Induction Motor Objective Questions

41. Synchronous-induction motors

(a) are used where high starting torque is required.
(b) can be operated on full load on any desired pf.
(c) are very often installed along with other induction motors in order to improve the overall power factor of the system.
(d) have been made for rating up to 30,000 kW.
(e) all of the above statements regarding synchronous induction motors are correct.

42. The secondary of a linear induction motor normally consists of a

(a) concentrated single phase winding.
(b) distributed single phase winding.
(c) solid conducting plate.
(d) distributed three phase winding.

43. Linear induction motor is used in

(a) traction.
(b) magnetic attraction.
(c) mechanical workshops.
(d) textile mills.

44. A Schrage motor is a variable speed commutator type three phase induction motor and has three windings,

(a) all the three windings located in stator.
(b) all the three windings located in rotor.
(c) two windings in rotor and one winding in stator.
(d) two windings in stator and one winding on rotor.

45. The primary winding of a Schrage motor is located

(a) in stator.
(b) in lower part of the rotor.
(c) in upper part of the rotor.
(d) partly in stator and partly in rotor.

46. For starting a Schrage motors 3-phase supply is connected to

(a) rotor via slip-rings.
(b) secondary winding via brushes.
(c) regulating windings.
(d) stator windings as in an ordinary induction motor.

47. Which of the following statements about Schrage motor is incorrect?

(a) it gives normally speed range of 3 to 1 but much speed ranges upto 15 to 1 and higher may also be obtained.
(b) It is cheaper than the slip-ring induction motor.
(c) Its efficiency is higher than that of a slip-ring induction motor at all speeds except synchronous speed and is very much high at lower speeds.
(d) While operating exactly at synchronous speed it behaves, in a sense, like a synchronous motor with compensating winding functioning as a dc exciter.

48. Which of the following statements associated with a Schrage motor is/are correct ?

(a) The magnitude of the slip frequency voltage injected into the secondary circuit depends upon the spacing between the two sets of brushes, and the phase depends on their angular position with respect to centre of the rotor winding.
(b) When the two sets of brushes of each phase are making contact with the same commutator bar, the secondary is short-circuited and the motor operates as an ordinary induction motor.
(c) Its power factor can be varied by rocking the brushes as a whole round the commutator.
(d) all of the above.

49. A 3-phase slip-ring induction motor is fed from the rotor side with stator winding short circuited. The frequency of the currents in the short-circuited stator is

(a) slip frequency.
(b) supply frequency.
(c) frequency corresponding to rotor speed.
(d) zero. [GATE E.E. 1953]

50. A 3-phase induction motor is fed from a balanced 3-phase supply on the rotor side, short-circuiting the stator terminals. The speed of the rotating magnetic field is

(a) zero.
(b) equal to rotor speed.
(c) equal to synchronous speed. [AMIE. Sec B. Winter 1994]
(d) equal to slip speed.

51. In particular motor there is a stator 3 phase winding and a short-circuited rotor winding. The motor runs at synchronous speed but does not need any dc excitation. The motor is

(a) 3-phase induction motor.
(b) 3-phase synchronous motor.
(c) 3-phase reluctance motor.
(d) hysteresis motor.

52. When the stators of two slip-ring induction motors are fed from a common 3-phase ac supply in parallel and the rotor windings are connected in opposition, they are called

(a) power selsyns.
(b) position selsyns.
(c) power synchros.
(d) position synchros.

53. Position synchros normally have ……… phase stator winding and …….. phase rotor winding.

(a) three, single
(b) single, three
(c) three, three
(d) single, single

54. Phase advancers are employed with large induction motors to

(a) regulate their speeds.
(b) reduce copper losses.
(c) improve power factor.
(d) reduce noise and vibrations.

55. Which of the following statements about a phase advancer is incorrect?

(a) It is a particular type of ac exciter which may be connected in the rotor circuit of an induction motor to improve power factor.
(b) It uses the principle of injection through slip-rings of the motor a current leading with regard to the rotor voltage.
(c) It resembles with an induction motor in construction.
(d) Its kVA capacity needs be only 5% or less of kVA correction affected in the main supply circuit.
(e) it has compensating windings on the stator to ensure good commutation.

 

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