DC MOTOR OBJECTIVE QUESTION WITH EXPLANATIONS : 1 to 75
Q NO 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 ) resultant force on conductor
correct answer : a ) current in the conductor
Q NO 2 . The type of D . C motor used for shears and punches is :
a ) shunt motor b ) series motor
c ) differential compound dc motor d) cumulative compound dc motor
correct answer : d) cumulative compound dc motor
Q NO 3 . The speed of dc 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
correct answer : a ) decreasing the field current
Q NO 4 . In a dc 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
correct answer : a ) independent of armature current
Q no 5. In a DC 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
explanations
correct answer D ) more than full load current
correct answer D ) more than full load current
Q no 6. In case the conditions for maximum power for a DC motor are established, the efficiency of the motor will be
A ) 100 %
B ) around 100 %
C ) anywhere between 75 % and 90 %
D ) less than 50 %
explanations
correct answer D ) less than 50 %
correct answer D ) less than 50 %
Q no 7. The ratio of starting torque to full load torque is least in case of
A ) series motor
B ) shunt motor
C ) compound motor
D ) none of the above
explanations
correct answer B ) shunt motor
correct answer B ) shunt motor
Q no 8 . In DC motor which of the following can sustain the maximum temperature rise?
A ) slip ring
B ) commutator
C ) field winding
D ) armature winding
explanations
correct answer C ) field winding
correct answer C ) field winding
Q no 9 . Which of the following load normally needs starting torque more than the rated torque ?
A ) blowers
B ) conveyors
C ) air compressor
D ) centrifugal pumps
explanations
correct answer B ) conveyors
correct answer B ) conveyors
Q no 10. In a DC 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
explanations
correct answer C ) 25 % of the previous value
explanations
correct answer C ) 25 % of the previous value
Meri Aashiqui Tum Hi Ho nice song by career-iq Q no 11. 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
explanations
answer A ) The torque required
Q no 12. In ward leonard method of speed control of a D.C motor , change in speed of motor is obtained by the
A ) change in armature voltage of D.C motor
B ) change in the field excitation of the D.C motor
C ) change in armature current of D.C motor
D ) change in supply voltage
explanations
answer A ) change in armature voltage of D.C motor
Q no 13. Which D.C motor has got maximum self loading property ?
A ) series motor
B ) shunt motor
C ) cumulatively compound motor
D ) differentially compound motor
explanations
answer D ) differentially compound motor
Q no 14. For the same H.P rating and full load speed , following motor has poor starting torque
A ) shunt
B ) series
C ) differentially compounded
D ) cumulatively compounded
explanations
answer C ) differentially compounded
Q no 15. In case of conductively compensated D.C series motors , the compensating winding is provided
A ) as separately wound unit
B ) in parallel with armature winding
C ) in series with armature winding
D ) in parallel with field winding
explanations
answer C ) in series with armature winding
Q no 16. 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
explanations
answer C ) full - load current
Q no 17. As compared to an induction motor , the air gap in a D.C motor is
A ) less than 50 %
B ) between 50 to 90 %
C ) same
D ) more
explanations
answer D ) more
Q no 18. Field winding of a D.C series motor is usually provided with thick wire
A ) to provide large flux
B ) to reduce the use of insulating materials
C ) as it carries large load current
D ) in order to reduce eddy current
explanations
answer C ) as it carries large load current
Q no 19. Which 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 uniformity
D ) to reduce the reluctance of the magnetic path
explanations
answer C ) as it carries large load current
Q no 20 . Sparking is discouraged in a D.C motor because
A ) it increases the input power consumption
B ) commutator gets damaged
C ) both A and B
D ) none of the above
explanations
Meri Aashiqui Tum Hi Ho nice song by career-iq 21. 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
answer B ) commutator gets damaged
Meri Aashiqui Tum Hi Ho nice song by career-iq 21. 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
explanations
answer A ) reduce slightly
22. 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
A ) field flux only
B ) armature current only
C ) both A and B
D ) none of the above
explanations
answer B ) armature current only
23. 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 of the above method
explanations
A ) voltage control method
B ) field control method
C ) armature control method
D ) all of the above method
explanations
answer C ) armature control method
24. Usually wide and sensitive speed control is desired in case of
