MCQ 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 the 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 an 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 the D.C. motor?
A. Columbus’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 the 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.Switch
Answer: D

7. To reverse the rotation of the DC motor, the methods that can be taken_______
A. Change the direction of the current
B.boost current
vs. Change the direction of the magnetic field
D.Both A and C
Answer: D

8. The number of the pole in a Small Dc Motor Up to 5 H.P are
A. 2 poles
B. 4 poles
c.8 poles
D. 10 poles
Answer: A

9. 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 flow 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 increases 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. In 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 the 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

21. The rated power of the DC motor refers to_________
A. Mechanical power absorbed on the shaft
B. Mechanical power output on the shaft
C. the electric power absorbed by the armature port
D. Electric power output by armature port
Answer: B

22. The role of the commutator of the DC generator is equivalent to________
A. To rectify
B.Inverter
C. is not a rectifier nor an inverter
D. Sometimes rectified and sometimes reversed
Answer: A

23. When starting the DC motor, the magnetic circuit should be powered______
A. Connect with the armature circuit at the same time
B. Access before the armature circuit
C. connect after the armature circuit
D. None of these
Answer: B

24. To change the direction of the shunt DC motor, you can________
A. Change the connection between the excitation winding and the armature
B. Increase excitement
C. Change the polarity of the power supply
D. reduce excitement
Answer: A

25. For a DC generator, the speed of the rated operating state is reduced to 30% of the original speed, but the excitation current and armature current remain unchanged______
A.Ea drops by 30%
B. T decreased by 30%
C. Ea and T all drop by 30%
D. The terminal voltage drops by 30%
Answer: A

26. During the speed regulation process of the series resistance of the DC motor, if the load torque remains unchanged, it will remain unchanged_________
A. Input power
B.Output power
C. Electromagnetic power
D. The efficiency of the motor
Answer: A

27. A shunt DC generator wants to change the positive and negative polarities of both ends of the armature, the method used is________
A. To change the steering of the prime mover
B. Change the connection method of the excitation winding
C. Change the steering of the prime mover and change the connection method of the excitation winding
D. None of these
Answer: C

28. If the speed of the shunt DC generator increases by 20%, the terminal voltage Uo of the generator will be__________
A.increased by 20%
B. increased by more than 20%
C. the increase is less than 20%
D. to remain unchanged
Answer: B

29. When the DC motor is running under load when the brush is in the geometric neutral line, its armature reaction is________
A. quadrature axis armature reaction
B. Direct axis magnetizing armature reaction
C. direct axis demagnetization armature reaction
D. All of the above are true
Answer: A

30. Which of the following statements about DC motors is correct________
A. The rotor has no electromagnetic torque when the generator is running, but only when the motor is running
B. The armature has no electromotive force when the motor is running, but only when the generator is running
C. Both the generator and the motor have the same electromotive force during operation
D. Whether a generator or an electric motor, there is electromagnetic torque during operation, but the nature is different
Answer: D