Induction Motor Objective Questions

21. Circle diagram of an induction motor can be used to determine its

(a) power factor.
(b) efficiency.
(c) output.
(d) slip.
(e) all of the above.

22. Which one of the following is correct statement?

The output line in an induction motor circle diagram is the line joining the tip of the
(a) no load current phasor to the point corresponding to slip = 0.
(b) no load current phasor to the point corresponding to slip 1.
(c) short circuit current phasor to the point corresponding to slip = 1.
(d) short circuit current phasor to the point corresponding to slip = 0. [I.E.S. E.E.- II, 2007]

23. If the rotor power factor of a 3-phase, induction motor is 0.866, the spatial displacement between the stator magnetic field and the rotor magnetic field will be

(a) 30°
(b) 90°
(c) 120°
(d) 150° [U.P.S.C. I.E.S. E.E.-II, 2000]

24. A 3-phase induction motor when started picks up speed but runs stably at about half the normal speed. This is because of

(a) unbalance in the supply voltages.
(b) non-sinusoidal nature of the supply voltage.
(c) stator circuit asymmetry.
(d) rotor circuit asymmetry. [U.P.S.C. I.E.S. E.E.- II, 1998]

25. An induction motor is said to be crawling when

(a) it runs at one-seventh of rated speed.
(b) it accelerates too fast.
(c) it is subjected to fluctuating loads.
(d) it is started on full load.

26. Zero sequence impedance of a 3-phase cage induction motor is due to

(a) stator circuit 3^rd time harmonic.
(b) stator circuit 3^rd space harmonic.
(c) rotor circuit 3^rd time harmonic.
(d) rotor circuit 3^rd space harmonic. [U.P.S.C. I.E.S. E.E.-II, 1998]

27. The stator of a 6-pole, 3-phase induction motor is fed from a 3-phase 50 Hz supply which contains a pronounced fifth time harmonic. The speed of the fifth space harmonic field produced by the fifth time harmonic in the stator supply will be

(a) 200 rpm.
(b) 1.500 rpm.
(c) 1,000 rpm.
(d) 5,000 rpm.

28. The synchronous speed for the seventh space harmonic mmf wave of a 3-phase, 8-pole, 50 Hz induction machine is

(a) 107.14 rpm in forward direction.
(h) 107.14 rpm in reverse direction.
(c) 5,250 rpm in forward direction.
(a) 5,250 rpm in reverse direction. [GATE E.E. 2004]

29. An induction motor when started on load does not accelerate up to full speed but runs at 1/7^th of the rated speed. The motor is said to be

(a) locking.
(b) plugging.
(c) crawling.
(d) cogging. [U.P.S.C. I.E.S. E.E.-II, 2001]

30. The presence of a dominant 7th harmonic in the winding distribution of a 3-phase, 6-pole, 50 Hz induction motor may cause the motor to crawl at a speed of about

(a) 750 rpm.
(b) 500 rpm.
(c) 242 rpm.
(d) 143 rpm. [U.P.S.C. I.E.S. E.E.-II, 1993]

31. Crawling in an induction motor is due to

(a) time harmonics in supply.
(b) slip ring rotor.
(c) space harmonics produced by winding currents.
(d) insufficient starting torque. [U.P.S.C. I.E.S. E.E.-II, 1999]

32. The crawling in the induction motor is caused by

(a) improper design of stator laminations.
(b) low voltage supply.
(c) high loads.
(d) harmonics developed in motor. [U.P.S.C. I.E.S. 2003]

33. The phenomenon of crawling in a 3-phase induction motor may be due to

(a) unbalanced supply voltage.
(b) 7th space harmonics of air-gap field.
(c) 7th time harmonies of voltage wave.
(d) 5th space harmonics. [U.P.S.C. I.E.S. E.E.-II, 1998]

34. In an induction motor, when the number of stator slots is equal to an integral multiple of rotor slots,

(a) there may be a discontinuity in torque-slip characteristics.
(b) a high starting torque will be available.
(c) the maximum torque will be high.
(d) the machine will fail to start. [U.P.S.C. I.E.S. E.E.-II, 2003]

35. Crawling of induction motor occurs due to

(a) harmonic synchronous torques.
(b) harmonic induction torques.
(c) vibration torques.
(d) both (a) and (b).

36. A 3-phase induction machine draws active power ‘P’ and reactive power ‘ Q’ from the grid. If it is operated as a generator, P and Q will respectively be

(a) positive and negative.
(b) negative and negative.
(c) positive and positive.
(d) negative and positive. [A.M.I.E. Sec B. Winter 2003]

37. In a 3-phase induction machine, motoring, generating and braking operations take place in the range of slip “s” given by

(a) motoring: 1 > s > 0; generating : 0 > s > – 1; braking : s > 1.
(b) motoring : s > 1; generating 1 > s > 0; braking ; 0 > s > –1.
(c) motoring: s > 1; generating 0 > s > –1 braking : 1 > s > 0.
(d) motoring: 0 > s > — 1; generating : s > 1; braking : 1 > s > 0. [U.P.S.C. LE.S. E.E.-II, 1994]

38. In a 3-phase induction machine, motoring, generating and braking operations take place in the range of slip “s” is

(a) 1 > s > 0, 0 > s > –2 and s > 1
(b) s > 1, 1 > s > –1 and 0 > s > –1
(c) s > 1, 0 > s > –1 and 1 > s > 0
(d) 0 > s > –1, s > 1 and 1 > s > 0

39. Which of the following statements about synchronous-induction motors is incorrect?

(a) It is basically a wound rotor induction motor with fewer and larger slots on rotor.
(b) It has a very small air gap as expected inaplain induction motor.
(c) It is connected to 3-phase ac supply on stator side and to dc on rotor side.
(d) It starts as an induction motor and runs as a synchronous motor.
(e) It is provided with a heavy rotor winding.

40. The synchronous impedance of a synchronous-induction motor is much larger than that of a synchronous motor due to

(a) its larger air gap.
(b) presence of damper bars in it.
(c) its less magnetic reluctance.
(d) supply of dc excitation to its 3-phase rotor.