In this article, we shared the DC motors objective questions and answers for electrical and electronics engineers.
DC Motors Objective Questions
The below list provides all the multiple-choice questions related to DC motor.
Answers are available at the bottom of the article.
1. According to Fleming’s left-hand rule, when the forefinger points in the direction of the field or flux, the middle finger will point in the direction of
| A) | current, in the conductor |
| B) | movement of conductor |
| C) | none of these answers |
| D) | the resultant force on the conductor |
2. In a D.C. shunt motor, speed is
| A) | proportional to the square of the current |
| B) | inversely proportional to the armature current |
| C) | independent of armature current |
| D) | directly proportional to the armature current |
3. In variable speed motor
| A) | same commutating field is needed at low speed than at high speed |
| B) | a stronger commutating field is needed at low speed than at high speed |
| C) | none of these answers |
| D) | a weaker commutating field is needed at low speed than at higt speed |
4. The generated e.m.f. and the current are in the opposite direction in the case of
| A) | d.c. motors |
| B) | none of these answers |
| C) | d.c. generators |
| D) | both d.c. generators and d.c. motors |
5. In a D.C. generator, the iron losses mainly take place in
| A) | yoke |
| B) | armature rotor |
| C) | commutator |
| D) | armature conductors |
6. In a D.C. motor constant torque is produced due to
| A) | pole shoes |
| B) | end-plates |
| C) | rotor laminations |
| D) | commutator |
7. Speed control by the variation of flux, in a D.C. shunt motor, will give
| A) | constant torque drive |
| B) | variable torque drive |
| C) | constant power drive |
| D) | variable power drive |
8. What will happen if supply terminals of D.C. shunt motor are interchanged?
| A) | Motor will run at its normal speed in the same direction as it was running |
| B) | Motor will run at speed lower than the normal speed in the same direction |
| C) | Motor will stop |
| D) | The direction of rotation will reverse |
9. A series motor is started without load. The effect is that
| A) | the speed increases rapidly |
| B) | the torque increases rapidly |
| C) | the current drawn increases rapidly |
| D) | the back e.m.f. decreases |
10. Which of the following methods is most effective in finding out the no-load losses in a large D.C. shunt motor?
| A) | Ward-Leonard test |
| B) | Block rotor test |
| C) | Hopkinson’s test |
| D) | Swinburne’s test |
| E) | Field’s test |
11. Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of
| A) | induction motor |
| B) | D.C. shunt motor |
| C) | all of these answers |
| D) | D.C. series motor |
12. Which of the following motors one will choose to drive the rotary compressor?
| A) | Synchronous motor |
| B) | D.C. shunt motor |
| C) | Universal motor |
| D) | D.C. series motor |
13. The plugging gives the
| A) | smallest torque braking |
| B) | zero torque braking |
| C) | none of these answers |
| D) | highest torque braking |
14. Which of the following loss in a D.C. generator varies significantly with the load current?
| A) | Windage loss |
| B) | Field copper loss |
| C) | None of these answers |
| D) | Armature copper loss |
15. Why are the D.C. motors preferred for traction applications?
| A) | Torque and speed are inversely proportional to armature current |
| B) | Torque is proportional to armature current |
| C) | The speed is inversely proportional to the torque and the torque is proportional to square of armature current |
| D) | Torque is proportional to square root of armature current |
16. Which of the following D.C. motors has the least drop in speed between no-load and nominal load?
| A) | Shunt motor with commutating pole |
| B) | Compound motor without commutating poles |
| C) | Series motor without commutating poles |
| D) | Series motor with commutating poles |
17. Which of the following can be used to control the speed of a D.C. motor?
| A) | Thermistor |
| B) | Transistor |
| C) | Thyristor |
| D) | Thyratron |
18. Which of the following application requires high starting torque?
| A) | Locomotive |
| B) | Air blower |
| C) | Lathe machine |
| D) | Centrifugal pump |
19. Which of the following can be used for controlling the speed of a D.C. motor?
| A) | Thyratron |
| B) | Thermistor |
| C) | Thyristor |
| D) | Transistor |
20. If the terminals of armature of the D.C. motor are interchanged, this action will offer the following kind of braking
