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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)
- 7. TRANSFORMER MODELS Ideal Transformer ip(t) Np Ns is(t) + + Vp(t) - Vs(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
- 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 ip(t) Lp Rp Vp(t) Ls jXm Rc Np Ns • Copper losses (I^2 R) • Leakage Flux losses • Core losses Eddy currents Hysteresis losses Rs Vs(t) is(t)
- 11. EQUIVALENT CIRCUIT Leqp Vp Rc Reqp jXm Approximate Transformer Model referred to the primary side Is/a aVs
- 12. TYPES Auto-Transformer – Used to change a desirable voltage by only a small amount. For example: 120/132 V IH VSE + VL - IL + ISE NSE VH NC -
- 13. TYPES Three Phase Transformers 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. Reference : Electric Machinery and Power System Fundamentals, Stephen J. Chapman
- 14. 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
- 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 V O 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