Test: Torque in DC Machines


10 Questions MCQ Test Electrical Machines | Test: Torque in DC Machines


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QUESTION: 1

 What is the principle of torque production in a dc machine?

Solution:

Though all the mentioned laws contribute towards the torque generation, but primarily its the Lorentz’s forces which initiate and define the torque direction.

QUESTION: 2

 The torque induced beyond the pole shoes in the DC machine is

Solution:

The magnetic field vanishes at the pole terminals and thus the cross product with the magnetic field yields zero.

QUESTION: 3

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?

Solution:

 Initially the induced emf in the armature is zero. But the current will flow as there is terminal source voltage.

QUESTION: 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. What will be the magnitude of the following current at t=0+?

Solution:

 I=(V-E)/r
=(120-0)/0.3
=400 A.

QUESTION: 5

If the torque induced is zero in the dc machine, it can be said that

Solution:

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.

QUESTION: 6

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?

Solution:

 v = V/(2*r*l*B)
= 120/(2*0.0005*1*0.25)
= 480 rad/s.

QUESTION: 7

 If the torque is in the direction of rotation, the DC machine acts as

Solution:

The electromagnetic torque generated in the dc generator is in the direction in which the prime mover is rotated.It is produced due to the action of armature current and stator field.

QUESTION: 8

The cogging torque is absent in the permanent magnet dc machine is due to

Solution:

 It is due to the fact that no field poles are induced in PMDC to create magnetic flux leading to harmonics.

QUESTION: 9

DC Motor torque depends on

Solution:

The torque will indeed depend on the geometry as well the magnetic materials used in the construction of the dc machine. Though this remains constant with the machine.

QUESTION: 10

For a universal motor, the rated supply current is 10A giving a rated torque of 100 N-m. If a current of 5A is applied, what will be generated torque vary to?

Solution:

The torque is proportional to the square of the current.
So the new torque will be 100*(2^2)= 400 N-m.

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