Electrical Machines Questions and Answers_pag345

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d) 2 to -2 
View Answer 
Answer: a 
Explanation: It is either 1 or -1. 
7. A 200V dc machine has 4 poles and 40 coils, having simplex lap winding. The number of 
commutator segments which required in the given machine will be 
a) 40 
b) 20 
c) 80 
d) 26 
View Answer 
Answer: a 
Explanation: It is a simplex winding and so one segment of commutator will be needed for each 
coil. 
8. Commutator segments in a DC shunt machine is equal to the number of 
a) coil sides 
b) turns 
c) coils 
d) slots 
View Answer 
Answer: a 
Explanation: Commutator segments are placed as per the coil sides in the machine. 
9. We can employ dummy coils in a DC machine to 
a) compensate reactance voltage 
b) reduce armature reaction 
c) provide mechanical balance to the armature 
d) improve the waveforms generated inside the commutator 
View Answer 
Answer: a 
Explanation: Dummy coils are placed inside so that to provide mechanical support to dc 
machine and they do not take part in the emf development. 
10. In AC machines we should prefer double layer winding over single layer windings because 
a) it requires identical coils 
b) it is economical to use 
c) it offers lower leakage reactance 
d) all of the mentioned 
View Answer 
Answer: a 
Explanation: Double layer winding enables more usage of the space and lesser air gaps so the 
leakage flux also reduces and efficiency improves. 
11. For an electrical machine with C number of coils and P poles, the distance between the coils 
connected by an equalizer ring is 
a) C/P