor 2 for every 10 °C lower temperature). Common
    temperature classes are 120, 155 and 180 °C.

Energy efficient transformer                                                                                                                         49

    Energy efficient transformer
    In a typical power distribution grid, electric transformer power loss typically contributes about 40-50% of the total
    transmission & distribution loss. Energy efficient transformers are therefore an important means to reduce T&D
    loss[1] . With the improvement of electrical steel (silicon steel) properties, the losses of a transformer in 2010 can be
    half that of a similar transformer in the 1970s. With new magnetic materials, it is possible to achieve even higher
    efficiency. The amorphous metal transformer is a modern example[2] .

    Notes and references
    [1] B. Kennedy, “Energy Efficient Transformers” McGraw-Hill, 1998.
    [2] J. Li, “Use of Energy Efficient Transformers in Asia”, presented in Asian Energy Conference 2000, Hong Kong (http:/ / jerryli. 110mb. com/
        Asian_Energy_2000. pdf)

    • World's largest Amorphous Metal Power Transformer: 99.31 % Efficiency (
    • Amorphous Metals in Electric-Power Distribution Applications (

    Flyback transformer
    A flyback transformer (FBT), also called a line output
    transformer (LOPT), is a special transformer which is used
    to generate high voltage (HV) signals at a relatively high
    frequency. It was invented as a means to control the horizontal
    movement of the electron beam in a cathode ray tube (CRT).
    As with all step-up transformers, it receives low voltages and
    transforms them into high voltages; in this case, it does so at a
    relatively high frequency--much faster than the vertical
    movement of the electron beam (known as the vertical scan
                                                                                                     An old style flyback transformer.

    The flyback transformer is used in the operation of
    CRT-display devices such as television sets and CRT computer monitors, and in other HV devices such as the DIY
    plasma lamp. The voltage and frequency can each range over a wide scale depending on the device. For example, a
    large color TV CRT may require 20 to 50 kV with a horizontal scan rate of 15.734 kHz for NTSC devices. Unlike a
    power (or 'mains') transformer which uses an alternating current of 50 or 60 Hertz, a flyback transformer typically
    operates with switched currents at much higher frequencies in the range of 15 kHz to 50 kHz.
Flyback transformer                                                                                                            50

    How it works
    Unlike mains transformers and audio transformers, a LOPT is designed not just to transfer energy, but also to store it
    for a significant fraction of the switching period. This is achieved by winding the coils on a ferrite core with an air
    gap. The air gap increases the reluctance of the magnetic circuit and therefore its capacity to store energy.
    The primary winding of the LOPT is driven by a relatively low voltage sawtooth wave, which is ramped up (and
    sweeping the beam across the screen to draw a line) and then abruptly switched off (and causing the beam to quickly
    fly back from the right to the left of the display) by the horizontal output stage. This is a ramped and pulsed
    waveform that repeats at the horizontal (line) frequency of the display. The flyback (vertical portion of the sawtooth
    wave) is extremely useful to the flyback transformer: the faster a magnetic field collapses, the greater the induced
    voltage. Furthermore, the high frequency used permits the use of a much smaller transformer. In television sets, this
    high frequency is about 15 kilohertz (15.734 kHz for NTSC), and vibrations from the related circuitry can often be
    heard as a high-pitched whine. In modern computer displays the frequency can vary over a wide range, from about
    30 kHz to 150 kHz.
    The alternating current coming from the flyback transformer is converted to direct current by a high-voltage rectifier.
    If the output voltage of the LOPT is not high enough by itself, the rectifier is replaced by a voltage multiplier. Early
    color television sets (like the 1954 RCA CT-100) also used a regulator to control the high voltage. The rectified
    voltage is then used to supply the anode of the cathode ray tube.
    There are often auxiliary secondary windings that produce lower voltages for driving other parts of the display's
    circuitry — often the CRT's filament will be driven from the flyback. In tube sets, a two-turn filament winding is
    located on the opposite side of the core as the HV secondary, used to drive the rectifier tube's heater.

    Practical considerations
    In modern displays, the LOPT, voltage multiplier and rectifier are often integrated into a single package on the main
    circuit board. There is usually a thickly insulated wire from the LOPT to the anode terminal (covered by a rubber
    cap) on the side of the picture tube.
    One advantage of operating the transformer at the flyback frequency is that it can be much smaller and lighter than a
    comparable transformer operating at mains (line) frequency. Another advantage is that it provides a failsafe
    mechanism — should the horizontal deflection circuitry fail, the flyback transformer will cease operating and shut
    down the rest of the display, preventing the screen burn that would otherwise result from a stationary electron beam.

    The primary is wound first around a ferrite rod, and then the secondary is wound around the primary. This
    arrangement minimizes the leakage inductance of the primary. Finally, a ferrite frame is wrapped around the
    primary/secondary assembly, closing the magnetic field lines. Between the rod and the frame is an air gap, which
    reduces the remanence. The secondary is wound layer by layer with enameled wire, and Mylar film between the
    layers. In this way parts of the wire with higher voltage between them have more dielectric material between them.
    The outside of the winding sustains the highest voltage so insulation and screening will be needed to protect the
    surrounding components. In a variant, to avoid some stray capacitance, every layer of the windings is connected by a
    rectifying diode to the next layer. Windings go up the rod and the diodes go down. In this way the AC voltage
    increases along the rod (axial) and the DC voltage increases radial from inside to outside. When applied to tape
    wound coils this would mean each coil goes from inside to outside and the diode goes back to the inside.
Flyback transformer                                                                                                            51

    Flyback transformers are a frequent source of failure in CRT displays. Often, the CRT itself is blamed when the
    display has actually experienced a flyback transformer failure. The high voltage present in the many turns of wire,
    with the thin insulation required for the transformer to be of reasonable size, can result in leakage between the
    windings. As the leakage heats the insulation it carbonizes, increases conduction; in turn heat and carbonization
    continues a downward spiral until the leaked current is high enough for the high voltage to arc between the windings,
    and destroy the transformer (and sometimes other components in the display). As a result, replacement flyback
    transformers for almost every set on the market are available through dealers in electronic parts, typically for under
    $50. The problem is exacerbated by the tendency of the flyback to accumulate a coating of dust due to electrostatic
    attraction, which serves as a path to ground for leaks which might otherwise not be of sufficient magnitude to initiate
    the chain of events leading to destructive failure, as described.
    As a result, occasional cleaning of the accumulated dust from the high voltage circuitry inside a television can be
    beneficial if proper precautions are taken -- however the small amount of additional life that is gained for the flyback
    transformer rarely justifies the time and effort necessary. It is debatable among technicians if displays installed in
    dirty, dusty locations experience more failures than those in cleaner locations, but many do say that dirty conditions
    contribute to malfunctions.
    A flyback transformer and its associated circuitry operate at very high voltages at low currents (<1mA-15mA), far
    beyond mains voltage. While most flybacks do not supply enough power to kill directly, the voltage they employ can
    cause violent muscle spasms if touched; and such spasms usually cause injury. A common injury that occurs when
    one is shocked is actually to be injured not as much by the shock itself, but when the victim's hand or arm is thrown
    back against other internal components in the display device. Therefore, only trained persons should touch or modify
    these devices, after first ensuring that the transformer is switched off and any stored energy has been safely
    discharged. The CRT attached to the flyback has an inherent capacitance which can hold a high voltage charge for up
    to a week or more after the power is switched off. Often, a high-resistance bleeder resistor is connected internally
    within the flyback transformer to ensure the charge is safely grounded when not in use, but many sets lack this,
    especially older models.
    In many recent televisions, after replacing the flyback transformer, the control firmware must be recalibrated to
    account for slight differences in performance between transformers in order to maintain accurate picture
    reproduction. In older televisions and monitors, these needed adjustments were performed by turning potentiometers
    inside, or on the back of the set (sometimes called "tweaking" by those in the electronics trade) to achieve optimal
    picture quality. Also, when flyback transformers fail, they frequently will also take out the horizontal output
    transistor that drives the flyback transformer, and sometimes even blow fuses in the low voltage power supply
    Unless the owner of the display device is savvy enough to repair it themselves, the failure of a flyback transformer
    frequently condemns the device as unrepairable, because the cost of repair can be higher than the replacement cost.
    Although the cost of the flyback transformer, and other damaged parts is relatively inexpensive, the labor time
    needed to disassemble, replace the parts, and then re-adjust the display can make the repair job expensive.
Flyback transformer                                                                                                          52

    • Dixon, Lloyd H, Magnetics Design Handbook, Section 1, Introduction and Basic Magnetics, Texas Instruments,
      2001 [1]
    • Dixon, Lloyd H, Magnetics Design Handbook, Section 5, Inductor and Flyback Transformer Design, Texas
      Instruments, 2001 [2]

    • U.S. Patent 3665288 [3] - "Television sweep transformer" - Theodore J. Godawski

    [1] http:/ / focus. ti. com/ lit/ ml/ slup123/ slup123. pdf
    [2] http:/ / www-s. ti. com/ sc/ techlit/ slup127. pdf
    [3] http:/ / www. google. com/ patents?vid=3665288

    Growler (electrical device)
    A growler is an electrical device used for testing insulation of a motor for
    shorted coils. A growler consists of a coil of wire wrapped around an iron
    core and connected to a source of AC current. When placed on the stator core
    of a motor the growler acts as the primary of a transformer and the stator coils
    act as the secondary. A "feeler", a thin strip of steel (hacksaw blade) can be
    used as the short detector.

    The alternating magnetic flux set up by the growler passes through the
    windings of the armature coil, generating an alternating voltage in the coil. A
    short in the coil creates a closed circuit that will act like the secondary coil of
    a transformer, with the growler acting like the primary coil. This will induce
    an alternating current in the shorted armature that will in turn cause an
    alternating magnetic field to encircle shorted armature coil. A flat, broad,
    flexible piece of metal containing iron is used to detect the magnetic field
    generated by a shorted armature. A hacksaw blade is commonly used as a
    feeler. The alternating magnetic field induced by a shorted armature is strong
    at the surface of the armature, and when the feeler is lightly touched to the
    iron core of an armature winding, small currents are induced in the feeler that              Silver Beauty Growler
    generate a third alternating magnetic field surrounding the feeler.

    With the growler energized, the feeler is moved from slot to slot. When the feeler is moved over a slot containing the
    shorted coil, the alternating magnetic field will alternately attract and release the feeler, causing it to vibrate in
    synchronism with the alternating current. A strong vibration of the feeler accompanied by a growling noise indicated
    that the coil is shorted.
    Along with the standard application the growler can be used to:
    • test series and interpoles (commutating) fields from a DC motor
    • to determine phasing and polarity in multiwinding armatures
    • to test rotors in rotating frequency changers, as well as in wound rotors
    • to test shorts between turns in taped coils before installation into an armature or a stator
    • as a low voltage isolation transformer
Growler (electrical device)                                                                                                        53

     • as a high voltage auto-transformer bucking or boosting for numerous tests on various types of equipment
     • for preheating or baking armatures and rotors.
     It is considered by many to be one of the most versatile tools for electric motor service.

     • Hubert, Charles I., "Operating, Testing, and Preventive Maintenance of Electrical Power Apparatus", Pearson
       Education Inc., Upper Saddle River, NJ. 2003.
     • Samuel Heller "THE GROWLER Design and Application" ISBN 0911740066

     Hybrid coil
     A hybrid coil (or bridge transformer, or sometimes hybrid) is a transformer that
     has three windings, and which is designed to be configured as a circuit having four
     branches, (i.e. ports) that are conjugate in pairs.
     A signal arriving on one branch is divided between the two adjacent branches but
     does not appear at the opposite branch. In the schematic diagram, the signal into W
     splits between X and Z, and no signal passes to Y. Similarly, signals into X split to
     W and Y with none to Z, etc.
     Correct operation requires matched characteristic impedance at all four ports.               W and Y, X and Z are conjugate

                                             The primary use of a voiceband hybrid coil is to convert between 2-wire and
                                             4-wire operation in sequential sections of a communications circuit, for
                                             example in a four-wire terminating set. Such conversion was necessary when
                                             repeaters were introduced in a 2-wire circuit, a frequent practice at early 20th
                                             century telephony. Without hybrids, the output of one amplifier feeds directly
                                             into the input of the other, resulting in a howling situation (upper diagram).
                                             By using hybrids, the outputs and inputs are isolated, resulting in correct
                                             2-wire repeater operation. Late in the century, this practice became rare but
                                             hybrids continued in use in line cards.

                                             Hybrid coil circuit diagrams
          Using hybrids for bi directional
                                             Hybrids are realized using transformers. Two versions of transformer hybrids
                                             were used, the single transformer version providing unbalanced outputs with
                                             one end grounded, and the double transformer version providing balanced
Hybrid coil                                                                                                                       54

        Wiring diagram of a double transformer hybrid

    Single transformer hybrid

                                                        For use in 2-wire repeaters, the single transformer version suffices,
                                                        since amplifiers in the repeaters have grounded inputs and outputs. X,
                                                        Y, and Z share a common ground. As shown at left, signal into W, the
                                                        2-wire port, will appear at X and Z. But since Y is bridged from center
                                                        of coil to center of X and Z, no signal appears. Signal into X will
                                                        appear at W and Y. But signal at Z is the difference of what appears at
                                                        Y and, through the transformer coil, at W, which is zero. Similar
                                                        reasoning proves both pairs, W & Y, X & Z, are conjugates.

       Wiring diagram of a single transformer hybrid    Double transformer hybrid

                                                  When both the 2-wire and the 4-wire circuits must be balanced, double
    transformer hybrids are used, as shown at right. Signal into port W splits between X and Z, but due to reversed
    connection to the windings, cancel at port Y. Signal into port X goes to W and Y. But due to reversed connection to
    ports W and Y, Z gets no signal. Thus the pairs, W & Y, X & Z, are conjugates.

    Hybrids are used in telephones (see telephone hybrid) to reduce the sidetone, or volume of microphone output that
    was fed back to the earpiece. Without this, the phone user's own voice would be louder in the earpiece than the other
    party's. Such hybrids also had their windings so arranged as to act as an impedance matching transformer, matching
    the low-impedance carbon button transmitter to the higher impedance parts of the system. Today, the transformer
    version of the hybrid has been replaced by resistor networks and compact IC versions, which uses integrated circuit
    electronics to do the job of the hybrid coil.
    Radio-frequency hybrids are used to split radio signals, including television. The splitter divides the antenna signal
    to feed multiple receivers.
Hybrid coil                                                                                                                55

    External links
    • Modelling hybrids as 2 x 2 matrices [1]

        This article incorporates public domain material from websites or documents of the General Services
    Administration (in support of MIL-STD-188).

    [1] http:/ / www. dougrice. plus. com/ dougnapTheory/ index. htm

    Induction coil
    An induction coil or "spark coil" (archaically known as a Ruhmkorff coil after Heinrich Ruhmkorff) is a type of
    disruptive discharge coil. It is a type of electrical transformer used to produce high-voltage pulses from a
    low-voltage direct current (DC) supply. To create the flux changes necessary to induce voltage in the secondary, the
    direct current in the primary is repeatedly interrupted by a vibrating mechanical contact called an interrupter.
    Developed beginning in 1836 by Nicholas Callan and others, the induction coil was the first type of transformer.
    The term 'induction coil' is also used for a coil carrying high-frequency alternating current (AC), producing eddy
    currents to heat objects placed in the interior of the coil, in induction heating or zone melting equipment.

              Antique induction coil used in schools, Bremerhaven, Germany
Induction coil                                                                                                                            56

     How it works
     An induction coil consists of two coils of insulated
     copper wire wound around a common iron core.
     One coil, called the primary winding, is made from
     relatively few (tens or hundreds) turns of coarse
     wire. The other coil, the secondary winding,
     typically consists of many (thousands) turns of fine
     wire. An electric current is passed through the
     primary, creating a magnetic field. Because of the
     common core, most of the primary's magnetic field
     couples with the secondary winding. The primary
     behaves as an inductor, storing energy in the
     associated magnetic field. When the primary
     current is suddenly interrupted, the magnetic field
     rapidly collapses. This causes a high voltage pulse
     to be developed across the secondary terminals
     through electromagnetic induction. Because of the               Induction coil showing construction, from 1920.

     large number of turns in the secondary coil, the
     secondary voltage pulse is typically many thousands of volts. This voltage is often sufficient to cause an electric
     spark, to jump across an air gap separating the secondary's output terminals. For this reason, induction coils were
     called spark coils.

     The size of induction coils was usually specified by the length of spark it could produce; an '8 inch' (20 cm)
     induction coil was one that could produce an 8 inch arc.

     The interrupter
                                                             To operate the coil continuously, the DC supply current must be
                                                             broken repeatedly to create the magnetic field changes needed for
                                                             induction. Induction coils use a magnetically activated vibrating arm
                                                             called an interrupter or break to rapidly connect and break the current
                                                             flowing into the primary coil. The interrupters on small coils were
                                                             mounted on the end of the coil next to the iron core. The magnetic field
                                                             created by the current flowing in the primary attracts the interrupter's
                                                             iron armature attached to a spring, breaking a pair of contacts in the
                                                             primary circuit. When the magnetic field then collapses, the spring
       Waveforms in the induction coil, demonstrating
      how the interrupter works. The blue trace, i1 is the
                                                             closes the contacts again, and the cycle repeats.
      current in the coil's primary winding. It is broken
                                                             Opposite potentials are induced in the secondary when the interrupter
          periodically by the vibrating contact of the
          interrupter. The changes in current create a       'breaks' the circuit and 'closes' the circuit. However, the current change
      changing magnetic flux in the coil which induces       in the primary is much more abrupt when the interrupter 'breaks'. When
       a high voltage in the secondary coil v2 shown in      the contacts close, the current builds up slowly in the primary because
       red. Both the "make" and "break" of the current
                                                             the supply voltage has a limited ability to force current through the
      induce pulses of voltage in the secondary, but the
       current change is much more abrupt on "break",        coil's inductance. In contrast, when the interrupter contacts open, the
       and this generates the high voltage produced by       current falls to zero suddenly. So the pulse of voltage induced in the
                             the coil.                       secondary at 'break' is much larger than the pulse induced at 'close', it
Induction coil                                                                                                                         57

     is the 'break' that generates the coil's high voltage output. A "snubber" capacitor is used across the contacts to quench
     the arc on the 'break', which causes much faster switching and higher voltages. So the output waveform of an
     induction coil is a series of alternating positive and negative pulses, but with one polarity much larger than the other.

     Mercury and electrolytic interrupters
     The small 'hammer' interrupters described above were used on coils creating up to 8 inch (~120 kV) sparks. Larger
     coils used motor-driven interrupters.[1] The largest coils, used in radio transmitters, used either electrolytic or
     mercury turbine 'breaks'.

     Construction details
     To prevent the high voltages generated in the coil from breaking down the thin insulation and arcing between the
     secondary wires, the secondary coil uses special construction so as to avoid having wires carrying large voltage
     differences lying next to each other. The secondary coil is wound in many thin flat pancake-shaped sections (called
     "pies"), connected in series. The primary coil is first wound on the iron core, and insulated from the secondary with a
     thick paper or rubber coating. Then each secondary subcoil is coated with an insulating layer like paraffin, connected
     to the coil next to it, and slid onto the iron core, insulated from adjoining coils with paper disks. The voltage
     developed in each subcoil isn't large enough to jump between the wires in the subcoil. Large voltages are only
     developed across many subcoils in series, which are too widely separated to arc over.
     To prevent eddy currents, which flow perpendicular to the magnetic axis, and cause energy losses, the iron core is
     made of a bundle of parallel iron wires, individually coated with shellac to insulate them electrically.

     Michael Faraday discovered the principle of induction,
     Faraday's induction law, in 1831 and did the first
     experiments with induction between coils of wire.[2]
     The induction coil was invented by the Irish scientist
     and Catholic priest Nicholas Callan in 1836 at the St.
     Patrick's College, Maynooth[3] [4] and improved by
     William Sturgeon and Charles Grafton Page. The early
     coils had hand cranked interrupters, invented by Callan
     and Antoine Masson. The automatic 'hammer'
                                                                  Callan's largest induction coil (Model of 1863), showing 'pancake'
     interrupter was invented by C. E. Neef, P. Wagner, and
                                                                  secondary construction. It was 42 inches (106 cm) long and could
     J. W. M'Gauley. Hippolyte Fizeau introduced the use of        produce 15 inch (38 cm) sparks, corresponding to a potential of
     the quenching capacitor.[5] Heinrich Ruhmkorff                                   approximately 200,000 volts.
     generated higher voltages by greatly increasing the
     length of the secondary, in some coils using 5 or 6 miles (10 km) of wire. In the early 1850s, after examining an
     example of a Ruhmkorff coil, which produced a small spark of around 2 inches (50 mm) when energized, American
     inventor Edward Samuel Ritchie perceived that it could be made more efficient and produce a stronger spark by
     redesigning and improving its secondary insulation. His own design divided the coil into sections, each properly
     insulated from each other. Ritchie's induction coil proved superior to other designs of the day, initially producing a
     spark of 10 inches (25 cm) in length; later versions could produce an electrical bolt 24 inches (60 cm) or longer in
     length.[6] [7] The full story of Page's invention of the induction coil in its modern guise is told in Robert Post,
     "Physics, Patents, and Politics: A Biography of Charles Grafton Page" (Science History Publications, 1976. In 1857,
     one of Ritchie's induction coils was exhibited in Dublin, Ireland at a conference of the British Association,[8] and
Induction coil                                                                                                                                          58

     later at the University of Edinburgh in Scotland.[9] Ruhmkorff himself purchased a Ritchie induction coil, utilizing
     its improvements in his own work.[10] [11]
     Induction coils were used to provide high voltage for early gas discharge and Crookes tubes and other high voltage
     research. They were also used to provide entertainment (lighting Geissler tubes, for example) and to drive small
     "shocking coils", Tesla coils and violet ray devices used in quack medicine. They were used by Hertz to demonstrate
     the existence of electromagnetic waves, as predicted by James Maxwell and by Lodge and Marconi in the first
     research into radio waves. Their largest industrial use was probably in early wireless telegraphy spark-gap radio
     transmitters and to power early cold cathode x-ray tubes. By about 1920 they were supplanted in both these
     applications by vacuum tubes. However their largest use was as the ignition coil or spark coil in the ignition system
     of internal combustion engines, where they are still used, although the interrupter contacts are now replaced by solid
     state switches. A smaller version is used to trigger the flash tubes used in cameras and strobe lights.

                                                              Wireless charging
                                                              Toyota's heavy duty division, Hino Motors, is testing a new kind of
                                                              hybrid electric vehicle without a plug (hybrid outboard chargeable
                                                              vehicle). The energy in the batteries doesn't come from a plug and a
                                                              charging point, but it comes from a wireless charging system built into
                                                              the road. A series of induction coils built into the road resonate energy
                                                              at certain frequency, like radio waves. The bus is able to capture those
                                                              waves and store the energy in its batteries.[12]

                                                              Early patents
         Automobile ignition coil, the largest remaining
                    use for induction coils                   • U.S. Patent 52054 [13] The induction-coil, instead of being made
                                                                movable upon the magnet
     •   U.S. Patent 72616 [14] This compound coil is made like any ordinary induction-coil
     •   U.S. Patent 74905 [15] The inner end of the induction-coil are surrounded by the prime coil
     •   U.S. Patent 76654 [16] The induction-coil consists of a metallic conductor, copper is generally preferred
     •   U.S. Patent 78495 [17] Energizing the primary wire of the induction-coil, the iron core becomes magnetized
     •   U.S. Patent 90626 [18] Making use of an induction-coil
     •   U.S. Patent 734197 [19] a split-coil improvement (1903).
     •   U.S. Patent 1092417 [20] Induction coil comprising a soft iron core (Mar 5, 1913)

     [1] Collins, Archie F. (1908). The Design and Construction of Induction Coils (http:/ / books. google. com/ ?id=dJNPAAAAMAAJ& pg=PA98).
         New York: Munn & Co.. . p.98
     [2] Faraday, Michael (1834). "Experimental researches on electricity, 7th series". Phil. Trans. R. Soc. (London) 124: 77–122.
     [3] Fleming, John Ambrose (1896). The Alternate Current Transformer in Theory and Practice, Vol.2 (http:/ / books. google. com/
         ?id=17sKAAAAIAAJ& pg=PA16). The Electrician Publishing Co.. . p.16-18
     [4] http:/ / www. nuim. ie/ museum/ ncallan. html
     [5] Severns, Rudy. "History of soft switching, Part 2" (http:/ / www. switchingpowermagazine. com/ downloads/ Oct 01 soft. pdf). Design
         Resource Center. Switching Power Magazine. . Retrieved 2008-05-16.
     [6] American Academy of Arts and Sciences, Proceedings of the American Academy of Arts and Sciences, Vol. XXIII, May 1895 - May 1896,
         Boston: University Press, John Wilson and Son (1896), pp. 359-360
     [7] Page, Charles G., History of Induction: The American Claim to the Induction Coil and Its Electrostatic Developments, Boston: Harvard
         University, Intelligencer Printing house (1867), pp. 104-106
     [8] Rogers, W. B. (Prof.), Brief Account of the Construction and Effects of a very Powerful Induction Apparatus, devised by Mr. E.S. Ritchie, of
         Boston, United States, British Association for the Advancement of Science, Report of the Annual Meeting (1858), p. 15
Induction coil                                                                                                        59

     [9] American Academy, pp. 359-360
     [10] American Academy, pp. 359-360
     [11] Page, pp. 104-106
     [12] http:/ / www. ecogeek. org/ content/ view/ 1431/
     [13] http:/ / www. google. com/ patents?vid=52054
     [14] http:/ / www. google. com/ patents?vid=72616
     [15] http:/ / www. google. com/ patents?vid=74905
     [16] http:/ / www. google. com/ patents?vid=76654
     [17] http:/ / www. google. com/ patents?vid=78495
     [18] http:/ / www. google. com/ patents?vid=90626
     [19] http:/ / www. google. com/ patents?vid=734197
     [20] http:/ / www. google. com/ patents?vid=1092417

     Further reading
     • Norrie, H. S., "Induction Coils: How to Make, Use, and Repair Them". Norman H. Schneider, 1907, New York.
       4th edition.
     • Collins, Archie F. (1908). The Design and Construction of Induction Coils (
       ?id=dJNPAAAAMAAJ&pg=PA98). New York: Munn & Co..
     • Fleming, John Ambrose (1896). The Alternate Current Transformer in Theory and Practice, Vol.2 (http://books. The Electrician Publishing Co.. Has detailed history of
       invention of induction coil

     External links
     • Battery powered Driver circuit for Induction Coils (
     • The Cathode Ray Tube site (
Iron loss                                                                                                                                 60

     Iron loss
     A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields
     in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors,
     inductors and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such
     as ferrites. The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated
     in the core material. The magnetic field is often created by a coil of wire around the core that carries a current. The
     presence of the core can increase the magnetic field of a coil by a factor of several thousand over what it would be
     without the core.
     The use of a magnetic core can enormously concentrate the strength and increase the effect of magnetic fields
     produced by electric currents and permanent magnets. The properties of a device will depend crucially on the
     following factors:
     •   the geometry of the magnetic core.
     •   the amount of air gap in the magnetic circuit.
     •   the properties of the core material (especially permeability and hysteresis).
     •   the operating temperature of the core.
     • whether the core is laminated to reduce eddy currents.
     In many applications it is undesirable that the core itself retain magnetization and become magnetized by the external
     field. This property, called hysteresis can cause energy losses in applications such as transformers. Therefore 'soft'
     magnetic materials with low hysteresis, such as silicon steel, rather than the 'hard' magnetic materials used for
     magnets, are usually used in cores.

     Commonly used structures

     Air core
     A coil not containing a magnetic core is called an air core coil. This includes coils wound on a plastic or ceramic
     form in addition to those made of stiff wire that are self-supporting and have air inside them. Air core coils have
     lower inductance than similarly sized ferromagnetic core coils, but are used in radio frequency circuits to prevent
     energy losses called core losses that occur in magnetic cores. The absence of core losses permits a higher Q factor, so
     air core coils are used in high frequency resonant circuits.

     Straight cylindrical rod
     Most commonly made of ferrite or a similar material, and used in
     radios especially for tuning an inductor. The rod sits in the middle of
     the coil and small adjustments of the rod's position will fine tune the
     inductance. Often the rod is threaded to allow adjustment with a
     screwdriver. In radio circuits, a blob of wax or resin is used once the
     inductor has been tuned to prevent the core from moving.

     The presence of the high permeability core increases the inductance
     but the field must still spread into the air at the ends of the rod. The
     path through the air ensures that the inductor remains linear. In this            On the left, a non-adjustable ferrite rod with
     type of inductor radiation occurs at the end of the rod and                     connection wires glued to the ends. On the right,
     electromagnetic interference may be a problem in some circumstances.           a molded ferrite rod with holes, with a single wire
                                                                                               threaded through the holes.
Iron loss                                                                                                                                      61

     Single "I" core
     Like a cylindrical rod but square, rarely used on its own. This type of core is most likely to be found in car ignition

     "C" or "U" core
     U and C-shaped cores are used with I or another C or U' core to make a square closed core, the simplest closed core
     shape. Windings may be put on one or both legs of the core.

                    a U-shaped core, with sharp corners                                 the C-shaped core, with rounded corners

     "E" core
     E-shaped core are more symmetric solutions to form a closed magnetic system. Most of the time, the electric circuit
     is wound around the center leg, whose section area is twice that of each individual outer leg.

            Classical E core               The EFD' core allows for        The ER core has a cylindrical     the EP core is halfways between
                                          construction of inductors or             central leg.                     a E and a pot core
                                       transformers with a lower profile
Iron loss                                                                                                                             62

     "E" and "I" core
     Sheets of suitable iron stamped out in shapes like the (sans-serif) letters "E" and "I", are stacked with the "I" against
     the open end of the "E" to form a 3-legged structure. Coils can be wound around any leg, but usually the center leg is
     used. This type of core is much used for power transformers, autotransformers, and inductors.

                                                                          Pair of "E" cores

                                                                          Again used for iron cores. Similar to using an "E" and
                                                                          "I" together, a pair of "E" cores will accommodate a
                                                                          larger coil former and can produce a larger inductor or
                                                                          transformer. If an air gap is required, the centre leg of
                                                                          the "E" is shortened so that the air gap sits in the
                                                                          middle of the coil to minimise fringing and reduce
                                                                          electromagnetic interference.

         Construction of an inductor using two ER cores, a
      plastic bobbin and two clips. The bobbin has pins to be
                 soldered to a printed circuit board.

            Exploded view of the previous figure showing the structure

                                                    Pot core
                                                    Usually ferrite or similar. This is used for inductors and transformers. The
                                                    shape of a pot core is round with an internal hollow that almost completely
                                                    encloses the coil. Usually a pot core is made in two halves which fit together
                                                    around a coil former (bobbin). This design of core has a shielding effect,
                                                    preventing radiation and reducing electromagnetic interference.

                a pot core of 'RM' type
Iron loss                                                                                                                         63

     Toroidal core
     This design is based on a toroid (the same shape as a doughnut). The coil is
     wound through the hole in the torus and around the outside. An ideal coil is
     distributed evenly all around the circumference of the torus. The symmetry of
     this geometry creates a magnetic field of circular loops inside the core, and
     the lack of sharp bends will constrain virtually all of the field to the core
     material. This not only makes a highly efficient transformer, but also reduces
     the electromagnetic interference radiated by the coil.                                            A toroidal core

     It is popular for applications where the desirable features are: high specific power per mass and volume, low mains
     hum, and minimal electromagnetic interference. One such application is the power supply for a hi-fi audio amplifier.
     The main drawback that limits their use for general purpose applications, is the inherent difficulty of winding wire
     through the center of a torus.
     Unlike a split core (a core made of two elements, like a pair of E cores), specialized machinery is required for
     automated winding of a toroidal core. Toroids have less audible noise, such as mains hum, because the magnetic
     forces do not exert bending moment on the core. The core is only in compression or tension, and the circular shape is
     more stable mechanically.