A ) centrifugal pumps
B ) elevator
C ) steel rolling mills
D ) colliery winders
A ) centrifugal pumps
B ) elevator
C ) steel rolling mills
D ) colliery winders
explanations
answer D ) colliery winders
25. 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 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
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
explanations
answer A ) a resistance is connected in series with armature
26. 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 field circuit
D ) reducing the resistance in the armature circuit
A ) increasing the resistance in armature circuit
B ) increasing the resistance in field circuit
C ) reducing the resistance in field circuit
D ) reducing the resistance in the armature circuit
explanations
answer B ) increasing the resistance in field circuit
27. In case the back e.m.f and the speed of D.C motor are doubled , the torque developed by the motor will
A ) remain unchanged
B ) reduce to one - fourth value
C ) increase four folds
D ) be doubled
A ) remain unchanged
B ) reduce to one - fourth value
C ) increase four folds
D ) be doubled
explanations
answer A ) remain unchanged
28. At the instant of starting when a D.C motor is put on supply , it behaves like
A ) a highly resistive circuit
B ) a low resistance circuit
C ) a capacitive circuit
D ) none of the above
A ) a highly resistive circuit
B ) a low resistance circuit
C ) a capacitive circuit
D ) none of the above
explanations
answer B ) a low resistance circuit
29. 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
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
explanations
answer B ) torque will change but power will remain constant
30. The plugging gives the
A ) zero torque braking
B ) smallest torque braking
C ) highest torque braking
D ) none of the above
A ) zero torque braking
B ) smallest torque braking
C ) highest torque braking
D ) none of the above
explanations
answer C ) highest torque braking
Q no 31. 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
Meri Aashiqui Tum Hi Ho nice song by career-iq Q no 41. If Bmax is the maximum flux density, then eddy current loss will vary as
A ) D.C shunt motor
B ) D.C series motor
C ) universal motor
D ) Synchronous motor
explanations
answer D ) Synchronous motor
Q no 32. Why are the DC motor preferred for traction application ?
A ) torque and speed are inversely proportional to armature current
B ) torque is proportional to armature current
C ) torque is proportional to square of armature current
D ) the speed is inversely proportional to the square and the torque is proportional to square of armature current
explanations
answer D ) the speed is inversely proportional to the square and the torque is proportional to square of armature current
Q no 33. Which of the following motor have almost constant speed over their full load range ?
A ) AC series motor
B ) DC series motors
C ) DC shunt motors
D ) low resistance squirrel cage motor
E ) both C and D
explanations
answer E ) both C and D
Q no 34. Ward - Leonard control is basically a
A ) voltage control method
B ) field divertor method
C ) field control method
D ) armature resistance control method
explanations
answer A ) voltage control method
Q no 35. 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 generator together
D ) all of the above
explanations
answer A ) residual magnetism of the generator
Q no 36. The hysteresis loss in a DC machine least depends on
A ) frequency of the magnetic reversals
B ) maximum value of flux density
C ) volume and grade of iron
D ) rate of flow of ventilating air
explanations
answer D ) rate of flow of ventilating air
Q no 37. In a DC 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
explanations
answer C ) increase in terminal voltage
Q no 38. The losses occurring in a DC generator are given below . which loss is likely to have highest proportional at rated load of the generator ?
A ) hysteresis loss
B ) field copper loss
C ) armature copper loss
D ) eddy current loss
explanations
answer C ) armature copper loss
Q no 39. If ' t ' be the thickness of the lamination , then eddy current loss in a generator will vary as
A ) 1/t
B ) 1 / ( t ) 2
C ) t
D ) ( t )2
explanations
answer D ) ( t )2
Q no 40. Which of the following loss in a DC generator varies significantly with the load current ?
A ) field copper loss
B ) windage loss
C ) armature copper loss
D ) none of the above
explanations
answer C ) armature copper loss
Meri Aashiqui Tum Hi Ho nice song by career-iq Q no 41. If Bmax is the maximum flux density, then eddy current loss will vary as
A ) Bmax
B ) ( Bmax )2
C ) ( Bmax )1.2
D ) ( Bmax )2.4
explanations
answer B ) ( Bmax )2
Q no 42. The hysteresis loss in a DC generator varies with the frequency of magnetic reversals as
A ) 1/f
B ) f
C ) ( f )1.6
D ) ( f )2
explanations
answer B ) f
Q no 43. Which of the following braking is used in rolling mills ?
