DC Motors MCQs

Question 1

When starting a differentially connected compound motor, it is desirable to short-circuit the series field winding to

Accepted Answer

avoid excessive starting speed

Answer

avoid very high starting time

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(c) prevent the motor from starting in the reverse direction

Question 2

In a loaded dc generator, if the brushes are given a shift from the interpolar axis in the direction of rotation, then the commutation will

Accepted Answer

deteriorate with fall of terminal voltage

Answer

improve with fall of terminal voltage

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improve with rise in terminal voltage

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deteriorate with rise in terminal voltage

Question 3

A dc shunt motor is required to drive a constant power load at rated speed while drawing rated armature current.Neglecting saturation and all machine losses, if both the terminal voltage and the field current of the machine are halved, then

Accepted Answer

the speed remains at 1 pu but armature current becomes 2 pu

Answer

the speed becomes 2 pu (per unit) but armature current remains at 1 pu

Answer

both speed and armature current becomes 2 pu

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both speed and armature current remain at 1 pu

Question 4

The series winding of a cumulatively compounded dc motor is short-circuited while driving a load at rated torque. This results in

Accepted Answer

increase in both the armature current as well as the motor speed

Answer

reduction in both the armature current and the motor speed.

Answer

increase in the armature current and reduction in the motor speed

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reduction in the armature current and increase in the motor speed

Question 5

Consider the following statements regarding an electrical machine having cylindrical stator and salient-pole rotor : I. Reluctance torque is produced when rotor winding is excited. 2. Reluctance torque is produced when stator winding is excited.3. When both stator and rotor windings are excited, electromagnetic torque is produced.4. When both stator and rotor windings are excited, electromagnetic as well as reluctance torques are produced. Which of these statements are correct?

Accepted Answer

1 and 3

Answer

2 and 4

Answer

1 and 4

Answer

2 and 3

Question 6

Which one of the following statements for a dc machine which is provided with interpole windings. (Im) as well as compensating winding (CW) is correct?

Accepted Answer

Both IW and CW are connected in parallel with the armature winding

Answer

Both 1W and CW are connected in series with the armature winding

Answer

IW is connected in series but CW is connected in parallel with the armature winding

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IW is connected in parallel but CW is connected in series with the armature winding

Question 7

A differentially compounded d.c. motor with interpoles and with brushes on the neutral axis is to be driven as a generator in the same direction with the same polarity of the terminal voltage. It will when.

Accepted Answer

be a cumulatively compounded generator without reversing the interpole coil connections

Answer

be a cumulatively compounded generator but the interpole coil connections are to be reversed

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be a differentially compounded generator without reversing the interpole coil connections

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be a differentially compounded generator but the interpole coil connections are to be reversed

Question 8

Match the items in the left side column with the most appropriate items in Right side column. Power losses-Dependent Variable,(A) No load Rotational loss-(P) Square of load,(B) Hystersis and eddy current losses-(Q)Rotar Rotation,(C) Brush contact loss-(R) Flux Magnitude,(D) Stray load loss-(S) Load,optionas :

Accepted Answer

A - Q; B - R; C - S; D ? P

Answer

A - R; B - P; C - Q; D- S

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A - P; B - R; C - Q; D ? S

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A - P; B - Q;C R; D - S

Question 9

A 4 pole generator with 16 coil has a two layer lap winding. The pole pitch is

Accepted Answer

4

Answer

12

Answer

16

Answer

8