Electrical Machines Questions and Answers_pag49

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View Answer 
Answer: a 
Explanation: The auto transformer is best suited with the voltage levels of same rating for a two 
winding transformer. 
11. In the lab, two students X and Y conducted the experiments to verify the name plate ratings 
of the transformer.But student X conducted the same at higher frequency. The kVA reading of 
student X will be 
a) higher than Y’s reading 
b) lower than Y’s reading 
c) same as Y’s reading 
d) none of the mentioned 
View Answer 
Answer: a 
Explanation: As rating is directly proportional to the frequency of operation. 
This set of Electrical Machines online test focuses on “Cooling (Loss Dissipation)”. 
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 
_____ 
a) independent, dependent, magnetic reversal 
b) independent, dependent, operation 
c) dependent, independent, magnetic reversal