Thursday, 7 September 2017

120 TOP D.C. Motors Multiple choice Questions and Answers pdf free download

Real Time D.C Motors Online Quiz Questions and Answers

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1. No-load speed of which of the following motor will be highest ?
(a) Shunt motor
(b) Series motor
(c) Cumulative compound motor
(d) Differentiate compound motor
Ans: b

2. 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
Ans: b

D.C. Motors Objective type Questions

3. Which of the following application requires high starting torque ?
(a) Lathe machine
(b) Centrifugal pump
(c) Locomotive
(d) Air blower
Ans: c

4. If a D.C. motor is to be selected for conveyors, which rriotor would be preferred ?
(a) Series motor
(b) Shunt motor
(c) Differentially compound motor
(d) Cumulative compound motor
Ans: a

5. Which D.C. motor will be preferred for machine tools ?
(a) Series motor
(b) Shunt motor
(c) Cumulative compound motor
(d) Differential compound motor
Ans: b

6. Differentially compound D.C. motors can find applications requiring
(a) high starting torque
(b) low starting torque
(c) variable speed
(d) frequent on-off cycles
Ans: b

7. Which D.C. motor is preferred for elevators ?
(a) Shunt motor
(b) Series motor
(c) Differential compound motor
(d) Cumulative compound motor
Ans: d

8. 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 aovtaat of conductor
(c) resultant force on conductor
(d) none of the above
Ans: a

9. If the field of a D.C. shunt motor gets opened while motor is running
(a) the speed of motor will be reduced %
(b) the armature current will reduce
(c) the motor will attain dangerously high speed 1
(d) the motor will continue to nuvat constant speed
Ans: c

10. Starters are used with D.C. motors because
(a) these motors have high starting torque
(b) these motors are not self-starting
(c) back e.m.f. of these motors is zero initially
(d) to restrict armature current as there is no back e.m.f. while starting
Ans: d

11. In D.C. shunt motors as load is reduced
(a) the speed will increase abruptly
(b) the speed will increase in proportion to reduction in load
(c) the speed will remain almost/constant
(d) the speed will reduce
Ans: c

12. A D.C. series motor is that which
(a) has its field winding consisting of thick wire and less turns
(b) has a poor torque
(c) can be started easily without load
(d) has almost constant speed
Ans: a

13. For starting a D.C. motor a starter is required because
(a) it limits the speed of the motor
(b) it limits the starting current to a safe value
(c) it starts the motor
(d) none of the above
Ans: b

14. The type of D.C. motor used for shears and punches is
(a) shunt motor
(b) series motor
(c) differential compoutid D.C. motor
(d) cumulative compound D.C. motor
Ans: d

15. If a D.C. motor is connected across the A.C. supply it will
(a) run at normal speed
(b) not run
(c) run at lower speed
(d) burn due to heat produced in the field winding by .eddy currents
Ans: d

16. To get the speed of D.C, motor below the normal without wastage of electrical energy is used.
(a) Ward Leonard control
(b) rheostatic control
(c) any of the above method
(d) none of the above method
Ans: a

17. When two D.C. series motors are connected in parallel, the resultant speed is
(a) more than the normal speed
(b) loss than the normal speed
(c) normal speed
(d) zero
Ans: c

18. 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
Ans: a

19. In a D.C. shunt motor, speed is
(a) independent of armature current
(b) directly proportional to the armature current
(c) proportional to the square of the current
(d) inversely proportional to the armature current
Ans: a

20. A direct on line starter is used: for starting motors
(a) up to 5 H.P.
(b) up to 10 H.P.
(c) up to 15 H.P.
(d) up to 20 H.P.
Ans: a

21. What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?
(a) The motor will stop
(b) The motor will continue to run
(c) The armature may burn
(d) The motor will run noisy
Ans: c

22. In case of D.C. shunt motors the speed is dependent on back e.m.f. only because
(a) back e.m.f. is equal to armature drop
(b) armature drop is negligible
(c) flux is proportional to armature current
(d) flux is practically constant in D:C. shunt motors
Ans: d

23. In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be
(a) almost negligible
(b) rated full-load current
(c) less than full-load current
(d) more than full-load current
Ans: d

24. These days D.C. motors are widely used in
(a) pumping sets
(b) air compressors
(c) electric traction
(d) machine shops
Ans: c

