Transformers
TRANSFORMER THEORY
D-Y: primary phase current =
I
3
10.4
1.73
6 amps
Example 3:
Find the secondary line current and phase current for each type of transformer
connection, if primary line current is 20 amps, and the turns ratio is 4:1.
D-D: secondary line current = 4(20) = 80 amps
secondary phase current =
aI
3
4 (20)
1.73
46.2 amps
Y-Y: second line current = aI = 4(20) = 80 amps
second phase current = aI = 4(20) = 80 amps
Y-D: secondary line current =
aI = (1.73)(4)(20) = 138.4 amps
3
secondary phase current = aI = 4(20) = 80 amps
D-Y: secondary line current =
aI
3
4 (20)
1.73
46.2 amps
secondary phase current =
aI
3
4 (20)
1.73
46.2 amps
Transformer Losses and Efficiency
All transformers have copper and core losses. Copper loss is power lost in the primary and
secondary windings of a transformer due to the ohmic resistance of the windings. Copper loss,
in watts, can be found using Equation (13-7).
(13-7)
Copper Loss
I2P RP
I2S RS
where
IP = primary current
IS = secondary current
RP = primary winding resistance
RS = secondary winding resistance
Rev. 0
Page 11
ES-13