| A) | none of the answers |
| B) | any of these answers |
| C) | regenerative |
| D) | plugging |
| E) | dynamic braking |
21. Which of the following load normally needs starting torque more than the rated torque?
| A) | Blowers |
| B) | Air compressors |
| C) | Conveyors |
| D) | Centrifugal pumps |
22. A face plate starter is employed for starting
| A) | synchronous motor |
| B) | induction motor |
| C) | d.c. series motor |
| D) | universal motor |
23. The losses occurring in a D.C. generator are given below. Which loss is likely to have the least proportion?
| A) | Armature copper losses |
| B) | Mechanical losses |
| C) | Magnetic losses |
| D) | Field copper losses |
24. The armature shaft of a D.C. motor must be able to withstand
| A) | twisting strains due to transmission of torque |
| B) | any unbalanced magnetic pull on the armature core |
| C) | all of these answers |
| D) | bending moment due to the weight of the armature and commutator |
25. Which of the following methods of braking is used in rolling mills?
| A) | Regenerative braking |
| B) | Plugging |
| C) | Dynamic braking |
| D) | Mechanical brakes |
26. Speed control by varying the armature circuit resistance, in a D.C. motor, provides a
| A) | constant torque drive |
| B) | variable power drive |
| C) | variable torque drive |
| D) | constant power drive |
27. The power mentioned on the nameplate of an electric motor indicates
| A) | the power is drawn in kW |
| B) | the power is drawn in kVA |
| C) | the gross power |
| D) | the output power available at the shaft |
28. Voltage equation for D.C. motor is
| A) | V = Eb - IaRa |
| B) | Eb = V + IaRa |
| C) | Eb = V - Ia2Ra |
| D) | V = Eb + IaRa |
29. Which of the following motors have almost constant speed over their full load range?
| A) | D.C. series motors |
| B) | A.C. series motors |
| C) | Both D.C. shunt motors and Low resistance squirrel cage motors |
| D) | D.C. shunt motors |
| E) | Low resistance squirrel cage motors |
30. As there is no back e.m.f. at the instant of starting, a D.C. motor, in order to prevent a heavy current from flowing through the armature circuit
| A) | a high value resistor is connected across the field winding |
| B) | a resistance is connected in series with armature |
| C) | a resistance is connected parallel to the armature |
| D) | armature is temporarily open circuited |
31. Which of the following motor has high starting torque?
| A) | Induction motor |
| B) | D.C. series motor |
| C) | A.C. series motor |
| D) | Synchronous motor |
32. Which D.C. motor Will have the least percentage increase of input current for the same percentage increase in the torque?
| A) | Separately excited motor |
| B) | Cumulative compound motor |
| C) | Shunt motor |
| D) | Series motor |
33. When the electric train is moving down a hill the D.C. motor act as
| A) | D.C. shunt motor |
| B) | D.C. shunt generator |
| C) | D.C. series generator |
| D) | D.C. series motor |
34. In electric motors carbon brushes are used to
| A) | prevent sparking during commutation |
| B) | prevent overheating of armature windings |
| C) | provide a path for the flow of current |
| D) | brush off carbon deposits on the commutator |
35. Between Field’s test and Hopkinson’s test the main common thing is that both
| A) | are regenerative tests |
| B) | use negligible power |
| C) | need two similar mechanically coupled motors |
| D) | need two electrically coupled series motors |
36. Speed control of a cumulatively compounded D.C. motor can be effected through the change of
| A) | armature voltage |
| B) | armature resistance |
| C) | field resistance |
| D) | any of these answers |
37. Which D.C. motor has got maximum self-relieving property?
| A) | Cumulatively compounded motor |
| B) | Differentially compounded motor |
| C) | Shunt motor |
| D) | Series motor |
38. A wide and very sensitive speed control is usually required in case of
| A) | elevators |
| B) | all of these answers |
| C) | colliery winders |
| D) | steel rolling mills |
39. If the field circuit of a loaded shunt motor is suddenly opened
| A) | it would draw abnormally high armature current |
| B) | it would race to almost infinite speed |
| C) | torque developed by the motor would be reduced to zero |
| D) | the fuse or circuit breaker will open the circuit before too much damage is done to the motor |
40. The speed of a D.C. motor can be varied by varying
| A) | applied voltage |
| B) | any of these answers |
| C) | resistance in series with the armature |
| D) | field current |
41. The direction of rotation of a D.C. series motor can be changed by
| A) | either of interchanging supply terminals and interchanging field terminals |
