DC MOTOR

 1. The direction of rotation of a dc shunt motor can be reversed by interchanging

 A) The field terminals only 
 B) The armature terminals only 
 C) Either field or the armature terminals 
 D) The supply terminals 

ANSWER= (C) 
Explain:- The direction of rotation of a dc shunt motor can be reversed by interchanging either field or the armature terminals. 

 

2. The armature of d.c.motors is laminated to

 A) Reduce the hysteresis losses 
 B) Reduce the eddy current losses 
 C) Reduce the inductivity of armature 
 D) Reduce the copper losses of the armature 

ANSWER= (B) 
Explain:- The armature of d.c.motors is laminated to reduce the eddy current losses. 

 

3. If the field of a dc shunt motor is opened

 A) The motor will stop 
 B) It will continue to run at its rated speed 
 C) The speed of the motor will become very high 
 D) The motor will stop 

ANSWER= (C) 
Explain:- If the field of a dc shunt motor is opened then the speed of the motor will become very high. 

 

4. Which of the following d.c. motors has the least drop in the speed between no load to nominal load?

 A) Series motor without commutating poles 
 B) Series motor with commutating poles 
 C) Compound motor without commutating poles 
 D) Shunt motor with commutating poles 

ANSWER= (D) 
Explain :- DC Shunt motor with commutating poles has the least drop in the speed between no load to nominal load. 

 

5. The speed of a series motor at no load is

A) 3000 rpm
B) 1500 rpm
C) Infinity
D) Zero rpm

ANSWER= (C)
Explain :- The speed of a series motor at no load is infinity.

 

7. The function of a commutator in a d.c. motor is

 A) To change d.c. into a.c 
 B) To change a.c.into d.c 
 C) To collect current from conductors 
 D) To conduct the current to the brushes 

ANSWER= (A) 
Explain :- The function of a commutator in a d.c.motor is to change DC into AC. 

 

8. The current flowing in the conductors of a d.c. motor is

 A) AC 
 B) DC 
 C) AC as well as DC 
 D) Transient 

ANSWER= (A) 
Explain :- The current flowing in the conductors of a d.c. motor is AC. 

 

9. As the load is increased the speed of a d.c. shunt motor will

 A) Increase slightly 
 B) Reduce slightly 
 C) Remain constant 
 D) Increase proportionately 

ANSWER= (B) 
Explain :- As the load is increased the speed of a d.c. shunt motor will decrease slightly. 

 

10. The speed of a d.c. motor is

 A) Inversely proportional to back emf and directly proportional to flux 
 B) Directly proportional to emf as well as flux 
 C) Directly proportional to back emf and inversely proportional to flux 
 D) Inversely proportional to emf as well as flux 

ANSWER= (C) 
Explain :- Eb = (N*P* Φ*Z)/(60*A). 

 

11. The rotating part of a d.c. motors is known as

 A) Stator 
 B) Armature 
 C) Pole 
 D) Yoke 

ANSWER= (B) 
Explain :- The rotating part of a d.c. motors is known as Armature. 

 

12. Why is the airgap between the yoke and armature of an electric motor kept smaller?

 A) To avoid overheating of the machine 
 B) To achieve a stronger magnetic field 
 C) To achieve a weaker magnetic field 
 D) None of the above 

ANSWER= (B) 
Explain :- The airgap between the yoke and armature of an electric motor kept smaller to achieve a stronger magnetic field. 

 

13. Carbon brushes are used in electric motors to

 A) Provide a path for flow of current 
 B) Prevent overheating of armature windings 
 C) Brush off carbon deposits on the commutator 
 D) Prevent sparking during commutation 

ANSWER= (A) 
Explain :- Carbon brushes are used in electric motors to provide a path for flow of current. 

 

14. What is the effect produced by the electric current in an electric motor

 A) Magnetic effect only 
 B) Heating effect only 
 C) Chemical effect only 
 D) Both magnetic and heating effect 

ANSWER= (D) 
Explain :- Both magnetic and heating effects are produced by the electric current in an electric motor. 

 

15. If the speed of d.c. motot is increased, the back emf

 A) Decreases 
 B) Increases 
 C) Remains constant 
 D) Increases then Decreases 

ANSWER= (B) 
Explain :- If the speed of a d.c.motor is increased, the back emf increases. 

 

16. Why is the air gap between stator and armature of an electric motor kept as small as possible?

A) To get a stronger magnetic field
B) To reach a higher speed of rotation
C) To improve the air circulation
D) None of the above

ANSWER= (A)
Explain:- To get a stronger magnetic field the air gap between stator and armature of an electric motor kept as small as possible.

 

17. The most economic method of electrical braking is

A) Plugging
B) Regenerative braking
C) Dynamic braking with self excitation
D) Dynamic braking with separate excitation

ANSWER= (B)
Explain:- The most economic method of electrical braking is regenerative braking.

 

18. Plugging of dc motors is normally executed by

A) Reversing the field polarity
B) Reversing the armature polarity
C) Connecting a resistance across the armature
D) Reversing both armature and field polarity

ANSWER= (B)
Explain:- Plugging of dc motors is normally executed by reversing the armature polarity.

