Transformer Losses & Efficiency Lecture No. 06 By. Sajid Hussain Qazi

Transformer Ratings Kilo Volt Amps KVA Transformers are rated to supply a given output in Kilo Volt Amps or KVA at a specified frequency and terminal voltage.

Transformer Ratings They are NOT rated in Watts The load power factor is unknown Apparent Power

Power output at unity PF ? Problem S = 2 kVA V2 = 230 V V1 = 6,351 V Power output at unity PF ? Power = S x PF P = 2 kVA x 1 P = 2 kW

Full load secondary current at 0.8 PF ? Problem S = 2 kVA V2 = 230 V V1 = 6,351 V Full load secondary current at 0.8 PF ? I = 10.87 A

(a) Power output at unity power factor Problem S = 20 kVA V1 = 1270 V V2 = 200 V (a) Power output at unity power factor P = 20 kW

(b) Power output at 0.8 power factor Problem S = 20 kVA V1 = 1270 V V2 = 200 V (b) Power output at 0.8 power factor P = 16 kW

(c) Full-load secondary current at unity power factor Problem S = 20 kVA V1 = 1270 V V2 = 200 V (c) Full-load secondary current at unity power factor I = 100 A

Ratio between Input power and Output Power Efficiency Ratio between Input power and Output Power

Efficiency is normally expressed as a percentage

Transformer Efficiency Power In Power Out η = 100% Overcome Copper Losses Overcome Iron Losses Some Power is used to:

(a) Power output of transformer Problem S = 20 kVA V1 = 230 V η = 90% V2 = 32 V PF = 0.85 (a) Power output of transformer P = 85 W

Problem P = 94.4 W S = 20 kVA V1 = 230 V η = 90% V2 = 32 V PF = 0.85 (b) Power input P = 94.4 W

Problem P = 9.4W S = 20 kVA V1 = 230 V η = 90% V2 = 32 V PF = 0.85 (c) Losses P = 9.4W

Transformer Losses Copper Losses (Cu) Varies with load current Produces HEAT Created by resistance of windings Short circuit test supplies copper losses

Limited Supply Voltage ≈ 5-10 % Short Circuit Test Copper Losses (Cu) Limited Supply Voltage ≈ 5-10 % Secondary Short Circuited Wattmeter indicates Copper Losses (Cu)

Transformer loaded at 50% Short Circuit Test Copper Losses (Cu) Finds Cooper losses at full load Copper losses vary with the square of the load Full load Cu loss = 100 W Transformer loaded at 50% PCu = 25 W

Copper Losses (Cu) Cu Losses (W) % Load

Transformer Losses Iron Losses (Fe) Fixed Always present Related to transformers construction Eddy Currents Hysteresis Reduced by laminations Produces HEAT Reduced by using special steels in laminations

Wattmeter indicates Iron Losses (Fe) Open Circuit Test Finds Iron Losses (Fe) Full Supply Voltage Secondary Open Circuit Wattmeter indicates Iron Losses (Fe)