Single Phase Induction Motor MCQ

1. Single phase ac motors are used in largest number. This is due to

(a) their great variety of useful services in the home, office, the factory, in business establishment and on the farm.
(b) their wide applications in automatic control devices of various types.
(c) lower cost.
(d) both (a) and (b).

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2. A dual voltage single phase ac motor can be operated on either

(a) 230 or 400 V.
(b) 30 or 400 V
(c) 115 or 230 V.
(d) 160 or 320 V.

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3. Single phase ac motors are classified on the basis of

(a) construction.
(b) starting methods.
(c) size.
(d) rpm.
(e) both construction and starting methods.

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4. In a single phase ac induction motor

(a) stator is supplied with single phase ac.
(b) rotor it supplied with single phase ac.
(c) rotor is a cage rotor.
(d) rotor is a wound rotor.
(e) both (a) and (c).

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5. The rotor slots in an induction motor are, usually not kept parallel to the shaft as it

(a) helps the rotor teeth to remain under the stator teeth.
(b) helps in reducing the tendency of the rotor teeth to remain under the stator teeth.
(c) improves pf.
(d) improves efficiency.

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6. A single phase induction motor is

(a) self starting.
(b) not self starting.
(c) self starting with the help of an auxiliary winding.
(d) none of the above.

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7. Double revolving field theory is based on the idea that pulsating field produced in single phase motors can be resolved into two components of its ………. amplitude and rotating in ………. direction with synchronous speed.

(a) half, same
(b) half, opposite
(c) 1/√2 times of, same
(d) √2 times, opposite

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8. When a single phase induction motor is excited with single phase ac voltage, the magnetic field set up is equivalent to

(a) two fields, rotating in opposite directions with different speeds.
(b) two fields, rotating at synchronous speed in opposite directions.
(c) two fields, rotating at synchronous speed.
(d) two fields, rotating in the same direction but at different speeds. [U.P.S.C. LES. E.E.- II, 2004]

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9. A single phase induction motor with only the main winding excited would exhibit the following response at synchronous speed

(a) Rotor current is zero.
(b) Rotor current is non-zero and is at slip frequency.
(c) Forward and backward rotating fields are equal.
(d) Forward rotating field is more than backward rotating field. [GATE E.E. 2003]

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10. A single phase induction motor is running at N rpm. Its synchronous speed is Ns. If its slip with respect to forward field is s, what is the slip with respect to the backward field?

(a) s
(b) — s
(c) (1 — s)
(d) (2 — s) [U.PS.C. IES. E.E-II, 2006]

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11. In case of a split-phase motor, the phase shift between currents in the two windings is around

(a) 30°
(b) 70°
(c) 90°
(d) 120° [GATE E.E. 1996]

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12. An 8-pole, 50 Hz single phase induction motor is running at 690 rpm. Its slip w.r.t. forward field is (in pu)

(a) + 0.08
(b) + 1.92
(c) — 0.08
(d) 1.00 [A.M.I.E. Sec B. Elec. Machines Summer 1999]

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13. An 8-pole single phase induction motor is running at 690 rpm. What is its slip with respect to forward and backward fields, respectively?

(a) 0.08, 2.0
(b) 0.08, 1.92
(c) 1.92, 0.08
(d) 2.0, 0.08 [U.P.S.C. I.E.S. E.E.-II, 2007]

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14. In a comparative study of the torque-slip characteristic of a balanced polyphase induction motor and that of a single phase induction motor, it is found that for zero slip

(a) the torque for the former is zero and has a non-zero negative value for the latter.
(b) the torque has a non-zero positive value on the former and has a non-zero negative value on the latter.
(a) the torque is zero for both of the charactristics.
(d) the torque has a non-zero positive value for both of the charactristics.

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15. A rotating magnetic field is produced by current in two windings displaced by 90 electrical degrees. This is the principle of

(a) phase sequence.
(b) phase splitting.
(c) phase timing.
(d) none of these.

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16. In an single phase induction motor

(a) stator current, stator flux and rotor emf are in phase.
(b) Rotor current and rotor flux are in phase and both lag behind the rotor emf by 90°.
(c) Rotor current, rotor flux and rotor emf are in phase.
(d) both (a) and (b) are correct.

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17. Field created by rotor currents is at right angle to the field created by stator currents and is so called the crossfield. The crossfield is ………. stator field at actually operating speeds.

(a) somewhat weaker than
(b) somewhat stronger than
(c) of the same strength as that of
(d) any of the above

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18. In comparison to three phase induction motors, single phase induction motors have

(a) lower efficiency.
(b) lower pf.
(c) poor speed regulation.
(d) more slip at full load.
(e) all of the above.

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19. The torque developed by a single phase induction motor drops to zero at

(a) synchronous speed.
(b) a speed slightly above synchronous speed.
(c) a speed slightly below synchronous speed.
(d) none of the above.

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20. The no-load current of a single phase induction motor is around ……….. % of full load current.

(a) 10
(b) 20
(c) 40
(d) 80

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