# Objective Questions on Induction Motor | 4

1. Crawling in an induction motor is due to

Whenever rotor slots are integral multiple of stator slots then space harmonic waves are introduced developed in the gap along with the main flux on giving supply. These harmonics create additional torque fields. A common problem is with the seventh harmonic wave where it creates a forward rotating torque field at one seventh of the synchronous speed. There will be a maximum torque at a speed just below ( 1 / 7)Ns, and if this is high enough, the net torque produced can be higher than the torque due to the line frequency even though speed is ( 1 / 7)Ns. The slip all this while remaining high. This can cause the motor to crawl at a speed just below 1 / 7 of synchronous speed. Thus crawling of IM is due to space harmonics.

2. During no load test, an induction motor draws power

No load test is similar to the open-circuit test on a transformer. The motor is made run at no load, the input power is measured by the 2-wattmeter method. No load current is measured by an ammeter and the normal rated supply voltage is measured by a voltmeter. Since no load current is 20 - 30 % of full load current, the I2R losses in the primary may be neglected as they vary with the square of the current. Since the motor is running at no load, total input power is equal to constant iron loss, friction and windage losses of the rotor.

3. Unbalanced supply voltage given to a 3 - Φ, delta connected induction motor will cause

If the supply voltage to a 3 - Φ IM is unbalanced and the it is delta connected, then a counter rotating magnetic field is formed. This counter rotating field results in the formation of a negative sequence current.

4. Exciting electric current of an induction motor is

Unlike that of a power transformer the magnetic circuit of an induction motor has an air gap. Therefore the exciting electric current of an induction motor( 30 % - 40 % of full load current) is much higher than that of a power transformer.

5. If starting electric current of a 3 - Φ induction motor is 5 times the rated current, while rated slip is 4%, then ratio of starting torque to full load torque is

Tst / Tfl = ( Ist / Ifl )2 × sfl = 25 × 0.04 = 1

6. In travelling cranes for 'hoisting and lowering' which type of motor is preferred?

For hoisting and lowering of crane requires high starting torque. Hence ac slip ring induction motor, Ward Leonard controlled dc shunt motors are preferred.

7. In travelling cranes for crane travel, trolley and boom hoist which motor is preferred?

For 'crane travel, trolley travel and boom hoist' of traveling crane requires high starting torque and constant speed in operation. Hence slip ring induction motor is preferred.

8. In travelling cranes for swing action which motor is preferred?

AC slip ring or DC shunt motors are prefer for 'slew and swing action' of traveling cranes.

9. Which of following motor is preferred for belt conveyor?

Normal starting electric current and high starting torque double cage IM with DOL starter or slip ring ring induction motors are preferred for belt conveyors.

10. In jaw crushers which of following motors are preferred?

A crusher is a machine designed to reduce large rocks into smaller rocks, gravel, or rock dust. Belted slip ring induction motor are employed in jaw crushers because of their high starting torque.

11. In shapers and slotters which motor are preferred?

A shaper is another machine tool that is used to plane metal. It is smaller than a planer and has a cutting action opposite to that of the planer. Constant speed squirrel cage induction motors are prefer for shapers and slotters.

12. Which motors are prefer for refrigeration and air conditioning in larger units?

For larger units high torque squirrel cage induction motor, slip ring induction motor are preferred.

13. Belt conveyors offer _______________ starting torque.

Belt conveyors require high starting torque. Hence for belt conveyors slip ring IM will employ.

14. The application that need frequent starting and stopping is

Lifts and hoists are requires frequent starting and stopping because load is not constant.

15. Fifth harmonic component of the induced voltage in three phase AC generator can be eliminated by using a winding pitch of

Coil span factor nth harmonic, Kc = cos0.5 × n ×θ.
Where θ is a angle by which the coil is short of full pitch. Kc = - cos( 1 / 2 ) × 5 × { 1 - ( 4 / 5 )} = 0. Those fifth harmonic component gets eliminated by using a coil pitch of 4 / 5.

16. In escalator which motor is used?

In escalator 3 - Φ IM is used because it has higher starting torque.

17. The speed of rotation of 5th harmonic component in the stator of an induction motor is

The speed of rotation of 5th harmonic component in stator of an induction motor is 1 / 5 times of the speed of synchronous motor.

18. In an induction motor, the no-load slip is generally equal to

For no-load the reactance become almost negligible and the rotor impedance is now depends on only resistance. Further if the rotor resistance is small the rotor electric current is large, so that motor works with a large torque (since torque ∝ current) which brings the speed near to synchronous speed. The slip is thus reduced to 1% or less.

19. Star - delta method of starting of motor is not possible in case of

Star - delta winding is three phase winding, and is only applicable in case of 3 - Φ motors. ∴ A single phase induction motor canâ€™t be started by this method.

20. If s is the slip and f is the supply frequency of an induction motor, the frequency of rotor electric current is given by

The relative rotor speed is given by Ns − Nr = 120fr / p.
Where Nr is the rotor speed and Ns is the synchronous speed given by Ns = 120f / p.
Dividing 1 by 2, we get ( Ns - Nr) / Ns = fr / f
∴ s= fr / f. Thus fr = sf.