25. By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?
(a) Frame
(b) Shaft
(c) Commutator
(d) Stator
Ans: c

26. In which of the following applications D.C. series motor is invariably tried?
(a) Starter for a car
(b) Drive for a water pump
(c) Fan motor
(d) Motor operation in A.C. or D.C.
Ans: a

27. 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
Ans: d

28. A three point starter is considered suitable for
(a) shunt motors
(b) shunt as well as compound motors
(c) shunt, compound and series motors
(d) all D.C. motors
Ans: b

29. In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be
(a) 100%
(b) around 90%
(c) anywhere between 75% and 90%
(d) less than 50%
Ans: d

30. The ratio of starting torque to full-load torque is least in case of
(a) series motors
(b) shunt motors
(c) compound motors
(d) none of the above
Ans: b

32. In D.C. motor which of the following can sustain the maximum temperature rise?
(a) Slip rings
(b) Commutator
(c) Field winding
(d) Armature winding
Ans: c

33. Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?
(a) Lenz's law
(b) Faraday's law
(c) Coloumb's law
(d) Fleming's left-hand rule
Ans: d

34. Which of the following load normally needs starting torque more than the rated torque?
(a) Blowers
(b) Conveyors
(c) Air compressors
(d) Centrifugal pumps
Ans: b

35. The starting resistance of a D.C. motor is generally
(a) low
(b) around 500 Q
(c) 1000 Q
(d) infinitely large
Ans: a

36. The speed of a D.C. series motor is
(a) proportional to the armature current
(b) proportional to the square of the armature current
(c) proportional to field current
(d) inversely proportional to the armature current
Ans: d

37. 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) 50% of the previous value
(c) 25% of the previous value
(d) 10% of the previous value
(e) none of the above
Ans: c

38. The current drawn by the armature of D.C. motor is directly proportional to
(a) the torque required
(b) the speed of the motor
(c) the voltage across the terminals
(d) none of the above
Ans: a

39. The power mentioned on the name plate of an electric motor indicates
(a) the power drawn in kW
(b) the power drawn in kVA
(c) the gross power
(d) the output power available at the shaft
Ans: d

40. Which D.C. motor has got maximum self loading property?
(a) Series motor
(b) Shunt motor
(c) Cumulatively compounded 'motor
(d) Differentially compounded motor
Ans: d

41. Which D.C. motor will be suitable alongwith flywheel for intermittent light and heavy loads?
(a) Series motor
(b) Shunt motor
(c) Cumulatively compounded motor
(d) Differentially compounded motor
Ans: c

42. If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens
(a) nothing will happen to th£ motor
(b) this will make armature to take heavy current, possibly burning it
(c) this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed
Ans: c

43. D.C. series motors are used
(a) where load is constant
(b) where load changes frequently
(c) where constant operating speed is needed
(d) in none of the above situations.
Ans: d

44. For the same H.P. rating and full load speed, following motor has poor starting torque
(a) shunt
(b) series
(c) differentially compounded
(d) cumulativelyc'ompounded
Ans: c

45. In case of conductively compensated D.C. series motors, the compensating winding is provided
(a) as separately wound unit
(6) in parallel with armature winding
(c) in series with armature winding
(d) in parallel with field winding
Ans: c

46. Sparking at the commutator of a D.C. motor may result in
(a) damage to commutator segments
(b) damage to commutator insulation
(c) increased power consumption
(d) all of the above
Ans: d

47. Which of the following motor is preferred for operation in highly explosive atmosphere ?
(a) Series motor
(b) Shunt motor
(c) Air motor
(d) Battery operated motor
Ans: c

48. If the supply voltage for a D.C. motor is increased, which of the following will decrease ?
(a) Starting torque
(b) Operating speed
(c) Full-load current
(d) All of the above
Ans: c

49. Which one of the following is not the function of pole shoes in a D.C. machine ?
(a) To reduce eddy current loss
(b) To support the field coils
(c) To spread out flux for better unifor-mity
(d) To reduce the reluctance of the mag-netic path
Ans: a

50. The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is
(a) 4.0
(b) 2.0
(c) 1.0
(d) 0.5
Ans: d

51. The condition for maximum power in case of D.C. motor is
(a) back e.m.f. = 2 x supply voltage
(b) back e.m.f. = | x supply voltage
(c) supply voltage = | x back e.m.f.
(d) supply voltage = back e.m.f.
Ans: b

