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

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

Correct! Wrong!

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

#### 42. When the ratio of back E.MF to applied voltage is ___ than the mechanical power develop by the shunt generator is maximum?

Correct! Wrong!

```Maximum power in DC motor is generated when

Eb = 1/2 Vs

Eb/Vs = 0.5```

#### 43. If the current in the armature of d.c series motor is reduced to 5%, the torque of the motor will become

Correct! Wrong!

```In DC series Motor, the torque is directly proportional to the square of the armature current

T ∝ Ia2

So if the armature current is reduced to 5% then torque is

T = 52

T = 25%```

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

Correct! Wrong!

• 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 the 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. This method is known as field weakening or constant power mode of operation.

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

Correct! Wrong!

• Yoke forms a part of the magnetic circuit of a dc machine. In addition, it also serves as mechanical support for the entire assembly of the machine. The cross-section of the yoke should be such, as to support the weight of the machine and to withstand the magnetic pull of the poles on the armature.
• The yoke of a dc machine should be of magnetic material. The magnetic circuit, i.e. (the path of magnetic flux) of the DC machine is completed through the yoke.
• Yoke section provides two paths of equal reluctance to the flow of magnetic flux φp. As such the pole flux φp while entering the yoke section divides into two equal parts, i.e. φp/2 each flowing in the two paths. Hence the flux in the section of the yoke is φp/2.

#### 46. DC machine windings are

Correct! Wrong!

• 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.

#### 47. The function of the Dummy coil is to

Correct! Wrong!

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.

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

Correct! Wrong!

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

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

Correct! Wrong!

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

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

Correct! Wrong!

```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
```

1. Browin says:

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.

Yes you are right @browin the flux density increases in the case of dc generator under trailing pole tips but the question is about motor flux density … motor work opposite to that of DC generator so the answer is right

2. i have doubt in qstn no. 18..why ans is series not differential motor? if self loading and self releiving properties are same or different? pls explain..