TRANSFORMERS

MARK LEAKEY

KOK WAI, WONG

WAN HAZLIN ZAINI

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

DEFINITION• Device used to transfer energy from

primary winding to secondary winding by electromagnetic induction.

• Based on Faraday’s Law of induction

Where:-

ε – EMF (V)

ΦB – Magnetic flux (Wb)

TRANSFORMER USES

• Impedance matching

• Electrical Isolation

• AC power transmission STEP-UP Transformer

STEP-DOWN Transformer

HISTORY• 1831 – Michael Faraday invented the

Induction Ring.

• 1881 – Lucien Gaulard and John Gibbs exhibited a device called secondary generator.

• 1885 – William Stanley developed the fist commercially used practical device while working for Westinghouse Electric Company in US.

INDUCTION THEORY

• Transformers behaviour is based on Faraday’s Law of Induction

Where:-

ε – EMF (V)

N – No of turns of wire

ΦB – Magnetic flux (Wb)

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

INDUCTION THEORY

TRANSFORMER MODELSIdeal Transformer

-

Vs(t)

Np Ns+

Vp(t)

ip(t)

-

+

is(t)

Np = No of windings on the primary Ns = No of windings on the secondary

ip = Current into the primary is = Current out from the secondary

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

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, θP = θS = θ

LOSSES

• Transformers have losses and these losses must come into consideration.

• Copper losses (I^2 R)

• Leakage Flux losses

• Core losses Eddy currents

Hysteresis losses

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

EQUIVALENT CIRCUIT

jXm aVsRc

Is/aLeqp Reqp

Vp

Approximate Transformer Model referred to the primary side

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

TYPES

Three phase transformerscan be constructed in twodifferent 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

THREE PHASE TRANSFORMER CONNECTIONS

Delta – wye (Δ – Y)

Wye – delta (Y- Δ)

Delta – delta (Δ – Δ)

Wye – wye (Y – Y)

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

TYPES• Two types of special purpose transformers

used in power systems for taking measurements.– Potential Transformer – Current Transformer

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

EFFICIENCY

• Efficiency (η) is the ratio of the power out to the power in of a transformer.

– η in an Ideal transformer, no power losses• PIN = VPIP cos θP

• POUT = VSIS cos θS

• PIN = POUT = VPIP cos θP = VSIS cos θS

• SOUT = SIN = VPIP = VSIS

• ηIdeal = 100%

Efficiency for a single phase real transformer

• As mentioned previously, losses occur in a real transformer and these losses must be taken into count.

• Hence, ηReal = POUT x 100% PIN

= POUT x 100% POUT + PLOSS

= VSIS cos θS_________

VSIS cos θS + i2R + (VP/a)2 RC

• Voltage regulation (VR) is the ability of a system to 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