INDUCTION MOTOR QUESTION AND ANSWER

Q: State the types of Induction motor ?

1) Squirrel cage Induction motor

2) Wound or slip ring Induction motor.

Q: Why skewing is done in squirrel cage Induction motor?

i)    For more uniform torque

ii)   Reduction in noise

iii) To decrease magnetic locking tendency

Q:What is the purpose of connecting external resistors to slip ring Induction motor?

It increases starting torque and decreases starting current.

Q.Can we add extra resistance in series with squirrel cage rotor? State the reason? 

We cannot add extra resistance in series with the rotor because all the copper bars of the rotor are short circuited in both the sides by copper end rings to have a closed circuit.

Q:State the difference between squirrel cage Induction motor and slip ring Induction motor.

Parameters			Squirrel cage IM	Slip ring IM
Rotor winding			Copper bars are short circuited at both ends	Three phase winding
End ring			present	Absent
Efficiency			high	low
External resistance in rotor			Not possible	Possible
Starting torque			Moderate	High
Construction			Simple & robust	Complicated
Maintenance			less	High
Industrial use			95%	5%
Speed control from rotor side			Not possible	Possible
cost			Economical	Expensive
Applications			Lathes, drilling machine, water pumps, blowers, fans, printing machine	Lifts, compressors, hoists, cranes, elevators, belt, conveyor

Q:  Why wound rotor construction is adopted?

The wound rotor has the facility of increasing the rotor resistance through slip rings. High value of rotor resistance is needed during starting to get a high value of starting torque.

Q:What are the advantages of slip ring motor over squirrel cage motor?

A:1.  The starting torque can be varied by adding resistance to rotor.

2.  The speed of the machine can be varied by injecting an emf through slip

rings to the rotor.

Q.What are slip rings? 

The slip rings  are made of copper alloys and  are fixed around the shaft insulating it. Through these slip rings and brushes the rotor winding can be connected to external circuits. 

Q:  What are the materials used to manufacture the brushes for slip rings of an  induction motor?

A:The slip rings are made of brass and phosphor bronze. The brushes are made of metal graphite which is an alloy of copper and carbon.

Q:Name the materials used to insulate the laminations of the core of

induction motor.

The materials used to insulate the laminations are kaolin and varnish.

Q: State the principle of 3 phase IM?

When three phase supply is given to three phase winding of stator, a rotating magnetic field is produced. The rotor conductors cut the rotating magnetic field and emf is induced in it which results in flow of current. According to Lenz’ law flux is produced and it interacts with rotating magnetic field. This produces torque and hence the rotor rotates.

Q:  How the slip ring motor is started?

The slip ring motor is started by using rotor resistance starter. The starter consists of star connected resistance to slip rings. While starting the full resistance is included in the rotor circuit to get high starting torque. Once the rotor starts rotating the resistance is gradually reduced in steps. At running condition the slip rings are shorted and so it is equivalent to squirrel cage rotor.

Q:Why Induction motor is called rotating transformer?

The principle of operation of induction motor is similar to that a transformer. The stator winding is equivalent to primary of the transformer and the rotor winding is equivalent to short circuited secondary of a transformer. In transformer the secondary is fixed but in induction motor it is allowed to rotate. Hence the induction motor also called rotating transformer.

Q: What is Synchronous speed?

The speed at which rotating magnetic field of three Phase Induction motor rotates is called synchronous speed.

Ns=120f/p

Q. Why the induction motor is called asynchronous motor?

Since the induction motor runs always at a speed lesser than synchronous speed, it is called asynchronous motor.

Q:What is slip speed?

The difference between synchronous speed and rotor speed is called slip speed.

Q: What is Slip?

The difference between synchronous speed and rotor speed with respect to synchronous speed is called slip.

S=Ns-N/Ns*100

Q. What would happen if synchronous speed is equal to rotor speed? 

The rotor never obtained the synchronous speed because at that speed there would be no relative speed between rotor conductors and stator revolving field and induction phenomenon is not possible. Its speed is always less than synchronous speed. 

Q  How the magnitude of rotor emf is related to the slip in an I M?

Rotor circuit emf per phase E2’=SE2

Q  How the frequency of rotor emf is related to the slip in an I M?

Frequency of rotor emf/current fr=Sf   

Q   What is the normal value of slip of an I M operating at full load?

3 -   5%

Q: Give formula for starting torque.

Ts = k ФE2R2/ (R2²+X2²)

Q: State the condition for maximum starting torque?

R2=X2

That is rotor resistance equals rotor standstill reactance

Q: Give formula for maximum starting torque.

Ts = k ФE2/2R2

Q: State the condition for maximum running torque?

R2=sX2

Q:  List the undesirable effects produced by certain combination of rotor and stator slots.

1.  The motor may refuse to start (cogging)

2.  The motor may run at sub synchronous speed (Crawling)

3.  Severe vibrations may develop and the noise will be excessive.

Q:  What is crawling ?

The tendency of squirrel cage Induction motor to run at 1/7 times of its synchronous speed is called crawling.

Q: What is Cogging?

Sometimes at low voltage the squirrel cage motor fails to start. This is due to magnetic locking between stator and rotor as the number of stator slot is equal to number of rotor slot. This phenomena is cogging.

Q: What is the necessity of starter?

Its function is to reduce starting current  and also provide protection against over load and over voltage.

Q:What are the types of starters?

