# 100 Important MCQ Question of DC Motor | Objective type question of DC Motor

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
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 develop by the shunt generator is maximum?

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

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

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 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

Explanation:

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

Que 47. The function of Dummy coil is to

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

Explanation:

• Dummy coils are used in machines with wave winding, they 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

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

Explanation:

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

### 5 thoughts on “100 Important MCQ Question of DC Motor | Objective type question of DC Motor”

1. Answer of question number 48 and 49 are wrong.
Flux density is increasing for trailing pole tips in generator action and leading pole tips in motor action.