Chapter 7: Transformer- Single and Three phase
11. The ordinary efficiency of a given transformer is maximum when
A. it runs at half full-load
B. it runs at full-load
C. its Cu loss equals iron loss
D. it runs overload
...
Answer is (C)
12. A transformer has negative voltage regulation when its load power factor is
A. zero
B. unity
C. leading
D.lagging
13. The saving in Cu achieved by convert ing 2-winding transformer into an autotransformer is determined by
A. voltage transformation ratio
B. load on the secondary
C. magnetic quality of core material
D. size of the transformer core
...
Answer is (A)
14. Two transformers A and B having equal outputs
and voltage ratios but unequal percentage impedances of a 4 and 2 are
operating in parallel. Transformer A will be running overload by
?percent.
A.50
B.66
C.33
D.25
...
Answer is (C)
15. If three transformers in A ? A are delivering rated load and one trans former is removed, then overload on each of the remaining transformers is ? precent.
A.66.6
B.173.2
C.73.2
D.58
16. A T-T connection has higher ratio of utilization than a V-V connection only when
A. identical transformers are used
B. load power factor is leading
C. load power factor is unity
D. non-identical transformers are used
...
Answer is (D)
17. For 3-to-3 phase conversion, which of the following given polyphase connections would you select?
A. delta/double-delta
B. star/double-star
C. delta/interconnected-star
D. delta/double-star
18. When two identical transformers are available, then the best method to find their efficiency under load conditions is the test.
A. short-circuit
B. back-to-back
C. open-circuit
D. phase-sequence
19. Leakage fluxes of a transformer may be minimized by
A. keeping the magnesising current to the minimum
B. reducing the reluctance of the iron core to the minimum
C. minimizing the number of turns both on primary and secondary
D. sectionalizing and interleaving the primary and secondary windings
20. If full-load Cu loss of a transformer is 1600 W, its Cu loss at 75% full-load would be watt.
A.900
B.1200
C. 1600 x ,5 /2
D. 1600 x 2473
...
Answer is (A)
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