Page 70 - Solutions Manual to accompany Electric Machinery Fundamentals

and the voltage regulation is

1.054 1.00
VR   100% 5.4%

1.00
(f) The efficiency of the transformer at 50 Hz is almost the same as the efficiency at 60 Hz (just slightly
less), but the total apparent power rating of the transformer at 50 Hz must be less than the apparent power
rating at 60 Hz by the ratio 50/60. In other words, the efficiencies are similar, but the power handling
capability is reduced.
2-21. Prove that the three-phase system of voltages on the secondary of the Y- transformer shown in Figure 2-
37b lags the three-phase system of voltages on the primary of the transformer by 30°.
SOLUTION The figure is reproduced below:

-
B
+ + V '
V + +
V - - A V '
B
C
- -
-
V C V '
A
+
+

+ +
V V '
A A
- -

+ +
V B V '
B
- -

+ +
V V '
C C
- -

Assume that the phase voltages on the primary side are given by
V A  V  P 0  V B  V  P  120  V C  V  P 120 

Then the phase voltages on the secondary side are given by
  
V A  V S  0  V B  V S   120  V C  V S  120 

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