ΜΕΤΑΣΧΗΜΑΤΙΣΤΕΣ TRANSFORMERS Reference : 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) Φ B – 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 – Lucien Gaulard and John Gibbs exhibited a device called secondary generator – 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 Φ B – Magnetic flux (Wb) ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 5

Reference: INDUCTION THEORY ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 6

LARGE INDUSTRY SMALL INDUSTRY RESIDENTIAL POWER STATION POWER GENERATION AND TRANSMISSION 11 KV/240 V 132/33 KV 400/132 KV 132/11 KV ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 7

TRANSFORMER MODELS Ideal Transformer N p = No of windings on the primaryN s = No of windings on the secondary i p = Current into the primaryi s = Current out from the secondary V p = Voltage across the primaryV s = Voltage across the secondary ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 8

Primary and Secondary Relationship V P = N P = a = i S V S N S i P Note; a < 1 = Step up transformer a > 1 = Step down transformer Voltage and current angles are NOT affected hence, θ P = θ S = 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 Copper losses (I^2 R) Leakage Flux losses Core losses  Eddy currents  Hysteresis losses ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 11

EQUIVALENT CIRCUIT 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 IHIH VLVL V SE NCNC N SE VHVH I SE ILIL ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 13

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 ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 14

THREE PHASE TRANSFORMER CONNECTIONS Delta – wye (Δ – Y) Wye – delta (Y- Δ) Delta – delta (Δ – Δ) Wye – wye (Y – Y) ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 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 P IN = V P I P cos θ P P OUT = V S I S cos θ S P IN = P OUT = V P I P cos θ P = V S I S cos θ S S OUT = S IN = V P I P = V S I S η Ideal = 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, η Real = P OUT x 100% P IN = P OUT x 100% P OUT + P LOSS = V S I S cos θ S_________ V S I S cos θ S + i 2 R + (V P /a) 2 RC ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 18

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 V O at full load. VOLTAGE REGULATION An Ideal transformer has a voltage regulation, VR = 0% ΤΕΙ Κρήτης - Ηλεκτρικές Μηχανές Συλλιγνάκης Γιάννης 19

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