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

2. 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.

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

4. 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

5. 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 = A

6. (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

7. (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

8. (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

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

10. Efficiency is normally expressed as a percentage

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

12. (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

13. 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

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

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

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

17. 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

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

19. 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

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