Electrical Machines Questions and Answers_pag62

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Answer: a 
Explanation: The magnetic field vanishes at the pole terminals and thus the cross product with 
the magnetic field yields zero. 
4. The simple rotating loop between pole faces connected to a battery and resistor through a 
switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms 
and magnitude strength = 0.25T is supplied with 120V. Suddenly the switch is closed at t=0, 
what is observed in the circuit? 
a) Current will flow but zero induced EMF 
b) Current will not flow and zero induced EMF 
c) Current will not flow but EMF is induced 
d) Current will flow and EMF will also be induced 
View Answer 
Answer: a 
Explanation: Initially the induced emf in the armature is zero. But the current will flow as there is 
terminal source voltage. 
5. The simple rotating loop between pole faces connected to a battery and resistor through a 
switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms 
and magnitude strength = 0.25T is supplied with 120V. What will be the magnitude of the 
following current at t=0+? 
a) 400A 
b) 200A 
c) 0 
d) any of the mentioned 
View Answer 
Answer: a 
Explanation: I=(V-E)/r 
=(120-0)/0.3 
 =400 A. 
6. If the torque induced is zero in the dc machine, it can be said that 
a) current is zero 
b) flux can be zero 
c) current or flux=0 
d) any of the mentioned 
View Answer 
Answer: a 
Explanation: The torque at steady state of operation is zero as the emf induced is equal to the 
source voltage. Also the the very much starting the current will be zero which will not let the emf 
to get induced. 
7. The simple rotating loop between pole faces connected to a battery and resistor through a 
switch, the specifications of this machine are radius = 0.5m, length 1m, resistance = 0.3 ohms 
and magnitude strength = 0.25T is supplied with 120V. What is the steady state angular velocity 
at no-load? 
a) 480 rad/s