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Chapter: Electrical machines : Single Phase Induction Motor and Special Machines

Important Short Questions and Answers: Single Phase Induction Motor and Special Machines

Electrical machines - Single Phase Induction Motor and Special Machines - Important Short Questions and Answers: Single Phase Induction Motor and Special Machines




1.         Name the two winding of single phase induction motor?

Running and starting winding.


2.         What are methods available for making single phase induction motor a self starting?

By splitting the single phase, by providing shading coil in the poles.


3.         What is the function of capacitor in single phase induction motor?

To make phase difference between starting and running winding, to improve PF and to get more torque.


4. State any four use of single phase induction motor?

Fans, wet grinders, vacuum cleaner, small pumps, compressors, drills. Explain


5. Why single phase induction motor is not a self starting one?


When motor fed supply from single phase, its stator winding produces an alternating flux, which doesn’t develops any torque.


6. What kind of motors used in ceiling fan and wet grinders?


Ceiling fan # Capacitor start and capacitor run single phase induction motor, wet grinders# Capacitor start capacitor run single phase induction motor.


7. What is the application of shaded pole induction motor?


Because of its small starting torque, it is generally used for small toys, instruments, hair driers, ventilators.etc.


8. In which direction a shaded pole motor runs?

The rotor starts rotation in the direction from unshaded part to the shaded part.


9. Why single phase induction motor have low PF?


The current through the running winding lags behind the supply voltage by large angle so only single phase induction motor have low PF.


10.                        Differentiate between “capacitor start” & “Capacitor start capacitor run” single phase induction motor?


Capacitor start – capacitor is connected series with starting winding, but it will be disconnected from supply when motor pick up its speed. Capacitor start capacitor run# starting winding and capacitor will not be disconnected from supply even though motor pickup its speed.


11.                        What are the principal advantages of rotating field type construction?


Relatively small amount of power required for field system can easily supplied to rotating system using slip rings and brushes, more space is available in the stator part of the machine to provide more insulation, it is easy to provide cooling system, stationary system of conductors can easily be braced to prevent deformation.


12. Why an induction motor never runs at its synchronous speed?


If it runs at sy.speed then there would be no relative speed between the two, hence no rotor emf, so no rotor current, then no rotor torque to maintain rotation.


13. What are the advantages of cage motor?


Since the rotor has low resistance, the copper loss is low and efficiency is very high. On account of simple construction of rotor it is mechanically robust, initial cost is less; maintenance cost is less, simple starting arrangement.


14. Why an induction motor is called as rotating transformer?


The rotor receives same electrical power in exactly the same way as the secondary of a two winding transformer receiving its power from primary. That is why induction motor is called as rotating transformer.


15. What is the use of shading coil in the shaded pole motor?


In shaded pole motors the necessary phase –splitting is produced by induction. These motors have salient poles on stator and a squirrel cage type rotor. The poles are shaded ie each pole carries a copper band one of its unequally divided part is called shading band. When single phase ac supply is given to the stator winding due to shading provided to the poles a rotating magnetic field is generated.


16. Why capacitor –start induction motors advantageous?


In capacitor start induction motors capacitor is connected in series with the auxiliary winding. When speed of the motor approaches to 75 to80%of the synchronous speed the starting winding gets disconnected due to the operation of the centrifugal switch. The capacitor remains in the circuit only at start. The starting torque is proportional to phase angle ά and hence such motors produce very high starting torque.


17. List out 4 applications of shaded pole induction motor?


Shaded pole motors have very low starting torque, low power factor and low efficiency. The motors are commonly used for small fans, toy motors, advertising displays, film projectors, record players, gramophones, hair dryers , photocopying machines etc


18.                        What are the drawbacks of the presence of the backward rotating field in a single phase induction motor?


Due to cutting of flux, emf gets induced in the rotor which circulates rotor current .the rotor current produces rotor flux. This flux interacts with forward component φf to produce a torque in one particular direction say anticlockwise direction. While rotor flux interacts with backward component


φb to produce a torque in the clockwise direction. So if anti clock wise torque is positive then clockwise torque is negative thus net torque experienced by the rotor is zero at start.


19.                        Why is hysteresis motor free from mechanical and magnetic vibrations?


The stator of hysteresis motor carries main and auxiliary windings to produce rotating magnetic field or of shaded pole type also. The rotor is smooth cylindrical type made up of hard magnetic material. The torque in this motor is constant at all speeds it runs at synchronous speed. There is not relative motion between stator and rotor field so the torque due to eddy current vanishes. Only hysteresis torque is present which keeps rotor running at synchronous speeds .the high retentivity ensures continuous magnetic locking between stator and rotor. Hence it is free from magnetic vibrations

20. What types of motor is used in computer drives and wet grinders?


For computer drives permanent magnet dc motors are used while in wet grinder’s universal motor may be used.


21.                        Give two advantages and two applications of stepper motor.



*These motors are compatible with digital equipments and are flexible in operation. *The dynamic response is fast




Stepper motors are widely used in computer peripherals such as serial printers tape drives, floppy disk drivers. They are also used in control of machine tools. Robotics.


22.                        List some applications of linear induction motor?


They are used in machine tool industry and in robotics .They are used in trains operated on magnetic levitation , reciprocating compressors can also be driven by linear motors


23. What are the specific characteristic features of the repulsion motor?


Repulsion motors give excellent performance characteristics. A very high starting torque of about 300 to350% of full load can be obtained with starting currents of about 3 to 4 times the full load current. Thus it has got very good operating characteristics. The speed of the motor changes with load .with compensated type of repulsion motor the motor runs with improved power factor as the quadrature drop in the field winding is neutralized. Also the leakage between armature and field is reduced which gives better regulation.


24. Discuss characteristics of single phase series motor

* To reduce the eddy current losses, yoke and pole core construction is laminated


*The power factor can be improved by reducing the number of turns. But this reduces the field flux. But this reduction in flux increases the speed and reducing the torque. To keep the torque same it is necessary to increase the armature turns proportionately. This increases the armature inductance.

25.                        What are the demerits of repulsion motor?

*very expensive

*speed changes with load

* on no load speed is very high causing sparking at brushes*low power factor on no load


26.                        List four applications of reluctance motors?


This motor is used in signaling devices, control apparatus, automatic regulators, recording instruments, clocks and all kinds of timing devices, teleprinters, gramophones 

27. What is a universal motor?


There are small capacity series motors which can be operated on dc supply or single phase ac supply of same voltage with similar characteristics called universal motors. The construction of this motor is similar to that of ac series motor



27. Define step angle?


It is defined as angle through which the stepper motor shaft rotates for each command pulse.


28.What are different types of stepper motor?


1.                                     Variable reluctance(VR)motor

2.                                     Permanent magnet (PM) stepper motor

3.                                     Hybrid stepper motor

29.What is the advantage in using stepper motor?


1.                                        it can drive open loop without feedback

2.                                        it requires little or no maintenance.



30.Givethe applications of stepper motor?


1.                                                                               Robotics

2.                                                                               Computer peripherals

3.                                                                               Facsimile machine

4.                                                                               Aerospace



31.What are the adv. of reluctance m/c?


1.                                                                 Motor speed is constant

2.                                                                 Simple construction


32.What is Universal motor?


A Universal motor is defined as a motor, which may be operated either on direct current or single-phase ac supply.


33.State some application of universal motor.

Used for sewing machines, table fans, Vaccum  cleaners, hair driers, blowers etc



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