1. Transformers are rated in Kilovolt-Amps (KVA) at a specified frequency and voltage, not in Watts, as the load power factor is unknown. 2. Efficiency is the ratio of output power to input power, expressed as a percentage. Some power is lost overcoming iron losses like eddy currents and hysteresis, and copper losses from resistance in the windings. 3. Copper losses vary with the square of the load current and can be measured using a short circuit test, while iron losses are fixed and measured using an open circuit test.

1. TRANSFORMER
LOSSES &
EFFICIENCY
Lecture No. 06
By. Sajid Hussain Qazi

2. Transformer Ratings
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
I
V
S 

PF
S
r
ActivePowe 

PF
r
ActivePowe
S 
Apparent Power

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

5. Problem
PF
V
S
I


V1 = 6,351 V V2 = 230 V
S = 2 kVA
Full load secondary current at 0.8 PF ?
I = 10.87 A
0.8
230
2000
I



6. PF
S
Power 

V2 = 200 V
V1 = 1270 V
S = 20 kVA
0
.
1
000
,
20 

P
P = 20 kW
(a) Power output at unity power factor
Problem

7. PF
S
Power 

V2 = 200 V
V1 = 1270 V
S = 20 kVA
8
.
0
000
,
20 

P
P = 16 kW
(b) Power output at 0.8 power factor
Problem

8. VxPF
S
I 
V2 = 200 V
V1 = 1270 V
S = 20 kVA
I = 100 A
(c) Full-load secondary current at unity power factor
200x1.0
20,000
I 
Problem

9. Efficiency
Power
Input
Power
Output
η 
Losses
Output
Input 

Losses
Power
Output
Power
Output
η


Ratio between Input power and Output Power
Power
Input
Losses
Power
Input
η



10. Efficiency
100
Power
Input
Power
Output
η% 

Efficiency is normally expressed as a percentage

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

12. PF
S
Power 

V1 = 230 V V2 = 32 V
S = 20 kVA
η = 90% PF = 0.85
(a) Power output of transformer
0.85
100
P 

P = 85 W
Problem

13. In
Out
η 
V1 = 230 V V2 = 32 V
S = 20 kVA
η = 90% PF = 0.85
(b) Power input
P = 94.4 W
η
Out
In 
0.9
W
85
In 
Problem

14. Losses
Out
In 

V1 = 230 V V2 = 32 V
S = 20 kVA
η = 90% PF = 0.85
(c) Losses
P = 9.4W
Losses
85
94.4 

Problem

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

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

17. Short Circuit Test
  100
0.5
loss
Copper
2


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
100
0.25
loss
Copper 


18. 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
0 10 20 30 40 50 60 70 80 90 100 110
Copper Losses (Cu)
% Load
Cu
Losses
(W)

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

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