| B) | interchanging field terminals |
| C) | interchanging supply terminals |
| D) | None of these answers |
42. In the case of D.C. shunt motors the speed is dependent on the back e.m.f. only because
| A) | flux is proportional to armature current |
| B) | back e.m.f. is equal to armature drop |
| C) | flux is practically constant in ac. shunt motors |
| D) | armature drop is negligible |
43. If Ia be the armature current, then the speed of a D.C. shunt motor is
| A) | varies as 1 / Ia |
| B) | proportional to Ia |
| C) | independent of Ia |
| D) | varies as (Ia)2 |
44. D.C. generators are normally designed for maximum efficiency around
| A) | all of these answers |
| B) | rated r.p.m. |
| C) | full-load |
| D) | rated voltage |
45. The total losses in a well designed D.C. generator of 10 kW will be nearly
| A) | 100 W |
| B) | 1500 W |
| C) | 500 W |
| D) | 1000 W |
46. In a D.C. motor, unidirectional torque is produced with the help of
| A) | bushes |
| B) | both bushes and commutator |
| C) | end plates |
| D) | commutator |
47. For which of the following applications a D.C. motor is preferred over an A.C. motor?
| A) | Variable speed operation |
| B) | High speed operation |
| C) | Low speed operation |
| D) | Fixed speed operation |
48. Which of the following motor has the poorest speed regulation?
| A) | Differential compound motor |
| B) | Cumulative compound motor |
| C) | Shunt motor |
| D) | Series motor |
49. In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to
| A) | 100% of the previous value |
| B) | 25% of the previous value |
| C) | 50% of the previous value |
| D) | 10% of the previous value |
| E) | none of these answers |
50. In the hazardous areas of gassy mines, material transportation is done by vehicles powered by
| A) | batteries |
| B) | I.C. engines |
| C) | D.C. mains |
| D) | A.C. mains |
51. Which of the following statement is incorrect? If a starter is not used with a large D.C. motor, it will draw a starting current which