     Ring or bead
     The ring is essentially identical in shape and performance to the toroid,
     except that inductors commonly pass only through the center of the
     core, without wrapping around the core multiple times.
     The ring core may also be composed of two separate C-shaped
     hemispheres secured together within a plastic shell, permitting it to be
     placed on finished cables with large connectors already installed, that
     would prevent threading the cable through the small inner diameter of
     a solid ring.
                                                                                       A ferrite ring on a computer data cable.

                                            Planar core
                                            A planar core consists of two flat pieces of magnetic material, one above and
                                            one below the coil. It is typically used with a flat coil that is part of a printed
                                            circuit board. This design is excellent for mass production and allows a high
                                            power, small volume transformer to be constructed for low cost. It is not as
                                            ideal as either a pot core or toroidal core but costs less to produce.

                A planar 'E' core
Iron loss                                                                                                                                 64

                                                                             Core loss
                                                                             In a transformer or inductor, some of the power that
                                                                             would ideally be transferred through the device is lost
                                                                             in the core, resulting in heat and sometimes noise.
                                                                             There are various reasons for such losses, the primary
                                                                             ones being:

                                                                             Hysteresis loss
                                                                             When the magnetic field through the core changes, the
                                                                             magnetization of the core material changes by
                                                                             expansion and contraction of the tiny magnetic
                                  A planar inductor
                                                                             domains it is composed of, due to movement of the
                                                                             domain walls. This is a lossy process, because the
                                                                             domain walls get "snagged" on defects in the crystal
                                                                             structure and then "snap" past them, dissipating energy
                                                                             as heat. This is called hysteresis loss. It can be seen in
                                                                             the graph of the B field versus the H field for the
                                                                             material, which has the form of a closed loop. The
                                                                             amount of energy lost in the material in one cycle of
                                                                             the applied field is proportional to the area inside the
                                                                             hysteresis loop. Hysteresis loss increases with higher
                                                                             frequencies as more cycles are undergone per unit time.

                                                                             Eddy current loss
            Exploded view that shows the spiral track made directly on the
                                                                  The induction of eddy currents within the core causes a
                                printed circuit board
                                                                  resistive loss. The higher the resistance of the core
                                                                  material the lower the loss. Lamination of the core
     material can reduce eddy current loss, and also making the core of a nonconductive magnetic material like ferrite.

     Magnetic core materials
     Having no magnetically active core material (an "air core") provides very low inductance in most situations, so a
     wide range of high-permeability materials are used to concentrate the field. Most high-permeability material are
     ferromagnetic or ferrimagnetic.

     Soft iron
     "Soft" iron is used in magnetic assemblies, electromagnets and in some electric motors; and it can create a
     concentrated field that is as much as 50,000 times more intense than an air core.[1]
     Iron is desirable to make magnetic cores, as it can withstand high levels of magnetic field without saturating (up to
     2.16 teslas at ambient temperature.[2] )
     It is also used because, unlike "hard" iron, it does not remain magnetised when the field is removed, which is often
     important in applications where the magnetic field is required to be repeatedly switched.
     Unfortunately, due to the electrical conductivity of the metal, at AC frequencies a bulk block or rod of soft iron can
     often suffer from large eddy currents circulating within it that waste energy and cause undesirable heating of the
Iron loss                                                                                                                         65


     Laminated silicon steel
     Because iron is a relatively good conductor, it cannot be used in bulk form with a rapidly changing field, such as in a
     transformer, as intense eddy currents would appear due to the magnetic field, resulting in huge losses (this is used in
     induction heating).
     Two techniques are commonly used together to increase the resistivity of iron: lamination and alloying of the iron
     with silicon.


     Laminated magnetic cores are made of thin, insulated iron sheets.
     Using this technique, the magnetic core is equivalent to many
     individual magnetic circuits, each one receiving only a small fraction
     of the magnetic flux (because their section is a fraction of the whole
     core section). Furthermore, these circuits have a resistance that is
     higher than that of a non-laminated core, also because of their reduced
     section. From this, it can be seen that the thinner the laminations, the
     lower the eddy currents.

     Silicon alloying

     A small addition of silicon to iron (around 3%) results in a dramatic
     increase of the resistivity, up to four times higher. Further increase in                Typical EI Lamination.

     silicon concentration impairs the steel's mechanical properties, causing
     difficulties for rolling due to brittleness.
     Among the two types of silicon steel, grain-oriented (GO) and grain non-oriented (GNO), GO is most desirable for
     magnetic cores. It is anisotropic, offering better magnetic properties than GNO in one direction. As the magnetic
     field in inductor and transformer cores is static (compared to that in electric motors), it is possible to use GO steel in
     the preferred orientation.

     Carbonyl iron
     Powdered cores made of carbonyl iron, a highly pure iron, have high stability of parameters across a wide range of
     temperatures and magnetic flux levels, with excellent Q factors between 50 kHz and 200 MHz. Carbonyl iron
     powders are basically constituted of micrometer-size spheres of iron coated in a thin layer of electrical insulation.
     This is equivalent to a microscopic laminated magnetic circuit (see silicon steel, above), hence reducing the eddy
     currents, particularly at very high frequencies.
     A popular application of carbonyl iron-based magnetic cores is in high-frequency and broadband inductors and
Iron loss                                                                                                                         66

     Iron powder
     Powdered cores made of hydrogen reduced iron have higher permeability but lower Q. They are used mostly for
     electromagnetic interference filters and low-frequency chokes, mainly in switched-mode power supplies.

     Ferrite ceramics are used for high-frequency applications. The ferrite materials can be engineered with a wide range
     of parameters.

     Vitreous Metal
     Amorphous metal is a variety of alloys that are non crystalline or glassy. These are being used to create high
     efficiency transformers. The materials can be highly responsive to magnetic fields for low hysteresis losses and they
     can also have lower conductivity to reduce eddy current losses. China is currently making wide spread industrial and
     power grid usage of these transformers for new installations.

     External links
     • [3] - Online calculator for ferrite coil winding calculations.
     •     EMERF, the Electric Motor Education and Research Foundation [4]
     •     What are the bumps at the end of computer cables? [5]
     •     Understanding Ferrite Inductors [6], by Murata Manufacturing
     •     How to use ferrites for EMI suppression [7] by Fair-Rite
     •     Tekzilla Daily Episode 13 [8], Explanation on Short Podcast
     •     Transformer Cores [9] - All Possible Maufactured Variants

     [1]   Soft iron core Text - Physics Forums Library (http:/ / www. physicsforums. com/ archive/ index. php/ t-164613. html)
     [2]   Daniel Sadarnac, Les composants magnétiques de l'électronique de puissance, cours de Supélec, mars 2001 [in french]
     [3]   http:/ / hyperphysics. phy-astr. gsu. edu/ Hbase/ magnetic/ indtor. html
     [4]   http:/ / www. smma. org/ emerf. html
     [5]   http:/ / computer. howstuffworks. com/ question352. htm
     [6]   http:/ / www. murata. com/ emc/ knowhow/ pdfs/ te04ea-1/ 23to25e. pdf
     [7]   http:/ / www. fair-rite. com/ newfair/ pdf/ CUP%20Paper. pdf
     [8]   http:/ / revision3. com/ tzdaily/ 2007-12-26ferrite
     [9]   http:/ / www. technotron. cz/ AMT_programme2_en. aspx/
Isolation transformer                                                                                                      67

     Isolation transformer
     An isolation transformer is a transformer used to transfer electrical
     power from a source of alternating current (AC) power to some
     equipment or device while isolating the powered device from the
     power source, usually for safety. Isolation transformers provide
     galvanic isolation and are used to protect against electric shock, to
     suppress electrical noise in sensitive devices, or to transfer power
     between two circuits which must not be connected together.

     Suitably designed isolation transformers block interference caused by
     ground loops. Isolation transformers with electrostatic shields are used
     for power supplies for sensitive equipment such as computers or
     laboratory instruments.
     Strictly speaking any true transformer, whether used to transfer signals
                                                                                       A 230V isolation transformer
     or power, is isolating, as the primary and secondary are not connected
     by conductors but only by induction.
     However, only transformers whose primary purpose is to isolate circuits (opposed to the more common transformer
     function of voltage conversion), are routinely described as isolation transformers.
     Given this function, a transformer sold for isolation is often built with special insulation between primary and
     secondary, and is tested, specified, and marked to withstand a high voltage between windings, typically in the 1000
     to 4000 volt range.
     Sometimes the term is exceptionally used to clarify that some transformer, although not primarily intended for
     isolation, is a true transformer rather than an autotransformer (whose primary and secondary are not isolated from
     each other).[1] Even step-down power transformers required, amongst other things, to protect low-voltage equipment
     from mains voltage by isolating the secondary and primary such as are used in older "wall warts", are not usually
     described specifically as "isolation transformers".
     Some very small transformers—e.g. 4 transformers in one tiny dual in-line (DIL) chip package—used to isolate
     high-frequency low-voltage (logic) pulse circuits (e.g., 500V RMS primary–secondary for one second), are
     described as isolation transformers[2] [1]
     Isolation transformers are commonly designed with careful attention to capacitive coupling between the two
     windings. The capacitance between primary and secondary windings would also couple AC current from the primary
     to the secondary. A grounded Faraday shield between the primary and the secondary greatly reduces the coupling of
     common-mode noise. This may be another winding or a metal strip surrounding a winding.
Isolation transformer                                                                                                               68

                                                          Differential noise can magnetically couple from the primary to the
                                                          secondary of an isolation transformer, and must be filtered out if a
                                                          Sometimes a balanced secondary with an earthed center is used. This
                                                          can reduce earth leakage in equipment used in wet locations. The
                                                          maximum voltage above earth is halved, reducing the risk of shock if
                                                          anything live is touched.

                                                          In electronics testing and servicing an isolation transformer is a 1:1
                                                          (under load) power transformer used for safety. Without it, exposed
                                                          live metal in a device under test is at a hazardous voltage relative to
       A simple 1:1 isolation transformer with an extra
                                                          grounded objects such as a heating radiator or oscilloscope ground lead
         dielectric barrier and an electrostatic shield   (a particular hazard with some old vacuum-tube equipment with live
       between primary and secondary. The grounded        chassis). With the transformer, as there is no conductive connection
        shield prevents capacitive coupling between
                                                          between transformer secondary and earth, there is no danger in
              primary and secondary windings.
                                                          touching a live part of the circuit while another part of the body is

     Electrical isolation is considered to be particularly important on medical equipment, and special standards apply.
     Often the system must additionally be designed so that fault conditions do not interrupt power, but generate a
     Isolation transformers are also used for the power supply of devices not at ground potential. An example is the
     Austin transformer for the power supply of air-traffic obstacle warning lamps on radio antenna masts. Without the
     isolation transformer, the lighting circuits on the mast would conduct radio-frequency energy to ground through the
     power supply.
     Metal boats are subject to corrosion if they use earthed power from shore when moored, due to galvanic currents that
     flow through the water between shore earth and the hull. This can be avoided by using an isolation transformer with
     the primary and case connected to shore earth, and the secondary "floating"[4] . A metal safety screen between
     primary and secondary is connected to shore earth; in the event of a fault current in the primary (due, e.g., to
     insulation breakdown) it will cause the fault current to return and trip a shore-based circuit breaker rather than
     making the hull live.
Isolation transformer                                                                                                                                       69

     [1] Website of typical electronics distributor (http:/ / uk. farnell. com/ ) Transformers whose heading is "Transformer, isolation" are safety power
         transformers. Others are headed simply "Transformer", with "Type: isolating" in the specification. High-frequency "data-bus isolator"
         transformers also headed as isolation transformers.
     [2] Specification of a typical line of high-frequency electrically isolating pulse transformers in 16-pin DIL package (http:/ / www. murata-ps.
         com/ data/ magnetics/ kmp_1600. pdf) The manufacturer describes them as "data-bus isolators", but some distributors describe them as
         isolation transformers.
     [3] Hugh Nash et. al, (ed), IEEE Recommended Practice for Electric Systems in Health Care Facilities, IEEE Standard 602-1996, ISBN
     [4] Boat installation, connected to shore earth and secondary "floating" (http:/ / www. ecmweb. com/ mag/ 805ecmIPQfig3. jpg)

     Leakage inductance
     Leakage inductance is the property of
     an electrical transformer that causes a
     winding to appear to have some
     inductance in series with the
     mutually-coupled             transformer
     windings. This is due to imperfect
     coupling of the windings and creation
     of leakage flux which does not link
     with all the turns of the winding.

     The leakage flux alternately stores and
     discharges magnetic energy with each
     electrical cycle and thus effectively
     acts as an inductor in series in each of                                          Le1 and Le2 are the leakage inductance

     the primary and secondary circuits.
     Leakage inductance is primarily caused by the design of the core and the windings. Voltage is dropped across the
     leakage reactance, resulting in poorer supply regulation when the transformer is placed under load.

     Definition of leakage inductance
     The magnetic flux linked to both the primary winding
     and the secondary winding is said to be the main flux,
     (φ12 or φ21). The magnetic flux which interlinks only
     with the primary winding, and does not interlink with
     the secondary winding, is said to be the primary
     leakage flux, φσ1. The magnetic flux which interlinks
     with the secondary winding, and does not interlink with
     the primary winding is said to be the secondary leakage
     flux, φσ2. The primary side leakage flux becomes the
     primary side leakage inductance, and the secondary
     side leakage flux becomes the secondary side leakage
     inductance. Defining k to be the coupling coefficient,
                                                                                                     Magnetic flux of the transformer
     and denoting the leakage inductances of the primary
     side and the secondary side as Le1 and Le2 respectively,
     it follows that:
Leakage inductance                                                                                                            70

    If one winding of a transformer which has two
    windings is short-circuited, the inductance measured
    from the other winding is the leakage inductance. It is
    also called short-circuited inductance . Denoting the
    leakage inductances of the primary and secondary sides
    as Lsc1 and Lsc2 respectively, it follows that:
                                                                              measuring of the leakage inductance

    Applications of leakage inductance
    Leakage inductance can be an undesirable property, as it causes the
    voltage to change with loading. In many cases it is useful. Leakage
    inductance has the useful effect of limiting the current flows in a
    transformer (and load) without itself dissipating power (accepting the
    usual non-ideal transformer losses). Transformers are generally
    designed to have a specific value of leakage inductance such that the
    leakage reactance created by this inductance is a specific value at the
    desired frequency of operation.
    Power distribution transformers are usually designed with a leakage
    reactance of between 1% and 10% of the full load impedance. If the
    load is resistive and the leakage reactance is small (<10%) the output
    voltage will not drop by more than 0.5% at full load, ignoring other
    resistances and losses.
    Leakage reactance is also used for some negative resistance devices,                     Leakage transformer
    such as neon signs, where a transformer action is required as well as
    current limiting. In this case the leakage reactance is usually 100% of full load impedance, so even if the transformer
    is shorted out it will not be damaged. Without the leakage inductance, the negative resistance characteristic of these
    gas discharge lamps would cause them to conduct excessive current and be destroyed.
    Transformers with variable leakage inductance are used to control the current in arc welding sets. In these cases, the
    leakage inductance limits the current flow to the desired magnitude.
Leakage inductance                                                                                                                 71

    Further reading
    • Texas Instruments Magnetics Design Handbook [1] covers leakage inductance, its causes and effects as well as
      how to design it out of a transformer.

    [1] http:/ / focus. ti. com/ docs/ training/ catalog/ events/ event. jhtml?sku=SEM401014

    Linear variable differential transformer
    The linear variable differential transformer (LVDT) is a type of
    electrical transformer used for measuring linear displacement. The
    transformer has three solenoidal coils placed end-to-end around a
    tube. The center coil is the primary, and the two outer coils are the
    secondaries. A cylindrical ferromagnetic core, attached to the
    object whose position is to be measured, slides along the axis of
    the tube.

    An alternating current is driven through the primary, causing a
    voltage to be induced in each secondary proportional to its mutual
    inductance with the primary. The frequency is usually in the range
    1 to 10 kHz.
    As the core moves, these mutual inductances change, causing the
    voltages induced in the secondaries to change. The coils are        Cutaway view of an LVDT. Current is driven through
                                                                       the primary coil at A, causing an induction current to be
    connected in reverse series, so that the output voltage is the
                                                                              generated through the secondary coils at B.
    difference (hence "differential") between the two secondary
    voltages. When the core is in its central position, equidistant
    between the two secondaries, equal but opposite voltages are induced in these two coils, so the output voltage is
    When the core is displaced in one direction, the voltage in one coil increases as the other decreases, causing the
    output voltage to increase from zero to a maximum. This voltage is in phase with the primary voltage. When the core
    moves in the other direction, the output voltage also increases from zero to a maximum, but its phase is opposite to
    that of the primary. The magnitude of the output voltage is proportional to the distance moved by the core (up to its
    limit of travel), which is why the device is described as "linear". The phase of the voltage indicates the direction of
    the displacement.

    Because the sliding core does not touch the inside of the tube, it can move without friction, making the LVDT a
    highly reliable device. The absence of any sliding or rotating contacts allows the LVDT to be completely sealed
    against the environment.
    LVDTs are commonly used for position feedback in servomechanisms, and for automated measurement in machine
    tools and many other industrial and scientific applications.
Linear variable differential transformer                                                                                        72

     External links
     •     How LVDTs Work (interactive) [1]
     •     How LVDTs Work [2]
     •     Phasing Explanation [3]
     •     LVDT models and applications [4]

     [1]   http:/ / www. rdpe. com/ ex/ hiw-lvdt. htm
     [2]   http:/ / www. lvdt. co. uk/ howtheywork. html
     [3]   http:/ / www. allaboutcircuits. com/ vol_2/ chpt_9/ 4. html
     [4]   http:/ / www. metrolog. net/ lvdt

     Magnifying transmitter
     The magnifying transmitter is an advanced version of Tesla coil
     transmitter. It is a high power harmonic oscillator that Nikola Tesla
     intended for the wireless transmission of electrical energy.[1] In his
     autobiography, Tesla stated that "...I feel certain that of all my
     inventions, the Magnifying Transmitter will prove most important and
     valuable to future generations."[1] The magnifying transmitter is an
     air-core, multiple-resonant transformer that can generate very high

                                                                               The magnifying transmitter was designed to
                                                                               implement Wireless energy transmission by
                                                                              means of the disturbed charge of ground and air
Magnifying transmitter                                                                                                                           73

                                                                                                  The first 'Magnifier' was assembled in
                                                                                                  New York City between 1895 -
                                                                                                  1898.[1] In 1899 a larger magnifier was
                                                                                                  constructed in Colorado Springs,
                                                                                                  Colorado. This machine was used to
                                                                                                  conduct fundamental experiments in
                                                                                                  wireless     telecommunications      and
                                                                                                  electrical      power      transmission.
                                                                                                  Measuring fifty-one feet (15.5 m) in
                                                                                                  diameter, it developed a working
                                                                                                  potential estimated at 3.5 million to 4
                                                                                                  million volts and was capable of
                                                                                                  producing       electrical    discharges
                                                                                                  exceeding one hundred feet (30 m) in

       A publicity photo of Nikola Tesla sitting in the Colorado Springs experimental station
       with his "Magnifying Transmitter". The arcs are about 22 feet (7 m) long. (Tesla's notes
                                identify this as a double exposure.)

    Colorado arrival
    In 1899, Tesla moved his research to Colorado Springs. He chose this
    location because the polyphase alternating current power distribution
    system had been introduced there and he had associates who were
    willing to give him all the power he needed without charging for it.[3]
    He kept a handwritten diary of his experiments in the Colorado Springs
    lab where he spent nearly nine months. It consists of 500 pages of
    notes and nearly 200 drawings, recorded chronologically between June
    1, 1899 and January 7, 1900, as the work occurred, containing
    explanations of his experiments.

    Tuned electrical circuits
                                                                                                  Tesla's Colorado lab was located in a highly
    While in Colorado, Tesla constructed many smaller resonance                             geomagnetic location.

    transformers and conducted further research on concatenated tuned
    electrical circuits. Tesla also designed various sensitive devices for detecting received electrical energy, including
    rotating coherers. These used a clockwork mechanism of gears driven by a coiled spring-drive which rotated a small
    glass cylinder containing metal filings. These experiments were the final stage after years of work on synchronized
    tuned electrical circuits. These instruments were constructed to demonstrate how a wireless receiver could be "tuned"
    to respond to a specific complex signal while rejecting others. Tesla logged in his diary on January 2, 1900 that a
    separate resonance transformer tuned to the same high frequency as a larger high-voltage resonance transformer
    (which acted as a transmitter) received energy from the larger coil, one of many demonstrations of the wireless

    transmission of electrical energy. These experiments helped to confirm Tesla's priority in the invention of radio
    during later disputes in the courts. These air core high-frequency resonant coils were the predecessors of systems
Magnifying transmitter                                                                                                                         74

    ranging from radio to medical nuclear magnetic resonance imaging.

    Energy transmission
    On July 3, 1899, Tesla claimed to have discovered terrestrial stationary waves or standing waves extending across
    the earth to the antipode opposite his transmitter. He demonstrated that Earth behaves as a smooth polished
    conductor of very low resistance, and that it responds to certain predescribed frequencies of electrical vibrations. He
    conducted experiments that contributed to our understanding of electromagnetic propagation and the earth electrical
    Tesla researched ways to transmit energy wirelessly over long distances, first by transverse waves, and then,
    possibly, by longitudinal waves. He transmitted extremely low frequency current through the earth with associated
    electric field energy propagating along the space between the Earth's surface and the Kennelly–Heaviside layer. He
    received patents on wireless transceivers designed to develop terrestrial standing waves by this method.
    The magnifying transmitter was the basis for Tesla's Wardenclyffe Tower project. Although modern Tesla coils are
    designed to generate spark discharges, this system was designed for wireless telecommunications and electrical
    power transmission. In 1925, John B. Flowers advanced a proposal to test Tesla's system and to implement the
    system. H. L. Curtis, the chief of the Bureau of Standards Radio Laboratory in Washington D.C., and J. H. Dillinger,
    a physicist, reviewed the proposal but declined to implement the proposed plan. Flowers's mechanical analogy test
    was successful, though.[4]

    Electromechanical oscillator
    Tesla developed the reciprocating electro-mechanical oscillator as a source of frequency stable or isochronous
    alternating electric current used in conjunction with both wireless transmitting and receiving apparatus. This circuit
    element was applied in the same manner as quartz crystal oscillators are now. He also proposed the use of this device
    for geophysical exploration by means of seismology—a technique that he called telegeodynamics.

    Magnifying transmitter and the Wardenclyffe Tower
    See also: Wardenclyffe Tower

             Transmitter details

                    The electrical oscillator, cited by Dr. Tesla as his most important and greatest invention, consists of three inductors:
                •   an air-core transformer (two-coil master oscillator)
                •   a third coil (extra coil)
                    The "extra coil" operates as a base-driven quarter-wave helical resonator.

    The layout of the Wardenclyffe magnifying transmitter is well known, based upon Tesla's patents[5] [6] and various
    photographs[7] [8] in which the concept was implemented. The magnifying transmitter is not identical to the classic
    Tesla coil. It has the short thick primary and secondary inductor characteristic of the Tesla coil, although magnetic
    coupling between the two is tighter. Because of this, more aggressive measures have to be taken in terms of primary
    spark quenching and providing additional insulation between the primary and secondary. In addition to these two
    large-diameter coils that comprise the master oscillator, Tesla added a third inductor called the "extra coil."
Magnifying transmitter                                                                                                        75

    Construction and theory of operation
    In a classic Tesla coil the primary drives the ground end of the secondary coil to form the driver transformer, which
    resonates the entire secondary coil. In the magnifying transmitter the driving and resonating parts of the secondary
    are separate coils. From a circuit analysis standpoint, there is little difference between the classic coil and the
    The extra coil or helical resonator can be physically separated from the two close-coupled coils, which comprise the
    master oscillator or driver section. The power from the master oscillator is fed to the lower end of the extra coil
    resonator through a large diameter electrical conductor or pipe to minimize corona discharge. The magnifying
    transmitter's base-driven extra coil behaves as a slow-wave helical resonator, the axial disturbance propagating at a
    velocity of less than 1% up to around 10% the speed of light in free space. The axial velocity of the resonator's
    charge-coupled electromagnetic field is established by the coil pitch and electrical charge propagation speed through
    the circuit.

    At Colorado Springs Tesla used his magnifying transmitter in an attempt to artificially stimulate terrestrial standing
    waves. Based upon observations made with the device, Tesla reported that earth resonance modes involving an
    electric current flowing through the earth can be excited. He claimed to have discovered a fundamental earth
    resonance frequency of nearly 11.78 Hz, which is somewhat higher than the fundamantal earth-ionosphere cavity
    Schumann resonance found to exist by researchers in the 1950s in the general vicinity of 7.3 Hz.
    In normal operation the magnifying transmitter is relatively silent, generating a high power electric field, but if the
    output voltage exceeds the design voltage of the elevated terminal, high-voltage sparks will strike out from the
    electrode into the air.

    Related Tesla patents
    •   "System of Electric Lighting," U.S. Patent 454622 [9], 23 June 1891
    •   "Means for Generating Electric Currents," U.S. Patent 514168 [10], 6 February 1894
    •   "Electrical Transformer," U.S. Patent 593138 [11], 2 November 1897
    •   "Method of and Apparatus for Controlling Mechanism of Moving Vehicle or Vehicles ", U.S. Patent 613809 [12], 8
        November 1898
    •   "System of Transmission of Electrical Energy," U.S. Patent 645576 [13], Mar. 20, 1900
    •   "Apparatus for Transmission of Electrical Energy," U.S. Patent 649621 [14], 15 May 1900
    •   "System of Transmission of Electrical Energy," U.S. Patent 645576 [13], 20 March 1900
    •   "Apparatus for Utilizing Effects Transmitted from a Distance to a Receiving Device through Natural Media," U.S.
        Patent 685953 [15], Nov. 5, 1901
    •   "Method of Utilizing Effects Transmitted through Natural Media," U.S. Patent 685954 [16], Nov. 5, 1901
    •   "Apparatus for Utilizing Effects Transmitted From A Distance To A Receiving Device Through Natural Media,"
        U.S. Patent 685955 [17], Nov. 5, 1901
    •   "Apparatus for Utilizing Effects Transmitted through Natural Media," U.S. Patent 685956 [18], Nov. 5, 1901
    •   "Method Of Utilizing Radiant Energy," U.S. Patent 685958 [19], 5 November 1901
    •   "Method of Signaling," U.S. Patent 723188 [20], Mar. 17, 1903
    •   "System of Signaling," U.S. Patent 725605 [21], Apr. 14, 1903
    •   "Art of Transmitting Electrical Energy through the Natural Mediums," U.S. Patent 787412 [22], Apr. 18, 1905
    •   "Apparatus for Transmitting Electrical Energy," Jan. 18, 1902, U.S. Patent 1119732 [23], Dec. 1, 1914
    See also: List of Tesla patents
Magnifying transmitter                                                                                                                           76

    [1] My Inventions: The Autobiography of Nikola Tesla, Hart Brothers, 1982, Ch. 5, ISBN 0-910077-00-2
    [2] Nikola Tesla: Guided Weapons & Computer Technology, Leland I. Anderson, 21st Century Books, 1998, pp. 12-13, ISBN 0-9636012-9-6.
    [3] Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power,
        Leland I. Anderson, 21st Century Books, 2002, p. 109, ISBN 1-893817-01-6.
    [4] Valone, Thomas, Harnessing the Wheelwork of Nature. ISBN 1-931882-04-5
    [5] Apparatus for Transmission of Electrical Energy, U.S. Patent No. 649,621, 15 May 1900
    [6] Apparatus for Transmitting Electrical Energy, Jan. 18, 1902, U.S. Patent 1,119,732, Dec. 1, 1914
    [7] Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power,
        Leland I. Anderson, 21st Century Books, 2002, pp. 74, 89-90, 107, 111, ISBN 1-893817-01-6.
    [8] Nikola Tesla Colorado Springs Notes, 1899-1900, Nikola Tesla Museum, Beograd, 1978.
    [9] http:/ / www. google. com/ patents?vid=454622
    [10] http:/ / www. google. com/ patents?vid=514168
    [11] http:/ / www. google. com/ patents?vid=593138
    [12] http:/ / www. google. com/ patents?vid=613809
    [13] http:/ / www. google. com/ patents?vid=645576
    [14] http:/ / www. google. com/ patents?vid=649621
    [15] http:/ / www. google. com/ patents?vid=685953
    [16] http:/ / www. google. com/ patents?vid=685954
    [17] http:/ / www. google. com/ patents?vid=685955
    [18] http:/ / www. google. com/ patents?vid=685956
    [19]   http:/ / www. google. com/ patents?vid=685958
    [20]   http:/ / www. google. com/ patents?vid=723188
    [21]   http:/ / www. google. com/ patents?vid=725605
    [22]   http:/ / www. google. com/ patents?vid=787412
    [23]   http:/ / www. google. com/ patents?vid=1119732

    Further reading
    Tesla writings
    • Tesla, Nikola, "On the Transmission of Electricity Without Wires". Electrical World and Engineer, 5 March 1904.
    Electrical World
    • "The Development of High Frequency Currents for Practical Application"., The Electrical World, Vol 32, No. 8.
    • "Boundless Space: A Bus Bar". The Electrical World, Vol 32, No. 19.
    • "Mr. Tesla's Application of the Hertz-Wave Transmission". The Electrical World, Vol 32, No. 8.
    Other publications
    • Bass, Robert W., "Self-Sustained Non-Hertzian Longitudinal Wave Oscillations as a Rigorous Solution of
      Maxwell's Equations for Electromagnetic Radiation". Inventek Enterprises, Inc., Las Vegas, Nevada.
    • Bieniosek, F. M., "Triple Resonance Pulse Transformer Circuit". Review of Scientific Instruments, 61 (6).
    • Corum, J. F., and K. L. Corum, J. F. X. Daum "Spherical Transmission Lines and Global Propagation, An
      Analysis of Tesla's Experimentally Determined Propagation Model". 1987.
    • Corum, J. F., and K. L. Corum, "Disclosure Concerning the Operation of an ELF Oscillator". CPG
      Communications, Inc., Newbury, Ohio.
    • Corum, J. F., and K. L. Corum, "A Physical Interpretation of the Colorado Springs Data". CPG Communications,
      Inc., Newbury, Ohio.
    • Corum, J. F., and K. L. Corum, "Tesla's Colorado Spring Receivers (A Short Introduction)". 2003.
    • Corum, J. F., and K. L. Corum, "RF Coils, Helical Resonators and Voltage Magnification by Coherent Spatial
      Modes". IEEE, 2001.
    • de Queiroz, Antonio Carlos M., "Synthesis of Multiple Resonance Networks". Universidade Federal do Rio de
      Janeiro, Brazil. EE/COPE.
Magnifying transmitter                                                                                                77

    • de Queiroz, Antonio Carlos M., "Designing a Tesla Magnifier". Universidade Federal do Rio de Janeiro, Brazil.
    • Grotz, Toby, "Wireless Transmission of Power: An Attempt to Verify Nikola Tesla's 1899 Colorado Springs
      Experiment, Results of Research and Experimentation". TESLA, Inc., Craig Colorado.
    • Hartley, R. V. L., "Oscillations with Non-linear Reactances". Bell Systems Technical Journal, Sun Publishing.
    • Wait, James, R., "Electromagnetic Waves in Stratified Media". Pergammon Press, 1972. (2nd edition)
    • Reed,J.R.,"Analytical expression for the output voltage of the triple resonance Tesla transformer," Review of
      Scientific Instruments, 76, 104702,(2005).
    • Reed,J.R.,"Designing triple resonance Tesla transformers of arbitrary frequency ratio," Review of Scientific
      Instruments, 77, 033301, (2006).
    Other patents
    • Armstrong, E. H., U.S. Patent 1113149 (, "Wireless receiving
      system". 1914.
    • Armstrong, E. H., U.S. Patent 1342885 (, "Method of receiving
      high frequency oscillation". 1922.
    • Armstrong, E. H., U.S. Patent 1424065 (, "Signalling system".
    • Fessenden, R. A., U.S. Patent 1108895 (, "Signalling by sound
      and other longitudal elastic impulses". 1914.
    • Weyrich, R., U.S. Patent 2044413 (, "Transmitter and receiver
      for electromagnertic waves".
    • Leydorf, G. F., U.S. Patent 3278937 (, "Antenna near field
      coupling system". 1966.
    • Tanner, R. L., U.S. Patent 3215937 (, "Extremely low-frequency
      antenna". 1965.
    • Eastlund, Bernard J., U.S. Patent 5038664 (, "Method for
      producing a shell of relativistic particles at an altitude above the earths surface". 1991.
    • Hansell, Clarence W., U.S. Patent 2389432 (, "Communication
      system by pulses through the Earth".

