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					ELECTRICAL MACHINE
       EEP 3243

   Lt Cdr Ong Khye Liat RMN
          13 Jan 2010



                              1
RECAP/ADDITION




                 2
• The amount of magnetic field lines passing through the area near
  the magnet is called the magnetic flux, ø and in unit Wb.
• Magnetic flux density, B is defined as the ratio of magnetic flux per
  unit area. Its unit is Tesla or Wb per square meter.



• The magnetomotive force (MMF),F is defined as the product of
  current and the number of turns of the coil.
                             F = NI At




                                                                   3
• Magnetic field intensity is defined as the magnetomotive force per
  unit length.

• Relative permeability is the ratio of permeability of any medium (µ)
  to the permeability of the free space (µo)

• The magnetic flux density (B) created in a magnetic material is
  directly proportional to the amount of applied magnetic field
  intensity (H).
                               B = µr µoH




                                                                    4
• Reluctance (R) is the ratio of magnetomotive force to the flux
  through any cross section of magnetic circuit.



• Permeance (P) is defined as the ratio of magnetic flux to the
  magnetomotive force through any cross section of the magnetic
  circuit.




                                                                   5
• Electromagnetic force or Lorentz force, F
            F = BlI




• Hysteresis loss, Ph = khVcvolBm nf

• Eddy Current Loss, Pe = keVcvolt2Bm 2f2




                                              6
Hysteresis Loop
    • Current increasing from zero to some
      value then H will increasing, Ha.
    • Increase the current of its max value
      until H reach at the saturation point
      where magnetic material has reached
      to its saturation region, Hb.




                                       7
 Cont
• In the reverse way (because of AC),
  when current decrease the H will
  moves by the other path.
• But there will be some B remaining
  (OC, Br ,residual induction) if the H is
  reduced to zero.
• If increase the current in the reverse
  direction, then the H will finish at point
  D.


                                       8
 Cont
• OD, Hc, coercive force is necessary
  to demagnetize the magnetic
  material.
• Continue increasing current will end
  to point E and will be developed
  complete hysteresis curve in the
  opposite direction if continue the
  other steps.




                                    9
Eddy Current Loss
      • 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,
        but are more laborious and
        expensive to construct.



                                        10
             Magnetic Materials
• Ferromagnetic materials have high and +ve
  susceptibility to a magnetic field. µr > 1000s µo .
  Like iron, nickel, steel and cobalt.
• Diamagnetic materials have very weak and –ve
  susceptibility to a magnetic field. µr < µo . Like
  copper, gold and silver.



                                                        11
                       Cont.
• Paramagnetic materials have small and +ve
  susceptibility to a magnetic field. µr slightly higher
  than µo . Like magnesium, molybdenum and lithium.




                                                      12
TRANSFORMER




              13
             Introduction
• A transformer is a device that transfers electrical
  energy from one circuit to another
  through inductively coupled conductors . It has
  following points:
   – No moving parts.
   – No electrical connection btw windings.
   – Magnetically coupled winding.
   – Efficiency more than 95%.
   – No frequency change.


                                                   14
                            Classification
• 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 isolation;
• By end purpose: distribution, rectifier, arc furnace, amplifier output;
• By winding turns ratio: step-up, step-down, isolating (equal or near-equal ratio),
  variable.




                                                                                15
               Transformer Types
• Power transformers
  – Laminated core
  – Toroidal
  – Polyphase transformer
• Instrument transformers
  – CT
  – VT/PT




                                   16
                             Cont.
• Pulse transformers
• RF transformers (Steel laminations are not suitable)
   – Air-core transformers
   – Balun
• Audio transformers
   – Loudspeaker transformers
   – Output transformer (valve)



                                                         17
Circuit symbols




                  18
        Transformer Construction
• Depends on voltage & current rating.
• It consists of:
   – Magnetic core
   – Coils
   – Insulating materials
   – Tank & cooling system




                                         19
                    Magnetic Core
•   Normally made of laminated sheet steel.
•   L, I, E type laminated sheet steel.
•   Nonmagnetic material known as air core transformer.
•   Ferromagnetic material known as iron-core transformer.
•   2 types of construction core & shell type.




                                                        20
                            Coils
• Made by bare copper wire by # of turns and insulated
  from each other by insulating materials.
• The conducting material used for the windings depends
  upon the application.
  – Small power and signal transformers, the coils are often wound
    from enameled magnet wire, such as Formvar wire.




                                                              21
                         Cont.
– Larger power transformers wound with copper rectangular strip
  conductors insulated by oil-impregnated paper and blocks
  of pressboard.

– High-frequency transformers operating in the tens to hundreds
  of kilohertz often have windings made of braided Litz wire .




                                                           22
                Insulating Material
• Enamel is a common material for inter turn insulation.
• Other materials are insulating paper, cotton tape, winding
  leads, high or low voltage bushing.
• Transformer oil, Pyranol used for insulation purposes
  inside energized transformer.




                                                         23
             Tank & Cooling System
• Tank is used to hold coils, core and transformer oil.
• Tank also for safety and protection purposes.
• Transformer oil is used to reduce the heat which
  generated by transformer under loaded condition.
• Natural air cooling also used to reduce the heat.




                                                          24
25
            Operating Principle
• 2 coils (primary &
  secondary) in a 1 phase
  transformer.
• Primary coil connected to
  source, secondary coil
  delivers the power to load.




                                  26
Cont.
• A sinusoidal current will flow
  through primary coil when
  connected to AC source.
• Flux, Ø will generate in core &
  move from bottom to up of primary
  coil.
• Small % of the flux will cut the
  primary coil and 95% flux will move
  to secondary coil through the
  magnetic core.



                                27
Cont.
• The flux will cut by the secondary
  coil.
• A voltage will induce across
  secondary coil according to
  Faradays laws of electronmagnetic
  induction.
• The current will flow through the
  load if load is attached to
  secondary coil.



                                28
END OF PART 2




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