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TRANSFORMERS

MARK LEAKEYKOK WAI, WONG

WAN HAZLIN ZAINI

Reference : http://www.osha.gov/SLTC/etools/electric_power/images/transformer2.jpg

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DEFINITION

Device used to transfer energy fromprimary winding to secondary winding by

electromagnetic induction.

Based on Faradays Law of induction

Where:-

EMF (V)B Magnetic flux (Wb)

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TRANSFORMER USES

Impedance matching

Electrical Isolation

AC power transmissionSTEP-UP Transformer

STEP-DOWN Transformer

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HISTORY

1831 Michael Faraday invented theInduction Ring.

1881 Lucien Gaulard and John Gibbs

exhibited a device called secondarygenerator.

1885 William Stanley developed the fist

commercially used practical device whileworking for Westinghouse Electric

Company in US.

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INDUCTION THEORY

Transformers behaviour is based

on Faradays Law of Induction

Where:-

EMF (V)N No of turns of wire

B Magnetic flux (Wb)

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Reference:

http://www.tpub.com/doeelecscience/electrical%20science2_files/image1053.jpg

INDUCTION THEORY

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TRANSFORMER MODELS

Ideal Transformer

-

Vs(t)

Np Ns

+

Vp(t)

ip(t)

-

+

is(t)

Np = No of windings on the primary Ns = No of windings on the secondaryip = Current into the primary is = Current out from the secondary

Vp = Voltage across the primary Vs = Voltage across the secondary

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Primary and Secondary

Relationship

VP = NP = a = iS

VS NS iP

Note; a < 1 = Step up transformer

a > 1 = Step down transformer

Voltage and current angles are NOT affected

hence, P= S=

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LOSSES

Transformers have losses and these

losses must come into consideration.

Copper losses (I^2 R)

Leakage Flux losses

Core losses

Eddy currentsHysteresis losses

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Vp(t) jXm

Rp

Vs(t)Rc

is(t)Lp Ls

Np

Rsip(t)

Ns

REAL TRANSFORMER LOSSES

Copper losses (I^2 R)

Leakage Flux losses Core losses

Eddy currents

Hysteresis losses

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EQUIVALENT CIRCUIT

jXm aVsRc

Is/aLeqpReqp

Vp

Approximate Transformer Model referredto the primary side

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TYPES

Auto-Transformer Used to change a desirable voltage by only a small

amount. For example: 120/132 V

IH +

-

+

-

VL

VSE

NC

NSE

VH

ISE

IL

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TYPES

Three phase transformers

can be constructed in two

different ways i.e. :-

1. A three phase bank consists

of three single phase

transformers.

2. Three windings wrapped

around a common core.

Three Phase Transformers

Reference : Electric Machinery and Power System Fundamentals, Stephen J. Chapman

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THREE PHASE TRANSFORMER

CONNECTIONS

Deltawye ( Y)

Wye delta (Y- )

Deltadelta ( )

Wye wye (Y Y)

Reference : Electric Machinery and Power System Fundamentals, Stephen J. Chapman

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TYPES

Two types of special purpose transformersused in power systems for taking

measurements.

Potent ial Trans form er

Current Transformer

Reference : Electric Machinery and Power System Fundamentals, Stephen J. Chapman

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EFFICIENCY

Efficiency () is the ratio of the power out tothe power in of a transformer.

in an Ideal transformer, no power losses

PIN = VPIPcos P

POUT = VSIScos S

PIN = POUT = VPIPcos P = VSIScos S

SOUT = SIN = VPIP = VSIS Ideal = 100%

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Efficiency for a single phase

real transformer

As mentioned previously, losses occur in a realtransformer and these losses must be taken into

count.

Hence, Real = POUT x 100%PIN= POUT x 100%

POUT + PLOSS

= VSIScos S_________VSIScos S + i2R + (VP/a)2

RC

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Voltage regulation (VR) is the ability of a systemto provide near constant voltage over a wide

range of load conditions. Also it compares the VO

at no load to VO at full load.

VOLTAGE REGULATION

An Ideal transformer has a voltage regulation,

VR = 0%

Reference : http://en.wikipedia.org/wiki/Voltage_regulation