Friday, September 11, 2020

Chapter 7: Transformer- Single and Three phase-Page 2

 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

... Answer is (C)

 
 
 

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
  ...
Answer is (B)

 
 
 

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

... Answer is (C)

 
 

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

... Answer is (B)

 
 

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

 
  ... Answer is (D)

 
 

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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