Electrical Machines Questions and Answers_pag146

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Answer: b 
Explanation: Reactive power depends on the excitation of the synchronous machine. 
6. If the synchronous machine is loaded from floating conditions, the alternator begins to 
operate at 
a) leading p.f 
b) lagging p.f 
c) unity p.f 
d) zero p.f 
View Answer 
Answer: a 
Explanation: The resultant flux will be leading in nature, so after loading it from loading it from 
floating, alternator will operate at leading p.f. 
7. If the synchronous machine is loaded from floating conditions, the synchronous motor begins 
to operate at 
a) leading p.f 
b) lagging p.f 
c) unity p.f 
d) zero p.f 
View Answer 
Answer: b 
Explanation: The resultant flux will be lagging in nature, so after loading it from loading it from 
floating, alternator will operate at lagging p.f. 
8. Reactive power flow in synchronous machine can be controlled by 
a) varying field excitation 
b) varying field current 
c) varying field flux 
d) any of the mentioned 
View Answer 
Answer: d 
Explanation: All the alternatives are same, varying the field flux matters at the end to change the 
reactive power in a synchronous machine. 
9. If the 3-phase alternator has 4 poles and has synchronous speed of 120 rad/s. 
Then the mechanical speed in rad/sec is 
a) 60 
b) 240 
c) 120 
d) 30 
View Answer 
Answer: a 
Explanation: Mechanical speed = electrical speed*2/Number of Poles. 
10. The power load angle characteristic as maximum for a load angle of 
a) 90° 
b) 45°