Direct on line

  Autotransformer

 Star delta

Rotor resistance starter

Soft starter or Electronic starter

Q. Name the tests to be conducted for predetermining the performance of 3-phase induction machine.

(a) No load test

(b) Blocked rotor test

Q. What are the informations obtained from no-load test in a 3-phase I M?

(i) No – load input current per phase, Io

(ii) No load power factor and hence no load phase angle

(iii) Iron and mechanical losses together

(iv) elements of equivalent circuit shunt branch

Q:What are the informations obtained from blocked rotor test in a 3-phase I M?

(i)Blocked rotor input current per phase at normal voltage

(ii) Blocked rotor power factor and hence phase angle

(iii) Total resistance  and leakage reactance per phase of the motor as

refered to the stator

Q What is circle diagram of an I M?

When an I M operates on constant voltage and constant frequency source, the loci of stator current phasor is found to fall on a circle. This circle diagram is used to predict the performance of the machine at different loading conditions as well as mode of operation.

Q  What are the advantages and disadvantages of circle diagram method of predetermining the performance of 3 – phase I M?

The prediction can be carried out when any of the following information

is available

The input line current., the input power factor, The active power input, The

reactive power input, The apparent power input, The output power , The slip

of operation, The torque developed, The equivalent rotor current per phase,

Maximum output power, Maximum torque developed.

The only disadvantage is, being a geometrical solution, errors made during

measurements will affect the accuracy of the result.

Q  What are the advantages and disadvantages of direct load test for 3 – phase I M?

Advantages

Direct measurement of input and output parameters yield accurate results Aside from the usual performance other performances like mechanical vibration, noise etc can be studied.

By operating the motor at full load for a continuous period, the final steady temperature can be measured.

Disadvantages

Testing involves large amount of power and the input energy and the entire energy delivered is wasted Loading arrangement cannot be provided for motors of large power rating

Q: Explain why the no load current of an induction motor is much higher than that of an equivalent transformer.

In induction motor, due to the presence of the air gap, the magnetizing current that is required to set up the flux is much higher. The working component of the current has to meet the hysteresis loss, eddy current loss, friction and windage losses. Hence the no load current of induction motor is higher.

Q: List out the methods of speed control of cage type 3-phase induction motor?

a) By changing supply frequency

b) By changing the number of poles

c) By operating two motors in cascade

Q: Mention different types of speed control of slip ring induction motor?

a) By changing supply frequency

b) By changing the number of stator poles

c) By rotor rheostat control

d) By operating two motors in cascade

Q: What are the advantages of 3-phase induction motor?

a) It was very simple and extremely rugged, almost unbreakable construction b) Its cost is very low and it is very reliable

c) It has been suffiently high efficiency .No brushes are needed and hence

frictional losses are reduced

d) It requires minimum of maintenance.

Q: What are the different types of stator winding in induction motor?

The different types of stator windings are mush winding, lap winding and wave winding.

Q: Where mush windings are used?

The mush windings are used in small induction motors of ratings below 5 HP.

Q: What types of slots are preferred for the induction motor?

Semi enclosed slots are preferred for induction motor. It results in less air gap contraction factor giving a small value of magnetizing currents, low tooth pulsation loss and much quieter operation(less noise)

Q:  Which part of induction motor has the maximum flux density? What is the maximum flux density in that part?

The teeth of the stator and rotor core will have maximum flux density. The

maximum value of flux density in the teeth is 1.7 wb/m

Q: What are the advantages and disadvantages of large air gap length in induction motor?

Advantage: A large air gap length results in higher overload capacity, better cooling, reduction in noise and reduction in unbalanced magnetic pull.

Disadvantages: The disadvantage of large air gap length is that it results in high value of magnetizing current.

Q:  What happens if the air gap length is doubled?

If the air gap of an induction motor is doubled then the mmf and magnetizing current approximately doubles. Also increase in air gap length increases the overload capacity, offers better cooling, reduces noise and reduces unbalanced magnetic pull.

Q: List out the methods to improve the power factor of the induction motor.

The power factor of the induction motor can be improved by reducing the magnetizing current and leakage reactance. The magnetizing current can be reduced by reducing the length of air gap. The leakage reactance can be reduced by the depth of stator & rotor slots, by providing short chorded winding and reducing the overhang in stator winding.

Q: Why the air gap of an induction motor is made as small as possible?

The mmf and the magnetizing current are primarily decided by length of air gap. If air gap is small then mmf and magnetizing current will be low, which in turn increase the value of power factor. Hence by keeping small air gap, high power factor is achieved.

Q: What are the methods adopted to reduce harmonic torques?

The methods used for reduction or elimination of harmonic torques are chording, integral slot winding, skewing and increasing the length of air gap.

Q:In what respect does a 1-phase Induction motor differ from a 3-phase Induction motor?

Construction wise a plain 1-phase Induction motor is more or less similar to a 3-phase squirrel-cage Induction motor except that  its stator is provided with only 1-phase winding..

Q. What would happen if a 3 phase induction motor is switched on with one phase disconnected?

The motor is likely to burn .

Q. What happens if the air gap flux density in an induction motor increases?

The increase in air gap flux increases iron loss and hence efficiency decreases.

Q: What are the effects of increasing rotor resistance on starting current  and starting   torque?

The additional external resistance reduces the rotor current and hence the current drawn from the supply. It improves the starting torque developed by improving the power factor in high proportion to the decrease in rotor current..

Q: why the three phase induction motor draws heavy current at starting?

At the time of starting, the rotor speed is zero hence slip is 1, the emf induced in rotor winding is very high. Hence rotor current is also very high. The starting current is 5 to 7 times more than its full load current.