Page 72 - Solutions Manual to accompany Electric Machinery Fundamentals
P. 72
V a b V A V C V P 0 V P 120 3V P 30
Note that the line voltage on the secondary side lags the line voltage on the primary side by 30.
2-23. A single-phase 10-kVA 480/120-V transformer is to be used as an autotransformer tying a 600-V
distribution line to a 480-V load. When it is tested as a conventional transformer, the following values are
measured on the primary (480-V) side of the transformer:
Open-circuit test Short-circuit test
(measured on secondary side) (measured on primary side)
V OC = 120 V V SC = 10.0 V
I OC = 1.60 A I SC = 10.6 A
V OC = 38 W P SC = 25 W
(a) Find the per-unit equivalent circuit of this transformer when it is connected in the conventional
manner. What is the efficiency of the transformer at rated conditions and unity power factor? What
is the voltage regulation at those conditions?
(b) Sketch the transformer connections when it is used as a 600/480-V step-down autotransformer.
(c) What is the kilovoltampere rating of this transformer when it is used in the autotransformer
connection?
(d) Answer the questions in (a) for the autotransformer connection.
SOLUTION
(a) The base impedance of this transformer referred to the primary side is
2 480 V 2
V
Z base,P P 23.04
S 10 kVA
The base impedance of this transformer referred to the secondary side is
V 2 120 V 2
Z P 1.44
base,P
S 10 kVA
The open circuit test yields the values for the excitation branch (referred to the secondary side):
I 1.60 A
Y EX ,OC 0.01333 S
V ,OC 120 V
cos 1 P OC cos 1 38 W 78.6
V OC I OC 120 V 1.60 A
Y EX G C jB M 0.01333 78.6 0.002635 j 0.01307
R C 1/ G C 380
X 1/ B 76.5
M M
The excitation branch elements can be expressed in per-unit as
380 76.5
R C 263 pu X M 53.1 pu
1.44 1.44
The short circuit test yields the values for the series impedances (referred to the primary side):
V 10.0 V
Z SC 0.943
EQ
I SC 10.6 A
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