TRANSFORMER THEORY
Transformers
TABLE 1: Voltage and Current Ratings of Transformers
Transformer
Connection
(Primary to
Secondary)
Primary
Secondary
Line
Phase
Line
Phase
Volt.
Current
Volt.
Current
Volt.
*
Current
Volt.
Current
D-D
V
I
V
I
3
V
a
aI
V
a
aI
3
Y-Y
V
I
V
3
I
V
a
aI
V
3 a
aI
Y-D
V
I
V
3
I
V
3 a
3 aI
V
3 a
aI
D-Y
V
I
V
I
3
3 V
a
aI
3
V
a
aI
3
*a = N1/N2;
3
1.73
Example 1:
If line voltage is 440 V to a 3f transformer bank, find the voltage across each
primary winding for all four types of transformer connections.
D-D: primary voltage = V = 440 volts
Y-Y: primary voltage =
V
3
440
1.73
254.3 volts
Y-D: primary voltage =
V
3
440
1.73
254.3 volts
D-Y: primary voltage = V = 440 volts
Example 2:
If line current is 10.4 A in a 3f transformer connection, find the primary phase
current.
D-D: primary phase current =
I
3
10.4
1.73
6 amps
Y-Y: primary phase current = I = 10.4 amps
Y-D: primary phase current = I = 10.4 amps
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