Electrical Machines Multiple Choice Questions on “Torque in DC Machines”.
1. Choose the correct representation, which depict a non-zero torque in the armature?
A. electrical-machines-questions-answers-dc-machine-torque-q1a”>
B. electrical-machines-questions-answers-dc-machine-torque-q1b”>
C. electrical-machines-questions-answers-dc-machine-torque-q1c”>
D. any of the mentioned
Answer: A
Clarification: Torque is produced when the net force is zero and there is a couple generated. The directions must obey Lorentz’s law and vector product.
2. What is the principle of torque production in a dc machine?
A. Lorentz’s law
B. Lenz’s law
C. Faraday’s law
D. Self inductance
Answer: A
Clarification: Though all the mentioned laws contribute towards the torque generation, but primarily its the Lorentz’s forces which initiate and define the torque direction.
3. The torque induced beyond the pole shoes in the DC machine is?
A. 0
B. 2/pi *phi *i
C. 4/pi *phi *i
D. none of the mentioned
Answer: A
Clarification: 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
Answer: A
Clarification: 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
Answer: A
Clarification: 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
Answer: A
Clarification: 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
B. 960 rad/s
C. 320 rad/s
D. 490 rad/s
Answer: A
Clarification: v = V/(2*r*l*B.
= 120/(2*0.0005*1*0.25)
= 480 rad/s.
8. If the torque is in the direction of rotation, the DC machine acts as ________
A. generator
B. motor
C. amplidyne
D. any of the mentioned