TRANSFORMERS

1. TRANSFORMERS MARK LEAKEY KOK WAI, WONG WAN HAZLIN ZAINI Reference : http://www.osha.gov/SLTC/etools/electric_power/images/transformer2.jpg

2. 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)

3. TRANSFORMER USES • Impedance matching • Electrical Isolation • AC power transmission • STEP-UP Transformer • STEP-DOWN Transformer

4. 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.

5. 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)

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

7. TRANSFORMER MODELS Ideal Transformer 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

8. 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

9. 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

10. REAL TRANSFORMER LOSSES • Copper losses (I^2 R) • Leakage Flux losses • Core losses • Eddy currents • Hysteresis losses

11. EQUIVALENT CIRCUIT Approximate Transformer Model referred to the primary side

12. IH ISE VSE NSE IL VH VL NC TYPES Auto-Transformer • Used to change a desirable voltage by only a small amount. For example: 120/132 V

13. TYPES Three Phase Transformers • Three phase transformers • can be constructed in two • different ways i.e. :- • A three phase bank consists of three single phase transformers. • Three windings wrapped around a common core. Reference : Electric Machinery and Power System Fundamentals, Stephen J. Chapman

14. THREE PHASE TRANSFORMER CONNECTIONS Delta – wye (Δ – Y) • Delta – delta (Δ – Δ) Wye – delta (Y- Δ) • Wye – wye (Y – Y) Reference : Electric Machinery and Power System Fundamentals, Stephen J. Chapman

15. 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

16. 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%

17. 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

18. VOLTAGE REGULATION • 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. • An Ideal transformer has a voltage regulation, • VR = 0% Reference : http://en.wikipedia.org/wiki/Voltage_regulation