Electrical Machines Questions and Answers_pag40

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Explanation: The effect of short pitched coil is to reduce the generated EMF, and it improves 
waveform of EMF generated per phase, and as the windings are placed closed to each other, 
copper will be saved. 
18. The DC machines are designed with flat topped flux density waves because 
a) average value of brush voltage is more for a flat topped B-wave 
b) average value of brush voltage is less for a flat topped B-wave 
c) losses for flat topped B-wave are less 
d) none of the mentioned 
View Answer 
Answer: a 
Explanation: For the same value of peak flux density B​p​, it is easy to see that average value of 
brush voltage could be more for a flat topped B-wave than for a sinusoidal flux density wave. 
 
 
5. Questions & Answers on Rating and Loss Dissipation 
This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on 
“Losses and efficiency”. 
1. For the given traction application using a dc series motor for a starting time ’t’ is applied. If 
this method is replaced by a series-parallel control, giving 50% time for each series and parallel. 
The saving in the starting energy of 
a) 25% 
b) zero 
c) 50% 
d) 75% 
View Answer 
Answer: a 
Explanation: Initially let the energy utilized was 100% for time t.Then for 50% duration for series, 
energy used is 50%. For parallel operation for the same circuit, the resistance gets halved 
assuming the same machine. So, for 50% of the parallel operation, 25% of energy is only 
utilized. Hence total saving is 25%. 
2. If two series motors are identically coupled. One is running as motor while other as generator. 
For this combination,the iron losses and frictional losses are found identical for 
a) identical speed and excitation. 
b) identical speed. 
c) identical rating and construction 
d) None of the mentioned. 
View Answer 
Answer: a 
Explanation: For same iron and frictional losses, same frequency should be there. For that 
same speed must be run. 
3. Hysteresis loss in a dc machine is _____ of rate of flow of air and _____ on frequency of 
_____