300+ DC Motor MCQ [Free PDF] – Objective Question Answer for DC Motor Quiz

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.

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.

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.

Explanation:

According to the DC motor equation,

I_a = (V – E_b)/R_a

⇒ V = E_b + I_aR_a

From the above equation, it is clear that the Back EMF is always less than the applied voltage.

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 I_a = (V – E_b)/R_a
• 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 increase to a very high value, therefore a large amount of heat will be produced which may burn the armature winding.

Explanation:

• In the case of DC motors Speed ∝ Back emf(E_b)/flux(φ).
• The field winding is connected across the armature. The resistance offered by the field is constant so the current is constant. As current is constant flux also becomes constant b the cause flux is directly proportional to current.
• Therefore the speed of the DC shunt motor depends on the back EMF.

Explanation:

• In a DC shunt motor, if we cut the supply for the field winding, the speed would dangerously increase in order to maintain the back emf of the motor.
• In the case of DC motors Speed ∝ Back emf(E_b)/flux(φ).
• So if flux drops to ZERO theoretically speed will tend to infinity and practically it will damage the windings of the motor.

Explanation:

In a differential compound motor, because of the opposition of the series field to the shunt field, the flux decreases as the load and armature current increase.  As we know that the decrease in flux causes an increase in speed. However, because the speed is proportional to E_B/φ, if both factors vary in the same proportion, the speed will remain constant. This action may occur in the differential compound motor. If more turns are added to the series coil, it is possible to cause the motor to run faster as the Load is Increased.

•  With over compounding, a slight increase in speed is possible with an increase in load. This speed characteristic is achieved only with a loss in the rate at which torque increases with load.
• Because the field decreases with a load increase, a differential compound motor has a tendency for load instability.
• When starting a differential motor, it is recommended that the series field is shorted because the great starting current in this field may overbalance the shunt field and cause the motor to start in the opposite direction. 

Explanation:

• At the time of starting a DC Motor, the Back EMF developed is zero as its speed is zero.
• We know that general emf equation E = E_b + l_aR_a. At starting E_b= 0 so l_a = E/R_a
• So very high current flow (which is limited by resistance only but resistance itself is very low usually 1-2 ohms) through the armature of the machine which can destroy it completely.
• So to protect the machine from such large current, starters which can be simple resistors or the sophisticated 3 point or 4 point starters may be employed.

Explanation:

In the differential compound, motor two field windings i.e shunt and series windings oppose each other. This causes a reduction in flux and consequences a decrease in torque.