 

19. If the flux of a dc series motor approaches zero, its speed will approach

A) Zero
B) A stable value nearer to the rated speed
C) Infinity
D) None of the above

ANSWER= (B)
Explain:- If the flux of a dc series motor approaches zero, its speed will approach infinity.

 

20. A 230 V dc series motor is connected to 230 V ac supply it will

A) Not Run
B) Run slowly
C) Run with less efficiency
D) Burnt

ANSWER= (C)
Explain:- If A 230 V dc series motor is connected to 230 V ac supply it will run but with less efficiency.

 

21. Armature reaction is attributed to

A) The effect of magnetic field setup by field current
B) The effect of magnetic field setup by armature current
C) Copper loss in the armature
D) The effect of magnetic field setup by back emf

ANSWER= (B)
Explain:- Armature reaction is attributed to the effect of magnetic field setup by armature current.

 

22. What will happen if the supply terminals of dc shunt motor are interchanged?

A) Motor will stop
B) The direction of rotation will reverse
C) Motor will run at its normal speed in the same direction as it was running
D) Motor will run at a speed lower than the normal speed in the opposite direction

ANSWER= (C)
Explain:- If the supply terminals of dc shunt motor are interchanged motor will run at its normal speed in the same direction as it was running.

 

23. When the electric train is moving down a hill the dc motor acts as

A) DC shunt motor
B) DC shunt generator
C) DC series motor
D) DC series generator

ANSWER= (D)
Explain:- When the electric train is moving down a hill the dc motor acts as dc series generator.

 

24. Which of the following dc motor is suitable for high starting torque?

A) Long shunt motor
B) Series motor
C) Compound motor
D) Short shunt motor

ANSWER= (B)
Explain:- DC series motor is suitable for high starting torque.

 

25. Interpoles are meant for

A) Increasing the speed of the motor
B) Reducing sparking at the commutator
C) Converting armature current to dc
D) Decreasing counter emf

ANSWER= (B)
Explain:- Interpoles are meant for reducing sparking at the commutator.

 

26. The speed of d.c. series motor decreases if the flux in the field winding

A) Increases
B) Decreases
C) Remains Constant
D) None of the above

ANSWER= (A)
Explain:- The speed of d.c. series motor decreases if the flux in the field winding increases.

 

27. If the back emf in a d.c. motor is absent , then

A) Motor will run at very high speed
B) Motor will run at very low speed
C) Motor will burn
D) Motor will not run at all

ANSWER= (C)
Explain:- If the back emf in a d.c. motor is absent , then motor will burn.

 

28. The power stated on the name plate of any motor is always

A) The output power at the shaft
B) The power drawn in kVA
C) The power drawn in kW
D) The gross power

ANSWER= (A)
Explain:- The power stated on the name plate of any motor is always the output power at the shaft.

 

29. Which of the following is a correct statement about a series motor?

A) Its field winding consists of thicker wire and less turns
B) It can run easily without load
C) It has almost constant speed
D) It has poor torque

ANSWER= (A)
Explain:- The field winding of DC series motor consists of thicker wire and less turns.

 

30. The output power of any electrical motor is taken from the

A) Conductors
B) Poles
C) Armature
D) Coupling mounted on the shaft

ANSWER= (D)
Explain:- The output power of any electrical motor is taken from the coupling mounted on the shaft.

 

31. The dummy coil in d.c. machines is used to

A) Eliminate reactance voltage
B) Eliminate armature reaction
C) Bring out mechanical balance of armature
D) Eliminate harmonics developed in the machine

ANSWER= (C)
Explain:- The dummy coil in d.c. machines is used to bring out mechanical balance of armature.

 

32. The torque of a motor is

A) Force is N-m acting on the rotor
B) The electrical power in kW
C) The product of tangential force on the motor and its radius
D) The power given to load being driven by the motor

ANSWER= (C)
Explain:- The torque of a motor is the product of tangential force on the motor and its radius.

 

33. The speed of a d.c. shunt motor is required to be more than full load speed. This is possible by

A) Decreasing the armature current
B) Increasing the armature current
C) Decreasing the field current
D) Increasing the field current

ANSWER= (C)
Explain:- The speed of a d.c. shunt motor is required to be more than full load speed. This is possible by decreasing the field current.

 

34. The highest speed attained by a d.c. shunt motor is

A) Equal to infinity at rated flux
B) Higher than no load speed at rated flux
C) Lower than no load speed at rated flux
D) Equal to no load speed at rated flux

ANSWER= (D)
Explain:- The highest speed attained by a d.c. shunt motor is equal to no load speed at rated flux.

 

35. An external resistance is added in the series with the field of a d.c. shunt motor. When the motor runs, the effect of resistance is

A) To increase the speed of the motor
B) To reduce the speed of the motor
C) To reduce the armature current drawn by the motor
D) None of the above

ANSWER= (A)
Explain:- An external resistance is added in the series with the field of a d.c. shunt motor. When the motor runs, the effect of resistance is to increase the speed of the motor.