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

Que 41. Which of the following is not the function of pole shoe in DC motor?

  1. To reduce eddy current loss
  2. To reduce reluctance
  3. To spread out flux
  4. To support field coil

Answer 1.To reduce Eddy current loss

Explanation: Eddy current is reduced by providing laminating sheet not by using pole shoe.

 

Que 42. When the ratio of back E.MF to applied voltage is _____ than the mechanical power developed by the shunt generator is maximum?

  1. 1.0
  2. 0.5
  3. 2
  4. 3

Answer B. 0.5

Explanation:

Maximum power in DC motor is generated when
Eb = 1/2 Vs
Eb/Vs =0.5

 

Que 43. The condition for maximum power in D.C motor is

  1. Back emf (Eb) = Supply voltage (Vs)
  2. Eb = 1/2 Vs
  3. 1/2 Eb = Vs
  4. Eb = 2 × Vs

Answer B. Eb = 1/2 Vs

Explanation:

Maximum power in DC motor is generated when

Eb = 1/2 Vs
Eb/Vs = 0.5

 

Ques 44. In a DC shunt motor if field current is decreased then the speed of the motor is

  1. Decrease
  2. Increase
  3. Remain same
  4. None of the above

Answer 2 . Increase

Explanation:

  • This is known as field weakening or constant power mode of operation.
  • When the field winding current reduced the Field’s magnetic strength reduced which in turn reduced the back EMF and at the same time the rotor current increases
  • Due to reduced magnetic field pole intensity rotor starts rotating faster till it develops same Back EMF which should be equal to its original value.
  • So in simple words when shunt field current If decreases, φ decreases, and the speed rises as speed is inversely proportional to flux.

 

Que 45. The function of yoke in a DC machine is

  1. To provide mechanical protection
  2. To reduce eddy current
  3. Flux path completion
  4. Both 1 & 3

Answer 4. Both 1 & 3

Explanation:

  • The function of the yoke is to provide mechanical protection & flux path completion.
  • To know more about yoke visit Construction of DC Machine

 

Que 46. DC machine windings are

  1. Short pitched
  2. Full pitched
  3. Any of the above
  4. None of the above

Answer 2. Full pitched

Explanation:

  • For DC machines full pitch winding is preferred to generate maximum induced EMF.
  • Short pitch windings are generally used in AC alternators to reduce harmonics.

 

Que 47. The function of the Dummy coil is to

  1. Reduce eddy current
  2. Reduce hysteresis loss
  3. Increase flux
  4. Mechanical Balancing

Answer 4 . Mechanical Balancing

Explanation:

  • Dummy coils are used in machines with wave winding, they are meant for the mechanical balance of the armature since with wave winding, the periphery is not utilized fully.
  • To know more about dummy coil visit Construction of DC Machine.

 

Que 48. The flux density under leading pole tip in DC motor tends to

  1. Decrease
  2. Remain same
  3. Increase
  4. None of the above

Answer 3. Increase

Explanation:

Under leading pole tips main flux and armature flux will aid each other. Therefore flux density increases.

 

Que 49. The flux density under trailing pole tip in DC motor tends to

  1. Decrease
  2. Remain same
  3. Increase
  4. None of the above

Answer 3. Decrease

Explanation:

Under trailing pole tips main flux and armature flux will subtract each other. Therefore flux density decrease.

 

Que 50. In D.C motors Diverters are used in

  1. Cumulative compound motors
  2. Differentially compound motors
  3. Shunt motors
  4. Series motors

Answer 4. Series motor

Explanation:

The speed of a dc series motor can be controlled by changing armature voltage(speed is directly proportional) or field current (speed is inversely proportional).

There are three ways to control the flux of the DC series motor.

⇒By field diverter
⇒By motor diverter method
⇒By armature diverter

Why is diverter not used in DC shunt motor?

  • As we know that to limit the starting current in DC shunt motor Field winding has high resistance.
  • Now if we connect the diverter across the field winding the total current will flow from the diverter as the current follows the path of minimum resistance.
  • This will increase the speed of the motor to a dangerous value which may cause a short circuit in the motor.
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