Electrical Machines Questions and Answers_pag50

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d) none of the mentioned 
View Answer 
Answer: a 
Explanation: Hysteresis loss in a dc machine is independent of rate of flow of air and depends 
on frequency of magnetic reversal. 
4. In dc machine iron losses cause 
a) heating in core 
b) loss in efficiency 
c) rise in temperature of ventilating air 
d) all of the mentioned 
View Answer 
Answer: a 
Explanation: The iron losses cause heating of the core which causes reduction inefficiency and 
cooling air gets heated up. 
5. For squirrel cage and slip ring induction motor, cooling methods is efficient in 
a) squirrel cage induction motor 
b) slip ring induction motor 
c) both of the motors 
d) none of the mentioned 
View Answer 
Answer: a 
Explanation: As the squirrel cage induction motor has more ventilation and more space as 
compares to slip ring induction motor, there will be more efficient ways to cool down the squirrel 
cage motor. 
6. If one of the phases of the supply breaks down, then the connected three phase induction 
motor 
a) continues to run as a single phase induction motor, provided load does does not increase 
beyond 57.7% 
b) stops operating after few seconds 
c) continues to run as single phase induction motor 
d) continues to run as two phase induction motor 
View Answer 
Answer: a 
Explanation: When one of the phases breaks down then the other two phases will supply the 
rated current but the load should be reduced to 57.7%. 
7. If one of the phases of RYB supply gets broken, then the temperature rise of the induction 
motor 
a) remains same as before 
b) reduces as compared to the normal operating temperature 
c) reduces to temperature rise of corresponding single phase induction motor 
d) increases 
View Answer 
Answer: a