Induction Motor Objective Type Questions

40. A squirrel cage induction motor is not favoured when ……….. is the main consideration.

(a) initial cost
(b) maintenance cost
(c) high starting torque
(d) higher efficiency and higher power factor

41. A slip-ring induction motor is recommended for applications requiring

(a) high starting torque.
(b) variable speed operation.
(c) frequent starting stopping and reversing operations.
(d) all of the above features.

42. Induction motors, over synchronous motors, have the advantages of

(a) being self starting.
(b) having no requirement for dc excitation.
(c) possibility of speed control to small extent.
(d) simplicity and lower cost.
(e) all of the above.

43. Unbalanced 3-phase voltage supply to an induction motor causes excessive heating of

(a) rotor.
(b) stator.
(c) rotor shaft.
(d) none of these.

44. Unbalanced supply voltage given to a 3-phase, delta-connected induction motor will cause

(a) zero sequence currents.
(b) less heating of motor.
(c) negative sequence component current.
(d) all the above. [GATE E.E. 1996]

45. Single phase preventer

(a) suppresses negative sequence currents.
(b) compensates for voltage drops.
(c) provides protection in the event of non-availability of one of the phases.
(d) none of the above.

46. An induction motor is fed from a balanced three phase supply at rated voltage and frequency through a bank of three single phase transformers connected in delta-delta. One unit of the bank develops fault and is removed. Then

(a) single phasing will occur and the machine fails to start.
(b) single phasing will not occur but the motor terminal voltages will become unbalanced and the machine can be loaded to the extent of 57.7% of its rating.
(c) the machine can be loaded to the extent of 57.7% of its rating with balanced supply at its terminals.
(d) the machine can be loaded to the extent of 66% of its rating with balanced supply at its terminals. [GATE E.E. 1995]

47. If a 3-phase induction motor is started when one of the phases is not available, then the motor

(a) will hum but not start.
(b) will continue to operate satisfactorily on loaa below 57.7% of rated load if brought up to speed by some external means.
(c) will operate under reduced load but usually with considerable vibrations in case of wound rotor motor is brought up to speed by some external means.
(d) start and operate at a lower speed.
(e) all of the above except (d).

48. If one of the 3-phases of supply to an induction motor fails, may be due to any reason, when the motor is running normally, the motor will

(a) continue running at the same speed if it was running on light load.
(b) continue running but will draw more current if it was operating at rated load. (c) stop and carry heavy current causing permanent damage to the windings if it was operating overloaded.
(d) all of the above.

49. Consider the following statements: In a 3-phase induction motor connected to a 3-phase supply; if one of the lines suddenly gets disconnected, then the

1. motor will come to a standstill.
2. motor will continue to run at the same speed with line current unchanged.
3. motor will continue to run at a slightly reduced speed with increase in line current.
4. rotor current will have both of sf and (2—s)f component frequencies where s is the slip and f is the supply fre-quency. Of these statements

(a) 1 and 4 are correct.
(b) 1 and 2 are correct.
(c) 3 and 4 are correct.
(d) 2 and 3 are correct. [I.E.S. EE.-II, 1995]

50. Increase in supply voltage to a 3-phase induction motor will not cause

(a) decrease in input current and so reduction in copper losses.
(b) increase in torque.
(c) decrease in speed as well as in core loss.
(d) operating temperature abnormally higher owing to increase in core loss.

51. Decrease in supply voltage to a 3 phase induction motor will not cause

(a) any difficulty in accelerating the load.
(b) increase in torque.
(c) decrease in speed as well as in core loss.
(d) increase in stator as well as rotor currents.
(e) operating temperature abnormally high owing to increase in copper loss.

52. A 3-phase induction motor is operating at its rated torque. If the supply voltage falls to 75% of its normal value,

(a) the motor will get heated up to inadmissible extent after some time.
(b) the motor will stop.
(c) the slip will decrease.
(d) the stator and rotor current will decrease.

53. A squirrel cage induction motor having a rated slip of 4% of full-load has a starting torque same as the full-load torque. Which one of the following statements is correct? The starting current is

(a) equal to the full-load current.
(b) twice the full-load current.
(c) four times the full-load current.
(d) five times the full-load current.

54. An induction motor having full-load torque of 60 Nm when delta connected develops a starting torque of 120 Nm. For the same supply voltage, if the motor is changed to star-connection, the starting torque developed will be

(a) 40 Nm
(b) 60 Nm
(c) 90 Nm
(d) 120 Nm [GATE E.E. 1996]

55. A 3-phase, 480 V, 60 Hz induction motor is to be operated at 50 Hz supply. The most satisfactory supply voltage for the machine would be

(a) 480 V
(b) 400 V
(c) 600 V
(d) 440 V

56. If a 3-phase 350 V, 50 Hz, 1440 rpm induction motor is operated on 420 V, 60 Hz supply, then the torque will

(a) increase.
(b) decrease.
(c) remain unchanged.
(d) none of these.

57. The starting torque of an induction motor varies as

(a) f
(b) 1/f²
(c) 1/f
(d) f²

58. With the increase in supply frequency[/expander_maker] …….. decrease(s).

(a) starting current
(b) starting torque
(c) full load current
(d) maximum running torque
(e) starting and full load currents and starting and maximum running torque

59. This size of a high speed motor is in comparison to that of a low speed motor for the same kW output.

(a) smaller
(b) larger
(c) almost the same
(d) unpredictable