281.  A 400V/200V/200V, 50Hz three winding transformer is connected as shown in Fig. 8.1. The reading ofthe voltmeter, V,will be400V 50 Hz
A. 0 V
B. 400 V
C. 600 V
D. 800 V

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282.  A single phase transformer has a maximum efficiency of 90% at full load and unity power factor. Efficiency at half load at the same power factor is
A.86.70%
B.88.26%
C.88.90%
D.87.80%

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283.  A 500 kVA, 3-phase transformer has iron losses of 300 W and full load copper losses of 600 W. The percentage load at which the transformer isexpected to have maximum efficiency is
A.50.00%
B.70.70%
C.141.40%
D.200.00%

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284.  The equivalent circuit of a transformer has leakage reactances X1, X'2 and magnetizing reactance Xm. Their magnitudes satisfy
A. XI >> X'2 >> Xm
B. Xi ? X'2 ? Xm
C. X1 X'2 >> Xm
D. X1 X2 >>Xm

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285.  Which three-phase connection can be used in a transformer to introduce a phase difference of 30? between its out put and corresponding input time voltages
A. Star - Star
B. Star ? Delta
C. Delta. Delta
D. Delta - Zigzag

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286.  A bank of three identical single-phase 250 kVA, 11 kV/230 V transformers is used to provide 400 V low tension supply from a 11 kV, 3-phase sub-station. The effective kVA rating of the bank will be
A.250
B. 250 13
C.500
D.750

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287. .A 4 kVA, 230/115 volt, 400 Hz, single-phase transformer, originally designed for use in aircrafts, isavailable in a laboratory for obtaining 115 V supply from standard 230 V, 50 Hz mains.If overloading ofthe device is not permitted, thenthe net kVA output of the transformer in the laboratory cannot exceed
A.32
B.16
C.4
D.0.5

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