| A) | is many times its full-load current |
| B) | will produce very low starting torque |
| C) | will produce an excessive line voltage drop |
| D) | will damage the commutator |
52. These days D.C. motors are widely used in
| A) | electric traction |
| B) | machine shops |
| C) | pumping sets |
| D) | air compressors |
53. Which D.C. motor has got maximum self loading property?
| A) | Cumulatively compounded motor |
| B) | Differentially compounded motor |
| C) | Series motor |
| D) | Shunt motor |
54. Compensating winding in a D.C. machine is placed
| A) | on yoke in the interpolar gap |
| B) | on yoke in the pole faces |
| C) | on armature |
| D) | none of these answers |
55. A direct online starter is used for starting motors
| A) | up to 10 H.P. |
| B) | up to 15 H.P. |
| C) | up to 5 H.P. |
| D) | up to 20 H.P. |
56. In a D.C. motor, the ratio Eb/Va indicates
| A) | running torque of the motor |
| B) | the efficiency of the motor |
| C) | speed regulation of the motor |
| D) | starting torque of the motor |
57. The condition for maximum power in case of D.C. motor is
| A) | supply voltage = 1/2 x back e.m.f. |
| B) | back e.m.f. = 1/2 x supply voltage |
| C) | supply voltage = back e.m.f. |
| D) | back e.m.f. = 2 x supply voltage |
58. Which of the following motor is used in the locomotives motor drives?
| A) | A.C. series motor |
| B) | D.C. series motor |
| C) | Synchronous motor |
| D) | Induction motor |
59. For constant torque drive which speed control method is preferred?
| A) | Field control |
| B) | Armature voltage control |
| C) | Shunt armature control |
| D) | Mechanical loading system |
60. The variable resistor shunting the field of a D.C. series motor is called a
| A) | field divertor |
| B) | armature divertor |
| C) | potential divider |
| D) | voltage regulator |
61. In Ward-Leonard control the lower limit of speed is imposed by
| A) | mechanical losses of motor and generator together |
| B) | core losses of motor |
| C) | all of these answers |
| D) | residual magnetism of the generator |
62. The speed of a D.C. series motor is
| A) | proportional to the armature current |
| B) | proportional to field current |
| C) | proportional to the square of the armature current |
| D) | inversely proportional to the armature current |
63. In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will
| A) | increase four folds |
| B) | be doubled |
| C) | reduce to one-fourth value |
| D) | remain unchanged |
64. At the instant of starting when a D.C. motor is put on supply, it behaves like
| A) | a low resistance circuit |
| B) | a highly resistive circuit |
| C) | a capacitive circuit |
| D) | none of these answers |
65. All motors, basically, operate on the principle of either repulsion or
| A) | electro-magnetism |
| B) | semi-conduction |
| C) | induction |
| D) | capacitance |
66. The main disadvantage of Hopkinson’s test for finding efficiency of the shunt D.C. motors is that it
| A) | needs one motor and one generator |
| B) | ignores any change in iron loss |
| C) | requires full-load power |
| D) | requires two identical shunt machines |
67. In electric motors carbon brushes are used to
| A) | provide a path for flow of current |
| B) | prevent overheating of armature windings |
| C) | prevent sparking during commutation |
| D) | brush off carbon deposits on the commutator |
68. In D.C. machines the residual magnetism is of the order of
| A) | 10 to 15 percent |
| B) | 50 to 75 percent |
| C) | 2 to 3 percent |
| D) | 20 to 25 percent |
69. The armature torque of the D.C. shunt motor is proportional to
| A) | both field flux only and armature current only |
| B) | armature current only |
| C) | none of these answers |
| D) | field flux only |
70. Which of the following motor is preferred for operation in a highly explosive atmosphere?
| A) | Series motor |
| B) | Air motor |
| C) | Battery operated motor |
| D) | Shunt motor |
71. A series motor is best suited for driving
| A) | cranes and hoists |
| B) | none of these answers |
| C) | machine tools |
| D) | shear and punches |
72. The operation of electric generators and motors depends on the interaction between magnetic field and
| A) | electric current |
| B) | commutator |
| C) | electric field |
| D) | copper conductors |
73. Sparking at the commutator of a D.C. motor may result in
| A) | damage to commutator segments |
| B) | all of these answers |
| C) | increased power consumption |
| D) | damage to commutator insulation |
74. When two D.C. series motors are connected in parallel, the resultant speed is
| A) | normal speed |
| B) | zero |
| C) | less than the normal speed |
| D) | more than the normal speed |
75. If conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be
| A) | 80 to 90% |
| B) | 60 to 70% |
| C) | less than 50% |
| D) | 1 |
76. Which of the following losses are significantly reduced by laminating the core of a D.C. generator?
| A) | Eddy current losses |
| B) | Windage losses |
| C) | Hysteresis losses |
| D) | Copper losses |
77. The purpose of retardation test on D.C. shunt machines is to find out
| A) | eddy current losses |
| B) | field copper losses |
| C) | stray losses |
| D) | windage losses |
78. Which of the following loss in a D.C. generator is dissipated in the form of heat?
| A) | Mechanical loss |
| B) | All of these answers |
| C) | Copper loss |
| D) | Core loss |
79. Ward-Leonard control is basically a
| A) | field diverter method |
| B) | armature resistance control method |
| C) | voltage control method |
| D) | field control method |
80. The simplest form of a motor controller is
| A) | toggle switch |
| B) | relay |
| C) | magnetic switch |
| D) | drum switch |
81. In Ward Leonard method of speed control of a D.C. motor, change in speed of the motor is obtained by the
| A) | change in armature current of D.C. motor |
| B) | change in the field excitation of the D.C. motor |
| C) | change in armature voltage of D.C. motor |
| D) | change in supply voltage |
82. One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven
| A) | does not run as a generator |
| B) | comes to stop after sometime |
| C) | also runs as a motor |
| D) | runs as a generator |
83. A D.C. series motor, as compared to shunt and compound motors, has the highest torque at the start because of its comparatively
| A) | large armature current |
| B) | lower armature resistance |
| C) | stronger series field |
| D) | fewer series turns |
84. If Bmax is the maximum flux density, then eddy current loss will vary as
| A) | Bmax |
| B) | (Bmax)1.2 |
| C) | (Bmax)2 |
| D) | (Bmax)2.4 |
85. A D.C. motor can be easily identified by
| A) | size of conductor |
| B) | commutator |
| C) | winding |
| D) | yoke |
86. A 230 V D.C. shunt motor takes 32 A at full load. The back e.m.f. on full load, if the resistance of motor armature and shunt field windings are 0.2 Ω and 115 Ω respectively, will be
| A) | 215 V |
| B) | 224 V |
| C) | 220 V |
| D) | 210 V |
87. The type of D.C. motor used for shears and punches is
| A) | series motor |
| B) | differential compound D.C. motor |
| C) | cumulative compound D.C. motor |
| D) | shunt motor. |
88. Torque developed by a D.C. motor depends upon
| A) | all of these answers |
| B) | radius of armature |
| C) | number of conductors |
| D) | the active length of the conductor |
| E) | current flow through the conductors |
89. Usually wide and sensitive speed control is desired in the case of
| A) | steel rolling mills |
| B) | colliery winders |
| C) | elevators |
| D) | centrifugal pumps |
90. 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) | none of these answers |
| C) | reducing the field current |
| D) | increasing the armature current |
| E) | increasing the excitation current |
91. Which of the following tests can be conducted on all types of D.C. machines?
| A) | Brake test |
| B) | Block rotor test |
| C) | Field’s test |
| D) | Hopkinson’s test |
| E) | Running down test |
92. The speed of a D.C. motor, under constant load conditions, is affected by
| A) | back e.m.f. |
| B) | none of these answers |
| C) | armature current |
| D) | field flux |
93. If the no-voltage release of a D.C. motor starter fails to work on the resumption of supply after a break, the motor will
| A) | develop very low torque |
| B) | not start automatically |
| C) | get damaged |
| D) | start automatically without trouble |
94. If a D.C. motor is to be selected for conveyors, which motor would be preferred?
| A) | Shunt motor |
| B) | Cumulative compound motor |
| C) | Differentially compound motor |
| D) | Series motor |
95. Which of the following motors is most suitable for signaling devices and many kinds of timers?
| A) | D.C. shunt motor |
| B) | D.C. series motor |
| C) | Induction motor |
| D) | Reluctance motor |
96. In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be
| A) | almost negligible |
| B) | more than full-load current |
| C) | rated full-load current |
| D) | less than full-load current |
97. The speed of a D.C. shunt motor more than its full-load speed can be obtained by
| A) | decreasing the field current |
| B) | increasing the field current |
| C) | decreasing the armature current |
| D) | increasing the armature current |
98. In a motor, energy conversion would not have been possible but for the
| A) | input energy from supply |
| B) | use of commutator |
| C) | production of opposing back e.m.f. Eb in the armature |
| D) | application of Fleming’s left-hand rule |
99. Which D.C. motor will be preferred for machine tools?
| A) | Differential compound motor |
| B) | Series motor |
| C) | Shunt motor |
| D) | Cumulative compound motor |
100. The rated speed of a given D.C. shunt motor is 900 r.p.m. To run this machine at 1000 r.p.m., which of the following speed control scheme will be used?