    External links
    • Tesla Technology Research ( - Tesla Coils and the Failure of
      Lumped-Element Circuit Theory (
    • Practical Magnifier Construction Principles: Making it work (
    • Jean-Louis Naudin's Magnifying Transmitter (
    • Antonio Carlos M. de Queiroz "Designing a Tesla Magnifier" (
      magnifier.html) (Theoretical and practical approaches to Tesla magnifier design)
    • Nicholson, Paul The Tesla Secondary Simulation Project ( (theoretical simulation of
      Tesla Coil resonators, confirmed by experiment)
    • Nicholson, Paul, "The Real Science of "Non-Hertzian" Waves" (
      Non-Herzian_Waves.html). Does not address the terrestrial transmission-line propagation modes nor the
      possibility of longitudinal waves; author's focus is on transverse radio waves.
    • Cooper, John. F., "Defective Tesla coil transmitter circuit, diagram #1 (
      Magnifier 1.jpg), diagram #2 ( 2.jpg)", but showing Tesla's
      techniques for wave-complex production. (
Metadyne                                                                                                                      78

    A Metadyne is an electrical machine with three, or more, brushes. It can be used as an amplifier or rotary
    transformer. It is similar to a third brush dynamo but much more complex, having additional regulator or "variator"
    windings. it is similar to an amplidyne except that the latter has a compensating winding. The technical description is
    "a cross-field direct current machine designed to utilize armature reaction". A metadyne can convert a
    constant-voltage input into a constant current, variable voltage, output.

    Metadynes have been used to control the aiming of large guns and for speed control in electric trains, e.g. London
    Underground O Stock. The equipment in the latter case weighed three tons. They have been superseded by solid
    state devices. The concept and original patents were issued to the Macfarlane engineering company of Cathcart,
    Scotland. They were licensed to 'Met Vick' (Metropolitan-Vickers) and there was a cross licence or some form of
    agreement with GE USA who patented almost to the day the 'Amplidyne' which works the same way (the difference
    is simply in the amount of compensation that is made for armature reaction). Macfarlane used the concept in two
    main applications: constant current for electric welders and, in Amplidyne form, for alternator voltage and current

    The history of the metadyne is very complex. The name is believed to have been coined by Pestarini in a paper
    which he submitted to the Montefiore International Contest at Liège, Belgium in 1928. However, machines similar to
    the metadyne had been experimented with much earlier, e.g. by Rosenberg in 1904. British patent number 26,607 of
    1907 by Felton and Guilleaume also refers to a similar machine. Development work at Metropolitan-Vickers in the
    1930s was led by Arnold Tustin.

    • Duffy, M. C. (2000-2001). "The Metadyne in Railway Traction". Transactions of the Newcomen Society 72:
Multiple Gas Extractor                                                                                                           79

    Multiple Gas Extractor
    A Multiple Gas Extractor is a device for sampling transformer oil.
    During 2004, Central Power Research Institute, Bangalore, India introduced a novel method in which a same sample
    of transformer oil could be exposed to vacuum many times, until there was no increase in the volume of extracted
    This method was further developed by Dakshin Lab Agencies to provide a Transformer Oil Multiple Gas Extractor.
    In the apparatus a fixed volume of sample oil is directly drawn from a sample tube into a degassing vessel under
    vacuum, where the gases are released. These gases are isolated using a mercury piston to measure its volume at
    atmospheric pressure and subsequent transfer to a gas chromatograph using a gas-tight syringe or auto-sampler.

    Neon sign transformer
    A Neon Sign Transformer (NST) is a transformer made for the
    purpose of powering a neon sign. They convert line voltage from the
    120-347 V range up to high voltages, usually in the range of 2 to 15
    kV. Most of these transformers generate between 30-120 mA.

    Older NSTs are simply iron-cored transformers, usually embedded in
    asphalt for protection and insulation. The core has a magnetic shunt
                                                                               An iron cored neon sign transformer, with a 9 V
    which serves to current-limit the output, allowing them to run
                                                                                              battery for scale.
    indefinitely in short-circuit conditions. They can also run indefinitely
    with no load. Iron cored varieties are quite heavy, for example a 15 kV,
    60 mA device may weigh up to 20 kg.
    Since the 1990s, manufacturers have been producing switch mode power supplies to power neon signs. These
    generate the same voltage and current ranges as iron cored transformers, but in a much smaller, lighter, and more
    efficient designs at high frequency (not the common 50–60 Hz). They are gradually replacing iron cored
    transformers in neon signs.

    Other uses
    Besides the obvious purpose of powering neon signs, iron cored NST's are often used by hobbyists for:
    • Tesla coil power supplies - used in small to medium sized tesla coils as the main source of high voltage.
    • Jacob's Ladder - a climbing arc device often pictured in older horror films.
    • Charging Capacitors - an NST makes a useful high voltage power supply to charge high voltage capacitors.
      Although the output of an NST is AC, it can be rectified by the proper diode or bridge rectifier.
    Switch mode neon supplies are generally not suitable for these purposes, as they shut down on short or open circuit
Neon sign transformer                                                                                                         80

    • Electrocution - The shock from a neon sign transformer could be lethal. The high voltage allows a large current to
      flow, even with light contact against dry skin. The transformer is current-limited, but typically to a level well
      above the threshold for ventricular fibrillation. Disconnect power to the transformer before servicing. Use
      appropriate insulation around connections; typical insulation, including standard electrical tape and most insulated
      wire, is rated only for much lower voltages.
    • UV Light - The ultraviolet light emitted from the high voltage electrical arc can be harmful to ones eyes. It is
      recommended that the arc be viewed through the appropriate welding goggles or at the very least a high quality
      pair of sunglasses.
    • Ozone - the production of ozone can be noticeable when there are problems with a luminous tube transformer
      installation. Ozone usually indicates failed secondary wiring, loose connections, high capacitance coupling, or a
      failing transformer.
    • Fires - The arc length on 15 kV is in excess of 2 inches. Keep wiring inside grounded 1/2" metallic conduit
      (watertight if applicable). Be sure to use proper enclosures for transformers. Stray electric arcs will ignite
      combustible materials.

    Oil sample tube
    An Oil Sample Tube is a device for sampling transformer oil.
    It consists of a gas tight glass tube of capacity 150 ml or 250 ml, having two airtight Teflon valves on both the ends.
    The outlets of these valves are provided with a screw thread which provides a convenient connection for synthetic
    connecting tubes while drawing an oil sample from a transformer. This provision is useful in transferring the oil into
    a sample oil burette of the Multiple Gas Extractor without exposure to the atmosphere thereby retaining all its
    dissolved and evolved fault gas contents.
    Thermo foam boxes are used to transport Oil Sample Tubes without any exposure to sunlight
Oudin coil                                                                                                                           81

    Oudin coil
    An Oudin coil, also called an Oudin oscillator or
    Oudin resonator, is a disruptive discharge coil wired
    as a transformer designed to produce high voltage arcs
    and discharges, similar to a Tesla coil. It was invented
    by French physician Paul Marie Oudin and physicist
    Jacques d'Arsonval around 1899.

    The device is a high frequency current generator which
    uses the principles of resonant electrical circuits. It
    produces an antinode of high potential. The
    high-voltage, self-regenerative resonant transformer has
    the bottom ends of the primary and secondary coils
    connected together and to ground.

    Oudin coils generate high voltages at high frequency,
                                                                               Oudin coil used for medical 'electrotherapy', 1907.
    but produce lower currents than other disruptive
    discharge coils (such as the later version of the Tesla
    coil). The Oudin coil is modified for greater safety.

    External articles
    • Circuit diagram of demonstration device [1] (Glasgow University)
    • de Queiroz, Antonio Carlos M., " Oudin coil [2]". [3] (image)

    [1] http:/ / www. physics. gla. ac. uk/ ~kskeldon/ PubSci/ exhibits/ E8/
    [2] http:/ / www. coe. ufrj. br/ ~acmq/ tur200. jpg
    [3] http:/ / www. coe. ufrj. br/ ~acmq/
Padmount transformer                                                                                                           82

    Padmount transformer
    Padmount or Pad Mounted Transformer is a low kVA ground mounted transformer with self protection and
    switching options in the form of elbow connectors and built in sectionalizing switches. Appropriate for areas where
    underground high voltage electrical distribution terminals are located, pad mounted transformers step down voltage
    before supplying power to end user's electrical system.

    Pad mount Transformers are available in various electrical and mechanical configurations. They may be obtained in
    any voltage, phase or frequency which an application may demand. A Pad mount Transformer is a tank like structure
    that holds the core/coil assembly, built on top of a wiring cabinet. The wiring cabinet has high and low voltage
    wiring compartments. High and low voltage underground cables from below enter the terminal compartments
    directly. The top of the tank has a cover secured with carriage bolt-nut assemblies. The wiring cabinet has sidewalls
    on two ends with doors that open sideways to expose the high and low voltage wiring compartments.[1]
    Pad mount transformers have self protecting fusing comprising of the Bay-O-Net fuse placed in a high voltage
    compartment with a back-up high energy current limiting fuse in series to protect against secondary faults and
    transformer overload. The Bay-O-Net fuse protects against secondary faults and transformer overload and is a field
    replaceable device. The backup current limiting fuse operates only during transformer failure, therefore it is not field
    replaceable. These transformers also serve the conventional low voltage fusing requirements. The use of polymeric
    cable and load break elbows enable switching and isolation to be carried out in the HV chamber in what is known as
    a ‘Dead Front’ environment, i.e. all terminations are fully screened and watertight.[2]
    Single and three phase pad-mounted transformers are available, and can be used in ground level and underground
    industrial and residential power distribution systems, especially where there is a need for safe, reliable and
    aesthetically appealing transformer design. Its construction allows installation of pad mount transformers in public
    areas without the need of protective fencing. In residential areas, which are powered by underground distribution
    systems, pad mount transformers are usually located at street easements, to regulate electrical voltage requirements
    for multiple households.
    Pad mounted transformers are available in numerous sizes and designs suitable for underground installation as well.
    Three phase pad mounted transformers range in sizes from 75kVA up to 5000kVA with voltages ranging from 2,400
    up to 34,500 delta or wye. Low voltage pad mounted range in size from 208y/120 through 24,940y/14,000.
    While most traditional pad mount transformer are fixed on a concrete ‘pad’, today small 1Ø designs are also available
    with the transformer already mounted on a ‘polypad’ base so that they can be mounted on a hard ground, and
    connected and switched on.

    Tanks and Compartments
    Pad mount Transformers have tanks and compartments assembled on top of a flat, rigid surface, usually a concrete
    pad. The pad mounted transformer unit may be rolled, skidded or jacked into place, or raised using hooks.
    Underground cables are used to step down high voltage, which are connected to the bushings or to factory-installed
    auxiliary equipment. The high and low voltage compartments are separated by a metal barrier and open sideways.

    High and Low Voltage Terminators
    Compartmental type pad mounted transformers support underground entrance of primary and secondary conductors.
    Live and dead front primary termination on radial or loop feed service is provisioned for. Wet process electrical
    grade porcelain bushings with eyebolt terminals are provided for live front construction. For all voltage ratings, dead
    front construction with provisions for high voltage terminators is available. Secondary terminals are sealed into
Padmount transformer                                                                                                          83

    molded epoxy bushings and externally clamped to the tank wall.

    Primary Switch and Fuse Arrangement
    In a 3 phase pad mounted transformer, oil immersed switches that switch in 3 current ratings are available for radial
    and loop feed. Load break and latch operations are facilitated through spring load mechanism. A three phase gang
    operated switch is mounted near the core and coil assembly for low cable capacitance. Depending on the customers
    requirement, and transformer size and rating , the most common fuse options include weak link expulsion fuses;
    Bay-o-Net fuses; dry well canisters; arc strangler fused switch blades; clip mounted, full range, current limiting
    fuses; and S&C disconnects with E-rated power fuses.

    Pad mount transformers are constructed particularly for installation in public areas, where there is a need for a
    tamper -proof design. Depending on the connection and access type, pad mount transformers can be installed indoor
    and outdoor. Pad mount transformers for indoors require dry type construction.
    Usually inspection authorities require pad mount transformers to be located away from the main building. The
    transformer must have at least 8 feet of unobstructed space in front for the doors to the electrical equipment to open.
    Upon installation, there should be a padlock on the transformer and a clearly visible warning sign indicating
    electrical voltage. Areas where pad mount transformers are located should be well lit, safe and inaccessible to young

    Application [3]
    Pad mount transformers are used in various production applications for management of electrical voltage power
    systems, such as,
    •   Refineries
    •   Office Buildings
    •   Schools
    •   Windmill Farms
    •   Hospitals
    •   Residential Neighborhoods
    •   Warehouses
    •   Retail Stores and Shopping Malls
    •   Manufacturing Facilities
    •   Electrical Substations
    •   Large Chain Type Grocery Store

    American National Standards Institute/Institute of Electrical and Electronic Engineer (ANSI/IEEE)
    • ANSI® C57.12.00 – Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating
    • ANSI® C57.12.22 – Standard for Transformers - Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase
      Distribution Transformers with High-Voltage Bushings, 2500 kVA and Smaller: High Voltage, 34,500Grd/19,920
      Volts and Below; Low-Voltage, 480 Volts and Below - Requirements
    • ANSI® C57.12.26 – Standard for Transformers-Pad-Mounted, Compartmental-Type, Self-Cooled, Three-Phase
      Distribution Transformers for Use with Separable Insulated High- Voltage Connectors, 34,500 Grd/19,920 Volts
      and Below; 2500 kVA and Smaller
Padmount transformer                                                                                                                                  84

    • ANSI® C57.12.28 – Standard for Switchgear and Transformers, Pad-Mounted Equipment – Enclosure Integrity
    National Electrical Manufacturers Association (NEMA) Standards
    • NEMA TR 1-1993 (R2000) – Transformers, Regulators and Reactors, Table 0-2 Audible Sound Levels for
      Liquid-Immersed Power Transformers.
    • NEMA 260-1996 (2004) – Safety Labels for Pad-Mounted Switchgear and Transformers Sited in Public Areas

    Inspection and Maintenance
    Ageing and corrosion of pad mount transformers can result in outages, safety hazards, and customer complaints.
    Debris, insects, and pests can lead to possible electrical failure. Laser-guided infrared can be used to detect elevated
    temperatures on critical transformer components, such as primary elbow connections. To improve safety and
    reliability of pad mount transformers regular external/internal inspection and maintenance is conducted. Exterior
    Inspection / Maintenance
    •   Site clearing
    •   Assessment of pad condition
    •   Cabinet leveling
    •   Cabinet repair
    •   Replacement of security lock
    •   Replacement of penta bolt
    •   Painting
    •   Tag and decal replacement
    •   Cleaning and removal of debris, insects, and animals
    •   Pesticide application
    •   Tag and label replacement
    •   Infrared inspection
    •   Grounding assessment

    [1] "Padmount Transformer" (http:/ / www. electricityforum. com/ electrical-transformers/ padmount-transformer. html). The Electricity Forum. .
        Retrieved 2011-01-02.
    [2] Langley Engineering. The information contained within this site is copyrighted to Langley Engineering unless otherwise stated.. "Pad Mount
        Transformers" (http:/ / www. langley-eng. co. uk/ langley_products/ pad_mount_transformers_substations. html). Langley Engineering. .
        Retrieved 2011-01-02.
    [3] "Padmount Transformers" (http:/ / www. pacificcresttrans. com/ liquid-filled-distribution-transformers/ 1/ Padmount-Transformers. html).
        Pacific Crest Transformers. . Retrieved 2011-01-02.
Paraformer                                                                                                                  85

    The paraformer is a particular type of transformer. It transfers the power from primary to secondary winding not by
    mutual inductance coupling but by a variation of a parameter in its magnetic circuit.
    Assuming inductor's law

    it is possible to obtain a voltage at the secondary winding terminals also thank to a variation of the inductance, so

    This can be accomplished by for example modulating the saturation of the core by means of an applied variable
    magnetic field. It works even if primary and secondary windings magnetic coupling is zero (when fluxes are
    mutually Orthogonal).

    Polyphase coil
    Polyphase coils are electrical coils (phases) connected together in a polyphase system such as a generator or motor.
    In modern systems, the number of phases is usually three or a multiple of three. Each phase carries a sinusoidal
    alternating current whose phase is delayed relative to one of its neighbours and advanced relative to its other
    neighbour. The phase currents are separated in time evenly within each period of the alternating current. For
    example, in a three-phase system, the phases are separated from each other by one-third of the period.

    Coil construction
    Like all coils used in electrical machinery, polyphase coils (made from insulated conducting wire) are wound around
    ferromagnetic armatures with radial projections and maximum core-surface exposure to the magnetic field.
    The windings are physically separated around the circumference of an electrical machine. The result of such an
    arrangement is a rotating magnetic field that is used to convert electrical power to rotary mechanical work, or vice

    Polyphase motors and generators
    Compared to single-phase motors and generators, polyphase motors are simpler, because they do not require external
    circuitry (using capacitors and inductors) to produce a starting torque. Polyphase machines can deliver constant
    power over each period of the alternating current, eliminating the pulsations found in a single-phase machine as the
    current passes through zero amplitude.

    The use of polyphase coils in electrical power systems was pioneered by the engineers Nikola Tesla, Galileo
    Ferraris, and Michail Dolivo-Dobrovolsky.
Prolec GE                                                                                                                     86

    Prolec GE
    Prolec GE is a transformer manufacturer located in the
    city of Apodaca, Nuevo León, Mexico. They
    manufacture transformers for residential, commercial,
    industrial and power applications. It is a joint-venture                          Prolec GE Logo
    between the Mexican industrial group Xignux and
    General Electric.

    External links
    • Official Prolec GE page [1]
    • Official Xignux page [2]
    • Official General Electric (GE) page [3]

    [1] http:/ / www. prolecge. com
    [2] http:/ / www. xignux. com
    [3] http:/ / www. ge. com

    Quadrature booster
    A phase-shifting transformer, also quadrature booster (quad booster for short), is a specialised form of
    transformer used to control the flow of real power on three-phase electricity transmission networks.
    For an alternating current transmission line, power flow through the line is proportional to the sine of the difference
    in the phase angle of the voltage between the transmitting end and the receiving end of the line. Where parallel
    circuits with different capacity exist between two points in a transmission grid (for example, an overhead line and an
    underground cable), direct manipulation of the phase angle allows control of the division of power flow between the
    paths, preventing overload.[1] Quadrature boosters thus provide a means of relieving overloads on heavily laden
    circuits and re-routing power via more favorable paths.
    The capital cost of a quadrature booster can be high: as much as four to six million GBP (6-9 million USD) for a unit
    rated over 2 GVA. However, the utility to transmission system operators in flexibility and speed of operation, and
    particularly savings in permitting more economical dispatch of generation, can soon recover the cost of ownership.
Quadrature booster                                                                                                              87

    Method of operation
    By means of a voltage derived from
    the supply that is first phase-shifted by
    90° (hence is in quadrature), and then
    re-applied to it, a phase angle is
    developed across the quadrature
    booster. It is this induced phase angle
    that affects the flow of power through
    specified circuits.

    A quadrature booster typically consists
    of two separate transformers: a shunt
    unit and a series unit. The shunt unit
                                                             Simplified circuit diagram of a three-phase quadrature booster
    has its winding terminals connected so
    to shift its output voltage by 90° with
    respect to the supply. Its output is then applied as input to the series unit, which, because its secondary winding is in
    series with the main circuit, adds the phase-shifted component. The overall output voltage is hence the vector sum of
    the supply voltage and the 90° quadrature component.

    Tap connections on the shunt unit allow the magnitude of the quadrature component to be controlled, and thus the
    magnitude of the phase shift across the quadrature booster. The flow on the circuit containing the quadrature booster
    may be increased (boost tapping) or reduced (buck tapping). Subject to system conditions, the flow may even be
    bucked enough to completely reverse from its neutral-tap direction.

    Illustration of effect
    The one-line diagram below shows the effect of tapping a quadrature booster on a notional 100 MW generator-load
    system with two parallel transmission lines, one of which features a quadrature booster (shaded grey) with a tap
    range of 1 to 19.
    In the left-hand image, the quadrature booster is at its center tap position of 10 and has a phase angle of 0°. It thus
    does not affect the power flow through its circuit and both lines are equally loaded at 50 MW. The right-hand image
    shows the same network with the quadrature booster tapped down so to buck the power flow. The resulting negative
    phase angle has transferred 23 MW of loading onto the parallel circuit, while the total load supplied is unchanged at
    100 MW. (Note that the values used here are hypothetical; the actual phase angle and transfer in load would depend
    upon the parameters of the quadrature booster and the transmission lines.)
Quadrature booster                                                                                                             88

                                                          Effect of tapping a quadrature booster

    The intended effect is opposite: equalizing power on lines where naturally one would be heavily loaded and one
    would be lightly loaded.

    • Weedy, D. (1988). Electrical Power Systems. Wiley. ISBN 0-471-97677-6.
    • Guile, A. Paterson, W. (1977). Electrical Power Systems vol 1. Pergamon. ISBN 0-08-021729-X.
    [1] B. M. Weedy, Electric Power Systems Second Edition, John Wiley and Sons, London, 1972, ISBN 0471924458 pages 127-128
Repeating coil                                                                                                                 89

    Repeating coil
    In telecommunications, a repeating coil is a voice-frequency transformer characterized by a closed magnetic core, a
    pair of identical balanced primary (line) windings, a pair of identical but not necessarily balanced secondary (drop)
    windings, and low transmission loss at voice frequencies. It permits transfer of voice currents from one winding to
    another by magnetic induction, matches line and drop impedances, and prevents direct conduction between the line
    and the drop.
    It is a special application of an isolation transformer, and is often used to prevent ground loops or earth loops, which
    cause humming or buzzing in audio circuits. It also prevents low direct current voltages from passing.
        This article incorporates public domain material from websites or documents of the General Services
    Administration (in support of MIL-STD-188).

    Resolver (electrical)
    A resolver is a type of rotary electrical transformer used for measuring degrees of rotation. It is considered an analog
    device, and has a digital counterpart, the rotary (or pulse) encoder.

    The most common type of resolver is the brushless transmitter resolver (other types are described at the end). On the
    outside, this type of resolver may look like a small electrical motor having a stator and rotor. On the inside, the
    configuration of the wire windings makes it different. The stator portion of the resolver houses three windings: an
    exciter winding and two two-phase windings (usually labeled "x" and "y") (case of a brushless resolver). The exciter
    winding is located on the top, it is in fact a coil of a turning (rotary) transformer. This transformer empowers the
    rotor, thus there is no need for brushes, or no limit to the rotation of the rotor. The two other windings are on the
    bottom, wound on a lamination. They are configured at 90 degrees from each other. The rotor houses a coil, which is
    the secondary winding of the turning transformer, and a separate primary winding in a lamination, exciting the two
    two-phase windings on the stator.
    The primary winding of the transformer, fixed to the stator, is excited by a sinusoidal electric current, which by
    electromagnetic induction induces current in the rotor. As these windings are arranged on the axis of the resolver, the
    same current is induced no matter what its position. This current then flows through the other winding on the rotor,
    in turn inducing current in its secondary windings, the two-phase windings back on the stator. The two two-phase
    windings, fixed at right (90°) angles to each other on the stator, produce a sine and cosine feedback current. The
    relative magnitudes of the two-phase voltages are measured and used to determine the angle of the rotor relative to
    the stator. Upon one full revolution, the feedback signals repeat their waveforms. This device may also appear in
    non-brushless type, i.e., only consisting in two lamination stacks, rotor and stator.

    Basic resolvers are two-pole resolvers, meaning that the angular information is the mechanical angle of the stator.
    These devices can deliver the absolute angle position. Other types of resolver are multipole resolvers. They have 2*p
    poles, and thus can deliver p cycles in one rotation of the rotor: electrical angle = mechanical angle * p. where p is
    the no. of pole pairs. Some types of resolvers include both types, with the 2-pole windings used for absolute position
    and the multipole windings for accurate position. Two-pole resolvers can usually reach angular accuracy up to about
    +/-5′, whereas multipole resolver can provide better accuracy, up to 10′′ for 16-pole resolvers, to even 1′′, for
    instance for 128-pole resolvers.
Resolver (electrical)                                                                                                           90

     Multipole resolvers may also be used for monitoring multipole electrical motors. This device can be used in any
     application in which the exact rotation of an object relative to another object is needed, such as in a rotary antenna
     platform or a robot. In practice, the resolver is usually directly mounted to an electric motor. The resolver feedback
     signals are usually monitored for multiple revolutions by another device. This allows for geared reduction of
     assemblies being rotated and improved accuracy from the resolver system.
     Because the power supplied to the resolvers produces no actual work, the voltages used are usually low (<24 VAC)
     for all resolvers. Resolvers designed for terrestrial use tend to be driven at 50-60 Hz (mains power frequency), while
     those for marine or aeronautical use tend to operate at 400 Hz (the frequency of the on-board generator driven by the
     engines). Control systems tend to use higher frequencies (5 kHz).
     Other types of resolver include:
     Receiver resolvers
             These resolvers are used in the opposite way to transmitter resolvers (the type described above). The two
             diphased windings are energized, the ratio between the sine and the cosine representing the electrical angle.
             The system turns the rotor to obtain a zero voltage in the rotor winding. At this position, the mechanical angle
             of the rotor equals the electrical angle applied to the stator.
     Differential resolvers
             These types combine two diphased primary windings in one of the stacks of sheets, as with the receiver, and
             two diphased secondary windings in the other. The relation of the electrical angle delivered by the two
             secondary windings and the other angles is secondary electrical angle, mechanical angle, and primary
             electrical angle. These types were used, for instance, as analog trigonometric-function calculators.
     A related type is also the transolver, combining a two-phase winding like the resolver and a triphased winding like
     the synchro.

     External links
     • AMCI Resolver Tutorial [1]

     [1] http:/ / www. amci. com/ tutorials/ tutorials-what-is-resolver. asp
Rotary transformer                                                                                                                          91

    Rotary transformer

             The fixed portion of a 6 channel rotary
           transformer used in a six-head VCR. Two
        additional shorted turns improve the isolation of
        the two outermost windings from each other and
              from the other, innermost windings.

    Prior to the development of the rotary transformer, a slip-ring pickup was used, though this was prone to developing signal noise due
    to corrosion of the slip rings.

    A rotary (rotatory) transformer is a specialized transformer used to couple electrical signals between two parts
    which rotate in relation to each other.
    Slip rings could be used for the same purpose, but these would be subject to friction, wear, intermittent contact, and
    limitations on the rotational speed that can be accommodated without damage. By comparison, a rotary transformer
    has none of these limitations.
    Rotary transformers are constructed by winding the primary and secondary windings into separate halves of a cup
    core; these concentric halves face each other, with each half mounted to one of the rotating parts. Magnetic flux
    provides the coupling from one half of the cup core to the other, providing the mutual inductance that couples energy
    from the transformer's primary to its secondary.
    In brushless synchros, typical rotary transformers (in pairs) provide longer life than slip rings that are more
    commonly used. These have a cylindrical rather than a disc-shaped air gap between windings. The rotor winding is a
    spool-shaped ferromagnetic core, with the winding placed like thread on a spool. The flanges are the pole pieces.
    The stator winding is a ferromagnetic cylinder with the winding inside, and end poles that are discs with holes, like
    hardware washers.
Rotary transformer                                                                                                                   92

    The most common use of a rotary transformer is within videocassette
    recorders. Signals must be coupled from the electronics of the VCR to
    the fast-moving tape heads carried on the rotating head drum; a rotary
    transformer is ideal for this purpose. Most VCR designs require
    coupling more than one signal to the head drum. In this case, the cup
    core has more than one concentric winding isolated by individual
    raised portions of the core; the transformer used with the head drum
    shown to the right couples six individual channels.its used in electric
    traction where mechanical output to a generator then output from
                                                                                    The rotating portion of the rotary transformer
    generator to metadyne converter
                                                                                         showing three of the six tape heads
    Another use is to transmit the signals from rotary torque sensors
    installed on electric motors, to allow electronic control of motor speed and torque using feedback.
    Rotary transformers cannot be used in most DC motors instead of commutators as transformers can only transfer AC
    The so-called "brushless DC motors" as used in some washing machines are actually AC motors. House current is
    rectified in a power control module which generates variable frequency variable voltage AC.

    Rotary variable differential transformer
    A rotary variable differential transformer (RVDT) is a type of electrical transformer used for measuring angular
    More precisely, a Rotary Variable Differential Transformer (RVDT) is an electromechanical transducer that provides
    a variable alternating current (AC) output voltage that is linearly proportional to the angular displacement of its input
    shaft. When energized with a fixed AC source, the output signal is linear within a specified range over the angular
    RVDT’s utilize brushless, non-contacting technology to ensure long-life and reliable, repeatable position sensing
    with infinite resolution. Such reliable and repeatable performance assures accurate position sensing under the most
    extreme operating conditions.
    Most RVDT are composed of a wound, laminated stator and a salient two-pole rotor. The stator, containing four
    slots, contains both the primary winding and the two secondary windings. Some secondary windings may also be
    connected together.

