Electrical Machines Questions and Answers_pag404

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d) 3*k*I 
View Answer 
Answer: a 
Explanation: The phase current will be reduced by 1/1.73. 
10. If para magnetic core is used in the place of the ferromagnetic core of the transformer, then 
magnetostriction will 
a) be vanished 
b) reduce 
c) increase 
d) not be affected 
View Answer 
Answer: a 
Explanation: If the core is not ferromagnetic then the transformer will not operate at all so no 
humming sound. 
11. When we magnetize the ferromagnetic core of the transformer, core length_______ and it 
______ when demagnetized. 
a) decreases, increases 
b) decreases, decreases 
c) increases,increases 
d) none of the mentioned 
View Answer 
Answer: a 
Explanation: Due to aligned magnetic dipoles in the material, the size will reduce by a small 
margin and vice versa while demagnetization. 
12. A 400 V, 10 KVA transformer at 50 Hz, is operated at the frequency of 40 Hz, then the 
humming 
a) increases 
b) decreases 
c) remains same 
d) increases to very high 
View Answer 
Answer: a 
Explanation: If the frequency is reduced, the core flux density increases, so the noise also will 
increase. 
13. While using three phase transformers, it is taken care about the third harmonics current in 
the system. 
This is present when transformer set is delta on either side. 
a) True 
b) False 
View Answer 
Answer: a 
Explanation: Third harmonic current are co-phase in nature and require closed path which is 
present in delta connected only.