41. Cross-connecting of commutator bars in repulsion motors reduces circulating currents caused due to
(a) unequal air gaps between the stator and rotor.
(b) shorted stator winding.
(c) shorted brushes.
(d) all of these.
Answer: (a) unequal air gaps between the stator and rotor.
42. In repulsion motors, brush angle with respect to magnetic axis is kept
(a) 0° to 15°
(b) 15° to 45°
(c) 90°
(d) 180°
Answer: (b) 15° to 45°
43. The torque-speed characteristic of a repulsion motor resembles which of the following dc motor characteristic ?
(a) Separately excited.
(b) Shunt.
(c) Series.
(d) Compound. [GATE E.E. 1996].
Answer: (c) Series.
44. In a single-phase repulsion motor power factor is
(a) always leading.
(b) high at low speed.
(c) high at high speed.
(d) always unity. [A.M.I.E. Sec B. Elec. Machines Summer 1996]
Answer: (c) high at high speed.
45. In a repulsion motor by shifting the brush position
(a) speed and direction of rotation can be controlled.
(b) only speed can be controlled.
(c) speed, torque and direction of rotation can be controlled.
(d) only torque can be controlled by varying the speed at constant output power. [A.M.I.E. Sec B. Elec. Machines Winter 1996]
Answer: (c) speed, torque and direction of rotation can be controlled.
46. In a repulsion motor, if the angle of brush shift α from low impedance position is increased from 0 to 45° with constant current supply, then
(a) speed and power factor decrease but torque increases.
(b) speed and torque increase but pf decreases.
(c) speed, pf and torque increase.
(d) speed and pf increase but torque decreases. [A.M.I.E. Sec B. Summer 2002]
Answer: (a) speed and power factor decrease but torque increases.
47. In a repulsion motor, torque developed will be zero when brush axis is at electrical degrees to the field axis.
(a) 0
(b) 90
(c) 45
(d) both(a) and (b)
Answer: (d) both(a) and (b)
48. In a repulsion motor speed control is affected by varying the
(a) impressed voltage.
(b) position of brushes.
(c) supply frequency.
(d) either (a) or (b).
Answer: (d) either (a) or (b).
49. Direction of rotation of a repulsion motor is reversed by.
(a) interchanging the supply leads.
(b) shifting the brushes round the commutator.
(c) either (a) or (b).
(d) none of these.
Answer: (b) shifting the brushes round the commutator.
50. In a compensated repulsion motor, an additional stator winding is provided in order to
(a) improve power factor.
(b) provide better speed regulation.
(c) increase the output.
(d) improve mechanical balance.
(e) both (a) and (b).
Answer: (e) both (a) and (b).
51. A repulsion start induction motor operates as an induction motor when
(a) commutator segments are short circuited.
(b) brushes are short-circuited.
(c) stator is short-circuited.
(d) short-circuiting device is disconnected.
Answer: (a) commutator segments are short circuited.
52. The short-circuiting device in a repulsion start induction motor operates by
(a) magnetic attraction.
(b) centrifugal force.
(c) circulating currents.
(d) none of the above.
Answer: (b) centrifugal force.
53. The repulsion start induction motor has the advantage(s) of
(a) high starting torque (2.5 to 4.5 times of full-load torque).
(b) low starting current (about 3.5 times of full-load current).
(c) low cost, high efficiency and good power factor.
(d) both (a) and (b).
Answer: (d) both (a) and (b).
54. Which of the following statements regarding repulsion start induction motor is/are correct ?
(a) It runs below synchronous speeds on all loads.
(b) It needs more maintenance of commutator and other mechanical devices.
(c) It makes quite a bit noise on starting.
(d) It starts as a repulsion motor with its brushes set to give maximum torque.
(e) all of the above.
Answer: (e) all of the above.
55. Repulsion induction motor has
(a) single phase winding on its stator.
(b) two separate windings on its rotor in common slots.
(c) squirrel cage rotor and two separate windings on stator in common slots.
(d) both (a) and (b).
Answer: (d) both (a) and (b).
56. In a repulsion induction motor, the rotor has a
(a) squirrel cage winding with rotor bars permanently short-circuited and placed at the bottom of the slots.
(b) repulsion winding similar to dc armature winding placed over the squirrel cage winding.
(c) repulsion winding placed at the bottom of slots and squirrel cage winding placed above the repulsion winding.
(d) both (a) and (b).
Answer: (d) both (a) and (b).
57. Which of the following statements regarding repulsion induction motor is wrong ?
(a) At start squirrel cage winding does not produce any torque.
(b) At start and at low speeds, major part of the torque is developed by repulsion winding.
(c) At rated load it almost runs at synchronous speed.
(d) It employs centrifugally operated mechanism for short-circuiting the commutator.
Answer: (d) It employs centrifugally operated mechanism for short-circuiting the commutator.
58. The repulsion induction motor has
(a) fairly good speed regulation.
(b) starting torque 2.25 to 3 times the full-load torque.
(c) starting current 3 to 4 times the full-load current.
(d) all of the above.
Answer: (d) all of the above.
59. The repulsion induction motor does not have the drawback of
(a) higher cost.
(b) careful maintenance.
(c) poor starting torque.
(d) tendency of sparking.
Answer: (c) poor starting torque.
60. Single phase synchronous motors
(a) are self starting motors.
(b) do not need dc excitation.
(c) operate at constant speed.
(d) are built for a wider range of output.
(e) all of the above.
Answer: (e) all of the above.