A ) dynamic braking
B ) plugging
C ) regenerative braking
D ) mechanical brakes
explanations
answer B ) plugging
Q no 44. Compensating winding in a DC machine is placed
A ) on yoke in the pole faces
B ) on yoke in the interpolar gap
C ) on armature
D ) none of the above
explanations
answer A ) on yoke in the pole faces
Q no 45. DC 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 thick wire
D ) few number of turns of thin wire
explanations
answer C ) few number of turns of thick wire
Q no 46. In a manual shunt motor starter
A ) over load relay is connected in series and no volt relay in parallel with the load
B) over load relay is connected in parallel 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
explanations
answer A ) over load relay is connected in series and no volt relay in parallel with the load
Q no 47. Which of the following steps is likely to results in reducing of hysteresis loss in a DC generator ?
A ) providing lamination in armature core
B ) providing lamination in stator
C ) using non magnetic materials for frame
D ) using materials of low hysteresis coefficient for armature core materials
explanations
answer D ) using materials of low hysteresis coefficient for armature core materials
Q no 48. Which loss in a generator does not vary with load as well as flux density ?
A ) copper loss
B ) eddy current loss
C ) hysteresis loss
D ) windage loss
explanations
answer D ) windage loss
Q no 49. Which of the following loss in a DC generator is dissipated in the form of heat ?
A ) mechanical loss
B ) core loss
C ) copper loss
D ) all of the above
explanations
answer D ) all of the above
Q no 50. DC generator are normally designed for maximum efficiency around
A ) full load
B ) rated r.p.m
C ) rated voltage
D ) all of the above
explanations
answer A ) full load
Q no 51. In a DC generator, the iron losses mainly take place in
A ) yoke
B ) commutator
C ) armature conductor
D ) armature rotor
A ) yoke
B ) commutator
C ) armature conductor
D ) armature rotor
explanations
answer D ) armature rotor
Q no 52. DC generator are installed near the load centres to reduce
A ) iron losses
B ) line losses
C ) sparking
D ) corona losses
A ) iron losses
B ) line losses
C ) sparking
D ) corona losses
explanations
answer B ) line losses
Q no 53. 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
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
explanations
answer B ) a weaker commutating field is needed at low speed than at high speed
Q no 54. Where DC 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 used
D ) all above can be used
A ) drum type controller is used
B ) three point starter is used
C ) four point starter used
D ) all above can be used
explanations
answer A ) drum type controller is used
Q no 55. If a DC 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
B ) 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
A ) this will make armature to take heavy current , possibly burning it
B ) 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
explanations
answer A ) this will make armature to take heavy current , possibly burning it
Q no 56. DC 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
A ) series motor
B ) shunt motor
C ) compound motor
D ) any of the above
explanations
answer A ) series motor
Q no 57 . In a DC motor the iron losses occur in
Meri Aashiqui Tum Hi Ho nice song by career-iq A ) the field
B ) the armature
C ) the brushes
D ) the commutator
Meri Aashiqui Tum Hi Ho nice song by career-iq A ) the field
B ) the armature
C ) the brushes
D ) the commutator
explanations
answer B ) the armature
Q no 58 . The speed of a DC shunt is required to be more than full load speed. this is possible by
A ) reducing the field current
B ) decreasing the armature current
C ) increasing the armature current
D ) increasing the excitation current
E ) none of the above method
A ) reducing the field current
B ) decreasing the armature current
C ) increasing the armature current
D ) increasing the excitation current
E ) none of the above method
explanations
answer A ) reducing the field current
Q no 59. One DC motor drives another DC motor . The second DC motor when excited and driven
A ) runs as a generator
B ) does not run as generator
C ) also runs as a motor
D ) comes to stop after sometimes
A ) runs as a generator
B ) does not run as generator
C ) also runs as a motor
D ) comes to stop after sometimes
explanations
answer A ) runs as a generator
Q no 60. Which of the following DC motor has the least drop in speed between no load and nominal load ?
A ) series motor without commutating poles
B ) series motor with commutating poles
C ) shunt motor with commutating poles
D ) compound motor without commutating poles
A ) series motor without commutating poles
B ) series motor with commutating poles
C ) shunt motor with commutating poles
D ) compound motor without commutating poles
explanations
answer C ) shunt motor with commutating poles
Q no 61. What will happen if supply terminal of DC shunt motor are interchanged ?
A ) the direction of rotation will reverse
B ) motor will stop
C ) motor will run at speed lower than the normal speed in the same direction
D ) motor will run at its normal speed in the same direction as it was running
A ) the direction of rotation will reverse
B ) motor will stop
C ) motor will run at speed lower than the normal speed in the same direction
D ) motor will run at its normal speed in the same direction as it was running
explanations
answer D ) motor will run at its normal speed in the same direction as it was running
Q no 62. Which of the following method is most economical for finding the no load losses of a large DC shunt motor ?