| A) | Ward-leonard control |
| B) | None of these answers |
| C) | Field resistance control |
| D) | Armature current resistance control |
101. D.C. shunt motors are used for driving
| A) | cranes |
| B) | machine tools |
| C) | trains |
| D) | hoists |
102. When the armature of a D.C. motor rotates, e.m.f. induced is
| A) | none of these answers |
| B) | mutually induced e.m.f. |
| C) | back e.m.f. |
| D) | self-induced e.m.f. |
103. If a D.C. shunt motor is working at no load and if the shunt field circuit suddenly opens
| A) | motor will run at very slow speed |
| B) | this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses |
| C) | nothing will happen to the motor |
| D) | this will make armature to take heavy current, possibly burning it |
104. In which of the following tests only one motor is required?
| A) | Swinburne’s test |
| B) | Brake test |
| C) | Hopkinson’s test |
| D) | Field’s test |
105. Three point starter can be used for
| A) | both shunt and compound motor |
| B) | shunt motor only |
| C) | series motor only |
| D) | compound motor only |
106. A motor for punching machine is usually subjected to
| A) | no load |
| B) | intermittent load |
| C) | continuous full load |
| D) | continuous part load |
107. A brake test on D.C. motors is usually restricted to
| A) | variable speed motors |
| B) | small horse power motors |
| C) | open frame type motors. |
| D) | high speed motors |
108. If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will
| A) | become zero |
| B) | remain same |
| C) | increase |
| D) | decrease |
109. Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity?
