Download Transformer Losses & Efficiency

TRANSFORMER
LOSSES & EFFICIENCY
Lecture No. 06
By. Sajid Hussain Qazi
Transformer Ratings
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
S V I
ActivePower  S  PF
ActivePower
S
PF
Problem
S = 2 kVA
V1 = 6,351 V
V2 = 230 V
Power output at unity PF ?
Power = S x PF
P = 2 kVA x 1
P = 2 kW
Problem
S = 2 kVA
V1 = 6,351 V
V2 = 230 V
Full load secondary current at 0.8 PF ?
S
I
V  PF
2000
I
230  0.8
I = 10.87 A
Problem
S = 20 kVA
V1 = 1270 V
(a)
V2 = 200 V
Power output at unity power factor
Power  S  PF
P  20,000 1.0
P = 20 kW
Problem
S = 20 kVA
V1 = 1270 V
(b)
V2 = 200 V
Power output at 0.8 power factor
Power  S  PF
P  20,000  0.8
P = 16 kW
Problem
S = 20 kVA
V1 = 1270 V
(c)
V2 = 200 V
Full-load secondary current at unity power factor
S
I
VxPF
20,000
I
200x1.0
I = 100 A
Efficiency
Ratio between Input power and Output Power
Output Power
η
Input Power
Input  Output  Losses
Input Power  Losses
Output Power
η
η
Input Power
Output Power  Losses
Efficiency
Efficiency is normally expressed as a percentage
Output Power
η% 
 100
Input Power
Transformer Efficiency
Power
In
Power
Out
η = 100%
Some Power
is used to:
Overcome
Copper
Losses
Overcome
Iron
Losses
Problem
S = 20 kVA
η = 90%
V1 = 230 V
(a)
V2 = 32 V
Power output of transformer
Power  S  PF
P  100  0.85
P = 85 W
PF = 0.85
Problem
S = 20 kVA
η = 90%
V1 = 230 V
(b)
Out
η
In
Out
In 
η
V2 = 32 V
PF = 0.85
Power input
85 W
In 
0.9
P = 94.4 W
Problem
S = 20 kVA
η = 90%
V1 = 230 V
(c)
V2 = 32 V
Losses
In  Out  Losses
94.4  85  Losses
P = 9.4W
PF = 0.85
Transformer Losses
Copper Losses (Cu)
•Varies with load current
•Produces HEAT
•Created by resistance of windings
•Short circuit test supplies copper losses
Short Circuit Test
Copper Losses (Cu)
Limited
Supply
Voltage
≈ 5-10 %
Secondary
Short Circuited
Wattmeter indicates Copper Losses (Cu)
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%
Copper loss  0.5  100
2
Copper loss  0.25  100
PCu = 25 W
Cu Losses (W)
Copper Losses (Cu)
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
% Load
70
80
90
100
110
Transformer Losses
•Fixed
Iron Losses (Fe)
•Always present
•Related to transformers construction
Eddy Currents
Reduced by laminations
Produces HEAT
Hysteresis
Reduced by using special
steels in laminations
Open Circuit Test
Finds Iron Losses (Fe)
Full Supply
Voltage
Secondary
Open Circuit
Wattmeter indicates Iron Losses (Fe)