Induction Motor Objective Type Questions

60. Which of the following induction motors will operate on the lowest value of power factor?

(a) 7.5 kW, 950 rpm.
(b) 7.5 kW, 1440 rpm.
(c) 15 kW, 950 rpm.
(d) 15 kW, 1440 rpm.

61. For starting a 3-phase induction motor ………. winding is to be connected to 3-phase ac supply.

(a) rotor
(b) stator
(c) both stator and rotor
(d) none

62. The starting current of a 3-phase induction motor is about ………. of its full-load rated current.

(a) half
(b) twice
(c) 5 to 7 times
(d) 15 to 20 times

63. The following starting method for an induction motor is inferior from the point of view of poor starting torque per ampere of the line current drawn

(a) Direct-on-line starting.
(b) Auto transformer method of starting.
(c) Series inductor method of starting.
(d) Star-delta method of stating. [GATE E.E. 1999]

64. Direct-on-line starter can be used for small capacity ……….. induction motors.

(a) squirrel cage
(b) wound rotor
(c) squirrel cage and wound rotor both types of

65. For starting a small induction motor DOL starter is used in place of iron clad triple-pole (ICTP) switch as it

(a) provides protection against overload and undervolt.
(b) reduces the starting current.
(c) increases the starting torque.
(d) improves the power factor and efficiency.

66. Induction motors, when directly switched on across their normal rated supply voltage, the torque developed at starting is about……… of their full load torque.

(a) half
(b) 1.5 to 2.5 times
(c) 5 times
(d) 10 times

67 ……… starter is used for starting a 3-phase induction motor that needs frequent starting and operating in forward and reverse directions.

(a) Direct-on-line
(b) Star-delta
(c) Auto-transformer
(d) Reduced voltage

68. A squirrel cage induction motor having a rated slip of 2% on full-load has a starting torque of 50% of full-load torque. The starting current is

(a) two times the full-load current.
(b) four times the full-load current.
(c) five times the full-load current.
(d) equal to the full-load current. [IES. E.E.-II, 2010]

69. With DOL start of an induction motor the heating during acceleration of inertia load is proportional to

(a) slip at maximum torque.
(b) maximum torque.
(c) stored kinetic energy.
(d) electromagnetic power developed on the rotor. [A.M.I.E. Sec B. Winter 1996]

70. Primary series resistors are employed in the stator for starting a 3-phase squirrel cage induction motor to

(a) increase the starting torque.
(b) improve the pf and efficiency of the motor.
(c) reduce voltage applied across the motor terminals.
(d) increase the motor speed.

71. For starting 3-phase squirrel cage induction motors reactors are preferred over resistors as the reactors

(a) incur less power loss and effectively reduce the applied volt-age to the motor. (b) increase the starting torque.
(c) improve the power factor at start.
(d) all of the above.

72. If the applied voltage across a 3-phase induction motor is reduced to x times, the starting current and starting torque will be reduced to ……… and ……… times respectively.

(a) x, x
(b) x, x²
(c) x², x
(d) x², x²

73. A 3-phase squirrel-cage induction motor is started by means of a star/delta switch. What is the starting current of the motor?

(a) 3 times the current with direct on line starting.
(b) 1/3 times the current with direct on line starting.
(c) 1/ √3 times the current with direct on line starting.
(d) √3 times the current with direct on line starting. [I.E.S. E.E.-11, 2008]

74. The starting current and torque of three phase induction motor on direct line starting is 30 Amp and 300 Nm, respectively. What are the corresponding values with star-delta starter?

(a) 10 A and 100 Nm.
(b) 30 A and 300 Nm.
(c) 17.32 A and 173.2 Nm.
(d) 30 A and 173.3 Nm. [U.PS.C. IES. 2009]

75. In auto-transformer starting of a 3-phase induction motor, if the tapping used is x then the starting line current and starting torque will be ……… and ……… times respectively with direct-on-line starting.

(a) x, x
(b) x, x²
(c) x², x²
(d) x², x

76. A starting torque of 80 Nm is developed in an induction motor by an auto-transformer starter with a tapping of 30%. If the tapping of auto transformer starter is 60%, then what is the starting torque?

(a) 40 Nm.
(b) 160 Nm.
(c) 240 Nm.
(d) 320 Nm. [U.P.S.C. I.E.S. 2005]

77. For the purpose of starting an induction motor, star/delta starter is an equivalent to an auto-transformer of ratio

(a) 33.3%
(b) 57.7%
(c) 73.2%
(d) 100% [I.E.S. E.E.-II 2002]

78. Consider the following statements: Star-delta starter is used in 3-phase induction motor because it

1. prevents heating of the motor windings.
2. ensures permissible minimum starting current.
3. is regulated by electricity authorities.
4. ensures smooth run-up to full load.

Of these statements.
(a) 1, 2 and 3 are correct.
(b) 2, 3 and 4 are correct.
(c) 1, 3 and 4 are correct.
(d) 1 and 2 are correct. [I.E.S. 1998]

79. For slip-ring induction motors, rotor resistance starting is preferred over reduced voltage starting because it

(a) limits the speed.
(b) limits the starting current.
(c) increases the starting torque.
(d) improves the starting power factor.
(e) limits starting current, increases starting torque and also improves starting power factor.