52. 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
Ans: c

53. In D.C. machines the residual magnetism is of the order of
(a) 2 to 3 per cent
(6) 10 to 15 per cent
(c) 20 to 25 per cent
(d) 50 to 75 per cent
Ans: a

54. Which D.C. motor is generally preferred for cranes and hoists ?
(a) Series motor
(b) Shunt motor
(c) Cumulatively compounded motor
(d) Differentially compounded motor
Ans: a

55. Three point starter can be used for
(a) series motor only
(b) shunt motor only
(c) compound motor only
(d) both shunt and compound motor
Ans: d

56. Sparking, is discouraged in a D.C. motor because
(a) it increases the input power con-sumption
(b) commutator gets damaged
(c) both (a) and (b)
(d) none of the above
Ans: b

57. Speed control by Ward Leonard method gives uniform speed variation
(a) in one direction
(b) in both directions
(c) below normal speed only
(d) above normal speed only.
Ans: b

58. Flywheel is used with D.C. compound motor to reduce the peak demand by the motor, compound motor will have
to be
(a) level compounded
(b) under compounded
(c) cumulatively compounded
(d) differentially compounded
Ans: c
59. Following motor is used where high starting torque and wide speed range control is required.
(a) Single phase capacitor start
(b) Induction motor
(c) Synchronous motor
(d) D.C. motor
(e) None of the above
Ans: d

60. In a differentially compounded D.C. motor, if shunt field suddenly opens
(a) the motor will first stop and then run in opposite direction as series motor
(b) the motor will work as series motor and run at slow speed in the same direction
(c) the motor will work as series motor and run at high speed in the same direction
(d) the motor will not work and come to stop
Ans: a

61. Which of the following motor has the poorest speed regulation ?
(a) Shunt motor
(b) Series motor
(c) Differential compound motor
(d) Cumulative compound motor
Ans: b

62. Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of
(a) D.C. series motor
(b) D.C. shunt motor
(c) induction motor
(d) all of above motors
Ans: a

63. As -the load is increased the speed of D.C. shunt motor will
(a) reduce slightly
(b) increase slightly
(c) increase proportionately
(d) remains unchanged
Ans: a

64. The armature torque of the D.C. shunt motor is proportional to
(a) field flux only
(b) armature current only
(c) both (a) and (b)
(d) none of the above
Ans: b

65. Which of the following method of speed control of D.C. machine will offer minimum efficiency ?
(a) Voltage control method
(b) Field control method
(c) Armature control method
(d) All above methods
Ans: c

66. Usually wide and sensitive speed control is desired in case of
(a) centrifugal pumps
(b) elevators
(c) steel rolling mills
(d) colliery winders
Ans: d

67. The speed of a motor falls from 1100 r.p.m. at no-load to 1050 r.p.m. at rated load. The speed regulation of the motor is
(a) 2.36%
(6) 4.76%
(c) 6.77%
(d) 8.84%
Ans: b

68. The armature voltage control of D.C. motor provides
(a) constant torque drive
(b) constant voltage drive
(c) constant current drive
(d) none of the above
Ans: a

69. 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 though the armature circuit
(a) a resistance is connected in series with armature
(b) a resistance is connected parallel to the armature
(c) armature is temporarily open circuited
(d) a high value resistor is connected across the field winding
Ans: a

70. The speed of a D.C. shunt motor can be increased by
(a) increasing the resistance in armature circuit
(b) increasing the resistance in field circuit
(c) reducing the resistance in the field circuit
(d) reducing the resistance in the armature circuit
Ans: b

71. If I2 be the armature current, then speed of a D.C. shunt motor is
(a) independent of Ia
(b) proportional to la
(c) varies as (Ia)
(d) varies as la
Ans: a

72. 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) remain unchanged
(6) reduce to one-fourth value
(c) increase four folds
(d) be doubled
Ans: a

73. At the instant of starting when a D.C. motor is put on supply, it behaves like
(a) a highly resistive circuit
(6) a low resistance circuit
(c) a capacitive circuit
(d) none of the above
Ans: b

74. The speed of a D.C. motor can be varied by varying
(a) field current
(b) applied voltage
(c) resistance in series with armature
(d) any of the above
Ans: d

75. Which one of the following is not necessarily the advantage of D.C. motors over A.C. motors ?
(a) Low cost
(b) Wide speed range
(c) Stability
(d) High starting torque.
Ans: a

