DC Motor MCQ Part-01

DC Motor MCQ part-01

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#1. A d.c motor is used to ………………

a) Generated power

b) change mechanical energy to electrical energy

c) change electrical energy to mechanical energy

d) increase energy output into it

#2. Iron losses in a d.c. motor depend upon…………..

a) flux only

b) speed only

c) both flux and speed

d) none of the above

#3. Which motor should not be used for centrifugal pumps ?

a) shunt

b) series

c) cumulative compound

d) differential compound

#4. Which of the following is the fundamental principle of operation of a DC motor?

A. Lorentz Force Principle

B. Faraday's Law of Electromagnetic Induction

C. Lenz's Law

D. Joule's Law of Heating

The Lorentz Force principle states that a current-carrying conductor placed in a magnetic field experiences a mechanical force, which produces torque in the motor.

#5. The primary function of the commutator in a DC motor is to:

A. Increase the magnetic field strength.

B. Act as a mechanical rectifier to convert AC to DC.

C. Reduce friction losses.

D. Regulate the motor speed.

The commutator reverses the current direction in the armature conductors to maintain a continuous, unidirectional torque.

This constant reversal of current within the rotating armature ensures that the torque always pushes the motor in the same rotational direction. This action of mechanically switching the current’s direction is functionally a form of rectification.

#6. The Back EMF (Eb) generated in a DC motor is directly proportional to:

A. Speed and Flux.

B. Number of poles and total armature conductors only.

C. Terminal voltage and load torque.

D. Armature current and armature resistance.

Back EMF, or back electromotive force, is a voltage that’s induced in the rotating armature of a DC motor. As the armature spins within the magnetic field, the conductors cut through the magnetic flux, which in turn generates a voltage. According to Lenz’s law, this induced voltage opposes the main supply voltage, hence the term “back” EMF.

The formula for back EMF is:

E_{b} = ϕZNP / 60A

#7. . A DC Series motor is best suited for applications requiring:

A. Precise speed control over a wide range.

B. Operation under no-load conditions.

C. Constant speed under varying load.

D. High starting torque.

A DC series motor is unique because its field winding is connected in series with its armature winding. This means the same current flows through both the field and armature coils. This design gives it a distinct and very useful characteristic: the ability to produce a massive amount of starting torque.

#8. Why is a DC series motor never started without a load?

A. The starting torque will be zero.

B. The motor will run at a dangerously high speed.

C. The voltage drop across the series field winding will be excessive.

D. The armature current will be zero.

The correct answer is B. The motor will run at a dangerously high speed.

The design of a DC series motor, where the field winding is in series with the armature, is the key to both its greatest strength (high starting torque) and its greatest weakness (dangerous no-load speed).

#9. A DC shunt motor runs on a constant supply voltage. If the load is increased, the motor speed will:

A. Increase slightly.

B. Decrease to zero.

C. Remain almost the same.

D. Decrease significantly.

DC shunt motors are nearly constant speed motors, and the speed drops only slightly due to the increased I_aR_a drop.

#10. What is the primary function of a motor starter for a DC motor?

A. To maintain a constant field current.

B. To reverse the direction of rotation.

C. To limit the high starting current.

D. To increase the motor speed.

Explanation

The primary function of a motor starter for a DC motor is to limit the excessive armature current that flows when the motor is first connected to the supply.

#11. The torque (T) developed by a DC motor is proportional to:

A. Armature current only.

B. Flux (Φ) and armature current (Ia).

C. Square of the armature current.

D. None of these

The torque ( $T$ ) developed by a DC motor is directly proportional to the magnetic flux per pole ( $Φ$ ) and the armature current ( $I_{a}$ ).

#12. If the field current of a DC shunt motor is reduced, its speed will:

A. Decrease.

B. Drop to zero.

C. Increase.

D. Remain the same.

Since N∝ΦEb, reducing the flux (Φ) causes the speed (N) to increase, although the back EMF (Eb) will also momentarily drop, leading to a large inrush of armature current to compensate.

#13. Which type of DC motor is most suitable for traction applications like electric trains?

A. DC Shunt motor

B. DC Compound motor (Long shunt)

C. DC Series motor

D. Permanent Magnet DC motor

Right answer

DC series motors have a high starting torque and are suitable for heavy-duty applications like traction, hoists, and cranes.

#14. The total losses in a DC motor can be classified into:

A. Input losses and Output losses.

B. No-load losses and Full-load losses.

C. Windage losses and Stator losses.

D. Copper losses, Iron losses, and Mechanical losses.

Copper losses (I²R) are electrical, iron losses (hysteresis and eddy current) are magnetic, and mechanical losses (friction and windage) are rotational.

#15. Armature reaction in a DC motor causes the magnetic neutral axis (MNA) to shift:

A. Opposite to the direction of rotation.

B. There is no shift in a DC motor.

C. In the direction of

D. Perpendicular to the direction of rotation.

Right answer

The MNA shifts in the direction of rotation for a motor and opposite for a generator.

Finish