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MCQ on DC Motor Pdf

Multiple Choice Questions on DC Machines

  • A direct current (DC) motor is an electrical machine that converts electrical energy into mechanical energy. 
  • According to the most common industry naming conventions today, there are three DC motor subtypes: DC brushed motors, DC permanent magnet motors, and DC universal motors. 
  • Designers can control the speed by changing the rotor voltage (and its current) or by adjusting the field winding current to change the magnetic flux between the rotor and stator. 
  • Torque is highest when the motor is stopped because the armature (rotor) does not generate back EMF at rest. With no load, the motor accelerates to dangerous speeds. 
  • Series motors do not regulate speed well because speed control depends on regulation of supply voltage.
  • The main structure of the DC motor is similar to that of the AC motor, and it is also composed of the stator part, the rotor part and the air gap between the stator and the rotor, but the composition of the stator and the rotor is different.
  • The rotor part of the DC motor has parts such as iron core, armature winding, commutator, shaft and bearing.


DC Motor MCQ with Answers Pdf

1. Which of the following rule is used to determine the direction of rotation of D.C motor?

A. Columb’s Law

B. Lenz’s Law

C. Fleming’s Right-hand Rule

D. Fleming’s Left-hand Rule

Answer: D


2. The current drawn by the armature of DC motor is directly proportional to________________

A. Torque

B. Speed

C. The voltage across the terminals

D. Cannot be determined

Answer : A


3. The efficiency of the DC motor at maximum power is

A. 90%

B. 100%

C. Around 80%

D. Less than 50%

Answer: D


4. The following electrical appliances are manufactured by using the phenomenon that the current-carrying conductor will rotate under force in a magnetic field.

A. generator

B. DC motor

C. Electromagnetic relay

D. Telephone

Answer: B


5. In which of the following applications DC series motor is used?

A. Centrifugal Pump

B. Motor Operation in DC and AC

C. Water pump drive

D. Starter for car

Answer: D


6. Which part of the DC motor can sustain maximum temperature rise?

A. Armature Winding

B. Field winding

C. Slip Ring

D. Commutator

Answer: D


7. To reverse the rotation of the DC motor, the methods that can be taken are_______

A. Change the direction of the current

B. boost current

c. Change the direction of the magnetic field

D. Both A and C

Answer: D


8. The number of the pole in Small Dc Motor Up to 5 H.P are

A. 2 poles

B. 4 poles

C. 8 poles

D. 10 poles

Answer: A


9. In order to increase the speed of a DC motor, the following methods cannot achieve the purpose:

A. Increase the current in the

coil 

B. Increase the number of turns of the coil

C. Swap the magnetic poles of the magnet

D. Enhance the original magnetic field

Answer: C


10. To change the direction of rotation of the DC motor, the method that should be taken_________

A. Enhance the magnetism of the poles

C. Change the direction of the current in the coil

D. Swap the poles of the magnet

D. Both B and C

Answer: D


11. Regarding DC motors, which of the following statements is true___________

A. The principle of the DC motor is the magnetic effect of the current

B. When the DC motor is working normally, it converts electrical energy into mechanical energy

C. The commutator of the DC motor is composed of two half-rings made of copper insulated from each other

D. Both B and C

Answer: D


12. The function of the DC motor commutator is__________

A. Change the direction of current in a DC power supply

B. Change the direction of the motor coils

C. Change the direction of the current flowing through the motor

D. The above statement is not correct

Answer: C


13. The function of the sliding rheostat in the DC motor is________

A. Change the direction of rotation of the motor

B. Automatically change the N and S poles of the electromagnet

C. Automatically change the direction of current in the coil

D. Change the current in the coil, thereby changing the speed of the motor

Answer: D


14. Which D.C. motor is preferred for elevators ? 

A. Shunt motor 

B. Series motor 

C. Differential compound motor 

D. Cumulative compound motor 

Answer: D


15. In a DC motor, when the coil turns to what position, the commutator can automatically change the direction of the current in the coil________

A. Coil to any position

B. The coil plane is parallel to the magnetic field lines

C. The coil plane is perpendicular to the magnetic field lines

D. The coil plane is at a 45° angle to the magnetic field lines

Answer: C


16. In D.C. machines fractional pitch winding is used 

A. to improve cooling 

B. to reduce copper losses 

C. to increase the generated e.m.f. 

D. to reduce the sparking 

Answer: D


17. What will happen, with the increase in speed of a DC motor?

A. Back emf increase but line current falls.

B. Back emf falls and line current increase.

C. Both back emf as well as line current increase.

D. Both back emf as well as line current fall.

Answer: A


18. A fully assembled DC motor model, the motor does not rotate after the power is turned on, and the coil rotor rotates normally after the rotor is moved by hand, then the reason why it does not rotate at the beginning may be_______

A. Internal short circuit of coil

B. Poor contact between brush and commutator

C. The magnet's magnetism is not strong or the current in the coil is not large enough

D. The coil is exactly in the equilibrium position

Answer: D


19. For which of the following applications a D.C. motor is preferred over an A.C. motor ? 

A. Low speed operation 

B. High speed operation 

C. Variable speed operation 

D. Fixed speed operation 

Answer: C


20. The direction of rotation of a D.C. series motor can be changed by 

A. interchanging supply terminals 

B. interchanging field terminals 

C. either of (a) and (b) above 

D. None of the above 

Answer: B

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