A ) retardation test
B ) Swinburne's test
C ) hopkinson's test
D ) none of the above
A ) retardation test
B ) Swinburne's test
C ) hopkinson's test
D ) none of the above
explanations
answer B ) Swinburne's test
Q no 63. Which of the following statement is incorrect ? If a starter is not used with large DC motor , it will draw a starting current which
A ) will produce very low starting torque
B ) will produce excessive line voltage drop
C ) will damage the commutator
D ) is many times its full load current
A ) will produce very low starting torque
B ) will produce excessive line voltage drop
C ) will damage the commutator
D ) is many times its full load current
explanations
answer A ) will produce very low starting torque
Q no 64. The rated speed of a given DC 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 ) armature current resistance control
C ) field resistance control
D ) none of the above
A )Ward - Leonard control
B ) armature current resistance control
C ) field resistance control
D ) none of the above
explanations
answer C ) field resistance control
Q no 65. In a DC motor , the ratio Eb / Va indicates
A ) running torque of the motor
B ) starting torque of the motor
C ) efficiency of the motor
D ) speed regulations of the motor
A ) running torque of the motor
B ) starting torque of the motor
C ) efficiency of the motor
D ) speed regulations of the motor
explanations
answer C ) efficiency of the motor
Q no 66. In a DC motor , unidirectional torque is produced with the help of
A ) end plates
B ) bushes
C ) commutator
D ) both B and C
A ) end plates
B ) bushes
C ) commutator
D ) both B and C
explanations
answer D ) both B and C
Q no 67. The speed of a DC motor under constant load conditions , is affected by
A ) back emf
B ) field flux
C ) armature current
D ) none of the above
A ) back emf
B ) field flux
C ) armature current
D ) none of the above
explanations
answer B ) field flux
Q no 68. A DC shunt motor is rotating in clockwise direction as viewed from one end. The polarity of connection of the armature and field winding are reversed simultaneously. The motor will
A ) rotate in clockwise direction
B ) rotate in anti - clockwise direction
C ) will not rotate at all
D ) gain excessively high speed
A ) rotate in clockwise direction
B ) rotate in anti - clockwise direction
C ) will not rotate at all
D ) gain excessively high speed
explanations
answer A ) rotate in clockwise direction
Q no 69. If the field circuit of a loaded shunt motor is suddenly opened
A ) torque developed by the motor would be reduced to zero.
B ) it would race to almost infinite speed
C ) it would draw abnormally high armature current
D ) the fuse or circuit breaker will open the circuit before too much damage is done to the motor
A ) torque developed by the motor would be reduced to zero.
B ) it would race to almost infinite speed
C ) it would draw abnormally high armature current
D ) the fuse or circuit breaker will open the circuit before too much damage is done to the motor
explanations
answer D ) the fuse or circuit breaker will open the circuit before too much damage is done to the motor
Q no 70. While starting a differential compound motor , it is best to short the series field in order to avoid
A ) excessive starting period
B ) motor starting in wrong direction
C ) tripping of the circuit breaker
D ) larger in rush of current
A ) excessive starting period
B ) motor starting in wrong direction
C ) tripping of the circuit breaker
D ) larger in rush of current
explanations
answer B ) motor starting in wrong direction
Q no 71. In a DC motor constant torque is produced due to
A ) rotor lamination
B ) end - plates
C ) pole shoes
D ) commutator
explanations
answer D ) commutator
Q no 72. A face plate starter is employed for starting
A ) induction motor
B ) universal motor
C ) synchronous motor
D ) DC series motor
explanations
answer D ) DC series motor
Q no 73. If the no voltage release of a DC motor starter fails to work on resumption of supply after a break , the motor will
A ) not start automatically
B ) start automatically without trouble
C ) get damaged
D ) develop very low torque
explanations
answer C ) get damaged
Q no 74. A DC series motor , as compared to shunt and compound motor , has the highest torque at the start because of its comparatively
A ) stronger series field
B ) lower armature resistance
C ) large armature current
D ) fewer series turns
explanations
answer A ) stronger series field
Q no 75. The series parallel system of speed control of DC series motor widely used in traction work gives a speed range of about
A ) 1 : 10
B ) 1 : 8
C ) 1 : 6
D ) 1 : 4
explanations
answer D ) 1 : 4
Meri Aashiqui Tum Hi Ho nice song by career-iq
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