    Operation of RVDT's
    The two induced voltages of the secondary windings,        and     , vary linearly to the mechanical angle of the rotor,

    where     is the gain or sensitivity. The second voltage can be reverse determined by:

    The difference           gives a proportional voltage:

    and the sum of the voltages is a constant:
Rotary variable differential transformer                                                                                          93

     This constant gives the RVDT great stability of the angular information, independence of the input voltage or
     frequency, or temperature, and enables it to also detect a malfunction.
     Putting the above mathematical equations in some theoretical form, the working of RVDT can be explained as below
     Basic RVDT construction and operation is provided by rotating an iron-core bearing supported within a housed
     stator assembly. The housing is passivated stainless steel. The stator consists of a primary excitation coil and a pair
     of secondary output coils. A fixed alternating current excitation is applied to the primary stator coil that is
     electromagnetically coupled to the secondary coils. This coupling is proportional to the angle of the input shaft. The
     output pair is structured so that one coil is in-phase with the excitation coil, and the second is 180 degrees
     out-of-phase with the excitation coil. When the rotor is in a position that directs the available flux equally in both the
     in-phase and out-of-phase coils, the output voltages cancel and result in a zero value signal. This is referred to as the
     electrical zero position or E.Z. When the rotor shaft is displaced from E.Z., the resulting output signals have a
     magnitude and phase relationship proportional to the direction of rotation. Because RVDT’s perform essentially like
     a transformer, excitation voltages changes will cause directly proportional changes to the output (transformation
     ratio). However, the voltage out to excitation voltage ratio will remain constant. Since most RVDT signal
     conditioning systems measure signal as a function of the transformation ratio (TR), excitation voltage drift beyond
     7.5% typically has no effect on sensor accuracy and strict voltage regulation is not typically necessary. Excitation
     frequency should be controlled within +/- 1% to maintain accuracy
     Although the RVDT can theoretically operate between ±45°, accuracy decreases quickly after ±35°. Thus, its
     operational limits lie mostly within ±30°, but some up to ±40°. Certain types can operate up to ±60°.
     The advantages of the RVDT are :
     •   low sensitivity to temperature, primary voltage & frequency variations
     •   sturdiness
     •   low cost
     •   simple control electronics
     •   small size

     RVDT varieties
     An RVDT can also be designed with two laminations, one containing the primary and the other, the secondaries.
     These types can operate on larger rotations.
     A similar transformer is called the Rotary Variable Transformer and contains only one secondary winding giving
     only one voltage:
Rotary variable differential transformer                                                                                     94

     External links
     • RVDT Sensors and applications [1]
     • Selection of Rotary Sensors [2]

     [1] http:/ / www. metrolog. net/ transdutores/ rvdt. php?lang=en
     [2] http:/ / www. positek. com/ rotary. htm

     Scott-T transformer
     A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase (2-φ) current
     from a three-phase (3-φ) source or vice-versa. The Scott connection evenly distributes a balanced load between the
     phases of the source.
     The Scott 3-phase transformer was invented by a Westinghouse engineer, C. F. Scott, in the late 1890's to bypass
     Edison's more expensive rotary-converter and thereby permit 2-phase generator plants to drive Tesla's 3-phase
     At this time 2 phase motor loads also
     existed and the Scott connection
     allowed connecting them to newer 3
     phase supplies with the currents equal
     on the 3 phases. This was valuable for
     getting equal voltage drop and thus
     feasible regulation of the voltage from
     the generator (can’t vary the phases
     separately in a 3 phase machine). But it
     should be understood that:

     • 2 phase motors inherently draw
       pulsating power and the Scott
       connection does not change this, the
       power will just pulsate equally in
       the 3 currents of the supply.
       Likewise changing a 2 phase supply
       to drive a 3 phase motor will result
       in a pulsating output from the 3
       phase motor. Simple instantaneous
       conservation of energy requires
       these conclusions. Mechanical
       momentum will heavily dampen the
       pulsations but they will still show in
       large loads.
     • As the typical 2 phase load was a motor, equality of the current in the 2 phases was inherently presumed during
       the Scott development. In modern times people have tried to revive the Scott connection as a way to power single
       phase railways from 3 phase Utility supplies. This will not result in balanced current on the 3 phase side as it is
        unlikely that 2 different railway sections connected as the 2 phases will at all times conform to the Scott
        presumption of being equal. The instantaneous difference in loading on the 2 sections will be seen as an
Scott-T transformer                                                                                                                                        95

       imbalance in the 3 phase supply, there is no ability to smooth it out [2] .
    The Scott-T transformer connection may be also be used in a back to back T to T arrangement for a three-phase to 3
    phase connection. This is a cost saving in the smaller kVA transformers due to the 2 coil T connected to a secondary
    2 coil T in-lieu of the traditional three-coil primary to three-coil secondary transformer. In this arrangement the X0
    Neutral tap is part way up on the secondary teaser transformer (see below). The voltage stability of this T to T
    arrangement as compared to the traditional 3 coil primary to three-coil secondary transformer is questioned.
    Nikola Tesla's original polyphase power system was based on simple to build two-phase components. However, as
    transmission distances increased, the more transmission line efficient three-phase system became more prominent.
    Both 2-φ and 3-φ components coexisted for a number of years and the Scott-T transformer connection allowed them
    to be interconnected.
    Assuming the desired voltage is the same on the two and three phase sides, the Scott-T transformer connection
    (shown below) consists of a center-tapped 1:1 ratio main transformer, T1, and an 86.6% (0.5√3) ratio teaser
    transformer, T2. The center-tapped side of T1 is connected between two of the phases on the three-phase side. Its
    center tap then connects to one end of the lower turn count side of T2, the other end connects to the remaining phase.
    The other side of the transformers then connect directly to the two pairs of a two-phase four-wire system.

    [1] Harold C. Passer, The Electrical Manufacturers, 1875-1900, Harvard, 1953, p. 315.
    [2] AIEE Transactions Jan 1957 pg 432-445 is a GE paper which points out that railway unbalance, even via Scott-T transformers, affects
        generators, the motors of other customers and presumably delta connected transformers. Even small unbalances can cause heating. As electric
        systems have grown larger over the 20th century however, the paper suggests that the railways are now a tolerable load provided one has a
        confirming system analysis. Scott-T transformers may not even be relevant, line-to-line load connections may be sufficient. So this leaves a
        potential solution but the single phase load should then be viewed as being tolerated not balanced. Allowing it would also raise the question of
        what if other customers asked for the same toleration.
Synchro                                                                                                                                 96

    A synchro or "selsyn" is a type of rotary electrical transformer that is
    used for measuring the angle of a rotating machine such as an antenna
    platform. In its general physical construction, it is much like an electric
    motor (See below.) The primary winding of the transformer, fixed to
    the rotor, is excited by a sinusoidal electric current (AC), which by
    electromagnetic induction causes currents to flow in three
    star-connected secondary windings fixed at 120 degrees to each other
    on the stator. The relative magnitudes of secondary currents are
    measured and used to determine the angle of the rotor relative to the
    stator, or the currents can be used to directly drive a receiver synchro
    that will rotate in unison with the synchro transmitter. In the latter case,
                                                                                   Schematic of Synchro Transducer The complete
    the whole device (in some applications) is also called a selsyn (a                 circle represents the rotor. The solid bars
    portmanteau of self and synchronizing). U.S. Naval terminology used            represent the cores of the windings next to them.
    the term "synchro" exclusively (possible exception: steering                   Power to the rotor is connected by slip rings and
                                                                                   brushes, represented by the circles at the ends of
    gear—info. needed).
                                                                                    the rotor winding. As shown, the rotor induces
                                                                                    equal voltages in the 120° and 240° windings,
                                                                                   and no voltage in the 0° winding. [Vex] does not
                                                                                   necessarily need to be connected to the common
                                                                                            lead of the stator star windings.

                                                                                             Two simple synchros system
Synchro                                                                                                                          97

    Synchro systems were first used in the control system of the Panama
    Canal, to transmit lock gate and valve stem positions, and water levels,
    to the control desks.[1]

                                                                                        A picture of a synchro transmitter

    Fire-control system designs developed during World War II used
    synchros extensively, to transmit angular information from guns and
    sights to an analog fire control computer, and to transmit the desired
    gun position back to the gun location. Early systems just moved
    indicator dials, but with the advent of the amplidyne, as well as
    motor-driven high-powered hydraulic servos, the fire control system
    could directly control the positions of heavy guns. [2]                          View onto the connection description of a
                                                                                               synchro transmitter
    Smaller synchros are still used to remotely drive indicator gauges and
    as rotary position sensors for aircraft control surfaces, where the
    reliability of these rugged devices is needed. Digital devices such as the rotary encoder have replaced synchros in
    most other applications.
    Synchros designed for terrestrial use tend to be driven at 50 or 60 hertz (the mains frequency in most countries),
    while those for marine or aeronautical use tend to operate at 400 hertz (the frequency of the on-board electrical
    generator driven by the engines).
    Selsyn motors were widely used in motion picture equipment to synchronize movie cameras and sound recording
    equipment, before the advent of crystal oscillators and microelectronics.
    On a practical level, synchros resemble motors, in that there is a rotor, stator, and a shaft. Ordinarily, slip rings and
    brushes connect the rotor to external power. A synchro transmitter's shaft is rotated by the mechanism that sends
    information, while the synchro receiver's shaft rotates a dial, or operates a light mechanical load. Single and
    three-phase units are common in use, and will follow the other's rotation when connected properly. One transmitter
    can turn several receivers; if torque is a factor, the transmitter must be physically larger to source the additional
    current. In a motion picture interlock system, a large motor-driven distributor can drive as many as 20 machines,
    sound dubbers, footage counters, and projectors.
    Single phase units have five wires: two for an exciter winding (typically line voltage) and three for the output/input.
    These three are bussed to the other synchros in the system, and provide the power and information to precisely align
    by rotation all the shafts in the receivers. Synchro transmitters and receivers must be powered by the same branch
Synchro                                                                                                                        98

    circuit, so to speak; voltage and phase must match. Different makes of selsyns, used in interlock systems, have
    different output voltages.
    Three-phase systems will handle more power and operate a bit more smoothly. The excitation is often 208/240 V
    3-phase mains power.
    In all cases, the mains excitation voltage sources must match in voltage and phase. The safest approach is to bus the
    five or six lines from transmitters and receivers at a common point.
    Synchro transmitters are as described, but 50 and 60-Hz synchro receivers require rotary dampers to keep their shafts
    from oscillating when not loaded (as with dials) or lightly loaded in high-accuracy applications.
    Large synchros were used on naval warships, such as destroyers, to operate the steering gear from the wheel on the
    A different type of receiver, called a control transformer (CT), is part of a position servo that includes a servo
    amplifier and servo motor. The motor is geared to the CT rotor, and when the transmitter's rotor moves, the servo
    motor turns the CT's rotor and the mechanical load to match the new position. CTs have high-impedance stators and
    draw much less current than ordinary synchro receivers when not correctly positioned.
    Synchro transmitters can also feed synchro to digital converters, which provide a digital representation of the shaft

    Synchro variants
    So called brushless synchros use rotary transformers (that have no magnetic interaction with the usual rotor and
    stator) to feed power to the rotor. These transformers have stationary primaries, and rotating secondaries. The
    secondary is somewhat like a spool wound with magnet wire, the axis of the spool concentric with the rotor's axis.
    The "spool" is the secondary winding's core, its flanges are the poles, and its coupling does not vary significantly
    with rotor position. The primary winding is similar, surrounded by its magnetic core, and its end pieces are like thick
    washers. The holes in those end pieces align with the rotating secondary poles.
    For high accuracy in gun fire control and aerospace work, so called multi-speed synchro data links were used. For
    instance, a two-speed link had two transmitters, one rotating for one turn over the full range (such as a gun's bearing)
    , while the other rotated one turn for every 10 degrees of bearing. The latter was called a 36-speed synchro. Of
    course, the gear trains were made accurately. At the receiver, the magnitude of the 1X channel's error determined
    whether the "fast" channel was to be used instead. A small 1X error meant that the 36x channel's data was
    unambiguous. Once the receiver servo settled, the fine channel normally retained control.
    For very critical applications, three-speed synchro systems have been used.
    So called multispeed synchros have stators with many poles, so that their output voltages go through several cycles
    for one physical revolution. For two-speed systems, these do not require gearing between the shafts.
    Differential synchros are another category. They have three-lead rotors and stators like the stator described above,
    and can be transmitters or receivers. A differential transmitter is connected between a synchro transmitter {CX} and
    a receiver {CT}, and its shaft's position adds to (or subtracts from, depending upon definition) the angle defined by
    the transmitter. A differential receiver is connected between two transmitters, and shows the sum (or difference,
    again as defined) between the shaft positions of the two transmitters.
    A resolver is similar to a synchro, but has a stator with four leads, the windings being 90 degrees apart physically
    instead of 120 degrees. Its rotor might be synchro-like, or have two sets of windings 90 degrees apart. Although a
    pair of resolvers could theoretically operate like a pair of synchros, resolvers are used for computation. Both
    synchros and resolvers have an accurate sine-function relationship between shaft position and transformation ratio
    for any pair of stator connections. (Of course, there are angular offsets of 120 or 240 degrees for synchros, and
    multiples of 90 degrees for resolvers, depeding upon the specific pair of leads being considered.)
Synchro                                                                                                                                        99

    Resolvers, in particular, can perform very accurate analog conversion from polar to rectangular coordinates. Shaft
    angle is the polar angle, and excitation voltage is the magnitude. The outputs are the [x] and [y] components.
    Resolvers with four-lead rotors can rotate [x] and [y] coordinates, with the shaft position giving the desired rotation
    Resolvers with four output leads are general sine/cosine computational devices. When used with electronic driver
    amplifiers and feedback windings tightly coupled to the input windings, their accuracy is enhanced, and they can be
    cascaded ("resolver chains") to compute functions with several terms, perhaps of several angles, such as gun
    (position) orders corrected for ship's roll and pitch.
    There are synchro-like devices called transolvers, somewhat like differential synchros, but with three-lead rotors and
    four-lead stators.
    A special T-connected transformer arrangement invented by Scott ("Scott T") interfaces between resolver and
    synchro data formats; it was invented to interconnect two-phase AC power with three-phase power, but can also be
    used for precision applications.

    [1] Goethals, George W (1916). The Panama Canal; An Engineering Treatise. A Series Of Papers Covering In Full Detail The Technical
        Problems Involved In The Construction Of The Panama Canal - Geology, Climatology, Municipal Engineering; Dredging, Hydraulics, Power
        Plants, Etc. Prepared By Engineers And Other Specialists In Charge Of The Various Branches Of The Work And Presented At The
        International Engineering Congress, San Francisco, California. New York: McGraw Hill.
    [2] "Naval Ordnance and Gunnery, Volume 1", 1957, U.S. Navy Manual, Chapter 10. (http:/ / www. eugeneleeslover. com/ USNAVY/
        CHAPTER-10-D. html)


    Tap (transformer)
    A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be
    selected. By this means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the
    output. The tap selection is made via a tap changer mechanism.

    Voltage considerations
    If only one tap changer is required, tap points are usually made on the high voltage, or low current, side of the
    winding in order to minimize the current handling requirements of the contacts. However, a transformer may include
    a tap changer on each winding if there are advantages to do so. For example, in power distribution networks, a large
    step-down transformer may have an off-load tap changer on the primary winding and an on-load tap changer on the
    secondary winding. The high voltage tap is set to match long term system profile on the high voltage network and is
    rarely changed. The low voltage tap may be requested to change positions once or more each day, without
    interrupting the power delivery, to follow loading conditions on the low-voltage network.
    To minimize the number of windings and thus reduce the physical size of a transformer, a 'reversing' winding may be
    used, which is a portion of the main winding able to be connected in its opposite direction and thus oppose the
    voltage. Insulation requirements place the tap points at the low voltage end of the winding. This is near the star point
    in a star connected winding. In delta connected windings, the tappings are usually at the center of the winding. In an
    autotransformer, the taps are usually made between the series and common windings, or as a series 'buck-boost'
    section of the common winding.
Tap (transformer)                                                                                                                 100

    Tap changing

    Off-circuit designs (DETC)
    In low power, low voltage transformers, the tap point can take the form of a connection terminal, requiring a power
    lead to be disconnected by hand and connected to the new terminal. Alternatively, the process may be assisted by
    means of a rotary or slider switch.
    Since the different tap points are at different voltages, the two connections can not be made simultaneously, as this
    would short-circuit a number of turns in the winding and produce excessive circulating current. Consequently, the
    power to the device must be interrupted during the switchover event. Off-circuit or de-energized tap changing
    (DETC) is sometimes employed in high voltage transformer designs, although for regular use, it is only applicable to
    installations in which the loss of supply can be tolerated. In power distribution networks, transformers commonly
    include an off-circuit tap changer on the primary winding to accommodate system variations within a narrow band
    around the nominal rating. The tap changer will often be set just once, at the time of installation, although it may be
    changed later during a scheduled outage in order to accommodate a long-term change in the system voltage profile.

    On-load designs
    For many power transformer applications, a supply interruption during
    a tap change is unacceptable, and the transformer is often fitted with a
    more expensive and complex on-load tap-changing (OLTC,
    sometimes LTC) mechanism. On-load tap changers may be generally
    classified as either mechanical, electronically assisted, or fully

    Mechanical tap changers

    A mechanical tap changer physically makes the new connection
    before releasing the old using multiple tap selector switches, but
    avoids creating high circulating currents by using a diverter switch to
    temporarily place a large diverter impedance in series with the
    short-circuited turns. This technique overcomes the problems with
    open or short circuit taps. In a resistance type tap changer, the
    changeover must be made rapidly to avoid overheating of the diverter.
    A reactance type tap changer uses a dedicated preventive
    autotransformer winding to function as the diverter impedance, and
    a reactance type tap changer is usually designed to sustain off-tap
    loading indefinitely.

    In a typical diverter switch powerful springs are tensioned by a low
    power motor (motor drive unit (MDU)), and then rapidly released to
    effect the tap changing operation. To reduce arcing at the contacts, the
    tap changer operates in a chamber filled with insulating transformer         A mechanical on-load tap changer (OLTC), also
                                                                                    known as under- load tap changer (ULTC)
    oil, or inside an SF6 vessel. Reactance-type tap changers, when
                                                                                    design, changing back and forth between tap
    operating in oil, must allow for with the additional inductive flyback                       positions 2 and 3
    generated by the autotransformer and commonly include a vacuum
    bottle in parallel with the diverter switch. During a tap-change operation, the flyback raises the potential between the
    two electrodes in the bottle, and some of the energy is dissipated in an arc discharge through the bottle instead of
    flashing across the diverter switch.
Tap (transformer)                                                                                                              101

    Some arcing is unavoidable, and both the tap changer oil and the switch contacts will slowly deteriorate with use. In
    order to prevent contamination of the tank oil and facilitate maintenance operations, the diverter switch usually
    operates in a separate compartment from the main transformer tank, and often the tap selector switches will be
    located in the compartment as well. All of the winding taps will then be routed into the tap changer compartment
    through a terminal array.
    One possible design (flag type) of on-load mechanical tap changer is shown to the right. It commences operation at
    tap position 2, with load supplied directly via the right hand connection. Diverter resistor A is short-circuited;
    diverter B is unused.
    In moving to tap 3, the following sequence occurs:
       1. Switch 3 closes, an off-load operation.
       2. Rotary switch turns, breaking one connection and supplying load current through diverter resistor A.
       3. Rotary switch continues to turn, connecting between contacts A and B. Load now supplied via diverter
          resistors A and B, winding turns bridged via A and B.
       4. Rotary switch continues to turn, breaking contact with diverter A. Load now supplied via diverter B alone,
          winding turns no longer bridged.
       5. Rotary switch continues to turn, shorting diverter B. Load now supplied directly via left hand connection.
          Diverter A is unused.
       6. Switch 2 opens, an off-load operation.
    The sequence is then carried out in reverse to return to tap position 2.

    Thyristor-assisted tap changers
    Thyristor-assisted tap changers use thyristors to take the on-load current while the main contacts change over from
    one tap to the next. This prevents arcing on the main contacts and can lead to a longer service life between
    maintenance activities. The disadvantage is that these tap changers are more complex and require a low voltage
    power supply for the thyristor circuitry. They also can be more costly.

    Solid state (thyristor) tap changers
    These are a relatively recent development which uses thyristors both to switch the load current and to pass the load
    current in the steady state. Their disadvantage is that all of the non-conducting thyristors connected to the unselected
    taps still dissipate power due to their leakage current and they have smaller short circuit withstand capacity. This
    power can add up to a few kilowatts which has to be removed as heat and leads to a reduction in the overall
    efficiency of the transformer, in exchange for a compact design that reduces the size and weight of the tap changer
    device. Solid state tap changers are typically employed only on smaller power transformers.

    Standards considering tap changers
Tap (transformer)                                                                                                102

                                             Name                     Status            Remark

                              IEC 60214-1:2003              Current                     -

                              IEC 60214-2:2004              Current                     -

                              IEEE Std C57.131-1995         Unknown                     -

                              ГОСТ 24126-80 (СТ СЭВ 634-77) Current                     -

                              IEC 214:1997                  Replaced by a later version -

                              IEC 214:1989                  Replaced by a later version -

                              IEC 214:1985                  Replaced by a later version -

    • Hindmarsh, J. (1984). Electrical Machines and their Applications, 4th ed.. Pergamon. ISBN 0-08-030572-5.
    • Central Electricity Generating Board (1982). Modern Power Station Practice: Volume 4. Pergamon.
      ISBN 0-08-016436-6.
    • Rensi, Randolph (June 1995). "Why transformer buyers must understand LTCs". Electrical World.
Tesla coil                                                                                                                    103

     Tesla coil
                                                                Tesla coil

                          Tesla coil at Questacon - the National Science and Technology center in Canberra, Australia

                          Uses           Application in educational demonstrations, novelty lighting, as well as music

                          Inventor       Nikola Tesla

                          Related items Electrical transformer, electromagnetic field

     A Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891.[1] It is used to produce
     high voltage, relatively high current, and high frequency alternating current electricity. Tesla experimented with a
     number of different configurations and they consist of two, or sometimes three, coupled resonant electric circuits.
     Tesla used these coils to conduct innovative experiments in electrical lighting, phosphorescence, x-ray generation,
     high frequency alternating current phenomena, electrotherapy, and the transmission of electrical energy without
     The early Tesla coil transformer design employs a medium- to high-voltage power source, one or more high voltage
     capacitor(s), and a spark gap to excite a multiple-layer primary inductor with periodic bursts of high frequency
     current. The multiple-layer Tesla coil transformer secondary is excited by resonant inductive coupling, the primary
     and secondary circuits both being tuned so they resonate at the same frequency (typically, between 25 kHz and 2
     MHz). The later and higher-power coil design has a single-layer primary and secondary. These Tesla coils are often
     used by hobbyists and at venues such as science museums to produce long sparks.
     Tesla coil circuits were used commercially in sparkgap radio transmitters for wireless telegraphy until the 1920s,[1]
     [2] [3]
             and in electrotherapy and pseudomedical devices such as violet ray (although Tesla circuits were not the first
     or the only ones used in spark transmitters). Today their main use is entertainment and educational displays. Tesla
     coils are built by many high-voltage enthusiasts, research institutions, science museums and independent
     experimenters. Although electronic circuit controllers have been developed, Tesla's original spark gap design is less
     expensive and has proven extremely reliable.
Tesla coil                                                                                                                            104


     Tesla's coil
     The "American Electrician"[4] gives a description of an early Tesla coil wherein a glass battery jar, 15 x 20 cm (6 x 8
     in) is wound with 60 to 80 turns of AWG No. 18 B & S magnet wire (0.823 mm²). Into this is slipped a primary
     consisting of eight to ten turns of AWG No. 6 B & S wire (13.3 mm²) and the whole combination immersed in a
     vessel containing linseed or mineral oil. (Norrie, pg. 34-35)

     Tesla Coil Theory
     A Tesla coil transformer operates in a significantly different fashion than a conventional (i.e., iron core) transformer.
     In a conventional transformer, the windings are very tightly coupled, and voltage gain is limited to the ratio of the
     numbers of turns in the windings.
     However, unlike a conventional transformer, which may couple 97%+ of the magnetic fields between windings, a
     Tesla coil's windings are "loosely" coupled, with the primary and secondary typically sharing only 10–20% of their
     respective magnetic fields and instead the coil transfers energy (via loose coupling) from one oscillating resonant
     circuit (the primary) to the other (the secondary) over a number of RF cycles.
     As the primary energy transfers to the secondary, the secondary's output voltage increases until all of the available
     primary energy has been transferred to the secondary (less losses). Even with significant spark gap losses, a well
     designed Tesla coil can transfer over 85% of the energy initially stored in the primary capacitor to the secondary
     circuit. Thus the voltage gain of a Tesla coil can be significantly greater than a conventional transformer, since it is
     instead proportional to the square root of the ratio of secondary and primary inductances.
     In addition, because of the large gap between the primary and secondary that loose coupling makes possible, the
     insulation between the two is far less likely to break down, and this permits coils to run extremely high voltages
     without damage.

     Modern day Tesla coils
     Modern high voltage enthusiasts usually build Tesla coils that are
     similar to some of Tesla's "later" air core designs. These typically
     consist of a primary tank circuit, a series LC (inductance-capacitance)
     circuit composed of a high voltage capacitor, spark gap and primary
     coil, and the secondary LC circuit, a series resonant circuit consisting
     of the secondary coil plus a terminal capacitance or "top load." In
     Tesla's more advanced design, the secondary LC circuit is composed of
     an air-core transformer secondary coil placed in series with a helical
     resonator. The helical coil is then connected to the terminal
     capacitance. Most modern coils use only a single helical coil
                                                                                      Electric discharge showing the lightning-like
     comprising both the secondary and primary resonator. The terminal                     plasma filaments from a Tesla coil.
     capacitance actually forms one 'plate' of a capacitor, the other 'plate'
     being the Earth (or "ground"). The primary LC circuit is tuned so that it resonates at the same frequency as the
     secondary LC circuit. The primary and secondary coils are magnetically coupled, creating a dual-tuned resonant
     air-core transformer. Earlier oil insulated Tesla coils needed large and long insulators at their high-voltage terminals
     to prevent discharge in air. Later version Tesla coils spread their electric fields over large distances to prevent high
     electrical stresses in the first place, thereby allowing operation in free air.
Tesla coil                                                                                                                        105

     Tesla's 1902 design for his advanced magnifying transmitter used a top terminal consisting of a metal frame in the
     shape of a toroid, covered with hemispherical plates (constituting a very large conducting surface). The top terminal
     has relatively small capacitance, charged to as high a voltage as practicable.[5] The outer surface of the elevated
     conductor is where the electrical charge chiefly accumulates. It has a large radius of curvature, or is composed of
     separate elements which, irrespective of their own radii of curvature, are arranged close to each other so that the
     outside ideal surface enveloping them has a large radius.[6] This design allowed the terminal to support very high
     voltages without generating corona or sparks. Tesla, during his patent application process, described a variety of
     resonator terminals at the top of this later coil.[7] Most Modern Tesla coils use simple toroids, typically fabricated
     from spun metal or flexible aluminum ducting, to control the high electrical field near the top of the secondary and to
     direct spark outward and away from the primary and secondary windings.
     As pointed out above, more advanced Tesla coil transmitters involve a more tightly coupled air core resonance
     transformer network or "master oscillator" the output of which is then fed another resonator, sometimes called the
     "extra coil." The principle is that energy accumulates in the extra coil and the role of transformer secondary is played
     by the separate master oscillator secondary; the roles are not shared by a single secondary. In some modern three-coil
     Magnifying transmitter systems the extra coil is placed some distance from the transformer. Direct magnetic
     coupling to the upper secondary is not desirable, since the third coil is designed to be driven by injecting RF current
     directly into the bottom end.
     This particular Tesla coil configuration consists of a secondary coil in close inductive relation with a primary, and
     one end of which is connected to a ground-plate, while its other end is led through a separate self-induction coil
     (whose connection should always be made at, or near, the geometrical center of that coil's circular aspect, in order to
     secure a symmetrical distribution of the current), and of a metallic cylinder carrying the current to the terminal. The
     primary coil may be excited by any desired source of high frequency current. The important requirement is that the
     primary and secondary sides must be tuned to the same resonant frequency to allow efficient transfer of energy
     between the primary and secondary resonant circuits. The conductor of the shaft to the terminal (topload) is in the
     form of a cylinder with smooth surface of a radius much larger than that of the spherical metal plates, and widens out
     at the bottom into a hood (which is slotted to avoid loss by eddy currents). The secondary coil is wound on a drum of
     insulating material, with its turns close together. When the effect of the small radius of curvature of the wire itself is
     overcome, the lower secondary coil behaves as a conductor of large radius of curvature, corresponding to that of the
     drum. The top of the extra coil may be extended up to the terminal U.S. Patent 1,119,732 [8] and the bottom should
     be somewhat below the uppermost turn of the primary coil. This lessens the tendency of the charge to break out from
     the wire connecting both and to pass along the support.
     Modern day transistor or vacuum tube Tesla coils do not use a primary spark gap. Instead, the transistor(s) or
     vacuum tube(s) provide the switching or amplifying function necessary to generate RF power for the primary circuit.
     Solid-state Tesla coils use the lowest primary operating voltage, typically between 155 to 800 volts, and drive the
     primary winding using either a single, half-bridge, or full-bridge arrangement of bipolar transistors, MOSFETs or
     IGBTs to switch the primary current. Vacuum tube coils typically operate with plate voltages between 1500 and
     6000 volts, while most spark gap coils operate with primary voltages of 6,000 to 25,000 volts. The primary winding
     of a traditional transistor Tesla coil is wound around only the bottom portion of the secondary (sometimes called the
     resonator). This helps to illustrate operation of the secondary as a pumped resonator. The primary induces alternating
     voltage into the bottommost portion of the secondary, providing regular "pushes" (similar to provided properly timed
     pushes to a playground swing). Additional energy is transferred from the primary to the secondary inductance and
     topload capacitance during each "push", and secondary output voltage builds (called ring-up). An electronic
     feedback circuit is usually used to adaptively synchronize the primary oscillator to the growing resonance in the
     secondary, and this is the only tuning consideration beyond the initial choice of a reasonable topload.
     In a dual resonant solid-state Tesla coil (DRSSTC), the electronic switching of the solid-state Tesla coil is combined
     with the resonant primary circuit of a spark-gap Tesla coil. The resonant primary circuit is formed by connecting a
Tesla coil                                                                                                                   106

     capacitor in series with the primary winding of the coil, so that the combination forms a series tank circuit with a
     resonant frequency near that of the secondary circuit. Because of the additional resonant circuit, one manual and one
     adaptive tuning adjustment are necessary. Also, an interrupter is usually used to reduce the duty cycle of the
     switching bridge, in order to improve peak power capabilities; similarly, IGBTs are more popular in this application
     than bipolar transistors or MOSFETs, due to their superior power handling characteristics. Performance of a
     DRSSTC can be comparable to a medium power spark gap Tesla coil, and efficiency (as measured by spark length
     versus input power) can be significantly greater than a spark gap Tesla coil operating at the same input power.



                                                     Typical Tesla Coil Schematic
                                            This example circuit is designed to be driven by
                                           alternating currents. Here the spark gap shorts the
                                             high frequency across the first transformer. An
                                            inductance, not shown, protects the transformer.
                                            This design is favoured when a relatively fragile
                                                 Neon Sign Transformer (NST) is used.

