Electrical Machines Questions and Answers_pag149

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4. The V-curves of synchronous motor is plotted between ________ 
a) Ia Vs If with constant shaft load 
b) If Vs Ia with constant shaft load 
c) power factor vs If 
d) power factor vs Ia 
View Answer 
Answer: a 
Explanation: The V-curves of synchronous motor is plotted between Ia Vs If with constant shaft 
load. 
5. When a constant power output is desired to maintain 
a) Ef*sinδ and Ia*cosθ are to be maintained constant 
b) Ef*sinδ is be maintained constant 
c) Ia*cosθ is to be maintained constant 
d) Ef*sinδ and Ia*sinθ are to be maintained constant 
View Answer 
Answer: a 
Explanation: For constant real power output the component of the excitation voltage lagging to 
Vt should remain constant as per the phasor diagram. 
6. When the excitation voltage is increased from 1 to 1.3 pu of a 3-phase synchronous motor. 
Then load angle for the constant power operation 
a) must be decreased 
b) can also be increased 
c) increased 
d) decreased 
View Answer 
Answer: a 
Explanation: Efsinδ is to be maintained constant. so if excitation voltage increases then the load 
angle must decrease. 
7. The armature current is ________ at UPF when compared to leading power factor for a 
motor. 
a) minimum 
b) maximum 
c) equal 
d) none of the mentioned 
View Answer 
Answer: a 
Explanation: At leading pf armature current will be high as the emf will be high. 
8. The armature current is _______ at UPF than motor at lagging power factor. 
a) minimum 
b) maximum 
c) zero 
d) none of the mentioned 
View Answer