76. For a D.C. shunt motor if the excitation is changed
(a) torque will remain constant
(b) torque will change but power will remain constant
(c) torque and power both will change
(d) torque, power and speed, all will change
Ans: b

77. Which motor has the poorest speed control?
(a) Differentially compounded motor
(b) Cumulatively compounded motor
(c) Shunt motor
(d) Series motor
Ans: d

78. The plugging gives the
(a) zero torque braking
(b) smallest torque braking
(c) highest torque braking
(d) none of the above
Ans: c

79. The armature voltage control of D.C. motor provides
(a) constant voltage drive
(b) constant current drive
(c) constant torque drive
(d) none of the above
Ans: c

80. If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor
(a) of lower H.P. should be selected
(6) of higher H.P. should be selected
(c) can be used for 50°C ambient temperature also
(d) is to be derated by a factor recom-mended by manufacturer and select the next higher H.P. motor
Ans: d

81. If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking
(o) regenerative
(b) plugging
(c) dynamic braking
(d) none of the above
(e) any of the above
Ans: b

82. Which of the following motors one will choose to drive the rotary compressor ?
(a) D.C. shunt motor
(b) D.C. series motor
(c) Universal motor
(d) Synchronous motor
Ans: d

83. If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will
(a) increase
(b) decrease
(c) remain same
(d) become zero
Ans: a

84. 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) Torque is proportional to square root of armature current
(d) The speed is inversely proportional to the torque and the torque is proportional to square of armature current
Ans: d

85. Which of the following motors is usually used in house-hold refrigerators ?
(a) D.C. shunt motor
(b) D.C. series motor
(c) Single phase induction motor (split phase start or induction run motor)
(d) Reluctance motor
(e) Synchronous motor
Ans: c

86. Which of the following motors is most suitable for signalling devices and many kinds of timers ?
(a) D.C. shunt motor
(b) D.C. series motor
(c) Induction motor
(d) Reluctance motor
Ans: d

87. Which motor should not be started on no-load ?
(a) Series motor
(b) Shunt motor
(c) Cumulatively compounded motor
(d) Differentially compounded motor.
Ans: a

88. Ward-Leonard control is basically a
(a) voltage control method
(b) field divertor method
(c) field control method
(d) armature resistance control method
Ans: a

89. 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
Ans: b

90. In Ward-Leonard control the lower limit of speed is imposed by
(a) residual magnetism of the generator
(b) core losses of motor
(c) mechanical losses of motor and gen¬erator together
(d) all of the above
Ans: a

91. The main disadvantage of the Ward-Leonard control method is
(a) high initial cost
(b) high maintenance cost
(c) low efficiency at Hght loads
(d) all of the above
Ans: d

92. Regenerative method of braking is based on that
(a) back e.m.f. is less than the applied voltage
(b) back e.m.f. is equal to the applied voltage
(c) back e.m.f. of rotor is more than the applied voltage
(d) none of the above
Ans: b

93. The hysteresis loss in a D.C. machine least depends on
(a) Frequency of magnetic reversals
(b) Maximum value of flux density
(c) Volume and grade of iron
(d) Rate of flow of ventilating air
Ans: d

94. In a D.C. generator all of the following could be the effects of iron losses except
(a) Loss of efficiency
(b) Excessive heating of core
(c) Increase in terminal voltage
(d) Rise in temperature of ventilating air
Ans: c

95. The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ?
(a) hysteresis loss
(b) field copper loss
(c) armature copper loss
(d) eddy current loss
Ans: c

96. Which of the following loss in a D.C. generator varies significantly with the load current ?
(a) Field copper loss
(b) Windage loss
(c) Armature copper loss
(d) None of the above
Ans: c

97. Torque developed by a D.C. motor depends upon
(a) magnetic field
(b) active length of the conductor
(c) current flow through the conductors
(d) number of conductors
(e) radius of armature
(f) all above factors
Ans: f

98. D.C. shunt motors are used for driving
(a) trains
(b) cranes
(c) hoists
(d) machine tools
Ans: d

99. In a manual shunt motor starter
(a) over load relay is connected in series and no volt relay in parallel with the load
(6) over load relay is connected in paral¬lel and no volt relay in series with the load
(c) over load relay and no volt relay are both connected in series with the load
(d) over load relay and no volt relay are both connected in parallel with the load
Ans: a

100. Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ?
(a) Providing laminations in armature core
(b) Providing laminations in stator
(c) Using non-magnetic material for frame
(d) Using material of low hysteresis co-efficient for armature core material
Ans: d

