D.C. Motors MCQ Questions & Answers | Electrical Engineering

1

The condition for maximum efficiency for a D.C. generator is

A) Eddy current losses = stray losses B) Hysteresis losses = eddy current losses C) Copper losses = 0 D) Variable losses = constant losses

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Answer & Solution

Answer: Option D

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2

If a D.C. motor is connected across the A.C. supply it will

A) Run at normal speed B) Not run C) Run at lower speed D) Burn due to heat produced in the field winding by eddy currents

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Answer & Solution

Answer: Option D

Solution:

If a D.C. motor is connected across an A.C. supply, it will burn due to the heat produced in the field winding by eddy currents.

The field windings in a D.C. motor are designed for direct current, and when subjected to alternating current, the changing magnetic fields induce eddy currents in the windings.

These eddy currents generate excessive heat, leading to potential damage or burning of the motor.

3

The speed of a D.C. shunt motor can be increased by

A) Increasing the resistance in armature circuit B) Increasing the resistance in field circuit C) Reducing the resistance in the field circuit D) Reducing the resistance in the armature circuit

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Answer & Solution

Answer: Option B

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4

The armature voltage control of D.C. motor provides

A) Constant torque drive B) Constant voltage drive C) Constant current drive D) None of the above

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Answer & Solution

Answer: Option A

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5

Starters are used with D.C. motors because

A) These motors have high starting torque B) These motors are not self-starting C) Back e.m.f. of these motors is zero initially D) To restrict armature current as there is no back e.m.f. while starting

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Answer & Solution

Answer: Option D

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6

If I_a be the armature current, then speed of a D.C. shunt motor is

A) Independent of I_a B) Proportional to I_a C) Varies as I_a D) None of these

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Answer & Solution

Answer: Option A

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7

In case of D.C. shunt motors the speed is dependent on back e.m.f. only because

A) Back e.m.f. is equal to armature drop B) Armature drop is negligible C) Flux is proportional to armature current D) Flux is practically constant in D.C. shunt motors

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Answer & Solution

Answer: Option D

Solution:

In the case of D.C. shunt motors, the speed is dependent on back e.m.f. only because

Flux is practically constant in D.C. shunt motors.

In a shunt motor, the field winding is connected in parallel with the armature winding. This configuration ensures that the flux (magnetic field) remains relatively constant, regardless of changes in load or speed.

As a result, the back electromotive force (e.m.f.) generated in the armature is primarily determined by the speed of the motor and not significantly affected by changes in the flux.

Therefore, the correct option is Flux is practically constant in D.C. shunt motors.

8

A direct on line starter is used: for starting motors

A) Up to 5 H.P. B) Up to 10 H.P. C) Up to 15 H.P. D) Up to 20 H.P.

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Answer & Solution

Answer: Option A

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9

The ratio of starting torque to full-load torque is least in case of

A) Series motors B) Shunt motors C) Compound motors D) None of the above

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Answer & Solution

Answer: Option B

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10

For starting a D.C. motor a starter is required because

A) It limits the speed of the motor B) It limits the starting current to a safe value C) It starts the motor D) None of the above

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Answer & Solution

Answer: Option B

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