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```					ELECTRICAL MACHINE
EEP 3243

Lt Cdr Ong Khye Liat RMN
13 Jan 2010

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• 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

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• 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

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• 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.

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• Electromagnetic force or Lorentz force, F
F = BlI

• Hysteresis loss, Ph = khVcvolBm nf

• Eddy Current Loss, Pe = keVcvolt2Bm 2f2

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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.

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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.

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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.

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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.

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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.

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

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TRANSFORMER

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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.

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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.

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

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

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Circuit symbols

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

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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.

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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.

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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 .

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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.

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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.

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Operating Principle
• 2 coils (primary &
secondary) in a 1 phase
transformer.
• Primary coil connected to
source, secondary coil

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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.

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Cont.
• The flux will cut by the secondary
coil.
• A voltage will induce across
secondary coil according to
induction.
• The current will flow through the
secondary coil.

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END OF PART 2

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