                                                  Alternate Tesla Coil Configuration
                                             This circuit also driven by alternating currents.
                                            However, here the AC supply transformer must
                                            be capable of withstanding high voltages at high
Tesla coil                                                                                                                      107

     High voltage production
     A large Tesla coil of more modern design often operates at very high peak power levels, up to many megawatts
     (millions of watts[9] ). It should therefore be adjusted and operated carefully, not only for efficiency and economy,
     but also for safety. If, due to improper tuning, the maximum voltage point occurs below the terminal, along the
     secondary coil, a discharge (spark) may break out and damage or destroy the coil wire, supports, or nearby objects.
     Tesla experimented with these, and many other, circuit configurations (see right). The Tesla coil primary winding,
     spark gap and tank capacitor are connected in series. In each circuit, the AC supply transformer charges the tank
     capacitor until its voltage is sufficient to break down the spark gap. The gap suddenly fires, allowing the charged
     tank capacitor to discharge into the primary winding. Once the gap fires, the electrical behavior of either circuit is
     identical. Experiments have shown that neither circuit offers any marked performance advantage over the other.
     However, in the typical circuit (above), the spark gap's short circuiting action prevents high frequency oscillations
     from 'backing up' into the supply transformer. In the alternate circuit, high amplitude high frequency oscillations that
     appear across the capacitor also are applied to the supply transformer's winding. This can induce corona discharges
     between turns that weaken and eventually destroy the transformer's insulation. Experienced Tesla coil builders
     almost exclusively use the top circuit, often augmenting it with low pass filters (resistor and capacitor (RC)
     networks) between the supply transformer and spark gap to help protect the supply transformer. This is especially
     important when using transformers with fragile high voltage windings, such as Neon-sign transformers (NSTs).
     Regardless of which configuration is used, the HV transformer must be of a type that self-limits its secondary current
     by means of internal leakage inductance. A normal (low leakage inductance) high voltage transformer must use an
     external limiter (sometimes called a ballast) to limit current. NSTs are designed to have high leakage inductance to
     limit their short circuit current to a safe level.

     Tuning precautions
     The primary coil's resonant frequency should be tuned to that of the secondary, using low-power oscillations, then
     increasing the power until the apparatus has been brought under control. While tuning, a small projection (called a
     "breakout bump") is often added to the top terminal in order to stimulate corona and spark discharges (sometimes
     called streamers) into the surrounding air. Tuning can then be adjusted so as to achieve the longest streamers at a
     given power level, corresponding to a frequency match between the primary and secondary coil. Capacitive 'loading'
     by the streamers tends to lower the resonant frequency of a Tesla coil operating under full power. For a variety of
     technical reasons, toroids provide one of the most effective shapes for the top terminals of Tesla coils.
Tesla coil                                                                                                                             108

     Air discharges

     While generating discharges, electrical energy from the secondary and
     toroid is transferred to the surrounding air as electrical charge, heat,
     light, and sound. The electric currents that flow through these
     discharges are actually due to the rapid shifting of quantities of charge
     from one place (the top terminal) to other places (nearby regions of
     air). The process is similar to charging or discharging a capacitor. The
     current that arises from shifting charges within a capacitor is called a
     displacement current. Tesla coil discharges are formed as a result of
     displacement currents as pulses of electrical charge are rapidly
     transferred between the high voltage toroid and nearby regions within
     the air (called space charge regions). Although the space charge
     regions around the toroid are invisible, they play a profound role in the
     appearance and location of Tesla coil discharges.

     When the spark gap fires, the charged capacitor discharges into the
     primary winding, causing the primary circuit to oscillate. The               A small, later-type "Tesla coil" in operation. The
     oscillating primary current creates a magnetic field that couples to the     output is giving 17-inch sparks. The diameter of
                                                                                 the secondary is three inches. The power source is
     secondary winding, transferring energy into the secondary side of the
                                                                                      a 10000 V, 60 Hz current limited supply.
     transformer and causing it to oscillate with the toroid capacitance. The
     energy transfer occurs over a number of cycles, and most of the energy
     that was originally in the primary side is transferred into the secondary side. The greater the magnetic coupling
     between windings, the shorter the time required to complete the energy transfer. As energy builds within the
     oscillating secondary circuit, the amplitude of the toroid's RF voltage rapidly increases, and the air surrounding the
     toroid begins to undergo dielectric breakdown, forming a corona discharge.

     As the secondary coil's energy (and output voltage) continue to increase, larger pulses of displacement current
     further ionize and heat the air at the point of initial breakdown. This forms a very conductive "root" of hotter plasma,
     called a leader, that projects outward from the toroid. The plasma within the leader is considerably hotter than a
     corona discharge, and is considerably more conductive. In fact, it has properties that are similar to an electric arc.
     The leader tapers and branches into thousands of thinner, cooler, hairlike discharges (called streamers). The
     streamers look like a bluish 'haze' at the ends of the more luminous leaders, and it is the streamers that actually
     transfer charge between the leaders and toroid to nearby space charge regions. The displacement currents from
     countless streamers all feed into the leader, helping to keep it hot and electrically conductive.
     The primary break rate of sparking Tesla coils is slow compared to the resonant frequency of the resonator-topload
     assembly. When the switch closes, energy is transferred from the primary LC circuit to the resonator where the
     voltage rings up over a short period of time up culminating in the electrical discharge. In a spark gap Tesla coil the
     primary-to-secondary energy transfer process happens repetitively at typical pulsing rates of 50–500 times per
     second, and previously formed leader channels don't get a chance to fully cool down between pulses. So, on
     successive pulses, newer discharges can build upon the hot pathways left by their predecessors. This causes
     incremental growth of the leader from one pulse to the next, lengthening the entire discharge on each successive
     pulse. Repetitive pulsing causes the discharges to grow until the average energy that's available from the Tesla coil
     during each pulse balances the average energy being lost in the discharges (mostly as heat). At this point, dynamic
     equilibrium is reached, and the discharges have reached their maximum length for the Tesla coil's output power
     level. The unique combination of a rising high voltage Radio Frequency envelope and repetitive pulsing seem to be
     ideally suited to creating long, branching discharges that are considerably longer than would be otherwise expected
     by output voltage considerations alone. High voltage discharges create filamentary multi-branched discharges which
     are purplish blue in colour. High energy discharges create thicker discharges with fewer branches, are pale and
Tesla coil                                                                                                                       109

     luminous, almost white, and are much longer than low energy discharges, because of increased ionisation. There will
     be a strong smell of ozone and nitrogen oxides in the area. The important factors for maximum discharge length
     appear to be voltage, energy, and still air of low to moderate humidity. However, even more than 100 years later
     after the first use of Tesla coils, there are many aspects of Tesla coil discharges and the energy transfer process that
     are still not completely understood.

     Wireless transmission and reception
     The Tesla coil can also be used for wireless transmission. In addition to the positioning of the elevated terminal well
     above the top turn of the helical resonator, another difference from the sparking Tesla coil is the primary break rate.
     The optimized Tesla coil transmitter is a continuous wave oscillator with a break rate equaling the operating
     frequency. The combination of a helical resonator with an elevated terminal is also used for wireless reception.[10]
     [11] [12] [13] [14] [15]
                              The Tesla coil receiver is intended for receiving the non-radiating electromagnetic field energy
     produced by the Tesla coil transmitter. The Tesla coil receiver is also adaptable for exploiting the ubiquitous vertical
     voltage gradient in the Earth's atmosphere. Tesla built and used various devices for detecting electromagnetic field
     energy. His early wireless apparatus operated on the basis of Hertzian waves or ordinary radio waves,
     electromagnetic waves that propagate in space without involvement of a conducting guiding surface.[16] During his
     work at Colorado Springs, Tesla believed he had established electrical resonance of the entire Earth using the Tesla
     coil transmitter at his "Experimental Station."[17]
     Tesla stated one of the requirements of the World Wireless System was the construction of resonant receivers.[18]
     The related concepts and methods are part of his wireless transmission system (US1119732 — Apparatus for
     Transmitting Electrical Energy — 1902 January 18). Tesla made a proposal that there needed to be many more than
     thirty transmission-reception stations worldwide.[19] In one form of receiving circuit the two input terminals are
     connected each to a mechanical pulse-width modulation device adapted to reverse polarity at predetermined intervals
     of time and charge a capacitor.[20] This form of Tesla system receiver has means for commutating the current
     impulses in the charging circuit so as to render them suitable for charging the storage device, a device for closing the
     receiving-circuit, and means for causing the receiver to be operated by the energy accumulated.[21]
Tesla coil                                                                                                                          110

     A Tesla coil used as a receiver is referred to as a Tesla receiving
     transformer.[22] [23] [24] [25] The Tesla coil receiver acts as a step-down
     transformer with high current output.[26] The parameters of a Tesla coil
     transmitter are identically applicable to it being a receiver (e.g.., an antenna
     circuit), due to reciprocity. Impedance, generally though, is not applied in
     an obvious way; for electrical impedance, the impedance at the load (e.g..,
     where the power is consumed) is most critical and, for a Tesla coil receiver,
     this is at the point of utilization (such as at an induction motor) rather than
     at the receiving node. Complex impedance of an antenna is related to the
     electrical length of the antenna at the wavelength in use. Commonly,
     impedance is adjusted at the load with a tuner or a matching networks
     composed of inductors and capacitors.

     A Tesla coil can receive electromagnetic impulses[27] from atmospheric
     electricity[28] [29] [30] and radiant energy,[13] [31] besides normal wireless
     transmissions. Radiant energy throws off with great velocity minute
     particles which are strongly electrified and other rays falling on the
     insulated-conductor connected to a condenser (i.e., a capacitor) can cause
     the condenser to indefinitely charge electrically.[32] The helical resonator
     can be "shock excited" due to radiant energy disturbances not only at the
                                                                                           Tesla coil in one experiment of many
     fundamental wave at one-quarter wave-length but also is excited at its
                                                                                         conducted in Colorado Springs. This is a
     harmonics. Hertzian methods can be used to excite the Tesla coil receiver           grounded tuned coil in resonance with a
     with limitations that result in great disadvantages for utilization, though.[33]   nearby transmitter; Light is glowing near
     The methods of ground conduction and the various induction methods can                              the bottom.

     also be used to excite the Tesla coil receiver, but are again at a
     disadvantages for utilization.[34] The charging-circuit can be adapted to be energized by the action of various other
     disturbances and effects at a distance. Arbitrary and intermittent oscillations that are propagated via conduction to
     the receiving resonator will charge the receiver's capacitor and utilize the potential energy to greater effect.[35]
     Various radiations can be used to charge and discharge conductors, with the radiations considered electromagnetic
     vibrations of various wavelengths and ionizing potential.[32] The Tesla receiver utilizes the effects or disturbances to
     charge a storage device with energy from an external source (natural or man-made) and controls the charging of said
     device by the actions of the effects or disturbances (during succeeding intervals of time determined by means of such
     effects and disturbances corresponding in succession and duration of the effects and disturbances).[36] The stored
     energy can also be used to operate the receiving device. The accumulated energy can, for example, operate a
     transformer by discharging through a primary circuit at predetermined times which, from the secondary currents,
     operate the receiving device.[36]

     While Tesla coils can be used for these purposes, much of the public and media attention is directed away from
     transmission-reception applications of the Tesla coil since electrical spark discharges are fascinating to many people.
     Regardless of this fact, Tesla did suggest that this variation of the Tesla coil could utilize the phantom loop effect to
     form a circuit to induct energy from the Earth's magnetic field and other radiant energy sources (including, but not
     limited to, electrostatics[37] ). With regard to Tesla's statements on the harnessing of natural phenomena to obtain
     electric power, he stated:
             Ere many generations pass, our machinery will be driven by a power obtainable at any point of the
             universe. — "Experiments with Alternate Currents of High Potential and High Frequency" (February
Tesla coil                                                                                                                           111

     Tesla stated that the output power from these devices, attained from Hertzian methods of charging, was low,[38] but
     alternative charging means are available. Tesla receivers, operated correctly, act as a step-down transformer with
     high current output.[39] There are, to date, no commercial power generation entities or businesses that have utilized
     this technology to full effect. The power levels achieved by Tesla coil receivers have, thus far, been a fraction of the
     output power of the transmitters.

     High frequency electrical safety

     The 'skin effect'
     The dangers of contact with high frequency electrical
     current are sometimes perceived as being less than at
     lower frequencies, because the subject usually doesn't
     feel pain or a 'shock'. This is often erroneously
     attributed to skin effect, a phenomenon that tends to
     inhibit alternating current from flowing inside
     conducting media. It was thought that in the body,
     Tesla currents travelled close to the skin surface,
     making them safer than lower frequency electric
     currents. In fact, in the early 1900s a major use of Tesla
     coils was to apply high frequency current directly to the
     body in electrotherapy.

     Although skin effect limits Tesla currents to the outer
     fraction of an inch in metal conductors, the 'skin depth'
     of human flesh at typical Tesla coil frequencies is still
     of the order of 60 inches (150 cm) or more.[40] [41] [42]
     [43] [44]
               This means that high frequency currents will
     still preferentially flow through deeper, better
     conducting, portions of an experimenter's body such as
     the circulatory and nervous systems. The reason for the        Student conducting Tesla coil streamers through his body, 1909
     lack of pain is that a human being's nervous system
     does not sense the flow of potentially dangerous electrical currents above 15–20 kHz; essentially, in order for nerves
     to be activated, a significant number of ions must cross their membrane before the current (and hence voltage)
     reverses. Since the body no longer provides a warning 'shock', novices may touch the output streamers of small Tesla
     coils without feeling painful shocks. However, there is anecdotal evidence among Tesla coil experimenters that
     temporary tissue damage may still occur and be observed as muscle pain, joint pain, or tingling for hours or even
     days afterwards. This is believed to be caused by the damaging effects of internal current flow, and is especially
     common with continuous wave (CW), solid state or vacuum tube type Tesla coils. It is, however, of note that certain
     transformers can be used to provide alternating current with a frequency high enough so that the skin depth becomes
     small enough for the voltage to be safe. As this number is inversely proportional to the root of the frequency, this is
     fairly high; the number is in the megahertz.

     Large Tesla coils and magnifiers can deliver dangerous levels of high frequency current, and they can also develop
     significantly higher voltages (often 250,000–500,000 volts, or more). Because of the higher voltages, large systems
     can deliver higher energy, potentially lethal, repetitive high voltage capacitor discharges from their top terminals.
     Doubling the output voltage quadruples the electrostatic energy stored in a given top terminal capacitance. If an
     unwary experimenter accidentally places himself in path of the high voltage capacitor discharge to ground, the low
Tesla coil                                                                                                                         112

     current electric shock can cause involuntary spasms of major muscle groups and may induce life-threatening
     ventricular fibrillation and cardiac arrest. Even lower power vacuum tube or solid state Tesla coils can deliver RF
     currents that are capable of causing temporary internal tissue, nerve, or joint damage through Joule heating. In
     addition, an RF arc can carbonize flesh, causing a painful and dangerous bone-deep RF burn that may take months to
     heal. Because of these risks, knowledgeable experimenters avoid contact with streamers from all but the smallest
     systems. Professionals usually use other means of protection such as a Faraday cage or a chain mail suit to prevent
     dangerous currents from entering their body.
     The most serious dangers associated with Tesla coil operation are associated with the primary circuit. It is the
     primary circuit that is capable of delivering a sufficient current at a significant voltage to stop the heart of a careless
     experimenter. Because these components are not the source of the trademark visual or auditory coil effects, they may
     easily be overlooked as the chief source of hazard. Should a high frequency arc strike the exposed primary coil
     while, at the same time, another arc has also been allowed to strike to a person, the ionized gas of the two arcs forms
     a circuit that may conduct lethal, low-frequency current from the primary into the person.
     Further, great care should be taken when working on the primary section of a coil even when it has been
     disconnected from its power source for some time. The tank capacitors can remain charged for days with enough
     energy to deliver a fatal shock. Proper designs should always include 'bleeder resistors' to bleed off stored charge
     from the capacitors. In addition, a safety shorting operation should be performed on each capacitor before any
     internal work is performed.[45]

     Instances and devices

                                                 Magnifier Configurations

                                                Classically driven        Later-type
                                                configuration.              driven
                                                                        Pancake may
                                                                        be horizontal;
                                                                            lead to
                                                                         resonator is
                                                                         kept clear of

     Tesla's Colorado Springs laboratory possessed one of the largest Tesla coils ever built, known as the "Magnifying
     Transmitter". The Magnifying Transmitter is somewhat different from classic 2-coil Tesla coils. A Magnifier uses a
Tesla coil                                                                                                                       113

     2-coil 'driver' to excite the base of a third coil ('resonator') that is located some distance from the driver. The
     operating principles of both systems are similar. The world's largest currently existing 2-coil Tesla coil is a
     130,000-watt unit, part of a 38-foot-tall (12 m) sculpture. It is owned by Alan Gibbs and currently resides in a private
     sculpture park at Kakanui Point near Auckland, New Zealand.[48]
     The Tesla coil is an early predecessor (along with the induction coil) of a more modern device called a flyback
     transformer, which provides the voltage needed to power the cathode ray tube used in some televisions and computer
     monitors. The disruptive discharge coil remains in common use as the ignition coil[49] [50] or spark coil in the
     ignition system of an internal combustion engine. These two devices do not use resonance to accumulate energy,
     however, which is the distinguishing feature of a Tesla coil. They do use inductive "kick", the forced, abrupt decay
     of the magnetic field, such that a voltage is provided by the coil at its primary terminals that is much greater than the
     voltage that was applied to establish the magnetic field, and it is this higher voltage that is then multiplied by the
     transformer turns ratio. Thus, they do store energy, and a Tesla resonator stores energy. A modern, low power
     variant of the Tesla coil is also used to power plasma globe sculptures and similar devices.
     Scientists working with a glass vacuum line (e.g. chemists working with volatile substances in the gas phase, inside a
     system of glass tubes, taps and bulbs) test for the presence of tiny pin-holes in the apparatus (especially a newly
     blown piece of glassware) using a Tesla coil. When the system is evacuated and the discharging end of the coil
     moved over the glass, the discharge travels through any pin-hole immediately below it and thus illuminates the hole,
     indicating points that need to be annealed or re-blown before they can be used in an experiment.

     Tesla coils are very popular devices among certain electrical engineers and electronics enthusiasts. Builders of Tesla
     coils as a hobby are called "coilers". A very large tesla coil, designed and built by Syd Klinge, is shown every year at
     the Coachella Valley Music and Arts Festival, in Coachella, Indio, California, USA. There are "coiling" conventions
     where people attend with their home-made Tesla coils and other electrical devices of interest.
     Low power Tesla coils are also sometimes used as a high voltage source for Kirlian photography.[51]
     Tesla coils can also be used to create music by modulating the system's effective "break rate" (i.e., the rate and
     duration of high power RF bursts) via MIDI data and a control unit. The actual MIDI data is interpreted by a
     microcontroller which converts the MIDI data into a PWM output which can be sent to the Tesla coil via a fiber
     optic interface.[52] [53] The YouTube video Super Mario Brothers theme in stereo and harmony on two coils [54]
     shows a performance on matching solid state coils operating at 41 kHz. The coils were built and operated by
     designer hobbyists Jeff Larson and Steve Ward. The device has been named the Zeusaphone, after Zeus, Greek god
     of lightning, and as a play on words referencing the Sousaphone.

     In popular culture
     Tesla coils are popular devices in films and videos games, such as Fallout 3 and many other computer games. It has
     played a role in novels and have even appeared on stage in opera:
     • The Jim Jarmusch film Coffee and Cigarettes (2003) featured a segment starring Jack and Meg White from the
       band The White Stripes entitled "Jack shows Meg his Tesla coil". In the segment, the pair are having a coffee.
       Jack explains the work of Nikola Tesla to Meg and demonstrates the coil he has by his side.
     • A Tesla coil was used to produce all of the V'Ger lightning effects for "Star Trek: The Motion Picture" (1979).
       Production was carried out at an airfield by teams of crew members working around the clock in order to make
       the very-cramped schedule; so much so that other members of the production crew (including Special Visual
       Effects director/supervisor Douglas Trumbull) were called in to staff it.
     • A Tesla disk was used to produce all of the 'headboards' in the Borg regeneration stations in the movie Star Trek:
       First Contact, as well as in all the Borg-related scenes of Star Trek: The Next Generation and Star Trek: Voyager.
Tesla coil                                                                                                                  114

     • In the opera Tesla - Lightning in His Hand, a huge Tesla coil appears on stage, enclosed in a Faraday cage. As the
       character of Tesla walks towards the coil, the voltage that comes off the top of the coil with a huge cracking
       sound forms a corona that looks like a bolt of lightning and appears to illuminate the globe in Tesla's hand. The
       installation of the coil took two people seven days, and was managed by a retired head physicist of Australia's
       telecommunications company Telstra.
     • The performance group ArcAttack, the first group to utilize musical Tesla Coils as an instrument in their act,[55]
       originate from the USA and have been touring locally and internationally since March 2006. They have also
       appeared on NBC's America's Got Talent.[56]
     • The musical group, Man or Astro-man? uses a spark gap Tesla coil as a lighting effect during their
       performances.[57] [58]
     • Command & Conquer Red Alert 1,2 and 3 feature an in-game defense system using tesla coils.
     • A Fallout 3 Broken Steel [59] quest [60] requires you to search an abandoned power plant for a pre-war Tesla coil.
     • In the movie The Sorcerer's Apprentice (2010 film), Dave uses a Tesla coil to impress Becky and to defeat Maxim
       Horvath and Morgana le Fay.

     Related patents
     Tesla's patents
             See also: List of Tesla patents
     •   "Electrical Transformer Or Induction Device". U.S. Patent No. 433,702, August 5, 1890[61]
     •   "Means for Generating Electric Currents", U.S. Patent No. 514,168, February 6, 1894
     •   "Electrical Transformer", Patent No. 593,138, November 2, 1897
     •   "Method Of Utilizing Radiant Energy", Patent No. 685,958 November 5, 1901
     •   "Method of Signaling", U.S. Patent No. 723,188, March 17, 1903
     •   "System of Signaling", U.S. Patent No. 725,605, April 14, 1903
     •   " Apparatus for Transmitting Electrical Energy" [62], January 18, 1902, U.S. Patent 1,119,732, December 1, 1914
         (available at U.S. Patent 1,119,732 [8]
     Others' patents
     • J. S. Stone, U.S. Patent 714,832 [63], "Apparatus for amplifying electromagnetic signal-waves". (Filed January 23,
       1901; Issued December 2, 1902)
     • A. Nickle, U.S. Patent 2125804 [64], "Antenna". (Filed May 25, 1934; Issued August 2, 1938)
     • William W. Brown, U.S. Patent 2059186 [65], "Antenna structure". (Filed May 25, 1934; Issued October 27,
     • Robert B. Dome, U.S. Patent 2101674 [66], "Antenna". (Filed May 25, 1934; Issued December 7, 1937)
     • Armstrong, E. H., U.S. Patent 1,113,149 [67], "Wireless receiving system". 1914.
     • Armstrong, E. H., U.S. Patent 1,342,885 [68], "Method of receiving high frequency oscillation". 1922.
     • Armstrong, E. H., U.S. Patent 1,424,065 [69], "Signalling system". 1922.
     • Gerhard Freiherr Du Prel, U.S. Patent 1675882 [70], "High frequency circuit". (Filed August 11, 1925; Issued July
       3, 1928)
     • Leydorf, G. F., U.S. Patent 3,278,937 [71], "Antenna near field coupling system". 1966.
     • Van Voorhies, U.S. Patent 6,218,998 [72], "Toroidal helical antenna"
     • Gene Koonce, U.S. Patent 6933819 [73], "Multifrequency electro-magnetic field generator". (Filed October 29,
       2004; Issued August 23, 2005)[74]
Tesla coil                                                                                                                                                115

     [1] Uth, Robert (December 12, 2000). "Tesla coil" (http:/ / www. pbs. org/ tesla/ ins/ lab_tescoil. html). Tesla: Master of Lightning. .
         Retrieved 2008-05-20.
     [2] Tilbury, Mitch (2007). The Ultimate Tesla Coil Design and Construction Guide (http:/ / books. google. com/ books?id=o39HRjMqTuwC&
         pg=PT27& dq="tesla+ coil"+ spark+ transmitter& hl=en& ei=Qt_xS8reHInKsAO09M2SDA& sa=X& oi=book_result& ct=result&
         resnum=1& ved=0CC4Q6AEwAA#v=onepage& q="tesla coil" spark transmitter& f=false). New York: McGraw-Hill Professional. pp. 1.
         ISBN 0071497374. .
     [3] Ramsey, Rolla (1937). Experimental Radio, 4th Ed. (http:/ / books. google. com/ books?id=AGrPAAAAMAAJ& q="oscillating+
         transformer"+ "Tesla+ coil"& dq="oscillating+ transformer"+ "Tesla+ coil"& hl=en& ei=XybzS7eeG4yIswP5-7R_& sa=X&
         oi=book_result& ct=result& resnum=1& ved=0CDIQ6AEwAA). New York: Ramsey Publishing. pp. 175. .
     [4] This is an early electronics magazine.
     [5] N. Tesla, US patent No. 1,119,732. "I employ a terminal of relatively small capacity, which I charge to as high a pressure as practicable."
         (emphasis added) Tesla's lightning rod, U.S. Patent 1266175 (http:/ / www. google. com/ patents?vid=1266175), goes more into this subject.
         The reader is also referred to the U.S. Patent 645576 (http:/ / www. google. com/ patents?vid=645576), U.S. Patent 649621 (http:/ / www.
         google. com/ patents?vid=649621), U.S. Patent 787412 (http:/ / www. google. com/ patents?vid=787412), and U.S. Patent 1119732 (http:/ /
         www. google. com/ patents?vid=1119732).
     [6] Patent 1119732, lines 53 to 69; In order to develop the greatest energy in the circuit, Tesla elevated the conductor with a large radius of
         curvature or was composed of separate elements which in conglomeration had a large radius.
     [7] In "Selected Patent Wrappers from the National Archives", by John Ratzlaff (1981; ISBN 0-9603536-2-3), there was a variety of terminals
         described by Tesla. Besides the torus shaped terminal, he applied for hemi-spherical and oblate termininals. A total of 5 different terminals
         were applied for, but four were rejected.
     [8] http:/ / www. google. com/ patents?vid=1,119,732
     [9] This is equivalent to hundreds of thousands of horsepower
     [10] Tesla, Nikola, "The True Wireless". Electrical Experimenter, May 1919. ( Available at (http:/ / www. pbs. org/ tesla/ res/ res_art06.
     [11] U.S. Patent 645576 (http:/ / www. google. com/ patents?vid=645576)
     [12] U.S. Patent 725605 (http:/ / www. google. com/ patents?vid=725605)
     [13] U.S. Patent 685957 (http:/ / www. google. com/ patents?vid=685957), Apparatus for the utilization of radiant energy, N. Tesla
     [14] U.S. Patent 685958 (http:/ / www. google. com/ patents?vid=685958), Method of utilizing of radiant energy, N. Tesla
     [15] "Apparatus for Transmitting Electrical Energy", Jan. 18, 1902, U.S. Patent 1,119,732, December 1, 1914 (available at U.S. Patent 1,119,732
         (http:/ / www. google. com/ patents?vid=1,119,732) and 21st Century Books' Apparatus for Transmitting Electrical Energy (http:/ / www.
         tfcbooks. com/ patents/ 1119732. htm))
     [16] Definition of "Hertzian" (http:/ / www. ul. com/ international/ india. html)
     [17] John J. O'Neill, Prodigal Genius: The Life of Nikola Tesla. Page 192.
     [18] Marc J. Seifer, Wizard: The Life and Times of Nikola Tesla. Page 228.
     [19] Marc J. Seifer, Wizard: The Life and Times of Nikola Tesla. Page 472. (cf. "Each tower could act as a sender or a receiver. In a letter to
         Katherine Johnson, Tesla explains the need for well over thirty such towers".)
     [20] U.S. Patent 0685956
     [21] U.S. Patent 0685955 Apparatus for Utilizing Effects Transmitted From A Distance To A Receiving Device Through Natural Media
     [22] G. L. Peterson, Rediscovering the Zenneck Surface Wave (http:/ / www. tfcbooks. com/ articles/ tws4. htm).
     [23] ' Energy-sucking' Radio Antennas (http:/ / amasci. com/ tesla/ tesceive. html), N. Tesla's Power Receiver.
     [24] William Beaty, " Tesla invented radio (http:/ / amasci. com/ tesla/ tradio. html)?". 1992.
     [25] Nikola Tesla's Contributions to Radio Developments (http:/ / www. tesla-symp06. org/ papers/ Tesla-Symp06_Aca. pdf).
     [26] A. H. Taylor, " Resonance in Aërial Systems (http:/ / prola. aps. org/ abstract/ PRI/ v18/ i4/ p230_1)". American Physical Society. Physical
         review. New York, N.Y.: Published for the American Physical Society by the American Institute of Physics. (cf. The Tesla coil in the receiver
         acts as a step-down transformer, and hence the current is greater than in the aerial itself.)
     [27] This would include being able to be "shock excited" by all electrical phenomena of transverse waves (those with vibrations perpendicular to
         the direction of the propagation) and longitudinal waves (those with vibrations parallel to the direction of the propagation). Further
         information can be found in U.S. Patent 685953 (http:/ / www. google. com/ patents?vid=685953), U.S. Patent 685954 (http:/ / www. google.
         com/ patents?vid=685954), U.S. Patent 685955 (http:/ / www. google. com/ patents?vid=685955), U.S. Patent 685956 (http:/ / www. google.
         com/ patents?vid=685956), U.S. Patent 685957 (http:/ / www. google. com/ patents?vid=685957) and U.S. Patent 685958 (http:/ / www.
         google. com/ patents?vid=685958).
     [28] Marc J. Seifer, Wizard: The Life and Times of Nikola Tesla. Page 221 (cf. "The inventor had tuned his equipment so carefully that “in one
         instance the devices recorded effects of lightning discharges fully 500 miles away […]"
     [29] Hermann Plauson, U.S. Patent 1540998 (http:/ / www. google. com/ patents?vid=1540998), "Conversion of atmospheric electric energy".
         Jun. 1925.
Tesla coil                                                                                                                                                   116

