Objective tests on single-phase motors

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OBJECTIVE TESTS

1. The starting winding of a single-phase mo- tor is placed in the

(a) rotor

(b) stator

(c) armature

(d) field.

2. One of the characteristics of a single- phase motor is that it

(a) is self-starting

(b) is not self-starting

(c) requires only one winding

(d) can rotate in one direction only.

3. After the starting winding of a single- phase induction motor is disconnected from supply, it continues to run only on ............winding.

(a) rotor

(b) compensating

(c) field

(d) running

4. If starting winding of a single-phase induction motor is left in the circuit, it will

(a) draw excessive current and overheat

(b) run slower

(c) run faster

(d) spark at light loads.

5. The direction of rotation of a single-phase mo- tor can be reversed by

(a) reversing connections of both windings

(b) reversing connections of starting winding

(c) using a reversing switch

(d) reversing supply connections.

6. If a single-phase induction motor runs slower than normal, the more likely defect is

(a) improper fuses

(b) shorted running winding

(c) open starting winding

(d) worn bearings.

7. The capacitor in a capacitor-start induction- run ac motor is connected in series with ...... winding.

(a) starting

(b) running

(c) squirrel-cage

(d) compensating

8. A permanent-split single-phase capacitor motor does not have

(a) centrifugal switch

(b) starting winding

(c) squirrel-cage rotor

(d) high power factor.

9. The starting torque of a capacitor-start induction-run motor is directly related to the angle a between its two winding currents by the relation

(a) cos a

(b) sin a

(c) tan a

(d) sin a/2.

10. In a two-value capacitor motor, the capacitor used for running purposes is a/an

(a) dry-type ac electrolytic capacitor

(b) paper-spaced oil-filled type

(c) air-capacitor

(d) ceramic type.

11. If the centrifugal switch of a two-value capacitor motor using two capacitors fails to open, then

(a) electrolytic capacitor will, in all probabil- ity, suffer breakdown

(b) motor will not carry the load

(c) motor will draw excessively high current

(d) motor will not come upto the rated speed.

12. Each of the following statements regarding a shaded-pole motor is true except

(a) its direction of rotation is from un- shaded to shaded portion of the poles

(b) it has very poor efficiency

(c) it has very poor p.f.

(d) it has high starting torque.

13. Compensating winding is employed in an ac series motor in order to

(a) compensate for decrease in field flux

(b) increase the total torque

(c) reduce the sparking at brushes

(d) reduce effects of armature reaction.

14. A universal motor is one which

(a) is available universally

(b) can be marketed internationally

(c) can be operated either on dc or ac supply

(d) runs at dangerously high speed on no-load.

15. In a single-phase series motor the main purpose of inductively-wound compensating winding is to reduce the

(a) reactance emf of commutation

(b) rotational emf of commutation

(c) transformer emf of commutation

(d) none of the above.

(Power App.-II, Delhi Univ. Jan. 1987)

16. A repulsion motor is equipped with

(a) a commutator

(b) slip-rings

(c) a repeller

(d) neither (a) nor (b).

17. A repulsion-start induction-run single- phase motor runs as an induction motor only when

(a) brushes are shifted to neutral plane

(b) short-circuiter is disconnected

(c) commutator segments are short- circuited

(d) stator winding is reversed.

18. If a dc series motor is operated on ac supply, it will

(a) have poor efficiency

(b) have poor power factor

(c) spark excessively

(d) all of the above

(e) none of the above.

19. An outstanding feature of a universal motor is its

(a) best performance at 50 Hz supply

(b) slow speed at all loads

(c) excellent performance on dc. supply

(d) highest output kW/kg ratio.

20. The direction of rotation of a hysteresis motor is determined by the

(a) retentivity of the rotor material

(b) amount of hysteresis loss

(c) permeability of rotor material

(d) position of shaded pole with respect to the main pole.

21. Speed of the universal motor is

(a) dependent on frequency of supply

(b) proportional to frequency of supply

(c) independent of frequency of supply

(d) none of the above.

(Elect. Machines, A.M.I.E. Sec. B, 1993)

22. In the shaded pole squirrel cage induction motor the flux in the shaded part always

(a) leads the flux in the unshaded pole segment

(b) is in phase with the flux in the unshaded pole segment

(c) lags the flux in the unshaded pole segment

(d) none of the above.

(Elect. Machines, A.M.I.E. Sec. B, 1993)

23. Which of the following motor is an interesting example of beneficially utilizing a phenomenon that is often considered undesirable ?

(a) hysteresis motor

(b) reluctance motor

(c) stepper motor

(d) shaded-pole motor.

24. Usually, large motors are more efficient than small ones. The efficiency of the tiny motor used in a wrist watch is approximately.......... per cent.

(a) 1

(b) 10

(c) 50

(d) 80

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