| A) | Brake test |
| B) | Swinburne’s test |
| C) | Hopkinson’s test |
| D) | Field test |
110. In the case of conductively compensated D.C. series motors, the compensating winding is provided
| A) | in series with armature winding |
| B) | in parallel with field winding |
| C) | as separately wound unit |
| D) | in parallel with the armature winding |
111. Which D.C. motor is preferred for elevators?
| A) | Series motor |
| B) | Cumulative compound motor |
| C) | Differential compound motor |
| D) | Shunt motor |
112. If the supply voltage for a D.C. motor is increased, which of the following will decrease ?
| A) | Full-load current |
| B) | Operating speed |
| C) | All of these answers |
| D) | Starting torque |
113. While starting a differential compound motor, it is best to short the series field in order to avoid
| A) | motor starting in wrong direction |
| B) | excessive starting period |
| C) | large in rush of current |
| D) | tripping of the circuit breaker |
114. D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for a short duration.
| A) | compound motor |
| B) | series motor |
| C) | shunt motor |
| D) | any of these answers |
115. In a D.C. generator, all of the following could be the effects of iron losses except
| A) | Increase in terminal voltage |
| B) | Rise in temperature of ventilating air |
| C) | Loss of efficiency |
| D) | Excessive heating of core |
116. To get the speed of the D.C. motor below the normal without wastage of electrical energy ____ is used.
| A) | none of these answers |
| B) | Ward Leonard control |
| C) | rheostatic control |
| D) | either rheostatic control or Ward Leonard control |
117. In case the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be
| A) | around 90% |
| B) | anywhere between 75% and 90% |
| C) | 1 |
| D) | less than 50% |
118. By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?
| A) | Frame |
| B) | Commutator |
| C) | Shaft |
| D) | Stator |
119. The function of a field regulator for compound motors is to
| A) | demagnetize the field partially |
| B) | control the flux |
| C) | none of these answers |
| D) | limit the armature current |
120. Which motor should be used for centrifugal pumps?
| A) | Shunt motor |
| B) | Series motor |
| C) | Either Series motor or Shunt motor |
| D) | None of these answers |
121. The hysteresis loss in a D.C. generator varies with the frequency of magnetic reversals as
| A) | f1.6 |
| B) | 1/6 |
| C) | f2 |
| D) | f |
122. The condition for maximum efficiency for a D.C. generator is
| A) | eddy current losses = stray losses |
| B) | hysteresis losses = eddy current losses |
| C) | variable losses = constant losses |
| D) | copper losses = 0 |
123. Which of the following law/rule can be used to determine the direction of rotation of D.C. motor?
| A) | Faraday’s law |
| B) | Lenz’s law |
| C) | Coloumb’s law |
| D) | Fleming’s left-hand rule |
124. Which of the following motor has high starting torque?
| A) | D.C. series motor |
| B) | Synchronous motor |
| C) | A.C. series motor |
| D) | none of these answers |
125. Which loss in a D.C. generator does not vary with the load as well as flux density?
| A) | Copper loss |
| B) | Hysteresis loss |
| C) | Eddy current loss |
| D) | Windage loss |
126. Which of the following motors, on the removal of load, will run at the highest speed?
| A) | Shunt motor |
| B) | Cumulative compound |
| C) | Differential compound |
| D) | Series motor |
127. In D.C. shunt motors as load are reduced
| A) | the speed will remain almost constant |
| B) | the speed will reduce |
| C) | the speed will increase abruptly |
| D) | the speed will increase in proportion to reduction in load |
128. Which of the following, in a D.C. motor, can sustain the maximum temperature rise?
| A) | Armature windings |
| B) | Field windings |
| C) | Slip rings |
| D) | Commutator |
129. A three point starter is considered suitable for
| A) | all D.C. motors |
| B) | shunt motors |
| C) | shunt, compound and series motors |
| D) | shunt as well as compound motors |
130. In a D.C. motor if the back e.m.f. is absent
| A) | motor will burn |
| B) | motor will run at very slow speed |
| C) | motor will not run at all |
| D) | motor will run at very high speed |
131. A D.C. series motor is that which
| A) | can be started easily without load |
| B) | has a poor torque |
| C) | has almost constant speed |
| D) | has its field winding consisting of thick wire and less turns |
132. D.C. generators are installed near the load centers to reduce
| A) | sparking |
| B) | corona losses |
| C) | line losses |
| D) | iron losses |
133. In Field’s test for series motors, one motor drives the other machine as
| A) | differential compound generator |
| B) | series generator |
| C) | separately excited generator |
| D) | cumulative compound generator |
134. If the speed of a D.C. shunt motor is increased the back e.m.f. of the motor will
| A) | become zero |
| B) | decrease |
| C) | increase |
| D) | remain same |
135. Differentially compound D.C. motors can find applications requiring
| A) | low starting torque |
| B) | variable speed |
| C) | frequent on-off cycles |
| D) | high starting torque |
136. If a D.C. motor is connected across the A.C. supply it will
| A) | run at normal speed |
| B) | burn due to heat produced in the field winding by eddy currents |
| C) | not run |
| D) | run at lower Speed |
137. In a manual shunt motor starter
| A) | overload relay is connected in series and no volt relay in parallel with the load |
| B) | overload relay and no volt relay are both connected in parallel with the load |
| C) | overload relay is connected in parallel and no volt relay in series with the load |
| D) | overload relay and no volt relay are both connected in series with the load |
138. When the speed of a D.C. motor increases
| A) | back e.m.f. increases and current drawn decreases |
| B) | back e.m.f. as well as current drawn both decrease |
| C) | back e.m.f. decreases and current drawn increases |
| D) | back e.m.f. as well as current drawn both increase |
139. Starters are used with D.C. motors because
| A) | these motors have high starting torque |
| B) | to restrict armature current as there is no back e.m.f. while starting |
| C) | back e.m.f. of these motors is zero initially |
| D) | these motors are not self-starting |
140. Which loss in a D.C. generator varies with load?
| A) | Copper loss |
| B) | Windage loss |
| C) | Eddy current loss |
| D) | Hysteresis loss |
141. For a D.C. shunt motor if the excitation is changed
| A) | torque, power and speed, all will change |
| B) | torque will remain constant |
| C) | torque will change but power will remain constant |
| D) | torque and power both will change |
142. D.C. motor is to drive a load which is almost nil for certain part of the load cycle and peak value for a short duration. We will select this
| A) | any of these answers |
| B) | series motor |
| C) | shunt motor |
| D) | compound motor |
143. A shearing machine has a cyclic load consisting of intermittent light and henry loads. Which of the following D.C. motor will be suitable for this purpose?
| A) | Series motor |
| B) | Differential compound motor |
| C) | Shunt motor |
| D) | Cumulative compound motor |
144. Which D.C. motor will be preferred for constant speed line shafting ?
| A) | Shunt motor |
| B) | Cumulative compound motor |
| C) | Series motor |
| D) | Differentially compound motor |
145. The hysteresis loss in a D.C. machine least depends on
| A) | Volume and grade of iron |
| B) | Maximum value of flux density |
| C) | Frequency of magnetic reversals |
| D) | Rate of flow of ventilating air |
146. The losses occurring in a D.C. generator are given below. Which loss is likely to have the highest proportion at rated load of the generator?