     [30] Nikola Tesla, " Tuned Lightning (http:/ / www. tfcbooks. com/ tesla/ 1907-03-08. htm)", English Mechanic and World of Science, March 8,
     [31] U.S. Patent 685958 (http:/ / www. google. com/ patents?vid=685958), Method of utilizing of radiant energy, N. Tesla
     [32] US685957 Utilization of Radiant Energy
     [33] U.S. Patent 0685953 Apparatus for Utilizing Effects Transmitted from a Distance to a Receiving Device through Natural Media
     [34] U.S. Patent 0685953 Apparatus for Utilizing Effects Transmitted from a Distance to a Receiving Device through Natural Media
     [35] U.S. Patent 0685953 Apparatus for Utilizing Effects Transmitted from a Distance to a Receiving Device through Natural Media
     [36] U.S. Patent 0685954 Method of Utilizing Effects Transmitted through Natural Media
     [37] Bell, Louis (1901). Electric Power Transmission; a Practical Treatise for Practical Men (http:/ / books. google. com/
         books?id=hSYKAAAAIAAJ& pg=RA3-PA110& lpg=RA3-PA110& dq="electric+ power+ transmission+ a+ practical+ treatise+ for+
         practical+ men"& source=web& ots=FTKTW8smJm& sig=8kcwxaAWKmm5-1ysBFR52oQiRik#PRA1-PA10,M1). p. 10. . Retrieved
         2007-02-15. "Both kinds of strains exist in radiant energy, […] The stresses in electro-magnetic energy are at right angles both to the
         electrostatic stresses and to the direction of their motion or flow."
     [38] U.S. Patent 0685953 "Apparatus for Utilizing Effects Transmitted from a Distance to a Receiving Device through Natural Media"
     [39] A. H. Taylor, " Resonance in Aërial Systems (http:/ / prola. aps. org/ abstract/ PRI/ v18/ i4/ p230_1)". American Physical Society. Physical
         review. New York, N.Y.: Published for the American Physical Society by the American Institute of Physics. (cf. The Tesla coil in the receiver
         act as a step-down transformer, and hence the current is greater than in the aerial itself.)
     [40] General Tesla coil construction plans (http:/ / users. tkk. fi/ ~jwagner/ tesla/ tc-plans. htm#skineffect)
     [41] Skin Effect/Material Constants (http:/ / web. archive. org/ web/ 20041216165015/ http:/ / www. geocities. com/ capecanaveral/ hangar/
         6160/ skineffect. html)
     [42] Re: skin depth in round conductors Re: 8 kHz Tesla Coil (http:/ / www. pupman. com/ listarchives/ 2005/ Sep/ msg00726. html)
     [43] Re: Mini Tesla. Dangerous Stunts (http:/ / www. pupman. com/ listarchives/ 1999/ January/ msg00096. html)
     [44] An independent analysis for a small coil yields 2.5 inches in normal saline, which is just as serious a health hazard as 60 inches for practical
     [45] Tesla Coils Safety Information (http:/ / www. pupman. com/ safety. htm)".
     [46] Cooper, John. F., "Magnifying Transmitter 1.jpg circuit diagram (http:/ / www. tesla-coil. com/ images/ Magnifier)". (http:/
         / www. Tesla-Coil. com).
     [47] Cooper, John. F., "Magnifying Transmitter 2.jpg alternate circuit diagram (http:/ / www. tesla-coil. com/ images/ Magnifier)". (http:/ / www. Tesla-Coil. com).
     [48] The Electrum Project (http:/ / www. lightninglab. org/ Projects/ electrum/ index. htm), Lightning On Demand, Brisbane CA
     [49] Ignitions circuit, H. B. Holthouse. U.S. Patent 2117422 (http:/ / www. google. com/ patents?vid=2117422)
     [50] Method and apparatus for producing ignition, Donald W. Randolph, U.S. Patent 2093848 (http:/ / www. google. com/
     [51] " Corona Discharge Electrographic Imaging Technology (http:/ / www. cebunet. com/ kirlian/ )"
     [52] Interview with ArcAttack on Odd Instruments (http:/ / oddstrument. com/ 2008/ 05/ 20/ arcattacks-musical-tesla-coils-shocking/ )
     [53] Duckon 2007-Steve Ward's Singing Tesla Coil video (http:/ / www. hauntedfrog. com/ gt/ movies/ 2007/ duckon/ SingingTeslaShow. html)
     [54] http:/ / www. youtube. com/ watch?v=B1O2jcfOylU
     [55] http:/ / news. cnet. com/ 8301-17938_105-9696332-1. html?tag=mncol
     [56] http:/ / www. thenational. ae/ apps/ pbcs. dll/ article?AID=/ 20090719/ ART/ 707189984
     [57] http:/ / www. rollingstone. com/ artists/ manorastroman
     [58] http:/ / www. bbc. co. uk/ music/ artists/ 28c5d97f-4321-4ef4-8ac2-d9d93b0eb16c
     [59] http:/ / fallout. wikia. com/ wiki/ Broken_Steel
     [60] http:/ / fallout. wikia. com/ wiki/ Shock_Value
     [61] History of Wireless By Tapan K. Sarkar, et al. ISBN 0471783013
     [62] http:/ / www. tfcbooks. com/ patents/ 1119732. htm
     [63] http:/ / www. google. com/ patents?vid=714,832
     [64] http:/ / www. google. com/ patents?vid=2125804
     [65] http:/ / www. google. com/ patents?vid=2059186
     [66] http:/ / www. google. com/ patents?vid=2101674
     [67] http:/ / www. google. com/ patents?vid=1,113,149
     [68] http:/ / www. google. com/ patents?vid=1,342,885
     [69] http:/ / www. google. com/ patents?vid=1,424,065
     [70] http:/ / www. google. com/ patents?vid=1675882
     [71] http:/ / www. google. com/ patents?vid=3,278,937
     [72] http:/ / www. google. com/ patents?vid=6,218,998
     [73] http:/ / www. google. com/ patents?vid=6933819
     [74] A Multifrequency electro-magnetic field generator that is capable of generating electro-magnetic radial fields, horizontal fields and spiral
         flux fields that are projected at a distance from the device and collected at the far end of the device by an antenna.
Tesla coil                                                                                                                   117

     Further reading
     Operation and other information
     • Armagnat, H., & Kenyon, O. A. (1908). The theory, design and construction of induction coils (http://books. New York: McGraw.
     • Haller, G. F., & Cunningham, E. T. (1910). The Tesla high frequency coil, its construction and uses (http:// New York: D. Van
       Nostrand Co.
     • Iannini, R. E. (2003). Electronic gadgets for the evil genius: 21 build-it-yourself projects (
       com/books?vid=ISBN0071426094&id=AAwboUmMZmEC). TAB electronics. New York: McGraw-Hill. Pages
       137 – 202.
     • Corum, Kenneth L. and James F. " Tesla Coils and the Failure of Lumped-Element Circuit Theory (http://www."
     • Nicholson, Paul, " Tesla Secondary Simulation Project (" (Current state of the art in
       rigorously describing Tesla coil secondary behavior through theoretical analysis, simulation and testing of results
       in practice)
     • Bill Beaty " Nikola Tesla Coil Information (".
     • Vujovic, Ljubo, " Tesla Coil (". Tesla Memorial Society of New
     • Hickman, Bert, " Tesla Coil Information Center ("
     • Cooper, John. F., "Magnifying Transmitter early-type circuit diagram (
       Magnifier 1.jpg); Later-type circuit diagram ( 2.jpg)". (
     Electrical World
     • "The Development of High Frequency Currents for Practical Application"., The Electrical World, Vol 32, No. 8.
     • "Boundless Space: A Bus Bar". The Electrical World, Vol 32, No. 19.
     Other publications
     • A. L. Cullen, J. Dobson, "The Corona Breakdown of Aerials in Air at Low Pressures". Proceedings of the Royal
       Society of London. Series A, Mathematical and Physical Sciences, Vol. 271, No. 1347 (February 12, 1963),
       pp. 551–564
     • Bieniosek, F. M., "Triple Resonance Pulse Transformer Circuit". Review of Scientific Instruments, 61 (6).
     • Corum, J. F., and K. L. Corum, "RF Coils, Helical Resonators and Voltage Magnification by Coherent Spatial
       Modes". IEEE, 2001.
     • de Queiroz, Antonio Carlos M., "Synthesis of Multiple Resonance Networks". Universidade Federal do Rio de
       Janeiro, Brazil. EE/COPE.
     • Haller, George Francis, and Elmer Tiling Cunningham, "The Tesla high frequency coil, its construction and uses".
       New York, D. Van Nostrand company, 1910.
     • Hartley, R. V. L., "Oscillations with Non-linear Reactances". Bell Systems Technical Journal, Sun Publishing.
     • Norrie, H. S., "Induction Coils: How to make, use, and repair them". Norman H. Schneider, 1907, New York. 4th
     • Reed, J. L., "Greater voltage gain for Tesla transformer accelerators", Review of Scientific Instruments, 59,
       p. 2300, (1988).
     • Curtis, Thomas Stanley, High Frequency Apparatus: Its Construction and Practical Application (http://books. Everyday
       Mechanics Co., 1916.
Tesla coil                                                                                                                    118

     External links
     • Tesla coil (
       ) at the Open Directory Project

     Toroidal inductors and transformers

                         Several small toroidal inductors. The major scale is in inches. A small toroidal transformer.

     Toroidal inductors and transformers are electronic components, typically consisting of a circular ring-shaped
     magnetic core of iron powder, ferrite, or other material around which wire is coiled to make an inductor. Toroidal
     coils are used in a broad range of applications, such as high-frequency coils and transformers. Toroidal inductors can
     have higher Q factors and higher inductance than similarly constructed solenoid coils. This is due largely to the
     smaller number of turns required when the core provides a closed magnetic path. The magnetic flux in a high
     permeability toroid is largely confined to the core; the confinement reduces the energy that can be absorbed by
     nearby objects, so toroidal cores offer some self-shielding.
     In the geometry of torus-shaped magnetic fields, the poloidal flux direction threads the "donut hole" in the center of
     the torus, while the toroidal flux direction is parallel the core of the torus.
Toroidal inductors and transformers                                                                                                                   119

    Total B Field Confinement by Toroidal Inductors
    In some circumstance, the current in the winding of a toroidal inductor contributes only to the B field inside the
    windings and makes no contribution to the magnetic B field outside of the windings.

    Sufficient conditions for total internal confinement of the B field

    Fig. 1. Coordinate system. The Z axis is the nominal axis of symmetry. The X Fig. 2. An axially symmetric toroidal inductor with no circumferential
     axis chosen arbitrarily to line up with the starting point of the winding. ρ is                            current.
          called the radial direction. θ is called the circumferential direction.

    The absence of circumferential current [1] (please refer to figure 1 of this section for definition of directions) and the
    axially symmetric layout of the conductors and magnetic materials [1] [2] [3] are sufficient conditions for total internal
    confinement of the B field. (Some authors prefer to use the H field). Because of the symmetry, the lines of B flux
    must form circles of constant intensity centered on the axis of symmetry. The only lines of B flux that encircle any
    current are those that are inside the toroidal winding. Therefore, from Ampere's circuital law, the intensity of the B
    field must be zero outside the windings.[3]
    Figure 3 of this section shows the most
    common toroidal winding. It fails both
    requirements for total B field confinement.
    Looking out from the axis, sometimes the
    winding is on the inside of the core and
    sometimes it is on the outside of the core. It
    is not axially symmetric in the near region.
    However, at points a distance of several
    times the winding spacing, the toroid does
    look symmetric[4] . There is still the problem
    of the circumferential current. No matter
    how many times the winding encircles the
    core and no matter how thin the wire, this
    toroidal inductor will function as a one coil
    loop in the plane of the toroid. This winding
    will also produce and be susceptible to an E                               Fig. 3. Toroidal inductor with circumferential current

    field in the plane of the inductor.
Toroidal inductors and transformers                                                                                                                          120

      Figures 4-6 show different ways to neutralize the circumferential current. Figure 4 is the simplest and has the
      advantage that the return wire can be added after the inductor is bought or built.

                                                                                                                   Fig. 6. Circumferential current countered with a split
                                                                                                                                     return winding.

 Fig. 4. Circumferential current countered with a return Fig. 5. Circumferential current countered with a return
 wire. The wire is white and runs between the outer rim                         winding.
  of the inductor and the outer portion of the winding.

      E Field in the Plane of the Toroid

                 Fig. 7. Simple toroid and the E-field produced. +/- 100             Fig. 8. Voltage distribtion with return winding. +/- 100
                                Volt excitation assumed.                                            Volt excitation assumed.

      There will be a distribution of potential along the winding. This can lead to an E-Field in the plane of the toroid and
      also a susceptibility to an E field in the plane of the toroid as shown in figure 7. This can be mitigated by using a
      return winding as shown on figure 8. With this winding, each place the winding crosses itself, the two parts will be at
      equal and opposite polarity which substantially reduces the E field generated in the plane.
Toroidal inductors and transformers                                                                                                      121

    Torroidal Inductor/Transformer and Magnetic Vector Potential
                                                                                               See Feynman chapter 14[5] and 15[6]
                                                                                               for a general discussion of magnetic
                                                                                               vector potential. See Feynman page
                                                                                               15-11 [7] for a diagram of the magnetic
                                                                                               vector potential around a long thin
                                                                                               solenoid which also exhibits total
                                                                                               internal confinement of the B field, at
                                                                                               least in the infinite limit.

       Showing the development of the magnetic vector potential around a symmetric torroidal

    The A field is accurate when using the assumption                          . This would be true under the following assumptions:
    • 1. the Coulomb gauge is used
    • 2. the Lorenz gauge is used and there is no distribution of charge,
    • 3. the Lorenz gauge is used and zero frequency is assumed

    • 4. the Lorenz gauge is used and a non-zero frequency that is low enough to neglect                      is assumed.

    Number 4 will be presumed for the rest of this section and may be referred to the "quasi-static condition".
    Although the axially symmetric toroidal inductor with no circumferential current totally confines the B field within
    the windings, the A field (magnetic vector potential) is not confined. Arrow #1 in the picture depicts the vector
    potential on the axis of symmetry. Radial current sections a and b are equal distances from the axis but pointed in
    opposite directions, so they will cancel. Likewise segments c and d cancel. In fact all the radial current segments
    cancel. The situation for axial currents is different. The axial current on the outside of the toroid is pointed down and
    the axial current on the inside of the toroid is pointed up. Each axial current segment on the outside of the toroid can
    be matched with an equal but oppositely directed segment on the inside of the toroid. The segments on the inside are
    closer than the segments on the outside to the axis, therefore there is a net upward component of the A field along the
    axis of symmetry.
Toroidal inductors and transformers                                                                                                            122

       Representing the magnetic vector potential (A), magnetic flux (B), and current density (j)
       fields around a toroidal inductor of circular cross section. Thicker lines indicate field lines
         of higher average intensity. Circles in cross section of the core represent B flux coming
        out of the picture. Plus signs on the other cross section of the core represent B flux going
                               into the picture. Div A = 0 has been assumed.

    Since the equations                           , and                           (assuming quasi-static conditions, i.e.           ) have

    the same form, then the lines and contours of A relate to B like the lines and contours of B relate to j. Thus, a
    depiction of the A field around a loop of B flux (as would be produced in a toroidal inductor) is qualitatively the
    same as the B field around a loop of current. The figure to the left is an artist's depiction of the A field around a
    totoidal inductor. The thicker lines indicate paths of higher average intensity (shorter paths have higher intensity so
    that the path integral is the same). The lines are just drawn to look good and impart general look of the A field.

    Toroidal Transformer Action in the Presence of Total B field Confinement
    The E and B fields can be computed from the A and                          (scalar electric potential) fields
                                 [8]                                     [8]
                                       and :                                   and so even if the region outside the windings is devoid of B

           field, it is filled with non-zero E field.

           The quantity                is responsible for the desirable magnetic field coupling between primary and secondary

           while the quantity                  is responsible for the undesirable electric field coupling between primary and
          secondary. Transformer designers attempt to minimize the electric field coupling. For the rest of this section,
                will assumed to be zero unless otherwise specified.
    Stokes theorem applies[9] , so that the path integral of A is equal to the enclosed B flux, just as the path integral B is
    equal to a constant times the enclosed current
    The path integral of E along the secondary winding gives the secondary's induced EMF (Electro-Motive Force).

    which says the EMF is equal to the time rate of change of the B flux enclosed by the winding, which is the usual
Toroidal inductors and transformers                                                                                                         123

    Toroidal Transformer Poynting Vector Coupling from Primary to Secondary in the
    Presence of Total B field Confinement

                                                                                            Explanation of the Figure

                                                                                                 This figure shows the half section of a
                                                                                                 toroidal     transformer.   Quasi-static
                                                                                                 conditions are assumed, so the phase of
                                                                                                 each field is everywhere the same. The
                                                                                                 transformer, its windings and all things
                                                                                                 are distributed symmetrically about the
                                                                                                 axis of symmetry. The windings are
       In this figure, blue dots indicate where B flux from the primary current comes out of the
                                                                                                 such that there is no circumferential
                      picture and plus signs indicate where it goes into the picture.            current. The requirements are met for
                                                                                                 full internal confinement of the B field
    due to the primary current. The core and primary winding are represented by the gray-brown torus. The primary
    winding is not shown, but the current in the winding at the cross section surface is shown as gold (or orange)
    ellipses. The B field caused by the primary current is entirely confined to the region enclosed by the primary winding
    (i.e. the core). Blue dots on the left hand cross section indicate that lines of B flux in the core come out of the left
    hand cross section. On the other cross section, blue plus signs indicate that the B flux enters there. The E field
    sourced from the primary currents is shown as green ellipses. The secondary winding is shown as a brown line
    coming directly down the axis of symmetry. In normal practice, the two ends of the secondary are connected
    together with a long wire that stays well away from the torus, but to maintain the absolute axial symmetry, the entire
    apparatus is envisioned as being inside a perfectly conductive sphere with the secondary wire "grounded" to the
    inside of the sphere at each end. The secondary is made of resistance wire, so there is no separate load. The E field
    along the secondary causes current in the secondary (yellow arrows) which causes a B field around the secondary
    (shown as blue ellipses). This B field fills space, including inside the transformer core, so in the end, there is
    continuous non-zero B field from the primary to the secondary, if the secondary is not open circuited. The cross
    product of the E field (sourced from primary currents) and the B field (sourced from the secondary currents) forms
    the Poynting vector which points from the primary toward the secondary.

    • US 4127238 [10], Potthoff, Clifford M., "Toroidal Core Winder", issued November 28, 1978

    External links
    • Approximate inductance of a toroid [11] includes formula, but assumes circular windings
    • toroid inductance calculator [12] more practical, allows rectangular winding

    [1] Griffiths (1989, p. 222)
    [2] Reitz, Milford & Christy (1993, p. 244)
    [3] Halliday & Resnick (1962, p. 859)
    [4] Hayt (1989, p. 231)
    [5] Feynman (1964, p. 14_1-14_10)
    [6] Feynman (1964, p. 15_1-15_16)
    [7] Feynman (1964, p. 15_11)
    [8] Feynman (1964, p. 15_15)
Toroidal inductors and transformers                                                                                             124

    [9] Purcell (1963, p. 249)
    [10] http:/ / v3. espacenet. com/ textdoc?DB=EPODOC& IDX=US4127238
    [11] http:/ / hyperphysics. phy-astr. gsu. edu/ hbase/ magnetic/ indtor. html
    [12] http:/ / www. mantaro. com/ resources/ impedance_calculator. htm#toroid_inductance

    • Griffiths, David (1989), Introduction to Electrodynamics, Prentice-Hall, ISBN 0134813677
    • Halliday; Resnick (1962), Physics, part two, John Wiley & Sons
    • Hayt, William (1989), Engineering Electromagnetics (5th ed.), McGraw-Hill, ISBN 0070274061
    • Purcell, Edward M. (1965), Electricity and Magnetism, Berkeley Physics Course, II, McGraw-Hill,
      ISBN 978-0070048591
    • Reitz, John R.; Milford, Frederick J.; Christy, Robert W. (1993), Foundations of Electromagnetic Theory,
      Addison-Wesley, ISBN 0201526247

    Transformer oil
    Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has
    excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors,
    fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate,
    suppress corona and arcing, and to serve as a coolant.

    The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live
    parts, transformer oil must remain stable at high temperatures for an extended period. To improve cooling of large
    power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural
    convection. Very large or high-power transformers (with capacities of thousands of KVA) may also have cooling
    fans, oil pumps, and even oil-to-water heat exchangers.
    Large, high voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of
    a vacuum, or both to ensure that the transformer is completely free of water vapor before the cooling oil is
    introduced. This helps prevent corona formation and subsequent electrical breakdown under load.
    Oil filled transformers with a conservator (an oil tank above the transformer) tend to be equipped with Buchholz
    relays. These are safety devices that detect the build up of gases (such as acetylene) inside the transformer (a side
    effect of corona or an electric arc in the windings) and switch off the transformer. Transformers without conservators
    are usually equipped with sudden pressure relays, which perform a similar function as the Buchholz relay.
    The flash point (min) and pour point (max) are 140 °C and −6 °C respectively. The dielectric strength of new
    untreated oil is 12 MV/m (RMS) and after treatment it should be >24 MV/m (RMS).

    Oil transformer
    Large transformers for indoor use must either be of the dry type, that is, containing no liquid, or use a less-flammable
    Well into the 1970s, polychlorinated biphenyls (PCB)s were often used as a dielectric fluid since they are not
    flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan. Starting
    in the early 1970s, concerns about the toxicity of PCBs have led to their banning in many countries.
    Today, non-toxic, stable silicon-based or fluorinated hydrocarbons are used, where the added expense of a
    fire-resistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based
Transformer oil                                                                                                              125

    dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly
    common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and
    have a lower volatility and a higher flash points than mineral oil.

    Transformer Oil Testing
    Transformer oils are subject to electrical and mechanical stresses while a transformer is in operation. In addition
    there are contaminations caused due to chemical interactions with windings and other solid insulations, catalyzed by
    high operating temperature. As a result the original chemical properties of transformer oil changes gradually,
    rendering it ineffective for its intended purpose after many years. Hence this oil has to be periodically tested to
    ascertain its basic electrical properties, and make sure it is suitable for further use or necessary actions like
    filtration/regeneration has to be done. These tests can be divided into:
    1.   Dissolved gas analysis
    2.   Furan analysis
    3.   PCB analysis
    4.   General electrical & physical tests:
    • Color & Appearance
    • Breakdown Voltage
    •    Water Content
    •    Acidity (Neutralization Value)
    •    Dielectric Dissipation Factor
    •    Resistivity
    •    Sediments & Sludge
    •    Interfacial Tension
    •    Flash Point
    •    Pour Point
    •    Density
    •    Kinematic Viscosity
    The details of conducting these tests is available in standards released by IEC, ASTM, IS, BS, and testing can be
    done by either of the methods. The Furan and DGA tests are specifically not for determining the quality of
    transformer oil, but for determining any abnormalities in the internal windings of the transformer or the paper
    insulation of the transformer, which cannot be otherwise detected without a complete overhaul of the transformer.
    Suggested intervals for these test are:
    • General and physical tests - bi-yearly
    • Dissolved gas analysis - yearly
    • Furan testing - once every 2 years, subject to the transformer being in operation for min 5 years.

    On-site transformer oil testing
    As in most countries transformer oil testing is mandatory, suppliers of test equipment have developed portable
    devices for on-site transformer oil testing.
    To determine the insulating property of the dielectric oil, an oil sample is taken from the device under test, and its
    breakdown voltage is measured on-site according the following test sequence:
    • In the vessel, two standard-compliant test electrodes with a typical clearance of 2.5 mm are surrounded by the
      insulating oil.
    • During the test, a test voltage is applied to the electrodes. The test voltage is continuously increased up to the
      breakdown voltage with a constant slew rate of e.g. 2 kV/s.
    • Breakdown occures in an electric arc, leading to a collapse of the test voltage.
Transformer oil                                                                                                              126

    • Immediately after ignition of the arc, the test voltage is switched off automatically.
    • Ultra fast switch off is crucial, as the energy that is brought into the oil and is burning it during the breakdown,
      must be limited to keep the additional pollution by carbonisation as low as possible.
    • The root mean square value of the test voltage is measured at the very instant of the breakdown and is reported as
      the breakdown voltage.
    • After the test is completed, the insulating oil is stirred automatically and the test sequence is performed
    • The resulting breakdown voltage is calculated as mean value of the individual measurements.
    The lower the resulting breakdown voltage, the poorer the quality of the oil!

    • Less and nonflammable liquid-insulated transformers, approval standard class Number 3990, Factory Mutual
      Research Corporation, 1997.
    • McShane C.P. (2001) Relative properties of the new combustion-resistant vegetable oil-based dielectric coolants
      for distribution and power transformers. IEEE Trans. on Industry Applications, Vol.37, No.4, July/August 2001,
      pp.1132-1139, No. 0093-9994/01, 2001 IEEE.
    • “The Environmental technology verification program”, U.S. Environmental Protection Agency, Washington, DC,
      VS-R-02-02, June 2002. [1]
    • IEEE Guide for loading mineral-oil-immersed transformers, IEEE Standard C57.91-1995, 1996.

    [1] http:/ / www. epa. gov/ etv/ pdfs/ vrvs/ 06_vs_cooper. pdf
Transformer oil testing                                                                                                                127

     Transformer oil testing
     The insulation oil of voltage- and current-transformers fulfills the purpose of insulating as well as cooling. Thus, the
     dielectric quality of transformer oil is a matter of secure operation of a transformer.
     Since transformer oil deteriorates in its isolation and cooling behaviour due to ageing and pollution by dust particles
     or humidity, and due to its vital role, transformer oil must be subject to oil tests on a regular basis.
     In most countries such tests are even mandatory. Transformer oil
     testing sequences and procedures are defined by various international
     Periodic execution of transformer oil testing is as well in the very
     interest of energy supplying companies, as potential damage to the
     transformer insulation can be avoided by well timed substitution of the
     transformer oil. Lifetime of plant can be substantially increased and the
     requirement for new investment may be delayed.

                                                                                    Voltage breakdown during transformer oil testing

     Transformer oil testing procedure
     To assess the insulating property of dielectric transformer oil, a sample of the transformer oil is taken and its
     breakdown voltage is measured.
     • The transformer oil is filled in the vessel of the testing device. Two standard-compliant test electrodes with a
       typical clearance of 2.5 mm are surrounded by the dielectric oil.
     • A test voltage is applied to the electrodes and is continuously increased up to the breakdown voltage with a
       constant, standard-compliant slew rate of e.g. 2 kV/s.
     • At a certain voltage level breakdown occures in an electric arc, leading to a collapse of the test voltage.
     • An instant after ignition of the arc, the test voltage is switched off automatically by the testing device. Ultra fast
       switch off is highly desirable, as the carbonisation due to the electric are must be limited to keep the additional
       pollution as low as possible.
     • The transformer oil testing device measures and reports the root mean square value of the breakdown voltage.
     • After the transformer oil test is completed, the insultaion oil is stirred automatically and the test sequence is
       performed repeatedly. (Typically 5 Repetitions, depending on the standard)
     • As a result the breakdown voltage is calculated as mean value of the individual measurements.
     Conclusion: The lower the resulting breakdown voltage, the poorer the quality of the transformer oil!
Transformer oil testing                                                                                                    128

     On-site transformer oil testing
     Recently time consuming testing procedures in test labs have been replaced by on-site oil testing procedures. There
     are various manufacturers of portable oil testers.
     With low weight devices in the range of 20 to 40 kg tests up to 100 kV rms can be performed and reported on-site
     automatically. Some of them are even battery-powered and come with all sorts of accessories.

     International transformer oil testing standards
     •   VDE370-5/96
     •   OVE EN60156
     •   IEC 60156/95,
     •   ASTM1816-04-1
     •   ASTM1816-04-2
     •   ASTM877-02A
     •   ASTM877-02B
     •   AS1767.2.1
     •   BS EN60156
     •   NEN 10 156
     •   NF EN60156
     •   PA SEV EN60156
     •   SABS EN60156
     •   UNE EN60156


     External links
     • Portable transformer oil testers (
     • Battery-powered tranfsormer oil tester 100kV rms (
Transformer types                                                                                                                           129

    Transformer types
                                                          Circuit symbols
                        Transformer with two windings and iron core.

                        Step-down or step-up transformer. The symbol shows which winding has more turns, but not usually the exact ratio.

                        Transformer with three windings. The dots show the relative configuration of the windings.

                        Transformer with electrostatic screen preventing capacitive coupling between the windings.

    A variety of types of electrical transformer are made for different purposes. Despite their design differences, the
    various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key
    functional parts.

    Power transformers

    Laminated core
    This is the most common type of transformer, widely used in
    appliances to convert mains voltage to low voltage to power electronics
    • Widely available in power ratings ranging from mW to MW
    • Insulated lamination minimizes eddy current losses
    • Small appliance and electronic transformers may use a split bobbin,
      giving a high level of insulation between the windings
    • Rectangular core
    • Core laminate stampings are usually in EI shape pairs. Other shape
      pairs are sometimes used
    • Mu-metal shields can be fitted to reduce EMI (electromagnetic
    • A screen winding is occasionally used between the 2 power
    • Small appliance and electronics transformers may have a thermal
      cut out built in
    • Occasionally seen in low profile format for use in restricted spaces
    • Laminated core made with silicon steel with high permeability

                                                                                                      Laminated Core Transformer
Transformer types                                                                                                            130

    Doughnut shaped toroidal transformers are used to save space
    compared to EI cores, and sometimes to reduce external magnetic
    field. These use a ring shaped core, copper windings wrapped round
    this ring (and thus threaded through the ring during winding), and tape
    for insulation.
    Toroidal transformers compared to EI core transformers:
    • Lower external magnetic field
    • Smaller for a given power rating
    • Higher cost in most cases, as winding requires more complex and
      slower equipment
    • Less robust
    • Central fixing is either
      • bolt, large metal washers and rubber pads
      • bolt and potting resin
                                                                                            Toroidal Transformer
    • Over-tightening the central fixing bolt may short the windings
    • Greater inrush current at switch-on

    An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed
    voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion
    of the same winding. The higher voltage will be connected to the ends of the winding, and the lower voltage from
    one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 V across
    the entire winding, and 115 volts between one end and the tap. It can be connected to a 230 V supply to drive 115 V
    equipment, or reversed to drive 230 V equipment from 115 V. Since the current in the windings is lower, the
    transformer is smaller, lighter cheaper and more efficient. For voltage ratios not exceeding about 3:1, an
    autotransformer is cheaper, lighter, smaller and more efficient than an isolating (two-winding) transformer of the
    same rating. Large three-phase autotransformers are used in electric power distribution systems, for example, to
    interconnect 33 kV and 66 kV sub-transmission networks.

    By exposing part of the winding coils of an autotransformer, and making the secondary connection through a sliding
    carbon brush, an autotransformer with a near-continuously variable turns ratio can be obtained, allowing for wide
    voltage adjustment in very small increments.

    Induction regulator
    The induction regulator is similar in design to a wound-rotor induction motor but it is essentially a transformer
    whose output voltage is varied by rotating its secondary relative to the primary i.e. rotating the angular position of
    the rotor.
    It can be seen as a power transformer exploiting rotating magnetic fields.
    The major advantage of the induction regulator is that unlike variacs, they are practical for transformers over 5 kVA.
    Hence, such regulators find windspread use in high-voltage laboratories. [1]
Transformer types                                                                                                                    131

    Stray field transformer
    A stray field transformer has a significant stray field or a (sometimes adjustable) magnetic bypass in its core. It can
    act as a transformer with inherent current limitation due to its lower coupling between the primary and the secondary
    winding, which is unwanted in most other cases. The output and input currents are low enough to prevent thermal
    overload under each load condition - even if the secondary is shorted.
    Stray field transformers are used for arc welding and high voltage discharge lamps (cold cathode fluorescent lamps,
    series connected up to 7.5 kV AC working voltage). It acts both as voltage transformer and magnetic ballast.

    Polyphase transformers
    For three-phase power, three separate single-phase transformers can be
    used, or all three phases can be connected to a single polyphase
    transformer. The three primary windings are connected together and
    the three secondary windings are connected together. The most
    common connections are Y-Delta, Delta-Y, Delta-Delta and Y-Y. A
    vector group indicates the configuration of the windings and the phase
    angle difference between them. If a winding is connected to earth
    (grounded), the earth connection point is usually the center point of a Y
    winding. If the secondary is a Delta winding, the ground may be                   Example of Y Y Connection
    connected to a center tap on one winding (high leg delta) or one phase
    may be grounded (corner grounded delta). A special purpose polyphase transformer is the zigzag transformer. There
    are many possible configurations that may involve more or fewer than six windings and various tap connections.

    Resonant transformers
    A resonant transformer operates at the resonant frequency of one or
    more of its coils and (usually) an external capacitor. The resonant coil,
    usually the secondary, acts as an inductor, and is connected in series
    with a capacitor. When the primary coil is driven by a periodic source
    of alternating current, such as a square or sawtooth wave at the
    resonant frequency, each pulse of current helps to build up an
    oscillation in the secondary coil. Due to resonance, a very high voltage
    can develop across the secondary, until it is limited by some process
    such as electrical breakdown. These devices are used to generate high                A 25 kV flyback transformer being used to
                                                                                                     generate an arc.
    alternating voltages, and the current available can be much larger than
    that from electrostatic machines such as the Van de Graaff generator or
    Wimshurst machine.

