Electrical Machines Questions and Answers_pag48

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c) no third harmonic current in its phase but has flat-topped flux 
d) third harmonic current in its phase but has sinusoidal flux 
View Answer 
Answer: a 
Explanation: Star connection in three phase has no closed path for the flow of the third 
harmonic current. 
7. For a separately excited dc machine of 25 kW, 250 V and armature resistance of 0.25 ohms, 
is running at 3000 rpm with supply of 255 V. 
The electromagnetic power produced at the armature in kilo watts is 
a) 5 
b) 10 
c) 0.5 
d) 8 
View Answer 
Answer: a 
Explanation: The current in armature, Ia = 255-250/0.25 = 20 A 
Power = Eb*Ia = 250*20 = 5000 W 
8. If two alternators A and B are operated in parallel having per unit synchronous reactance as 
0.02 ohms and 0.05 ohms respectively.Then what the ratio of contributing loading of machine A 
to B? 
a) 2.5 
b) 0.4 
c) 1 
d) can not be operated in parallel 
View Answer 
Answer: a 
Explanation: The load shared by the alternators is inversely related to their pu reactance. 
9. A 200V, 10 hp, 4 poles, 60 Hz, Y-connected induction motor had full load slip of 5%. Then the 
rotor speed of the motor with respect to stator is 
a) 1710 rpm 
b) 1800 rpm 
c) 90 rpm 
d) 3510 rpm 
View Answer 
Answer: a 
Explanation: Synchronous speed, Ns = 120*f/P = 120*60/4 = 1800 rpm 
Rotor speed = (1-slip)*Ns = 0.95*1800 = 1710 rpm. 
10. Auto transformer rating is chosen so as to have best performance, when the analogous two 
winding transformer has 
a) two voltage levels fairly close to each other 
b) very high transformation ratio. 
c) any voltage levels 
d) step down purpose.