Electrical Machines Questions and Answers_pag84

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View Answer 
Answer: a 
Explanation: We need the angular acceleration and the moment of inertia of the machine. 
5. ______ test is used for determining the efficiency of a traction motor 
a) Field 
b) Retardation 
c) Hopkinson 
d) Swineburne’s 
View Answer 
Answer: a 
Explanation: Field test is used for finding efficiency of a traction motor. 
6. If the field current and armature current are reversed, then 
a) direction of rotation remains same 
b) direction of rotation reverses 
c) stops 
d) none of the mentioned 
View Answer 
Answer: a 
Explanation: When both field current and armature current are reversed then the direction will 
not change. 
7. Mark the possible causes of overheating of commutator in a DC machine. 
a) Overload 
b) Restricted ventilation 
c) Shorted winding 
d) Any of the mentioned 
View Answer 
Answer: a 
Explanation: All the factors can cause the overheating of commutator. 
8. In the hopkinson’s test on two DC machines. Machine A has field current of 1.4A and B has 
field current of 1.3A. Which machine acts as motor? 
a) B 
b) A 
c) A,B 
d) None of the mentioned 
View Answer 
Answer: a 
Explanation: Ea/Eb = 1.4/1.3 
Ea= 1.077 Eb > Eb 
So, the machine will act as motor. 
9. In the Hopkinson’s test on two DC machines. Machine A has field current of 1.4A and B has 
field current of 1.3A. Which machine acts as generator? 
a) B 
b) A