    •   Tesla coil
    •   Oudin coil (or Oudin resonator; named after its inventor Paul Oudin)
    •   D'Arsonval apparatus
    •   Ignition coil or induction coil used in the ignition system of a petrol engine
    •   Flyback transformer of a CRT television set or video monitor.
    • Electrical breakdown and insulation testing of high voltage equipment and cables. In the latter case, the
      transformer's secondary is resonated with the cable's capacitance.
    Other applications of resonant transformers are as coupling between stages of a superheterodyne receiver, where the
    selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.
Transformer types                                                                                                                    132

    Constant voltage transformer
    By arranging particular magnetic properties of a transformer core, and installing a ferro-resonant tank circuit (a
    capacitor and an additional winding), a transformer can be arranged to automatically keep the secondary winding
    voltage relatively constant for varying primary supply without additional circuitry or manual adjustment.
    Ferro-resonant transformers run hotter than standard power transformers, because regulating action depends on core
    saturation, which reduces efficiency. The output waveform is heavily distorted unless careful measures are taken to
    prevent this. Saturating transformers provide a simple rugged method to stabilize an AC power supply.

    Ferrite core
    Ferrite core power transformers are widely used in switched-mode power supplies (SMPSs). The powder core
    enables high-frequency operation, and hence much smaller size-to-power ratio than laminated-iron transformers.
    Ferrite transformers are not used as power transformers at mains frequency since laminated iron cores cost less than
    an equivalent ferrite core.

    Planar transformer

    Manufacturers etch spiral patterns on a printed circuit
    board to form the "windings" of a planar transformer.
    (Manufacturers literally wind pieces of wire on some
    core or bobbin to form the windings of other kinds of
    Some planar transformers are commercially sold as
    discrete components—the transformer is the only thing
    on that printed circuit board. Other planar transformers
    are one of many components on one large printed
    circuit board.
    • much thinner than other transformers, for
      low-profile applications (even when several PCBs                                   A planar transformer
      are stacked)[2]
    • almost all use a ferrite planar core

    Oil cooled transformer
    For large transformers used in power distribution or
    electrical substations, the core and coils of the
    transformer are immersed in oil which cools and
    insulates. Oil circulates through ducts in the coil and
    around the coil and core assembly, moved by
    convection. The oil is cooled by the outside of the tank
    in small ratings, and in larger ratings an air-cooled
    radiator is used. Where a higher rating is required, or
    where the transformer is used in a building or
    underground, oil pumps are used to circulate the oil and   Exploded view: the spiral primary "winding" on one side of the PCB
                                                                  (the spiral secondary "winding" is on the other side of the PCB)
    an oil-to-water heat exchanger may also be used.[3]
    Formerly, indoor transformers required to be
    fire-resistant used PCB liquids; since these are now banned, substitute fire-resistant liquids such as silicone oils are
    instead used.
Transformer types                                                                                                                     133

    Cast resin transformers
    Cast-resin power transformers encase the windings in epoxy resin. These transformers simplify installation since
    they are dry, without cooling oil, and so require no fire-proof valut for indoor installations. The epoxy protects the
    windings from dust and corrosive atmospheres. However, because the molds for casting the coils are only available
    in fixed sizes, the design of the transformers is less flexible, which may make them more costly if customized
    features (voltage, turns ratio, taps) are required.

    Isolating Transformer
    Most transformers isolate, meaning the secondary winding is not connected to the primary. But this isn't true of all
    However the term 'isolating transformer' is normally applied to mains transformers providing isolation rather than
    voltage transformation. They are simply 1:1 laminated core transformers. Extra voltage tappings are sometimes
    included, but to earn the name 'isolating transformer' it is expected that they will usually be used at 1:1 ratio.

    Instrument transformers

    Current transformers
    A current transformer (CT) is a measurement device
    designed to provide a current in its secondary coil
    proportional to the current flowing in its primary.
    Current transformers are commonly used in metering
    and protective relays in the electrical power industry
    where they allow safe measurement of large currents,
    often in the presence of high voltages. The current
    transformer safely isolates measurement and control
                                                                Current transformers used in metering equipment for three-phase 400
    circuitry from the high voltages typically present on the                        ampere electricity supply
    circuit being measured.

    Current transformers are often constructed by passing a single primary turn (either an insulated cable or an
    uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. The CT is typically
    described by its current ratio from primary to secondary. For example, a 4000:5 CT would provide an output current
    of 5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or have
    several tap points to provide a range of ratios. Care must be taken that the secondary winding is not disconnected
    from its load while current flows in the primary, as this will produce a dangerously high voltage across the open
    secondary and may permanently affect the accuracy of the transformer.

    Specially constructed wideband CTs are also used, usually with an oscilloscope, to measure high frequency
    waveforms or pulsed currents within pulsed power systems. One type provides a voltage output that is proportional
    to the measured current; another, called a Rogowski coil, requires an external integrator in order to provide a
    proportional output.

    Voltage transformers
    Voltage transformers (VT) or potential transformers (PT) are another type of instrument transformer, used for
    metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being
    measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective
    relay equipment can be operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69
    V or 120 V at rated primary voltage, to match the input ratings of protective relays.
Transformer types                                                                                                                134

    The transformer winding high-voltage connection points are typically labeled as H1, H2 (sometimes H0 if it is
    internally grounded) and X1, X2 and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1,
    Y2, Y3) may also be available on the same voltage transformer. The high side (primary) may be connected phase to
    ground or phase to phase. The low side (secondary) is usually phase to ground.
    The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This applies to current transformers as
    well. At any instant terminals with the same suffix numeral have the same polarity and phase. Correct identification
    of terminals and wiring is essential for proper operation of metering and protective relays.
    Some meters operate directly on the secondary service voltages at or below 600 V. VTs are typically used for higher
    voltages (for example, 765 kV for power transmission) , or where isolation is desired between the meter and the
    measured circuit.

    Pulse transformers
    A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses
    with fast rise and fall times and a relatively constant amplitude). Small versions called signal types are used in digital
    logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power
    versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the
    electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power
    semiconductors. Special high voltage pulse transformers are also used to generate high power pulses for radar,
    particle accelerators, or other high energy pulsed power applications.
    To minimise distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and
    distributed capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling
    capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from
    high-powered transients created by the load. For the same reason, high insulation resistance and high breakdown
    voltage are required. A good transient response is necessary to maintain the rectangular pulse shape at the secondary,
    because a pulse with slow edges would create switching losses in the power semiconductors.
    The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is
    often used to characterise pulse transformers. Generally speaking, the larger this product, the larger and more
    expensive the transformer.
    Pulse transformers by definition have a duty cycle of less than 0.5, whatever energy stored in the coil during the
    pulse must be "dumped" out before the pulse is fired again.

    RF transformers
    There are several types of transformer used in radio frequency (RF) work. Steel laminations are not suitable for RF.

    Air-core transformers
    These are used for high frequency work. The lack of a core means very low inductance. Such transformers may be
    nothing more than a few turns of wire soldered onto a printed circuit board.

    Ferrite-core transformers
    Widely used in intermediate frequency (IF) stages in superheterodyne radio receivers. are mostly tuned transformers,
    containing a threaded ferrite slug that is screwed in or out to adjust IF tuning. The transformers are usually canned
    for stability and to reduce interference.
Transformer types                                                                                                                  135

    Transmission-line transformers
    For radio frequency use, transformers are sometimes made from configurations of transmission line, sometimes
    bifilar or coaxial cable, wound around ferrite or other types of core. This style of transformer gives an extremely
    wide bandwidth but only a limited number of ratios (such as 1:9, 1:4 or 1:2) can be achieved with this technique.
    The core material increases the inductance dramatically, thereby raising its Q factor. The cores of such transformers
    help improve performance at the lower frequency end of the band. RF transformers sometimes used a third coil
    (called a tickler winding) to inject feedback into an earlier (detector) stage in antique regenerative radio receivers.

    Baluns are transformers designed specifically to connect between balanced and unbalanced circuits. These are
    sometimes made from configurations of transmission line and sometimes bifilar or coaxial cable and are similar to
    transmission line transformers in construction and operation.

    Audio transformers
                                                        Audio transformers are usually the factor which limit sound quality
                                                        when used; electronic circuits with wide frequency response and low
                                                        distortion are relatively simple to design.
                                                        Transformers are also used in DI boxes to convert high-impedance
                                                        instrument signals (e.g. bass guitar) to low impedance signals to enable
                                                        them to be connected to a microphone input on the mixing console.
                                                        A particularly critical component is the output transformer of an audio
                                                        power amplifier. Valve circuits for quality reproduction have long been
                                                        produced with no other (inter-stage) audio transformers, but an output
                                                        transformer is needed to couple the relatively high impedance (up to a
                                                        few hundred ohms depending upon configuration) of the output
          Transformers in a tube amplifier. Output      valve(s) to the low impedance of a loudspeaker. (The valves can
       transformers are on the left. The power supply
                                                        deliver a low current at a high voltage; the speakers require high
               toroidal transformer is on right.
                                                        current at low voltage.) Most solid-state power amplifiers need no
                                                        output transformer at all.

    For good low-frequency response a relatively large iron core is required; high power handling increases the required
    core size. Good high-frequency response requires carefully designed and implemented windings without excessive
    leakage inductance or stray capacitance. All this makes for an expensive component.
    Early transistor audio power amplifiers often had output transformers, but they were eliminated as designers
    discovered how to design amplifiers without them.

    Loudspeaker transformers
    In the same way that transformers are used to create high voltage power transmission circuits that minimize
    transmission losses, loudspeaker transformers can be used to allow many individual loudspeakers to be powered
    from a single audio circuit operated at higher-than normal loudspeaker voltages. This application is common in
    industrial public address applications. Such circuits are commonly referred to as constant voltage speaker systems,
    although the audio waveform is a changing voltage. Such systems are also known by other terms such as 25-, 70-
    and 100-volt speaker systems, referring to the nominal voltage of the loudspeaker line.
    At the audio amplifier, a large audio transformer may be used to step-up the low impedance, low-voltage output of
    the amplifier to the designed line voltage of the loudspeaker circuit. At the distant loudspeaker location, a smaller
Transformer types                                                                                                           136

    step-down transformer returns the voltage and impedance to ordinary loudspeaker levels. The loudspeaker
    transformers commonly have multiple primary taps, allowing the volume at each speaker to be adjusted in discrete

    Output transformer
    Valve (tube) amplifiers almost always use an output transformer to match the high load impedance requirement of
    the valves (several kilohms) to a low impedance speaker.

    Small signal transformers
    Moving coil phonograph cartridges produce a very small voltage. In order for this to be amplified with a reasonable
    signal-noise ratio, a transformer is usually used to convert the voltage to the range of the more common
    moving-magnet cartridges.
    Microphones may also be matched to their load with a small transformer, which is mumetal shielded to minimise
    noise pickup. These transformers are less widely used today, as transistorized buffers are now cheaper.

    Interstage and coupling transformers
    In a push-pull amplifier, an inverted signal is required and is obtained from a transformer with a center-tapped
    winding, used to drive two active devices in opposite phase. These phase splitting transformers are not much used

    Homemade and obsolete transformers

    Transformer kits
    Transformers may be wound at home using commercial transformer kits, which contain laminations & bobbin.
    Alternatively, ready made transformers may be disassembled and rewound. These approaches are occasionally used
    by home constructors but are usually avoided where possible due to the number of hours required to hand wind a
    Firm clamping of laminations and varnish help to avoid buzz.

    100% homemade
    It is possible to make the transformer laminations by hand too. Such transformers are encountered at times in 3rd
    world countries, using laminations cut from scrap sheet steel, paper slips between the laminations, and string to tie
    the assembly together. The result works, but is usually noisy due to poor clamping of laminations.
    • picture [4]
    • device in use [5]

    Hedgehog transformers are occasionally encountered in homemade 1920s radios. They are homemade audio
    interstage coupling transformers.
    Enamelled copper wire is wound round the central half of the length of a bundle of insulated iron wire (eg florists'
    wire), to make the windings. The ends of the iron wires are then bent around the electrical winding to complete the
    magnetic circuit, and the whole is wrapped with tape or string to hold it together.
Transformer types                                                                                                                       137

    Variocouplers (sometimes called variometers) are RF transformers with two windings and variable coupling between
    the windings. They were standard equipment in 1920s radio sets.
    Pancake coil variocouplers were common in 1920s radios for variable RF coupling. The two planar coils were
    arranged to swing away from each other and for the angle between them to increase to 90 degrees, thus giving wide
    variation in coupling. No core was used. These were mostly used to control reaction. The pancake structure was a
    means to minimize stray capacitance.
    In another design of variocoupler, two coils were wound on two circular bands, and housed one inside the other, with
    provision for rotating the inner coil. Coupling varies as one coil is rotated between 0 and 90 degrees from the other.
    These had higher stray capacitance than the pancake type.

    Not transformers
    Items which may be mistaken for transformers, but which are not always transformers.
    Wall warts: small power supplies with integral mains plug. These can contain a transformer and other circuitry. Most
    use a laminated iron transformer, but an increasing number now contain a small switched-mode power supply. These
    are smaller and much lighter.
    Halogen lighting transformers: Toroidal transformers are sometimes used for this task, but most halogen
    'transformers' are switched-mode power supplies.
    Transformers rely on a linear relationship between the currents in primary and secondary circuits. Interesting and
    useful power control devices such as the saturable reactor and the magnetic amplifier rely on controlled saturation of
    a ferromagnetic core. Such devices can provide considerable power amplification without use of transistors or
    vacuum tubes. Although they resemble transformers with cores and sets of windings, the operating principles and
    purposes are different.

    [1]   "High Voltage - Measurement, Testing and Design", ISBN: 0 471 90096 6
    [2]   "Planar Transformer on a PCB" (http:/ / www. comsol. com/ showroom/ gallery/ 4329/ )
    [3]   ANSI IEEE Standard C57.12.00 General Requirements for Liquid-Immersed Distribution, Power and Regulating Transformers, 2000
    [4]   http:/ / www. zen40166. zen. co. uk/ image006. jpg
    [5]   http:/ / www. zen40166. zen. co. uk/ image005. jpg
Trigger transformer                                                                                                                 138

    Trigger transformer
    A Trigger transformer is a small, usually ferrite cored transformer used in applications requiring a high voltage
    pulse, typically to start ionization of a gas to allow a current to pass.[1]

    A commonly used device requiring a trigger transformer are strobe lights.
    Strobe lights consist of a tube containing an inert gas, such as xenon.
    Capacitors inside the light are charged up to a relatively high voltage,
    roughly 300 Volts for small strobes. This voltage itself is not capable of
    ionizing the gas in the flash tube, and the tube will not fire.[2] If the
    capacitors were charged to a voltage high enough to ionize the tube, the
    result would be no longer a flash, but a continuous arc as the voltage is
    sufficient to maintain ionization in the flash tube.

    Once the main storage capacitor has finished charging, a smaller capacitor is
    discharged into the trigger transformers primary coil. A large magnetic pulse
                                                                                       Various small trigger transformers. The
    is generated inside the transformer, which is coupled to the secondary coil         primary coil is connected to the 2 axial
    through the core. The secondary coil consists of hundreds, or even thousands      leads out either side, and the high voltage
    of windings of very fine copper wire, much thinner than human hair. This          is delivered through the smaller lead, and
                                                                                               one of the primary leads.
    creates a huge voltage differential spike, in the order of 2 kilovolts (2000
    volts) or more, depending on the length of the flash tube, and the gas
    contained within the tube. This high voltage spike overcomes the resistance of the gas, and allows ionization.

    Once this ionization exists, the tube offers a much lower path of resistance, for which the main storage capacitor can
    now release its energy into the tube extremely quickly. By this point, the main storage capacitor is recharging and the
    trigger transformer ready to create another high voltage spike, and the cycle repeats.
    Although strobe lights are the most common use of trigger transformers, other devices such as arc lamps, gas laser
    tubes and even the common fluorescent light employ trigger transformers, however usually in a different form, such
    as using an electrical ballast to provide both the high voltage spike, as well as current limiting to prevent the tube
    from drawing excess current.
    Inductors are also commonly used in place of a trigger transformer, however are not considered transformers
    themselves, although similar in operation.

    [1] Amglo Kemlite Laboratories (http:/ / amglo. com/ trigger_transformers. html)
    [2] Elliot Sound Products (http:/ / sound. westhost. com/ project65. htm)
Vector group                                                                                                                  139

    Vector group
    In electrical engineering, a vector group is the International Electrotechnical Commission (IEC) method of
    categorizing the primary and secondary winding configurations of three-phase transformers. It indicates the windings
    configurations and the difference in phase angle between them.
    The phase windings of a polyphase transformer can be connected together internally in different configurations,
    depending on what characteristics are needed from the transformer. For example, in a three-phase power system, it
    may be necessary to connect a three-wire system to a four-wire system, or vice versa. Because of this, transformers
    are manufactured with a variety of winding configurations to meet these requirements.
    Different combinations of winding connections will result in different phase angles between the voltages on the
    windings. This limits the types of transformers that can be connected between two systems, because mismatching
    phase angles can result in circulating current and other system disturbances.

    Symbol designation
    The vector group provides a simple way of indicating how the internal connections of a particular transformer are
    arranged. In the system adopted by the IEC, the vector group is indicated by a code consisting of two or three letters,
    followed by one or two digits. The letters indicate the winding configuration as follows:
    • D: Delta winding, also called a mesh winding. Each phase terminal connects to two windings, so the windings
      form a triangular configuration with the terminals on the points of the triangle.
    • Y: Wye winding, also called a star winding. Each phase terminal connects to one end of a winding, and the other
      end of each winding connects to the other two at a central point, so that the configuration resembles a capital
      letter Y. The central point may be connected outside of the transformer.
    • Z: Zigzag winding, or interconnected star winding. Basically similar to a star winding, but the windings are
      arranged so that the three legs are "bent" when the phase diagram is drawn. Zigzag-wound transformers have
      special characteristics and are not commonly used where these characteristics are not needed.
    • III: Independent windings. The three windings are not interconnected inside the transformer at all, and must be
      connected externally.
    In the IEC vector group code, each letter stands for one set of windings. The HV winding is designated with a capital
    letter, followed by medium or low voltage windings designated with a lowercase letter. The digits following the
    letter codes indicate the difference in phase angle between the windings, with HV winding is taken as a reference.
    The number is in units of 30 degrees. For example, a transformer with a vector group of Dy1 has a delta-connected
    HV winding and a wye-connected LV winding. The phase angle of the LV winding lags the HV by 30 degrees.
    The point of confusion is in how to use this notation in a step-up transformer. As the IEC60076-1 standard has
    stated, the notation is HV-LV in sequence. For example, a step-up transformer with a delta-connected primary, and
    star-connected secondary, is not written as 'dY11', but 'Yd11'. The 11 indicates the LV winding leads the HV by 30
    Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and instead a
    vector diagram is given to show the relationship between the primary and other windings.
Wet Transformer                                                                                                                            140

    Wet Transformer
    A wet transformer is a special type of impedance matching transformer that is used to couple telephone equipment
    to electronic equipment. It is made with a primary winding that is designed to have a DC voltage impressed upon it.
    It is also made with a special type of magnetic core that is not seen in usual transformers. It has a specific 600Ω input
    impedance which matches the telephone equipment. The output of some of these transformers are at 1KΩ and at line

    Zigzag transformer
    A Zigzag transformer is a special purpose transformer with a zigzag arrangement. It has primary windings but no
    secondary winding. One application is to derive an earth reference point for an ungrounded electrical system.
    Another is to control harmonic currents.[1]

    As with other three-phase transformers, the zigzag transformer contains six coils on three cores. The first coil on
    each core is connected contrariwise to the second coil on the next core. The second coils are then all tied together to
    form the neutral and the phases are connected to the primary coils. Each phase, therefore, couples with each other
    phase and the voltages cancel out. As such, there would be negligible current through the neutral pole and it can be
    tied to ground.[2]
    If one phase, or more, faults to earth, the voltage applied to each phase of the transformer is no longer in balance;
    fluxes in the windings no longer oppose. (Using symmetrical components, this is Ia0 = Ib0 = Ic0.) Zero sequence
    (earth fault) current exists between the transformer’s neutral to the faulting phase. Hence, the purpose of a zigzag
    transformer is to provide a return path for earth faults on delta-connected systems. With negligible current in the
    neutral under normal conditions, engineers typically elect to under size the transformer; a short time rating is applied
    (i.e., the transformer can only carry full rated current for, say, 60 s). Ensure the impedance is not too low for the
    desired fault limiting. Impedance can be added after the secondaries are summed (the 3Io path)[3] .
    An application example: occasionally engineers use a combination of Y (wye or star), delta, and zigzag windings to
    achieve a vector phase shift. For example, an electrical network may have a transmission network of 110 kV/33 kV
    star/star transformers, with 33 kV/11 kV delta/star for the high voltage distribution network. If a transformation is
    required directly between the 110 kV/11 kV network the most obvious option is to use 110 kV/11 kV star/delta. The
    problem is that the 11 kV delta no longer has an earth reference point. Installing a zigzag transformer near the
    secondary side of the 110 kV/11 kV transformer provides the required earth reference point.

    [1] IEEE Xplore - Zigzag transformers for reducing harmonics (http:/ / ieeexplore. ieee. org/ xpl/ freeabs_all. jsp?arnumber=152320)
    [2] Post Glover - Zigzag Grounding Transformers (http:/ / www. postglover. com/ Literature/ GT210-08_Zigzag_Trans. pdf)
    [3] Blackburn, J. Lewis, Protective Relaying, Marcel Dekker, Inc., New York, 1998
Article Sources and Contributors                                                                                                                                                                         141

    Article Sources and Contributors
    Short circuit test  Source:  Contributors: Auntof6, Diannaa, Katharineamy, Malcolmxl5, Saurav1641, 5 anonymous edits

    Transformer  Source:  Contributors: 15.253, 29.12, A. Carty, AJim, Acather96, Adamantios, Aeolian, Aeon1006, Aforencich,, Aitias, Akidd dublin, Alai, Alamgir, Alansohn, AlexiusHoratius, Alfred Centauri,, Alientraveller, Allstarecho, Alphachimp, Altenmann, Anakin101,
    Andrejj, Andres, Andrewpmk, Andromodon, Anon user, Anthony Appleyard, Antonio Lopez, ApprenticeFan, Arch dude, Arnero, ArnoldReinhold, Arunnano, Asc3nti0n, AtheWeatherman,
    Atlant, Attilios, Atulfotedar, Auric, Awadewit, AxelBoldt, Baa, Bad News Live 1982-87, Badharlick, BananaManCanDance, BarretBonden, Bartimaeus, Begemotv2718, Belmond, Bert Hickman,
    BillC, Binksternet, Bjankuloski06en, Black Falcon, Blehfu, Bluesquareapple, Bobblehead, Bobblewik, Bobet, Bobo192, Bongwarrior, Boothy443, Brews ohare, BrokenSegue, Bryan Derksen,
    Bsadowski1, Bubba hotep, C J Cowie, Caltas, Calton, CanadianLinuxUser, CapitalR, Capricorn42, CatherineMunro, Cbdorsett, Ccrrccrr, Celebration1981, Chetvorno, Chris the speller,
    ChrisSloan, Clampert, Closedmouth, Cmdrjameson, Code-Binaire, Colonies Chris, Commander Keane, Conversion script, Correogsk, CosineKitty, Cowpriest2, Cpl Syx, Crazy Boris with a red
    beard, Crohnie, Crunchy Numbers, Curps, CyrilB, D. Recorder, DMChatterton, DS1953, DV8 2XL, DVD R W, Da monster under your bed, DabMachine, Daisyotis, DavidBlackwell, DavidCary,
    Deepdive217, Deglr6328, DerHexer, Dicklyon, Dina, Dipiete, Discospinster, Dispenser, Doriftu, Dougweller, Dtgriscom, Dual Freq, Easchiff, Edaudio, Eddie.willers, Editor at Large, Edward,
    Egil, Eguyle, ElBarto, Electron9, Elekronics, Ellywa, Energizer07, Epbr123, Ericg33, Everyking, Excirial, Faraaz ahmed khan lodhi, Ferengi, Fg2, Finalius, Fireex7777, Foobaz, FrancoGG,
    Fresheneesz, Frungi, Fumbams, Funandtrvl, Fusionmix, GWBeasley, Gaia Octavia Agrippa, Gbmaizol, Gene Nygaard, Geniac, Geometry guy, Georgia guy, Gerben49, Gerry Ashton, Giftlite,
    Glane23, Glenn, Glogger, Goatasaur, GoldenMeadows, Goodgerster, Gorm, GraemeL, Graham87, Gralo, GreenSpigot, Gregorydavid, GreyCat, Guanaco, Guenter1948, Gwernol, Gzuckier,
    Haham hanuka, Head, Hellbus, Hello32020, HenryLi, HereToHelp, Heron, Herr Gruber, Holofect, Hooperbloob, IJA, IanOfNorwich, Icairns, Ilgaitis, Ilikefood, Immunize, Ingolfson, Inigo75,
    Intgr, Iridescence, Islandboy99, Ismayil, J.delanoy, JIP, JaGa, Jak123, JamesBWatson, Jamesooders, Jauhienij, Jc3s5h, Jebba, Jerryobject, Jerzy, Jj137, Jo.Fruechtnicht, Joe Kress, Joe Shupienis,
    John of Reading, JohnTechnologist, Jojit fb, Joramar, JordanSamuels, Jorunn, Josh3580, Justin W Smith, Juzaf, Jwink3101, K Eliza Coyne, Kajasudhakarababu, Kallog, Karl-Erik, Karl-Henner,
    Kbthompson, Kephart, Kevin x1000, Keycard, Kf4bdy, Khukri, Kieff, Kilmer-san, Kimse, Kingpin13, Kmccoy, Kusma, Kvng, LOL, Lambrosus, Lchiarav, Lda523287, Lenilucho, Lerdsuwa,
    Light current, Linas, Lindosland, Liton 102, LittleOldMe, Ljiljan, Logion, Lommer, Lumpenke, MER-C, MJ94, MK8, Makkar.lalit, Manuel Anastácio, Marcus Brute, Martin TB, Maxis ftw,
    Mbell, Mboverload, Md koz, Mebden, Meelar, Meggar, Mehdi242004, MeltBanana, Mentifisto, Michael Hardy, Mike R, Mike Rosoft, Mike1024, Mikeo, Mild Bill Hiccup, Militoy, Minamti,
    Mindmatrix, Minna Sora no Shita, Mintguy, MisfitToys, Mmarci, Momirt, Moreschi, Msiddalingaiah, Muhandes, Myanw, Mystic Pixel, NJM, Nabla, Nancy, Natalie Erin, NawlinWiki, Nayvik,
    Neddy53, Neier, Nekura, Neo Piper, Neonumbers, Neotesla, Neparis, Nickj, Nikai, Nikola Smolenski, Nixdorf, Nkaufman, No such user, No1lakersfan, Nonforma, Novum, Nozog,
    NuclearWarfare, Ocaasi, Ohnoitsjamie, OlEnglish, Omaga99, Omegatron, Onorem, Opelio, OpenLoop, Overjive, OverlordQ, Paul A, Paverider, Pb30, Peter, Peter Wöllauer, Petr.adamek,
    Pfalstad, Phennessy, Philbertc, Philip Trueman, Plugwash, Pokrajac, Pol098, Pooattack, Postrach, Prolog, Protonk, Pyrosparker, QueenCake, Quintote, Quodfui, R U Bn, R. S. Shaw, RG2,
    Rabogliatti, Raghu3684, RandomP, Randomguy25, RaseaC, Ray G. Van De Walker, RazorICE, Reaper Eternal, Reddi, Reedy, Requestion, Res2216firestar, RexNL, Rich Farmbrough,
    Richmond8255, Richtom80, Richurford, Rico402, Rijkbenik, Rjwilmsi, Rrburke, Rsrikanth05, Ryanvang, Ryucloud, Ryulong, SCΛRECROW, SGBailey, Saga City, Salsb, Sam Hocevar, SamH,
    Sameerkk, Samuel Grant, Sarvagyana guru, ScAvenger lv, Scm83x, Scottfisher, Search4Lancer, Searchme, Sefarkas, Semperf, Senator Palpatine, Sfngan, Sfsfstrefred, Shaddack, Shanel, Shoefly,
    Siawase, Simetrical, Simmaeee2002, SimonP, Sjö, Slakr, Smack, Smedhaug, Smitster2007, SnappingTurtle, Sonett72, Special-T, Spinningspark, Sridharan eee, Srleffler, StaticGull, Stemonitis,
    Stroback, Stubes99, Suckindiesel, Suruena, Svick, Synchronism, TAU Jew Fro, TFOWR, Tabby, TerryKing, That Guy, From That Show!, Thayerhughes, The Anome, The Lightning Stalker, The
    Photon, The Rambling Man, The Rogue Penguin, The Thing That Should Not Be, The wub, Theresa knott, Thestick, Tim Starling, Timrollpickering, Todaywz, Tohd8BohaithuGh1, Topory,
    Trav123, TreeSmiler, True Pagan Warrior, Tsi43318, Tugjob, Tyrant670, Ukexpat, UncleDouggie, Unionhawk, Unyoyega, V.narsikar, Vegaswikian, Ventusa, Venu62, Versus22,
    VictorianMutant, VirtualEarth, Waggers, Welsh, Werdna, WillowW, WimdeValk, Wimt, Windharp, WirelessCello, Wolfkeeper, Woohookitty, Wtshymanski, XJamRastafire, Xenonice, Xorm,
    Yamamoto Ichiro, Yun-Yuuzhan, Zachpfeif, Zoicon5, Zolika76, Zorbatron, Zsinj, Zueignung, Zureks, 1132 anonymous edits

    Amorphous metal transformer  Source:  Contributors: EscapingLife, GunnsteinLye, Headbomb, Jerrycfli, LorenzoB, Malcolma,
    Markiewp, Nick Number, Redneckboi, Wtshymanski, 28 anonymous edits

    Austin transformer  Source:  Contributors: Arjayay, Electron9, LilHelpa, Marek69, Northstar1000, Stephan Leeds, Wtshymanski

    Autotransformer  Source:  Contributors: Adambro, Ale jrb, Alexwcovington, Atlant, BillC, Bjankuloski06en, C J Cowie, Chetvorno,
    Cremepuff222, DV8 2XL, Drmiles, Edward, FFMG, GreyCat, HeirloomGardener, Hooperbloob, HopeChrist, Hu12, Hydrargyrum, Jaho, Jamesofur, Knuck, Kvng, Lerdthenerd, Light current,
    MichaelFrey, Monedula, Nlilovic, Ocaasi, Ot, Philip Trueman, Plugwash, Pointbonita, Renacat, Rjamesbrown, Rjwilmsi, SatuSuro, Sfisher, Skinny McGee, Sladen, Taka76, Tim Starling,
    Trav123, Wtshymanski, Yintan, Zachlipton, Zhangzhe0101, 97 anonymous edits