| A) | armature copper loss |
| B) | eddy current loss |
| C) | hysteresis loss |
| D) | field copper loss |
147. Which one of the following is not the function of pole shoes in a D.C. machine?
| A) | To spread out flux for better uniformity |
| B) | To reduce eddy current loss |
| C) | To reduce the reluctance of the magnetic path |
| D) | To support the field coils |
148. Which of the following statements is correct, in the case of a D.C. series motor?
| A) | It cannot be used where high starting torque is required |
| B) | It should be directly connected to the load |
| C) | It may run away if its field becomes open |
| D) | It is a constant speed motor |
149. Which of the following tests can be used to determine no-load losses in a D.C. shunt motor?
| A) | Swinburne’s test |
| B) | Running down test |
| C) | Field test |
| D) | Brake test |
150. What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?
| A) | The armature may burn |
| B) | The motor will stop |
| C) | The motor will continue to run |
| D) | The motor will run noisy |
151. Where D.C. motor of H.P. 12 or more requires frequent starting, stopping, reversing, and speed control
| A) | four point starter is used |
| B) | all of these answers |
| C) | drum type controller is used |
| D) | three point starter is used |
152. In the case of a shunt motor if the supply voltage is increased by 10%, which of the following will decrease?
| A) | starting torque |
| B) | none of these answers |
| C) | full load speed |
| D) | full load current |
153. Sparking is discouraged in a D.C. motor because
| A) | it increases the input power consumption |
| B) | commutator gets damaged |
| C) | none of these answers |
| D) | both it increases the input power consumption and commutator gets damaged |
154. Which of the following steps is likely to result in a reduction of hysteresis loss in a D.C. generator?
| A) | Providing laminations in armature core |
| B) | Using non-magnetic material for frame |
| C) | Providing laminations in stator |
| D) | Using material of low hysteresis coefficient for armature core material |
155. In regenerative braking
| A) | motor energy is dissipated in armature heating |
| B) | motor energy is dissipated as heat |
| C) | motor energy is dissipated in windage losses |
| D) | motor is made to run as a generator |
156. Regenerative braking on shunt motors is used when
| A) | the load is variable |
| B) | the load is constantly decreasing |
| C) | the load acts as a braking force |
| D) | the load has overhauling characteristics. |
157. Which of the following loss of D.C. motor decreases with increase in load?
| A) | Friction and windage loss |
| B) | Brush contact loss |
| C) | Core loss |
| D) | none of these answers |
158. Which of the following methods is most economical fol. finding the no-load losses of a large D.C. shunt motor?
| A) | Retardation test |
| B) | Hopkinson’s test |
| C) | Swinburne’s test |
| D) | none of these answers |
159. If Ta is the torque and Ia is the armature current for a D.C. motor, then which of the following relation is valid before saturation ?
