DC Motor Questions

What is a DC motor?

A DC motor is an electrical machine that converts electrical energy into mechanical energy. In a DC motor, the input electrical energy is the direct current which is transformed into the mechanical rotation. A current-carrying conductor experiences a torque when placed in a magnetic field; also, it tends to move. Therefore, it can be concluded that a mechanical force is produced when a magnetic field and an electric field interact. This principle is known as the motoring action on which the DC motor works.

Flemingβs left-hand rule gives the direction of rotation of the motor. This rule states that if you stretch your index finger, middle finger, and thumb of your left hand equally perpendicular to each other, then if the middle finger designates the current direction, the index finger signifies the direction of the magnetic field. The thumb will point out the direction in which the DC motor shaft experiences the force.

Important Questions on DC Motor

1) What will happen if the DC shunt motor is linked across the AC supply?

a) Motor will run at normal speed

b) Motor will not run

c) Motor will run at a lower speed

d) Due to the heat produced in the field, the winding motor will burn

Correct Option: (d)

Explanation: The DC motor will be heated up, and it may burn because if the motor doesnβt rotate in case of the parallel field connection, it will start humming and produce vibrations. Due to this, the torque produced by negative and positive cycles will cancel out each other.

2) If the back emf of a DC motor is wiped out suddenly, then what will happen?

a) Motor will stop

b) Motor will continue to run

c) Armature may burn

d) None of these

Correct Option: (c)

Explanation: If back emf is wiped out suddenly, then the motor circuit will draw more current from the supply to retain back emf. Hence, excess current in the armature may heat up and burn the armature.

3) If the speed of a DC motor is increased, then _____.

a) Line current falls but back emf increase

b) Line current increase and back emf fall

c) Both back emf and line current increase.

d) None of these

Correct Option: (a)

Explanation: The speed is proportional to the back emf in the case of a DC motor. Therefore, back emf increases with the increase in speed. Hence, armature current decreases, and in the case of the series motor, armature current is equivalent to the line current.

4) Which of the following parts can determine the difference between a DC motor and an AC motor?

a) Field windings

b) Rotor

c) Stator

d) Commutator

Correct Option: (d)

Explanation: When outer looks are only taken into consideration, except brushes and commutator, all other parts are the same in AC machines. Commutators are not used in AC machines but in DC machines for mechanical modification.

5) Which among the following parts can withstand the maximum temperature rise in a DC motor?

a) Armature winding

b) Commutator

c) Carbon brushes

d) Field winding

Correct Option: (d)

Explanation: Since the field windings are not involved in rotary parts, maximum temperature rise can be sustained by them.

6) Which among the following laws is used to determine the direction of rotation?

b) Snellβs law

c) Newtonβs law

d) Flemingβs left-hand rule

Correct Option: (d)

Explanation: There will be a force acting on the conductor whenever it comes under a magnetic field; Flemingβs laws summarised this. And on the other hand, an induced current will be produced in a conductor if it is forcefully brought under a magnetic field.

7) The armature of the DC motor draws current, which is proportional to the _____.

a) Voltage

b) Speed

c) Torque

d) None of these

Correct Option: (c)

Explanation: According to the equation of torque produced in a DC machine, it can be concluded that a DC motor draws current, which is directly proportional to the torque required by the machine.

8) The armature shaft of a DC motor must be able to resist _____.

a) Bending moment produced due to the weight of the armature.

b) Any unbalanced magnetic pull

c) Twisting stains

d) Twisting stains, bending moment and unstable magnetic pull

Correct Option: d

Explanation: For a good and long run application of the motor, the armature shaft must be able to resist any unstable magnetic pull on the armature core, winding stains due to transmission of torque and bending moment due to the weight of the armature.

9) Sparking is avoided in a DC motor.

a) True

b) False

Correct Option: (a)

Explanation: Sparking leads to damage of commutators at brushes and commutator sections. Thus, it is sensible to avoid sparking and increase the commutation speed.

10) DC series motors are used in buses, trains, and cranes because _____.

a) DC series motor provides low starting torque

b) Efficient in cost

c) DC series motor provides high starting torque

d) As compared to other motors, it requires the minimum power

Correct Option: (c)

Explanation: Very high starting torque is delivered by the DC series motor, and also, the speed of the DC series motor can be kept constant at higher torques. Because of these two properties, DC series motors are used in buses, trains, and cranes.

Practice Questions

1) What is Fleming’s left hand rule?

2) What is Faraday’s law of electromagnetic induction?

3) What are the types of DC motors?

4) What is a universal motor?

5) What are the applications of DC motors?

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