Electrical Machines Questions and Answers_pag52

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6. Questions on Electrical Machine Applications/ Armature Reaction and Commutation 
This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on 
“Machine applications”. 
1. Synchronous-induction coupled motors are required for 
a) high starting torque applications 
b) high pull out torque applications 
c) high running torques applications 
d) none of the mentioned 
View Answer 
Answer: a 
Explanation: Induction motor provides high starting torque with proper values of slip ring 
terminal resistance. Coupled along with the synchronous motor it acts as an independent set 
and be employed in high torque application. 
2. Which of the following can be used for braking purposes in electric trains? 
a) Induction generator 
b) Induction motor 
c) Dc series motor 
d) Dc differentially compounded generator 
View Answer 
Answer: a 
Explanation: Induction generator runs at a speed greater than the synchronous speed. To apply 
in braking purposes the induction generator can be run in the opposite direction to that of the 
operating machine direction and stopped. 
3. A synchronous motor absorbing 60 kW is connected in parallel with an continuous load of 
240 kW operating at 0.8 power factor lag.What is the kVAR supplied by the synchronous motor 
to obtain a total power factor of 0.9 lagging? 
a) 35, leading 
b) 35, lagging 
c) 145, leading 
d) None of the mentioned 
View Answer 
Answer: a 
Explanation: Total power = 240 + 60 = 300 kW 
cos(phi) = 0.90. Total reactive power = P*tan(phi) = 300*tan(25.8) = 180 kVAR Load kVAR = 
240*tan(36.87) = 180 kVAR 
kVAR supplied by the motor = 145-180 = -35 or k3 kVAR leading. 
4. The most widely used application of ac series motor is 
a) electric traction upto 1600 kW 
b) paper mills 
c) welding 
d) none of the mentioned 
View Answer