Electrical Machines Questions and Answers_pag167

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View Answer 
Answer: a 
Explanation: Air gap voltage will lead the field flux by 90° in a cylindrical-rotor alternator. 
14. The power factor of an alternator under short circuit condition will be almost near 
a) zero lagging 
b) zero leading 
c) unity 
d) depends on the type of the alternator 
View Answer 
Answer: a 
Explanation: During short circuit condition, the magnetizing current flows to provide the core 
losses compensation. So the pf is slightly zpf lagging. 
15. In a 3-phase cylindrical rotor alternator 
a) field mmf leads air gap flux, air gap flux leads armature flux 
b) field mmf leads air gap flux, air gap flux lags armature flux 
c) armature mmf leads air gap flux and air gap flux leads field flux 
d) armature mmf leads air gap flux and air gap flux lags field flux 
View Answer 
Answer: a 
Explanation: Check the phasor diagram. 
 
12. Questions on Synchronous Generators 
This set of Electrical Machines Quiz focuses on “Phasor Diagram of a Cylindrical Rotor 
Alternator”. 
1. The flux addition is due to 
(i) sinusoidal distribution of mmfs 
(ii) zero relative velocity between two mmfs 
(iii) spatial distribution of three phases by 120° 
a) (i), (ii) 
b) (i),(ii), (iii) 
c) (iii) 
d) (ii), (iii) 
View Answer 
Answer: a 
Explanation: Flux addition in ac machines is due to sinusoidal flux distribution and they must be 
stationary with respect to each other. 
2. If the resultant flux will not be in phase with the resultant mmf Fr. This is due to 
a) hysteresis effect 
b) armature reaction 
c) hysteresis effect and armature reaction 
d) eddy currents 
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