    Balun  Source:  Contributors: A. B., Aaron.axvig, Abhityagi85, Acjohnson55, Adam850, Adamantios, AlexeyV, Andrewa, Angr, Arnero,
    Binksternet, Cadmium, Casliber, Catapult, Cfallin, Charlierepetti, Chrylis, CosineKitty, Cyfal, DV8 2XL, DaGizza, Daniel Christensen, Deville, Dicklyon, Eckhart Wörner, Father Goose, Gaius
    Cornelius, GandalfUK, Gbleem, Gerry Ashton, GreenSpigot, GyroMagician, Hohum, Hooperbloob, Jc3s5h, Jecowa, JerzyTarasiuk, Jim.henderson, Julesd, KD5TVI, Kgrr, Krsont, Kwamikagami,
    Lenilucho, Light current, Mavros, Metrax, Mhaeberli, Mkeroppi, Mlewis000, NoSnooz, OS2Warp, Omegatron, Pagingmrherman, Peter Karlsen, Phpone, PsyberS, Radiant chains, Radiojon,
    Radiondistics, RexNL, Richmond8255, Rjwilmsi, Rob.desbois, Rogerbrent, Romanski, S7evyn, Scarfboy, Shaddack, Silverxxx, SlipperyHippo, Ssd, Techbug, Techfixer, Thingg, Timwi,
    Toresbe, Utcursch, Vicarious, W.F.Galway, Wolfmankurd, Wtshymanski, Xaosflux, 83 anonymous edits

    Buchholz relay  Source:  Contributors: A. Carty, A2Kafir, Alfaomega, Amalas, BillC, Conscious, El Irlandés, Ericwb, Escherichia coli,
    Favonian, Hoogestraat, Hooperbloob, LorenzoB, MER-C, Monni1995, Mr.Z-man, Ot, Poco a poco, Shaddack, Steve carlson, Theotherskip, Wtshymanski, 26 anonymous edits

    Buck–boost transformer  Source:  Contributors: Akradecki, BabyBatter, Berserkfish, Closedmouth, DMahalko, Dennis Brown, Giftlite,
    Groggy Dice, HeirloomGardener, Hooperbloob, Hydrogen Iodide, Indefatigable, Kurt Shaped Box, Plrk, TedPavlic, Thisisborin9, Vitund, Wtshymanski, 14 anonymous edits

    Capacitor voltage transformer  Source:  Contributors: Cedars, OlEnglish, Ravisankar74, Raymondwinn, Sander123, Shoefly, Thorsten1,
    Wtshymanski, 15 anonymous edits

    Center tap  Source:  Contributors: 7severn7, BD2412, Biscuittin, Bmicomp, DMacks, Disneyfreak96, Hooperbloob, Jidan, Jonon, L
    Kensington, Light current, Purpy Pupple, RJFJR, Rjwilmsi, Torquil, VMS Mosaic, Whitepaw, Wtshymanski, 25 anonymous edits

    Compensation winding  Source:  Contributors: Allen3, Amalas, Andrew Maiman, Garyseven, Hooperbloob, Toffile, 3 anonymous edits

    Copper loss  Source:  Contributors: Amalas, Catslash, Ccrrccrr, KD5TVI, Kjkolb, Nillerdk, No such user, PaulHanson, Pearle, R. S. Shaw,
    Rjwilmsi, Smalljim, Some guy, StuFifeScotland, The Missing Piece, Toffile, Ze miguel, 16 anonymous edits

    Current transformer  Source:  Contributors: Alansohn, Aleenf1,, Almuqbel, Andres, Andy Dingley, Biezl, BillC,
    BokicaK, Dolovis, Doniago, Dreadstar, Enisbayramoglu, Gggh, Itl-uk, John of Reading, Jpcallan, Keilana, LilHelpa, Logion, MER-C, Materialscientist, Mbennett555, Mgbayer7, Mike6271,
    Mintleaf, Noinrush, Nrupalakolkar, Ojs, OwenX, Pb30, Rich Farmbrough, Rmu2, RussBlau, Spartaz, Sponsion, Steelpillow, Suffusion of Yellow, The Thing That Should Not Be, Thorsten1,
    Tosohal, Trev1942, Tugjob, Virtualphtn, Voidxor, Wtshymanski, 95 anonymous edits

    Delta-wye transformer  Source:  Contributors: A. Carty, Amalas, Biscuittin, Clicketyclack, DMahalko, Dhollm, Gargoyle888,
    Hooperbloob, Icairns, Interiot, Ipecac, Karada, LilHelpa, Linas, No such user, Schmiteye, StarDelta, Thiseye, Utkugenc, Wdwd, West.andrew.g, Wtshymanski, 14 anonymous edits

    Dissolved gas analysis  Source:  Contributors: Akumar48, Auntof6, Bearcat, Crayon2000, D4g0thur, DMS, Dakshinlab, Discospinster,
    Fabrictramp, GB fan, Gobonobo, Icairns, JIP, Jeff3000, Kernel Saunters, Philip Trueman, R'n'B, RJFJR, RobDe68, Skarebo, Vendettax, 12 anonymous edits

    Distribution transformer  Source:  Contributors: AvicAWB, Chetvorno, Chris 73, DOSGuy, Dhiraj1984, Fratrep, Gaius Cornelius, High
    Contrast, Larman, Linas, Lpgeffen, Mattbuck, Onore Baka Sama, Salsb, Whispering, Wizard191, Wtshymanski, 18 anonymous edits

    Enameled wire  Source:  Contributors: Alan Liefting, ChrisHodgesUK, Chuck Boot, Drbrain, Drbreznjev, GB fan, Gene Nygaard,
    Gerben49, Hellbus, Heron, Jag123, Micru, Neparis, Tabby,, Vishwas2008, Wdwd, 19 anonymous edits

    Energy efficient transformer  Source:  Contributors: BananaManCanDance, Jacquehori, Malcolma, Michelvoss, Pol098, Rjwilmsi,
    Spinningspark, Tabby, Toffile, 7 anonymous edits
Article Sources and Contributors                                                                                                                                                                   142

    Flyback transformer  Source:  Contributors: Arnero, Cisum.ili.dilm, Cmacd123, CyrilB, DrLove0378, Electron9, Evil saltine, Frotz, Gene
    Nygaard, Grafen, Gzuckier, Heron, Hooperbloob, KnightRider, Longbowman, Lordmoz, Lovecz, Mhbrady, Munozdj, OutOfWhack, Reddi, Requestion, SlamDiego, Speedevil, Srleffler, Tabby,
    Teravolt, The Original Wildbear, Tothwolf, Tylerni7, Vertigo Acid, Whitepaw, Wtshymanski, Zueignung, Zureks, 75 anonymous edits

    Growler (electrical device)  Source:  Contributors: Belovedfreak, Clicketyclack, DV8 2XL, Eabbazia, Ekotkie, IanOfNorwich, Korg, Matt
    Yeager, Mellery, Pjvpjv, Planetneutral, WjfIII, 4 anonymous edits

    Hybrid coil  Source:  Contributors: A. B., Britney1973, Catslash, Cbdorsett, Deville, GreenSpigot, Heron, Hooperbloob, Jim.henderson,
    Light current, LoopTel, MGlosenger, Omegatron, OscarJuan, PaulHanson, SimonP, TUF-KAT, Tabby, Timon, TubularWorld, 12 anonymous edits

    Induction coil  Source:  Contributors: AI, Aarchiba, Anemonella, Armstrong1113149, Bert Hickman, BobPost, CJLL Wright, Calsifer2,
    Chetvorno, Crtcollector, Cwkmail, Davehi1, Dcljr, Dellant, Dferrantino, Dmitri Lytov, Edison, Ensada, Gasperi, Glj1952, GrooveDog, Gzuckier, Hephaestos, Heron, Hertzian, Hooperbloob,
    Jamelan, K7jeb, KingJooba, Kkmurray, Mahogny, Meggar, Metricmike, Mild Bill Hiccup, Nukeless, Outback the koala, PatGallacher, Pieter Kuiper, Pillar Technologies, Poeloq, Prabhathh,
    Py4nf, R'n'B, Randall O, Reddi, Redrey, SDC, Sinalco, Tabletop, Thorseth, Tim Q. Wells, Tom harrison, Utcursch, 73 anonymous edits

    Iron loss  Source:  Contributors: Alai, Andrew M. Vachin, Arch dude, Areelle, ArglebargleIV, Atlant, BenFrantzDale, BillC, Binksternet,
    CRGreathouse, Ccrrccrr, Chetvorno, CultureDrone, CyrilB, DMahalko, Dual Freq, Gene Nygaard, Gregogil, IanOfNorwich, IronGargoyle, Jim Swenson, Leon7, Levin, Lovecz, Lucas the scot,
    Mdd, Mebden, Melaen, Mikiemike, Mindmatrix, Pol098, R'n'B, Rememberway, SCEhardt, Shaddack, Shankarkr, Sir Boris, Sjsprague, Some jerk on the Internet, Stillnotelf, StuFifeScotland,
    Thiseye, Tommassino, Wegsjac, Wizard191, Wolfkeeper, Wtshymanski, Zhangzhe0101, Zureks, 67 anonymous edits

    Isolation transformer  Source:  Contributors: Akidd dublin, Alfred Centauri, Atlant, BillC, Colonies Chris, Davidsxls, Hooperbloob, Ldo,, Maximus Rex, MichaelFrey, Omegatron, Pale blue dot, Patag2001, Pol098, Qwyrxian, ShawnKuo, Tabby, The Thing That Should Not Be, UnivEducator, Wdwd,
    Wtshymanski, 32 anonymous edits

    Leakage inductance  Source:  Contributors: Amalas, Atlant, BillC, Bushben, Caiaffa, Chetvorno, ElBarto, Gaius Cornelius, Glrx, Heron,
    Hooperbloob, Immibis, JerzyTarasiuk, LedgendGamer, Light current, Neotesla, Pjacobi, RJFJR, Richard Stephens, Salsb, Starblind, Thedatastream, Toffile, Underpants, W0lfie, Wdwd,
    Wtshymanski, Wvbailey, 11 anonymous edits

    Linear variable differential transformer  Source:  Contributors: Capricorn42, Dgnospam247, Email4mobile, Grabert, Heron,
    Hooperbloob, Jshadias, JuanMac, Mugs021, Nabla, Neo139, Panchhee, Pjvpjv, ReyBrujo, Rustyguts, Soap, SpaceFlight89, Tommycw1, Wapcaplet, Wtshymanski, Zvn, 44 anonymous edits

    Magnifying transmitter  Source:  Contributors: Akadruid, Alai, Angela, Auntof6, Bbq332, BenFrantzDale, Bert Hickman, Bryan
    Derksen, Carioca, Cmdrjameson, Dbenbenn, Deglr6328, Deville, Dthomsen8, Dukeofomnium, Dysprosia, GLPeterson, Gene Nygaard, GlassFET, Golbez, Heron, Hooperbloob, J.delanoy,
    Jammus, John of Reading, Joshua P. Schroeder, Kevin Rector, La goutte de pluie, LilHelpa, Maghnus, Mahalis, MasterofUnvrs314, Mattworld, Mblaze, Mditto, Michael H 34, Michael Hardy,
    Mlewis000, NeonMerlin, Neotesla, Ocatecir, Omegatron, Pjacobi, PoccilScript, Rboatright, Reddi, Rich Farmbrough, Robin S, Saif alsahra, Sanders muc, Sietse Snel, SimonP, Thue, Welsh,
    William M. Connolley, Wjbeaty, Wtshymanski, Yamamoto Ichiro, 145 anonymous edits

    Metadyne  Source:  Contributors: Andy Dingley, Atlant, Biscuittin, David7turner, Quackdave, 11 anonymous edits

    Multiple Gas Extractor  Source:  Contributors: Biscuittin, Dakshinlab, Eeekster, Icairns, Malcolma

    Neon sign transformer  Source:  Contributors: Billybobjoe, Chongkian, D6, DOSGuy, Deor, Gaius Cornelius, IceCreamAntisocial,
    Ixfd64, Maverick423, Msylvester, Nehrams2020, Nihiltres, RevelationDirect, Riflemann, Sadads, Tabletop, 20 anonymous edits

    Oil sample tube  Source:  Contributors: Bearcat, Dakshinlab, Eeekster, Malcolma, William Avery

    Oudin coil  Source:  Contributors: Amalas, Chetvorno, Dicklyon, FlyHigh, Gzuckier, Heron, Monni1995, Pjacobi, Pol098, Reddi, SDC,
    Utcursch, Wjbeaty, Wolfkeeper, 28 anonymous edits

    Padmount transformer  Source:  Contributors: Belmond, Cherkash, Malcolmxl5

    Paraformer  Source:  Contributors: JoshuaZ, N2e, RJFJR, Welsh, 3 anonymous edits

    Polyphase coil  Source:  Contributors:, Angela, Biscuittin, Bkell, DMChatterton, DigitalGhost, Docu, Dysprosia,
    Evanruga, Heron, Hooperbloob, Hydrogen Iodide, Linas, Omegatron, Pjvpjv, Reddi, Salsb, Utcursch, Wik, Wtshymanski, Zoicon5, 8 anonymous edits

    Prolec GE  Source:  Contributors: AlexandrDmitri, Colonies Chris, Graeme Bartlett, Hiramcillo

    Quadrature booster  Source:  Contributors: BillC, Jeff3000, Light current, MisfitToys, PieterJanR, PigFlu Oink, Rmgunsmith, SmithPet,
    Wdwd, Wtshymanski, 8 anonymous edits

    Repeating coil  Source:  Contributors: CaseInPoint, Cbdorsett, Deville, Heron, Hooperbloob, IslandHopper973, Jim.henderson,
    Mlewis000, Omegatron, Radiojon, Rpyle731, Sanguinity, Stewartadcock, Synchronism, The Anome, Toasteh, Utcursch, Zoicon5, 2 anonymous edits

    Resolver (electrical)  Source:  Contributors: Andres, Average Earthman, Diberri, Emilio Juanatey, Heron, Hooperbloob, Justin latham,
    Karlthegreat, Kohbra, Mosquit0, Nikevich, Onionmon, Pro crast in a tor, Reddi, Srleffler, Suruena, Wtshymanski, Zoggie50, 28 anonymous edits

    Rotary transformer  Source:  Contributors: Atlant, Biscuittin, Chrishmt0423, DMahalko, Hooperbloob, ILike2BeAnonymous, Light
    current, Michael Daly, Nikevich, Saimhe, Tommy2010, Wdl1961, 8 anonymous edits

    Rotary variable differential transformer  Source:  Contributors: Chris the speller, Clicketyclack, HamburgerRadio, Heron, Hooperbloob,
    Nikevich, Onionmon, Pearle, Pjbflynn, Positek, RussBlau, Srleffler, Suruena, 16 anonymous edits

    Scott-T transformer  Source:  Contributors: Atlant, CRGreathouse, Couposanto, Cpswarrior, DOSGuy, Echuck215, Gerry Ashton,
    Gilliam, Gökhan, Hooperbloob, Linas, Mcg410, Mild Bill Hiccup, PhaseBreak, Plugwash, Qviri, Reddi, Rmgremillion, Specs112, Voidxor, Wtshymanski, Zureks, 13 anonymous edits

    Synchro  Source:  Contributors: Alansohn, BPK, BlckKnght, Caknuck, Cck2008, Centrx, Charly Whisky, CrookedAsterisk, D6, Danhoar,
    Diberri, Epbr123, Glenn Willen, Heron, Hmwith, Hooperbloob, Intersofia, Janke, Jgm, Karn, MGeek, Nagle, Nikevich, NixonB, Obli, Onionmon, PigFlu Oink, Sanders muc, Sho Uemura,
    Skdeewan, Slowking Man, Suruena, Wiki alf, Winstonwolf33, Yuriybrisk, 17 anonymous edits

    Tap (transformer)  Source:  Contributors: Barticus88, BillC, CiaPan, Dimitrissaab, Edward, Edward Z. Yang, GreyCat, Hooperbloob, II
    MusLiM HyBRiD II, Light current, OLTC, Rs999, That Guy, From That Show!, Tombreck, Wtshymanski, Zinnmann, 28 anonymous edits

    Tesla coil  Source:  Contributors: 833nirassi, A Train, AJCham, ARTE, Aksi great, Alai, Alex.tan, AlexiJohannson, Andonic, Antandrus,
    Anthony Appleyard, Arjayay, Arnero, Arthena, ArthurDenture, Artur Buchhorn, Arundhati bakshi, Atlant, Atropos235, Austin512, AxelBoldt, Bainzy, Banpei, Barklund, Barras, Barticus88,
    Bbookk1, BenFrantzDale, BernardSumption, Bert Hickman, Betacommand, Bezapt, Biglovinb, Bioform 1234, Biscuts8, Blanchardb, Bluerasberry, Bobblehead, Bobblewik, Bobxii, Bondcar13,
    BorgQueen, BrenDJ, Brighterorange, Bryan Derksen, CJLL Wright, Cal 1234, Can't sleep, clown will eat me, CardinalDan, Carioca, Cartoonmaster, Cbdorsett, Chamal N, CharlesDexterWard,
    Chasebaur, Chetvorno, Chris Roy, Chris the speller, Chris9999100, Clayhalliwell, Cmdrjameson, Cof123123, Colonies Chris, ConradPino, Corvus coronoides, DARTH SIDIOUS 2, DG, DMS,
    DV8 2XL, Darac, Davehi1, David Gerard, David Schaich, Ddawson, Deewiant, DerbyLad, Derek Ross, Dicklyon, Discospinster, Dlloyd, Doctorfluffy, Dr Aleksandar Karanovic, Dr.enh,
    Dreadengineer, Editor at Large, Egil, Egocasiumamo, Einstein9073, Emurphy42, Enochlau, Epbr123, Epilectrik, Equazcion, Eric Urban, Essaregee, Evil Monkey, FETSmoke, Falcon8765,
    Fintan264, Flewis, Flominator, Foobaz, ForgottenHistory, Foxy2542, Frencheigh, Frenchman113, Furrykef, Fuzheado, GLPeterson, Gaius Cornelius, Gargaj, GaryPeterson, Gene Nygaard,
    Gerbrant, Geremia, Ghw777, GlassFET, Glaurung, Glenn, Glennwells, Gurch, Gutta Percha, Gzuckier, Hao2lian, Hernanca, Heron, Hightek02, Hike395, Honge, Hooperbloob, IPSOS, IRP,
    Iantresman, Incnis Mrsi, InvertRect, Iridescent, IronGargoyle, J-Star, JNW, Jacobe35, Jan247, Jeff3000, JeffJ, Jeffq, Jjregist, JoelC, John Vandenberg, JonathonReinhart, Jonkerz, Jtmendes,
    Julesd, Just Another Dan, K7jeb, Kalas Grengar, Kelly Martin, Kieff, Kirchsw, Koyaanis Qatsi, Krash, Kreachure, Kuru, Kwertii, LMB, LOL, Lawilkin, Leonard G., Leyh, Light current,
    LittleDan, Logan, LookingYourBest, LorenzoB, MER-C, Mahogny, Marshallsumter, MartinSFSA, Matthew Yeager, Matveims, Mecsoljae, Meduzaza, Metta Bubble, Michael Hardy, Miko3k,
    Milomedes, Miserlou, Mlewis000, Mmmchocolate, Modeha, Moeron, Morrand, Mousy, Murtasa, Nanouk, Natalie Erin, NawlinWiki, Nebuchadnezzar o'neill, Nelson50, Nem1yan, NeoJustin,
Article Sources and Contributors                                                                                                                                                                  143

    Neotesla, Netoholic, NewEnglandYankee, Nima Baghaei, Nimbusania, Nlu, Nuno Tavares, Oamaro, Omegatron, Onceler, Opelio, OpenToppedBus, OranL, Ottawa4ever, OverlordQ, Pakaran,
    Patton123, Pdn, Pegasus1138, Penubag, Pjacobi, Plugwash, Pmsyyz, Pol098, Postrach, Purple-plates, QVanillaQ, QuestionAuthority, R'n'B, RapidEye0, Read-write-services, Red devil 4 life,
    Reddi, Repliedthemockturtle, RepublicanJacobite, RexNL, Riana, Rich Farmbrough, Rjwilmsi, Robin S, Robomojo, Rossbrown, Ru48669, RyanF76, SCEhardt, SDC, Sam Hocevar, Searchme,
    Sfmixers, Shanes, Shermozle, Shimgray, Sietse Snel, Sintaku, Sjakkalle, Sk8trnate, Skitch13, Skunkboy74, Slike, SnakeEyesNinja, SoWhy, Sodium, Sonett72, Sonicblue4, Springnuts,
    Spydergwiz, Srleffler, StaticGull, SteveG23, Stevenyu, Sullview, SunCreator, Superraptor5, Suruena, Sveska, Swordsman04, Sysy, T3knomanser, THC Loadee, TTLLOGIC, Tavilis, Tawker,
    Tcncv, Template namespace initialisation script, TerraHikaru, Tesla coil, That Guy, From That Show!, The Nixinator, The Photon, The Ronin, The Thing That Should Not Be, Thebiggiantkevin,
    Thingg, Thiseye, Thumperward, Thunderbrand, TigerShark, Timwi, Tinton5, TonioAsura, Transity, Traumerei, Trevor Johns, TwoOneTwo, Tylerwhitedesign, Ultramince, Utcursch,
    Vancouverguy, Vitz-RS, Vocaro, Voorlandt, Vshade, WhiteCrane, WhyBeNormal, Wildthing61476, WillMak050389, Wimt, Winifred Lake, Winterheart, Wjbeaty, Wolfkeeper, Wtshymanski,
    X-Flare-x, Xen01, Xoloki, Xorian, Yincrash, Yourname13579, ZICO, ZX81, Zoicon5, ZyMOS, 772 anonymous edits

    Toroidal inductors and transformers  Source:  Contributors: A. Carty, Ccrrccrr, Constant314, Finavon, Giftlite, Glrx, Hooperbloob,
    Jkbw, Ki162, Lindorm, Lovecz, Pieter Kuiper, Scwerllguy, Sm00th101, Spinningspark, 7 anonymous edits

    Transformer oil  Source:  Contributors: Bert Hickman, Biscuittin, Brogandman, Cadmium, Dakshinlab, Danielrichardbond,
    DeadEyeArrow, Eamon03, Gary King, HVWeb, Hazard-SJ, Hooperbloob, Iridescent, J.delanoy, Joannacooper, Light current, LorenzoB, Mandarax, Maury Markowitz, Michael Hardy, Nfr-Maat,
    Ngch89, Oli Filth, Panpulha, Pgarg78, Philip Trueman, Piercetheorganist, Pmj, Shaddack, Shinkolobwe, Sonett72, Stepa, ThePerpetualStudent, Tommy2010, Wayaguo, Zachlipton, 50
    anonymous edits

    Transformer oil testing  Source:  Contributors: HVWeb, Paul A

    Transformer types  Source:  Contributors: 10metreh, A. Carty, Alan Liefting, Anaxial, BabyBatter, BillC, Binksternet, Brambleclawx,
    Chongkian, Chris the speller, Cooperised, CosineKitty, DMChatterton, Dffgd, Dhiraj1984, Dicklyon, Epbr123, Epibulbar, Fyyer, Iridescent, Joramar, Kurt Shaped Box, Kvng, LilHelpa, Lindorm,
    Lovecz, Materialscientist, Mdd, Mysterious Dragon, Okita2, Pleasantville, Pol098, ProperFraction, Punit86, Rico402, Rjwilmsi, Rmhermen, RockMagnetist, Seegoon, Silverxxx, Sonett72,
    Spookytimothy, Szzuk, Tabby, Tabletop, TreeSmiler, Tugjob, WimdeValk, Wtshymanski, Ww, 113 anonymous edits

    Trigger transformer  Source:  Contributors: Bearcat, LilHelpa, Lpfthings, 4 anonymous edits

    Vector group  Source:  Contributors: Barkeep, Can't sleep, clown will eat me, Druzhnik, DuncanHill, Mindmatrix, Minesweeper.007,
    Morrand, Pearle, Soumyasch, Wtshymanski, 9 anonymous edits

    Wet Transformer  Source:  Contributors: DabMachine, Elkman, Hooperbloob, Jim.henderson, Mellery, Repairscircuitboards, Tevildo,
    Wickethewok, 2 anonymous edits

    Zigzag transformer  Source:  Contributors: Callidior, Charles Matthews, CheekyMonkey, Hooperbloob, Jeepday, Keilana, Linas, Me
    whynot, MightyWarrior, Mild Bill Hiccup, Peripitus, Philip Trueman, Syd1435, YoswaSmith, 11 anonymous edits
Image Sources, Licenses and Contributors                                                                                                                                                         144

    Image Sources, Licenses and Contributors
    Image:Polemount-singlephase-closeup.jpg  Source:  License: unknown  Contributors: User:Glogger
    Image:Induction experiment.png  Source:  License: Public Domain  Contributors: J. Lambert
    Image:Faradays transformer.png  Source:  License: Public Domain  Contributors: Unknown
    Image:Trafo1885.jpg  Source:  License: Public Domain  Contributors: Original uploader was WHell at de.wikipedia
    Image:DBZ trafo.jpg  Source:  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Zátonyi Sándor, (ifj.)
    Image:StanleyTransformer.png  Source:  License: Public Domain  Contributors: Reddi
    Image:Transformer3d col3.svg  Source:  License: GNU Free Documentation License  Contributors: BillC at
    Image:Transformer under load.svg  Source:  License: Creative Commons Attribution-Sharealike 2.5  Contributors:
    Image:Transformer flux.gif  Source:  License: GNU Free Documentation License  Contributors: My self
    File:Transformer equivalent circuit.svg  Source:  License: Creative Commons Attribution-Sharealike 2.5
     Contributors: Me
    File:Magnify-clip.png  Source:  License: GNU Free Documentation License  Contributors: User:Erasoft24
    Image:Variable Transformer 01.jpg  Source:  License: GNU Free Documentation License  Contributors: Original
    uploader was C J Cowie at en.wikipedia
    Image:PoleMountTransformer02.jpg  Source:  License: Public Domain  Contributors: User:Light current
    File:Three-phase transformer EI core flux animation full pulse.gif  Source:
     License: Creative Commons Attribution-Sharealike 3.0  Contributors: FEMM author is Dr. David C. Meeker // Modelling and animation done by
    Image:Kvglr.jpg  Source:  License: Public Domain  Contributors: User:Ulfbastel
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    at de.wikipedia
    Image:Transformer.filament.agr.jpg  Source:  License: unknown  Contributors: Original uploader was
    ArnoldReinhold at en.wikipedia
    Image:Lamination eddy currents.svg  Source:  License: Creative Commons Attribution-Sharealike 2.5
     Contributors: User:BillC
    Image:Small toroidal transformer.jpg  Source:  License: Creative Commons Attribution-Sharealike 2.5
     Contributors: User:BillC
    Image:Transformer-hightolow smaller.jpg  Source:  License: GNU Free Documentation License
     Contributors: User:Mtodorov 69
    Image:transformer min stray field geometry.svg  Source:  License: Public Domain  Contributors:
    Image:Drehstromtransformater im Schnitt Hochspannung.jpg  Source:  License: GNU
    Free Documentation License  Contributors: Stahlkocher
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    anonymous edits
    Image:Tapped autotransformer.svg  Source:  License: Creative Commons Attribution-Sharealike 2.5  Contributors:
    Image:Variable Transformer MFrey 001.jpg  Source:  License: Creative Commons
    Attribution-Sharealike 2.0  Contributors: 06:32, 22 October 2006 (UTC)
    File:Balun-twisted-pair-to-coaxial-hdr-0a.jpg  Source:  License: Creative Commons
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    File:2 Balun matching transformers.jpg  Source:  License: Creative Commons Attribution 3.0  Contributors:
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    Image:Buchholz 2.JPG  Source:  License: GNU Free Documentation License  Contributors: Original uploader was Fluppe37 at
    Image:Buckboostransformer.jpg  Source:  License: Creative Commons Attribution 2.5  Contributors: User:Dennis
    Image:Buckboostransformer2.jpg  Source:  License: Creative Commons Attribution 2.5  Contributors: User:Dennis
    File:Buck-Boost Powerline Transformers 320x240 23sec noaudio vidqual 0.ogv  Source:  License: Creative Commons Attribution-Sharealike 3.0
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    Image:Leistungsschalter-110KV.jpg  Source:  License: GNU Free Documentation License  Contributors: Arnero,
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    File:Stromwandler Zeichnung.svg  Source:  License: Creative Commons Attribution 3.0  Contributors: User:Biezl
    Image:CurrentTransformers.jpg  Source:  License: Creative Commons Attribution-Sharealike 2.5  Contributors:
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    Image:DeltaWyeTransformer.svg  Source:  License: Creative Commons Attribution-Sharealike 3.0  Contributors:
    Image:Pylon transformer in Syria.jpg  Source:  License: unknown  Contributors: User:High Contrast
    Image:Ölgekühlter Transformator ohne Gehäuse.jpg  Source:Ölgekühlter_Transformator_ohne_Gehäuse.jpg  License: GNU Free
    Documentation License  Contributors: Chetvorno, Julo, Stahlkocher
    Image:Flyback.jpg  Source:  License: Creative Commons Attribution 3.0  Contributors: User:Teravolt
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Image Sources, Licenses and Contributors                                                                                                                                                           145

    Image:HybridConjugate.jpg  Source:  License: GNU Free Documentation License  Contributors: LoopTel
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    Image:DoubleTransformerHybrid.jpg  Source:  License: Creative Commons Attribution-Sharealike 3.0
     Contributors: User:LoopTel
    Image:SingleTransformerHybrid.jpg  Source:  License: Creative Commons Attribution-Sharealike 3.0
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    Image:PD-icon.svg  Source:  License: Public Domain  Contributors: User:Duesentrieb, User:Rfl
    File:Induktionsapparat hg.jpg  Source:  License: Creative Commons Attribution 3.0  Contributors: User:Hgrobe
    File:Induction coil cutaway.png  Source:  License: Public Domain  Contributors: Harry Winfield Secor
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    File:Original Tesla Coil.png  Source:  License: Public Domain  Contributors: Original uploader was Reddi at
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    Image:Colorado GeoMag Map.png  Source:  License: Public Domain  Contributors: Ciaurlec, Inductiveload
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    Image:Oudin coil and circuit diagram.png  Source:  License: Public Domain  Contributors: Mihran Krikor
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    Image:tesla-coil-discharge.jpg  Source:  License: Creative Commons Attribution 2.5  Contributors: User Iantresman on
Image Sources, Licenses and Contributors                                                                                                                                                           146

    Image:Tesla coil 3.svg  Source:  License: unknown  Contributors: User:Omegatron
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    Image:operating tesla coil.jpg  Source:  License: unknown  Contributors: Eric Urban, Nv8200p
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    Image:Tesla discharge through boys body.jpg  Source:  License: Public Domain  Contributors: Harry
    La Verne Twining
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    Image:Small Toroidal Inductors.jpg  Source:  License: Creative Commons Attribution-Sharealike 3.0
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    Image:Toroidal Inductor-Simple with Axes.JPG  Source:  License: Public Domain  Contributors:
    Image:Toroidal Inductor-with Fully Confined Magnetic B Field, Three View.JPG  Source:,_Three_View.JPG  License: Public Domain  Contributors: User:Constant314
    Image:Toroidal Inductor-Simple with Circumferential current.JPG  Source:
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    Image:Toroidal Inductor-Simple with Return Wire.JPG  Source:  License: Creative
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    Image:Toroidal Inductor-Simple with Split Return Winding.JPG  Source:
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    Image:Toroidal Transformer Poynting Vector.jpg  Source:  License: Creative Commons Zero
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    Image:BA75-b2hv-Breakdown-during-transformer-oil-testing.jpg  Source:  License:
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    was Richtom80 at en.wikipedia
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    was Richtom80 at en.wikipedia
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    Image:Laminated transformers.jpg  Source:  License: GNU Free Documentation License  Contributors: Tabby
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    File:Trigtransformer.jpg  Source:  License: Public Domain  Contributors: ShuangHeng Electronics
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