101. Which of the following loss in a D.C. generator is dissipated in the form of heat?
(a) Mechanical loss
(b) Core loss
(c) Copper loss
(d) All of the above
Ans: d

102. Which of the following losses are significantly reduced by laminating the core of a D.C. generator ?
(a) Hysteresis losses
(b) Eddy current losses
(c) Copper losses
(d) Windage losses
Ans: b

103. The total losses in a well designed D.C. generator of 10 kW will be nearly
(a) 100 W
(b) 500 W
(c) 1000 W
(d) 1500 W
Ans: b

104. The condition for maximum efficiency for a D.C. generator is
(a) eddy current losses = stray losses
(b) hysteresis losses = eddy current losses
(c) copper losses = 0
(d) variable losses = constant losses
Ans: d

105. D.C. generators are normally designed for maximum efficiency around
(a) full-load
(b) rated r.p.m.
(c) rated voltage
(d) all of the above
Ans: a

106. In a D.C. generator, the iron losses mainly take place in
(a) yoke
(b) commutator
(c) armature conductors
(d) armature rotor
Ans: d

107. D.C. generators are installed near the load centres to reduce
(a) iron losses
(b) line losses
(c) sparking
(d) corona losses
Ans: b

108. The purpose of retardation test on D.C. shunt machines is to find out
(a) stray losses
(b) eddy current losses
(c) field copper losses
(d) windage losses
Ans: a

109. Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?
(a) Swinburne's test
(b) Hopkinson's test
(c) Field test
(d) Brake test
Ans: c

110. Hopkinson's test on D.C. machines is conducted at
(a) no-load
(b) part load
(c) full-load
(d) overload
Ans: c

111. During rheostat braking of D.C. series motors
(a) motor is run as a generator
(b) motor is reversed in direction
(c) motor is run at reduced speed
Ans: a

112. For which types of D.C. motor, dynamic braking is generally used ?
(a) Shunt motors
(b) Series motors
(c) Compound motors
(d) All of the above
Ans: d

113. Which method of braking is generally used in elevators ?
(a) Plugging
(b) Regenerative braking
(c) Rheostatic braking
(d) None of the above
Ans: a

114. In variable speed motor
(a) a stronger commutating field is needed at low speed than at high speed
(b) a weaker commutating field is needed at low speed than at high speed
(c) same commutating field is needed at low speed than at high speed
(d) none of the above is correct
Ans: b

115. When the armature of a D.C. motor rotates, e.m.f. induced is
(a) self-induced e.m.f.
(b) mutually induced e.m.f.
(c) back e.m.f.
(d) none of the above
Ans: c

116. Where D.C. motor of H.P. 12 or more requires frequent starting, stopping, reversing and speed control
(a) drum type controller is used
(b) three point starter is used
(c) four point starter is used
(d) all above can be used
Ans: a

117. If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens
(a) this will make armature to take heavy current, possibly burning it
(6) this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
(c) nothing will happen to motor
(d) motor will come to stop
Ans: a

118. D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short
duration. We will select the
(a) series motor
(b) shunt motor
(c) compound motor
(d) any of the above
Ans: a

119. D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this
(a) series motor
(b) shunt motor
(c) compound motor
(d) any of the above
Ans: c

120. Which D.C. motor has got maximum self relieving property ?
(a) Series motor
(6) Shunt motor
(c) Cumulatively compounded motor
(d) Differentially compounded motor
Ans: a

Latest D.C Motors Objective type Questions for freshers

AC DC Motor Interview Questions and Answers pdf free download

=> Also Read DC Motors MCQ's

1. What is the principle of motor?
Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.

2. Types of dc generator?
DC Generators are classified into two types 1)separatly excited DC generator
2)self excited DC generator, which is further classified into; 1)series 2)shunt and 3)compound(which is further classified into cumulative and differential).

AC DC Motor Interview Questions

3. Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?
DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.

4. Define stepper motor. What is the use of stepper motor?
The motor which work or act on the applied input pulse in it, is called as stepper motor. This stepper motor is under the category of synchronous motor, which often does not fully depend of complete cycle. It likes to works in either direction related to steps. for this purpose it mainly used in automation parts.

5. What is 2 phase motor?
A two phase motor is a motor with the the starting winding and the running winding have a phase split. e.g;ac servo motor.where the auxiliary winding and the control winding have a phase split of 90 degree.

6. Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, padestal fan, bracket fan etc) which are find in the houses ?
Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

7. Give two basic speed control scheme of DC shunt motor?
1. By using flux control method:in this method a rheostat is connected across the field winding to control the field current.so by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled
2.armature control method:in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop(IaRa) can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled.

8. Difference between a four point starter and three point starter?
The shunt connection in four point starter is provided separately form the line where as in three point starter it is connected with line which is the drawback in three point starter.

9. What is the difference between synchronous generator & asynchronous generator?
In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing. This type of generators are used in windmills.

10. Why syn. generators are used for the production of electricity?
Synchronous machines have capability to work on different power factor (or say different imaginary power varying the field EMF. Hence syn. generators r used for the production of electricity.

11. Why is the starting current high in a DC motor?
In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage, Eb = Back emf in Motor, Ia = Armature current,Ra = Aramture resistance). At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra ,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.

12. What are the advantages of star-delta starter with induction motor?
The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online starting.(2). Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.

13. Why series motor cannot be started on no-load?
Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc.

14. Mention the methods for starting an induction motor?
The different methods of starting an induction motor
DOL:direct online starter
Star delta starter
Auto transformer starter
Resistance starter
Series reactor starter

15. What are Motor Generator Sets and explain the different ways the motor generator set can be used ?
Motor Generator Sets are a combination of an electrical generator and an engine mounted together to form a single piece of equipment. Motor generator set is also referred to as a genset, or more commonly, a generator The motor generator set can used in the following different ways:
1.Alternating current (AC) to direct current (DC)
2.DC to AC
3.DC at one voltage to DC at another voltage
4.AC at one frequency to AC at another harmonically-related frequency

16. Which type of motor is used in trains, what is the rating of supply used explain Working principal?
Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.

17. What are the Application of DC Motors in Thermal Power Plant?
In thermal power plants dc motors are employed for certain control and critical emergency operations which are supplied by dedicated batteries. DC motors operate as backup drives for normal ac drive systems when ac power supply to the plant is lost.
In thermal power plant, the dc motors finds applications for performing control functions such as

Turbine governor motor
Governor limit setting
Motor operated rheostats
Emergency lubrication for the turbines (main, boiler feed pumps)
Generator (H2 oil seal).
DC motor operated valves

DC motors employed in thermal plants are classified in to two types based on the type of application.
DC motors carrying out Control function
Dc motors carrying out Emergency function

Control functions:This category consists of the turbine governor motor, governor limiting setting, motor operated rheostats, etc. These motors are small, about 1/8 hp or less. They are operated quite infrequently for short duration.

Emergency functions:This category consists of turbine-generator emergency (lubrication) bearing oil pumps and emergency seal oil pumps. Such pumps may also be provided for steam turbine drives of feedwater pumps, fans, and other large loads. The lack of lubrication during a shutdown without ac power will ruin the linings of the bearings and damage the shaft.
Hydrogen seal oil pump is provided to prevent the escaping of hydrogen (for large turbine-generators hydrogen cooling is provided for efficient cooling) from the casing by providing a tight seal with high pressure oil

18. What are the Advantages & Disadvantages of Synchronous motors?
Advantage or Merits:
One of the major advantage of using synchronous motor is the ability to control the power factor. An over excited synchronous motor can have leading power factor and can be operated in parallel to induction motors and other lagging power factor loads thereby improving the system power factor.

In synchronous motor the speed remains constant irrespective of the loads. This characteristics helps in industrial drives where constant speed is required irrespective of the load it is driving. It also useful when the motor is required to drive another alternator to supply at a different frequency as in frequency changes.

Synchronous motors can be constructed with wider air gaps than induction motors which makes these motors mechanically more stable.

In synchronous motors electro-magnetic power varies linearly with the voltage.

Synchronous motors usually operate with higher efficiencies ( more than 90%) especially in low speed and unity power factor applications compared to induction motors

Disadvantages or Demerits:
Synchronous motors requires dc excitation which must be supplied from external sources.
Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing.
The cost per kW output is generally higher than that of induction motors.
These motors cannot be used for variable speed applications as there is no possibility of speed adjustment unless the incoming supply frequency is adjusted (Variable Frequency Drives).
Synchronous motors cannot be started on load. Its starting torque is zero.

