1. Nowadays DC motor is widely used in
(A) Electric Traction
(B) Air compressor
(C) Centrifugal Pump
(D) Machine shop
Answer (A). Electric Traction
Explanation:
DC series motors are used in traction applications due to their excellent starting torque properties.
Torque is directly proportional to the square of the armature current resulting in high starting torque.
2. Which DC motor has the least percentage increase in Input current for the same percentage increase in torque?
(A) Separately excited motor
(B) Series motor
(C) Shunt motor
(D) Compound motor
Answer (B). Series motor
Explanation:
In DC series motor both field and armature windings carry the same current from the source as both are connected in series.
Therefore torque is directly proportional to the product of current in field and armature windings.
For a given source current the torque produced is square of this current. So, even there is low current change, the torque change is great.
3. By looking at which particular part of the motor we can Identify a “DC motor”?
(A) Shaft
(B) Field winding
(C) Commutator
(D) Armature winding
Answer (C). Commutator
Explanation:
The commutator converts the alternating current generated in armature into the unidirectional current.
It is a mechanical rectifier, so the commutator collects induced EMF or current developed in the armature.
In the DC motor, the commutator is used to alternate the magnetic field created by the DC current.
For AC motor the magnetic field is already alternated by the nature of the AC current.
4. Counter EMF of the DC motor is
(A) Less than the applied voltage
(B) More than the applied voltage
(C) Equal to the applied voltage
(D) None of the above
Answer (A) Less than the applied voltage
Explanation:
According to the DC motor equation,
Ia = (V – Eb)/Ra
⇒ V = Eb + IaRa
From the above equation, it is clear that the Back EMF is always less than the applied voltage.
5. If the back EMF of DC motor vanishes then
(A) The motor continues to run
(B) Motor will stop
(C) Armature will burn
(D) The motor continues to run in slow speed
Answer (C) Armature will burn
Explanation:
The presence of back emf makes the d.c. motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load.
Armature current is Ia = (V – Eb)/Ra
When the motor is running on no-load, small torque is required to overcome the friction and windage losses.
As Eb is zero the armature current will increases to a very high value, therefore a large amount of heat will be produced which may burn the armature winding.