Page 74 - Solutions Manual to accompany Electric Machinery Fundamentals

(b) The autotransformer connection for 600/480 V stepdown operation is
+ +

N V
SE SE
-
600 V + +
N V
C C
480 V

- - -
(c) When used as an autotransformer, the kVA rating of this transformer becomes:

N  N 41

S  IO C SE S  W 10 kVA   50 kVA
N SE 1
(d) As an autotransformer, the per-unit series impedance Z EQ is decreased by the reciprocal of the

power advantage, so the series impedance becomes
0.00963
R   0.00193 pu
EQ
5
0.0398
X   0.00796 pu
EQ
5
while the magnetization branch elements are basically unchanged. At rated conditions and unity power
factor, the output power to this transformer would be P OUT = 1.0 pu. The input voltage would be

V  V I Z
P S S EQ
1 0 V 
V P   1 0  0.00193 j 0.00796   1.002 0.5 pu  
The core losses (in resistor R C ) would be

2 V 1.002 2
P    0.00382 pu
core
R C 263
The copper losses (in resistor R EQ ) would be

2
P  CU I R EQ   1.0 2  0.00193   0.0019 pu
The input power of the transformer would be




P  IN P OUT  P  CU P core  1.0 0.0019 0.00382 1.0057
and the transformer efficiency would be
P 1.0
  OUT  100%   100% 99.4%

P IN 1.0057
The voltage regulation of the transformer is


1.0057 1.00
VR   100% 0.6%

1.00

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