These motors have tendency to hunt.
When loading on the synchronous motor increases beyond its capability, the synchronism between rotor and stator rotating magnetic field is lost and motor comes to halt.

Collector rings and brushes are required resulting in increase in maintenance.

Synchronous motors cannot be useful for applications requiring frequent starting or high starting torques required.

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=> Also Read DC Motors MCQ's

1.Why Series motors cannot be started without load?
2.Advantages of DC motors over AC motors
3. What are the applications of DC series motors?
4. What is back emf and its significance in DC motor?
5. How speed is controlled in DC shunt motor?

DC machines LAB VIVA Questions

6. Why starter is required in DC motor for starting?
7. What is critical resistance in DC motor?
8. What are the different losses in DC motor?
9. What is Armature Reaction in DC motor and its impact?
10. What is Commutation failure?

11. Advantages of Dummy windings in DC motors?
12. On what principle does DC motor operates?
13. What are the applications of DC Shunt motors?
14. What is the typical brush drop in DC motors?
15. Draw different characteristic curves of DC motors?
16. By carrying out brake test on DC motors what we can understand?
17. Explain Fleming's Left hand and Right hand rule and which rule applies to motor and Generator

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20 TOP D.C Motor Interview Questions and Answers pdf download

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1. WHAT IS THE PRINCIPLE OF OPERATION OF A D.C MOTOR?
A. The operation of a D.C Motor based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. Basically, there is no constructional difference between a d.c. motor and a d.c generator. The same d.c. machine can be run as a generator or motor.

2. What is back e.m.f or counter e.m.f?
A. When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator. The induced e.m.f. acts in opposite direction to the applied voltage V (Lenz’s law) and in known as back or counter e.m.f Eb. It is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.

D.C Motor Interview Questions

3. What is speed regulation?
A. Speed regulation is the change in speed when the load on the motor is reduced from rated value to zero. It is expressed as a percentage of rated load speed.

4. Why series motor cannot be started on no-load?
A. Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc.

5. Which type of motor is used in trains, what is the rating of supply used?
A. Dc series is in the used in trains to get high starting torque while starting of the trains and the operating voltage is 1500v dc.

6. Explain different methods of speed control of d.c shunt motors?
A. Speed control of a d.c shunt motor can be done using
1. Flux control method ()
2. Armature control method
3. Voltage control method

7. Explain different methods of speed control of d.c series motors?
A. Speed control of a d.c series motor can be done using
1. flux control method
2. Armature-resistance control control method

8. Why is the starting current high in a DC motor (or) Why do we use a starter?
A. For DC motors, Voltage equation is
V=Eb-IaRa
Where V = Terminal voltage,
Eb = Back emf in Motor
Ia = Armature current
Ra = Aramture resistance.
At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra, where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased. The excessive current will damage commutator and brushes and eventually blow out the fuses.

9. How can you reverse the direction of rotation of a D.C Motor?
A. We can reverse the direction of rotation of a D.C Motor by either reversing the field current or armature current. If both the currents are reversed the motor will run in original direction.

10. Why the field of a D.C Shunt motor should not be open?
A. The shunt motor will achieve dangerously high speed and may destroy itself.

11. What will happen if the direction of current at the terminals of a D.C Motor is reversed?
A. The direction of rotation of motor remains the same as current in armature and field.

12. What happens when a D.C motor is connected across an A.C supply?
A. 1. The motor will run but it would not carry same load as it would on d.c supply.
2. More sparking at the brushes
3. Eddy currents will be high and will cause overheating and may eventually burn on a.c supply

13. Mention the methods for starting an induction motor?
A. 1. DOL – direct online starter
2. Star delta starter
3. Auto transformer starter
4. Resistance starter
5. Series reactor starter

14. What are the types of starters used for starting of D.C Motors?
A. There are two principal types of d.c. motor starters viz., three-point starter and four-point starter. As we shall see, the two types of starters differ only in the manner in which the no-volt release coil is connected.

15. What are the advantages of star-delta starter with induction motor?
A. The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting. As the starting current is reduced, the voltage drops during the starting of motor in systems are reduced.

16. What are the different tests conducted on D.C Machines?
A. 1. Brake test
2. Swinburne’s test or No-load test
3. Regeneration test or Hopkinson’s test
4. Retardation or Running down test

17. Why is Swinburne’s test conducted?
A. Swinburne’s test is conducted at No-load condition to predetermine the efficiency at different loads and to measure the losses separately.

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