| A) | Ta ∝ Ia |
| B) | Ta ∝ I / Ia |
| C) | Ta ∝ Ia2 |
| D) | Ta ∝ Ia2 |
160. As compared to an induction motor, the air gap in a D.C. motor is
| A) | less than 50% |
| B) | same |
| C) | between 50% and 90% |
| D) | more |
161. If a D.C. shunt motor is working at full load and if the shunt field circuit suddenly opens
| A) | this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses |
| B) | this will make armature to take heavy current, possibly burning it |
| C) | nothing will happen to motor |
| D) | motor will come to stop |
162. Following motor is used where high starting torque and wide speed range control is required.
| A) | D.C. motor |
| B) | Single phase capacitor start |
| C) | None of these answers |
| D) | Induction motor |
| E) | Synchronous motor |
163. D.C. series machine has field consisting of
| A) | many number of turns of thick wire |
| B) | many number of turns of thin wire |
| C) | few number of turns of thin wire |
| D) | few number of turns of thick wire |
164. Which motor should not be started on no-load?
| A) | Differentially compounded motor |
| B) | Cumulatively compounded motor |
| C) | Series motor |
| D) | Shunt motor |
165. The armature voltage control of the D.C. motor provides
| A) | constant voltage drive |
| B) | none of these answers |
| C) | constant torque drive |
| D) | constant current drive |
166. Regenerative braking on D.C. shunt motors is used when
| A) | the load has overhauling characteristics |
| B) | the load is constantly decreasing |
| C) | the load also acts as a braking force |
| D) | the load is variable |
167. Which of the following is not necessarily the advantage of D.C. motors over A.C. motors?
| A) | High starting torque |
| B) | Low cost |
| C) | Better speed control |
| D) | Wide speed range |
168. Which of the following method of speed control, of D.C. machine will offer minimum efficiency?
| A) | Voltage control method |
| B) | Armature control method |
| C) | Field control method |
| D) | All of these answers |
169. Hopkinson’s test on D.C. machines is conducted at
| A) | overload |
| B) | no-load |
| C) | part load |
| D) | full-load |
170. Ward-Leonard system of speed control is not recommended for
| A) | constant speed operation |
| B) | very low speeds |
| C) | wide speed range |
| D) | frequent motor reversals |
171. If the field of a D.C. shunt motor gets opened while the motor is running
| A) | the speed of motor will be reduced |
| B) | the motor will continue to run at constant speed |
| C) | the armature current will reduce |
| D) | the motor will attain dangerously high speed |
172. What is the effect produced by the electric current in an electric motor?
| A) | Heating as well as chemical effect |
| B) | Magnetic effect only |
| C) | Heating effect only |
| D) | Heating as well as magnetic effect |
173. The current drawn by the armature of D.C. motor is directly proportional to
| A) | the voltage across the terminals |
| B) | the speed of the motor |
| C) | the torque required |
| D) | none of these answers |
174. Speed control by Ward Leonard method gives uniform speed variation
| A) | above normal speed only |
| B) | in one direction |
| C) | in both directions |
| D) | below normal speed only |
175. In D.C. motor which of the following can sustain the maximum temperature rise?
| A) | Commutator |
| B) | Armature winding |
| C) | Field winding |
| D) | Slip rings |
176. In which of the following applications D.C. series motor is invariably tried?
| A) | Starter for a car |
| B) | Fan motor |
| C) | Motor operation in A.C. or D.C. |
| D) | Drive for a water pump |
177. During rheostatic braking the braking torque is proportional to
| A) | (speed)-1 |
| B) | (speed)2 |
| C) | (speed) |
| D) | (speed)-2 |
178. Small D.C. motors up to 5 H.P. usually have
| A) | 2 poles |
| B) | 4 poles |
| C) | 8 poles |
| D) | 6 poles |
179. D.C. series motors are used
| A) | where load changes frequently |
| B) | none of these answers |
| C) | where load is constant |
| D) | where constant operating speed is needed |
180. Which D.C. motor will be suitable along with flywheel for intermittent light and heavy loads?
| A) | Differentially compounded motor |
| B) | Shunt motor |
| C) | Cumulatively compounded motor |
| D) | Series motor |
181. Which losses can be determined by performing the retardation test?
| A) | Stray losses |
| B) | Eddy current losses |
| C) | Copper losses |
| D) | Friction losses |
182. For which types of D.C. motor, dynamic braking is generally used?
| A) | Compound motors |
| B) | Shunt motors |
| C) | All of these answers |
| D) | Series motors |
183. During rheostat braking of D.C. series motors
| A) | motor is reversed in direction |
| B) | motor is run at reduced speed |
| C) | motor is run as a generator |
| D) | none of these answers |
184. The speed of a series motor at no load is
| A) | 2000 r.p.m. |
| B) | infinity |
| C) | 1000 r.p.m. |
| D) | 4000 r.p.m. |
185. A D.C. shunt motor is rotating in a clockwise direction as viewed from one end. The polarity of connection of the armature and field winding is reversed simultaneously. The motor will
| A) | will not rotate at all |
| B) | gain excessively high speed |
| C) | rotate in clockwise direction |
| D) | rotate in anti-clockwise direction |
186. As the load is increased the speed of the D.C. shunt motor will
| A) | increase slightly |
| B) | increase proportionately |
| C) | remains unchanged |
| D) | reduce slightly |
187. Four point starter in the D.C. motor is used
| A) | not to effect the current passing through Hold on coil even if any change M the field current takes place |
| B) | to increase the field current |
| C) | all of these answers |
| D) | to decrease the field current |
188. The series parallel system of speed control of D.C. series motors widely used in traction work gives a speed range of about
| A) | 1 : 10 |
| B) | 1 : 6 |
| C) | 1 : 4 |
| D) | 1 : 8 |
189. The starting resistance of a D.C. motor is generally
| A) | infinitely large |
| B) | around 500 ohms |
| C) | low |
| D) | 1000 ohms |
190. For starting a D.C. motor a starter is required because
| A) | it limits the starting current to a safe value |
| B) | it limits the speed of the motor |
| C) | it starts the motor |
| D) | none of these answers |
191. Field winding of a D.C. series motor is usually provided with thick wire
| A) | as it carries large load current |
| B) | to provide large flux |
| C) | to reduce the use of insulating materials |
| D) | in order to reduce eddy current |
192. Which D.C. motor is generally preferred for cranes and hoists?
| A) | Shunt motor |
| B) | Differentially compounded motor |
| C) | Series motor |
| D) | Cumulatively compounded motor |
193. In D.C. machines fractional pitch winding is used
| A) | to increase the generated e.m.f. |
| B) | to reduce copper losses |
| C) | to improve cooling |
| D) | to reduce the sparking |
194. The retardation test is applicable to shunt motors and generators and is used to find
| A) | the eddy current losses |
| B) | the copper losses |
| C) | the friction losses |
| D) | the stray losses |
195. If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor
| A) | is to be derated by a factor recommended by the manufacturer and select the next higher H.P. motor |
| B) | of lower H.P. should be selected |
| C) | can be used for 50°C ambient temperature also |
| D) | of higher H.P. should be selected |
196. Which of the following D.C. motors has the least drop in speed between no-load and nominal load?
| A) | Series motor with commutating poles |
| B) | Compound motor without commutating poles |
| C) | Series motor without commutating poles |
| D) | Shunt motor with commutating pole |
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