# Objective Questions on Induction Motor | 3

1. The synchronous speed of an induction motor will be increased by

Ns = (120f) / P i.e., frequency is proportional to synchronous speed. Hence synchronous speed will increases as frequency increases.

2. When the rotor of a 3 phase induction motor is blocked then the slip is

When the motor is blocked i.e., rotor is at zero speed.
Slip = Ns - N / Ns as rotor speed N = 0 then slip = Ns / Ns = 1.

3. In induction motor the 7th harmonic torque may make the motor rotate at (1/7)th synchronous speed. This is called

It is found that induction motor particularly the squirrel cage type sometimes exhibit a tendency to runs stably at a speeds as low as ( 1 / 7 )th of synchronous speed. This phenomenon is called crawling of induction motor.

4. In induction motor especially the cage rotors slots are nearly parallel to shaft axis. This is called

In squirrel cage rotors consists of cylindrical laminated core with slots nearly parallel to shaft axis this process is called skewing.

5. Skewing of cage rotors offers the following advantages

1. More uniform torque is produced.
2. Less noise during operation.
3. It reduces or even entirely eliminates cogging.

6. If the no of rotor slots is equal to no. of stator slots, then motor refuses to start. Hence it is known as

During locking tendency of rotor and stator teeth attract each other due to magnetic attraction. During condition motor refuses to start. This is known as cogging. In case of an induction motor when the number of stator slots is equal to or an integral multiple of rotor slots staying parallel to each other strong alignment forces may be produced at the instant of starting due to variation of reluctance which may create an aligning torque stronger than the accelerating torque with consequent failure to start. This phenomenon of refusing to start sometimes observed is known as cogging.

7. In IM the rotor lamination can be ________________ than stator laminations.

Normally the lamination thickness depends up on frequency. In IM rotor is having lesser frequency than stator frequency because of slip i.e., Fr = sfs. Hence rotor lamination can be thicker than stator lamination because rotor frequency is less.

8. In induction motor wound rotor motor is mainly is used in application where

In IM the wound rotor winding are connected in star. The open end of the star is connected to slip rings. The slip rings mounted on the shaft with the brushes resting on them. The purpose of slip rings and brushes is to provide a means for connecting external resistance in the rotor circuit.
The resistors enable the variation of each rotor. The purpose of using resistors at ends
1. To increase the starting torque and decease a starting electric current from supply.
2. To control the speed.

9. The slip of an induction motor normally does not depend on

We know that slip = ( synchronous speed - rotor speed ) / synchronous speed. So slip is depends upon synchronous speed, rotor speed and hence depends on shaft torque also.

10. Commonly used starters of cage motor is

DOL direct online starter and star delta starter is used for starting of cage induction motor.

11. A 3 - Φ, 4 pole, 50 hz induction motor 4 % slip, calculate frequency of the rotor E.M.F?

f’ = s.f (f’ = frequency of rotor EMF, s = slip in %, f = supply frequency) = ( 0.04 × 50 ) = 2 Hz.

12. Which slots are used in the rotor of an induction motor?

Usually semi closed slots are used in the rotor of an IM, since open slots increase effective length of air gap and closed slots increase leakage reactance.

13. In double cage induction motor, the inner cage offers

Double cage induction motor consists of 2 stages.
1. Inner cage
2. Outer cage
Inner cage is designed in such way that having low resistance and high reactance. As if frequency is less then impedance is less in the inner cage. Hence, electric current flows through inner cage.

14. In double cage induction motor, the outer cage offers

Double cage induction motor consists of 2 stages.
1. Inner cage
2. Outer cage
Outer cage offers high resistance and low reactance. At high frequencies inner cage offers high impedance and outer cage offers low impedance. Hence, electric current flows through outer cage because it offers lower impedance.

15. Which of the following induction motor has maximum speed?

Speed Ns = (120f / P), where P is the number of poles. Therefore speed is inversely proportional with number of poles in the machine. So the answer will be 2 pole induction motor.

16. High starting torque and low starting electric current is the advantage of

In the slip ring induction motor slip rings are mounted on the shaft with brushes resting on them. The brushes are connected to three variable resistors connected in star. The purpose of slip rings and brushes is to provide external resistors in the rotor circuit. Varying the resistors we can vary each rotor phase resistance. As the starting torque and starting electric current can be controlled by rotor resistance, thus slip ring motor has that advantage. Hence the correct option is slip ring induction motor.

17. Which method is less recommendable for induction motor starting in the industries?

Stator resistance starting method will not be recommendable. This method suffers from two drawbacks. First, the reduced voltage applied to the motor during the starting period lowers the starting torque and hence increases the accelerating time. Secondly, a lot of power is wasted in the starting resistances. Therefore, this method is used for starting small motors only.

18. The air gap between stator and the rotor of the 3 - Φ induction motor is kept

The air gap between the rotor and stator of a 3 - Φ induction motor is made as small as possible in order to produce sufficient mutual flux with a minimum exciting electric current and thereby making leakage reactance are as small as possible.

19. What is power factor of a 3 - Φ induction motor at no load?

Because of air-gap, the reluctance of the magnetic circuit of 3 - Φ induction motor is very high. Consequently, the electric current drawn by the motor on no load is largely magnetizing current; the no load current lagging the applied voltage by a large angle. For this reason, the p.f of a lightly loaded 3-phase induction motor is very low.

20. For a 3 - Φ, 4 pole, 50 Hz IM rotating at full load speed of 1440 rpm, calculate speed of rotor field w.r.t rotor structure and stator structure respectively?

Ns= 120f / P = 1500 rpm
Nr = 1440 rpm
∴ s = 0.04
Rotor frequency f2 = sf = 0.04 X 50 = 2Hz
Speed of rotor field w.r.t rotor structure is (120f2) /P = 60 rpm
Speed of rotor field w.r.t stator structure = ( Speed of rotor structure w.r.t stator structure ) + ( Speed of rotor field w.r.t rotor structure )
= 1440 + 60 = 1500 rpm
∴ Ans is 60, 1500.