Electrical engineering practice mcq question : DC Motor (Part 2)

For previous parts, check the previous post.




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