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Chapter: Electrical machines : Three Phase Induction Motor

Important Short Questions and Answers: Three Phase Induction Motor

Electrical machines - Three Phase Induction Motor - Important Short Questions and Answers: Three Phase Induction Motor



1. What are types of 3- phase induction motor?


i. Squirrel cage induction motor ii. Slip ring induction motor


2. Why the rotor slots of a 3-phase induction motor are skewed?


The rotor slots of a three -phase induction motor are skewed


i. to make the motor run quietly by reducing the magnetic hum ii. to reduce the locking tendency of the rotor


3. 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.


4. 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.


5. State the difference between slip ring rotor and cage rotor of an induction motor?


Slip ring rotor has 3-phase windings. Three ends of which are stared and the other three ends are brought up and connected to 3 slip rings mounted in the shaft. Extra


resistance can be added in the rotor circuit. Squirrel cage rotor has short-circuited copper bars.

Extra resistance can’t be added as slip ring rotor.

6. Write an expression for the slip of an induction motor.


Percentage slip = (Ns -Nr) / Ns * 100.


7. What is cogging of an induction motor?


When the number of stator and rotor teeth’s is equal or integral multiple of rotor teeth


,they have a tendency to align themselves exactly to minimum reluctance position. Thus the rotor may refuse to accelerate. This phenomenon is known as cogging.


8. 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.


9. State the effect of rotor resistance on starting torque


Starting torque increases with increase in value of rotor resistance.


10. What are the advantages of cage motor?


Ø Since the rotor has very low resistance, the copper loss is low and efficiency is high


Ø On the account of simple construction of rotor, it is mechanically robust.


Ø Initial cost is less.


Ø Maintenance cost is less.


Ø Simple stating arrangement


11. Give the conditions for maximum torque for 3-phase induction motor?


The rotor resistance and rotor reactance should be equal for developing maximum torque i.e. R2 = s X2 where s is the slip –under running conditions.


R2 = X2 under starting conditions


12. What is reason for inserting additional resistance in rotor circuit of a slip ring induction motor?


Introduction of additional resistance in the rotor circuit will increase the starting torque as well as running torque. Also it limits the starting current, improves the power factor.


13. 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


14. 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


15. 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.


16. What does crawling of induction motor mean?


Squirrel cage type, sometimes exhibit a tendency to run stably at speeds as low as


1/7 the of their synchronous speed, because of the harmonics this phenomenon is known as crawling


17. State the application of an induction generator?


a) Used in windmill for generating electric power.


b) Used in regenerative breaking places like traction.


18. Name the two windings of a single-phase induction motor.


I. Running winding ii. Starting winding.


19. What are the various methods available for making a single-phase motor self-starting?


I. By splitting the single phase into 2 phases ii. By providing shading coil in the poles.


20. What is the function of capacitor in a single-phase induction motor?


I. To make more phase difference between the starting and running winding. ii. To improve the power factor and to get more torque.


21. Give the names of three different types of single-phase motor.


I. Split phase motor


ii. Shaded pole motor.


iii. Single phase series motor. iv. Repulsion motor.


22. What is the use of shading ring in a pole motor?

The shading coil causes the flux in the shaded portion to lag behind the flux in unshaded portion of pole. This gives in effect a rotation of flux across the pole face and under the influence of this moving flux a stating torque is developed.


23. State any four use of single-phase induction motor.

Fans, Wet grinders, Vacuum cleaners, small pumps, compressors, drills


24. Why is the efficiency of a 3-phase induction motor less than of a transformer?


In induction motor, there is a mechanical loss due to the rotation of the rotor. Hence the efficiency of an induction motor is less than that of the transformer.


25. What are the types of starters?


Stator rheostat, Autotransformer and Star to Delta switch


Rotor resistance starter.





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


(a)                                                    No load test

(b)                                                    Blockedrotor test


27.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


28.What are the informations obtained from blocked rotor test in a3-phase I M?


(i) Blocked rotor input current perphase at normal voltage

(ii)                                                     Blocked rotor power factor and hence phase angle

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

motor asrefered to the stator



29.What is circle diagram of an I M?


When an IM 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.



30.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.



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





Direct measurement of input and output parameters yield accurate Results Aside from the usual performance other performances like mechanical Vibration, noise etcc anbestudied. 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 can not be provided for motors of large power rating



32.What type of single phase induction motor would you use for the following applications?


(i)Ceiling fan

(ii)Wet grinder Ceiling fan– capacitor start and run motor Wet grinder–capacitor start motor


33.After servicing a single phase fan it was found to run inreverse direction. What could be the reason?


The connection to the starting/ auxiliary winding would have reversed.


34.What will be the direction of rotation of a shaded pole single phase induction motor?


The motor rotates in the direction specified by the unshaded to shaded region in the pole phase


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