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ΜΕΤΑΣΧΗΜΑΤΙΣΤΕΣ TRANSFORMERS Reference : http://www.osha.gov/SLTC/etools/electric_power/images/transformer2.jpg ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 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) ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 2 TRANSFORMER USES • Impedance matching • Electrical Isolation • AC power transmission STEP-UP Transformer STEP-DOWN Transformer ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 3 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. ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 4 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) ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 5 INDUCTION THEORY Reference: http://www.tpub.com/doeelecscience/electrical%20science2_files/image1053.jpg ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 6 TRANSFORMER MODELS Ideal Transformer ip(t) Np Ns is(t) + + Vp(t) Vs(t) - - Np = No of windings on the primary ip = Current into the primary Vp = Voltage across the primary Ns = No of windings on the secondary is = Current out from the secondary Vs = Voltage across the secondary ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 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 ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 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 ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 10 REAL TRANSFORMER LOSSES ip(t) Lp Rp Vp(t) Ls jXm Rc Np Ns is(t) Rs Vs(t) • Copper losses (I^2 R) • Leakage Flux losses • Core losses Eddy currents Hysteresis losses ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 11 EQUIVALENT CIRCUIT Leqp Vp Rc Is/a Reqp jXm aVs Approximate Transformer Model referred to the primary side ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 12 TYPES Auto-Transformer – Used to change a desirable voltage by only a small amount. For example: 120/132 V IH VSE + VL + ISE NSE IL VH NC - ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 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. ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 14 THREE PHASE TRANSFORMER CONNECTIONS Delta – wye (Δ – Y) Wye – delta (Y- Δ) ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης Delta – delta (Δ – Δ) Wye – wye (Y – Y) 15 TYPES • Two types of special purpose transformers used in power systems for taking measurements. – Potential Transformer – Current Transformer ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 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% ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 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 ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 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% ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 19 ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 20 ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 21 ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 22 ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 23 ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 24