Single Phase Induction Motor Interview Questions

Single Phase Induction Motor Interview Questions

Q. what do you mean by single phase induction motor?

A machine that converts 1 phase ac electrical power into mechanical power by using a electromagnetic induction phenomenon is called a 1 phase induction motor.

Q: 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 develop any torque.

Q: Where do we require single-phase induction motors?

Single-phase induction motors are required where

(i) 3 phase supply is not available

(ii) Efficiency is of lesser importance

(iii) Rating is less than one H.P.

(iv) Equipment is portable

Q: What is a fractional H.P. motor?

A small motor having H.P. less than unit is called fractional H.P. motor.

Q: Which type of rotor is used in single phase motors?

Squirrel cage type

Q: Which type of torque is developed in single phase motors?

Pulsating torque is produced.

Q:Name the two different theories with which principle of 1-phase induction motors are explained.

The two different theories are

  • Double revolving field theory

  • Cross field theory

Q: State double revolving field theory.

According to this theory, alternating flux can be resolved into 2 rotating components, which rotates in opposite direction to each other and magnitude of each component is equal to half of the maximum magnitude of alternating flux.

Q: What are the conclusions from Double revolving field theory?

Forward component of flux produce forward torque while backward component produce backward torque.

At start, magnitude of both torque are equal but their directions are opposite.

hence resultant torque is zero

Thus single phase Induction motor isnt self starting.

Q: Name types of 1-phase induction motors.

Based on the method of starting arrangement provided, the 1-phase Induction

motors are classified as follows

(i) Split-phase motor

(ii) Shaded pole motor

(iii) Resistor start Induction run single phase IM

(iv) capacitor start Induction run single phase IM

(v) capacitor start Capacitor run single phase IM

Q What are the inherent characteristics of plain 1-phase Induction motor ?

1-phase Induction motor is not used in practice due to the following inherent characteristics

· 1-phase Induction motor does not have any starting torque

· However, if the rotor is initially given a starting torque, by some means, the motor can pick up its speed in a direction at which the initial torque is given and deliver the required output

Q: What are methods available for making single phase induction motor a self starting?

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

Q: 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 staRting torque is developed.

Q: what do you mean by shaded pole motor?

These motor have projected pole with 1/3 pole part rapped with a copper strip. This portion of the pole is called shaped part of the pole because of this a revolving field is set up in the Stator and torque is developed.

Q: In which direction does a shaded pole motor run?

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

Q: What is the principle of operation of shaded-pole induction motor?

A shaded-pole motor is basically a small single-phase squirrel cage motor in which the starting winding is composed of short-circuited copper ring (called shading coil) surrounding one-third of each pole.

The effect of the shading coil is to cause a flux to sweep across the pole faces, from unshaded to shaded portion of the pole, producing a weak rotating magnetic field. As a result, the rotor is set in motion due to induction principle.

Q: what is the application of shaded pole induction motor?

Because of its small starting torque, it is generally used for small fans, toys, instruments, hair driers, tape recorder, advertising display, ventilators, electric clock etc.

Q: What is size of shaded-pole motor?

These are usually built in small fractional H.P, not exceed 1/4 H.P.

Q: Name the two winding of single phase induction motor?
Running and starting winding.

Q: Why are centrifugal switches provided on many 1-phase Induction motors?
Centrifugal switches are provided on many 1-phase Induction motors to disconnect the starting / auxiliary winding from the supply when the motor reaches about 70% of its synchronous speed.

Q: What is the function of centrifugal starting switch in a single-phase induction motor?

Ans: The centrifugal switch is connected in series with the starting winding. The primary function of the centrifugal switch is to produce rotating flux in conjunction with main winding at the time of starting.

When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross field effect. The starting winding now has no function to perform and is removed from the circuit by a centrifugally operated switch.

Q: What happens when the centrifugal starting switch fails to open?

Ans: If the starting switch fails to open when needed, then the starting winding will overheat and burn out and motor will not start next time.

Q: why single-phase induction motor has low power factor?

The current through the running winding lags behind the supply voltage by a very large angle. Therefore power factor is very low.

Q:How the starting winding produce rotation in a single phase resistance start induction motor?

The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.

Q: How the starting winding is made resistive?

It consists of only few turns of smaller diameter.

Q : Why are resistance split-phase inductions motors most popular?

These motors are most popular due to their low cost. They are used where moderate starting torque is required and where the starting periods are not frequent.

They drive fans, pumps, washing machines, small machine tools etc. They have power rating between 60 watts and 250 watts.

Q: What is the draw back of the resistance split-phase induction motor?

The starting winding has a relatively small number of turns of fine wire and its resistance is higher than that of the main winding.

Therefore the current density is high and the winding heats up quickly. If the starting period lasts for more than 5 seconds, the winding begins to smoke and may burn out unless the motor is protected by a built-in-thermal relay.

Q: Why is the starting torque of a resistance split-phase induction motor not high?

The starting torque is given as, Ts = K Im Is Sin Ф


        K = constant whose magnitude depends upon the design of the motor

(i) The angle between Is and Im is small (approximately 25 degree) in a resistance split-phase induction motor, so the starting torque is small.

(ii) Since currents Is and Im are not equal in magnitude, the rotating magnetic field is not uniform and the starting torque produced is small.

Q: Give applications of resistance start Induction run motor?

Fans, Lathe machines, blowers, refrigerator , separators, washing machine etc

Q: 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

Q: Why is the starting torque of a capacitor start induction motor high?

Ans: The capacitor C in the starting winding is so chosen that Is leads Im by 75 degree. Since the starting torque is directly proportional to Sin Ф, and it is quite high in capacitor-start induction motor.

Q: Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?

Ans : Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

Q: How is the direction of a capacitor start Induction motor be reversed?

The direction of rotation can be reversed by interchanging the terminals of either the main winding or the starting winding.

Q: Give applications of Capacitor start Induction run motors.

Drilling machine, centrifugal pump, grinders , refrigerator etc

Q: Give application of capacitor start capacitor run Induction motor

Ceiling fan, air conditioner, blowers, table fans, conveyors etc

Q: what is the type of induction motor used in wet grinders?

Capacitor start capacitor run single-phase induction motor.

Q: what kind of motor is used in mixie?

Single-phase ac series motor is used in mixie.

Q: Differentiate between “capacitor start “and “capacitor start capacitor run“ induction motor?

In capacitor start motor, capacitor is connected in series with the starting winding. But it will be disconnected from the supply, when the motor picks up its speed. But in capacitor start capacitor run motor starting winding and capacitor are not disconnected, but always connected in the supply .so it has high starting and running torque.

Q : Is there any relation between the capacitances of two capacitors used in two value capacitor start capacitor run motor?

Ans: Starting capacitor has about 10 – 15 times high capacity than the value of running capacitor.

Q: 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.

Q : 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 to 80% 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.

Q: If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?

Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip.

Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.

Q: How the speed of rotation of a split phase induction motor is reversed?

The terminal connections of the starting windings are reversed with respect to main running windings…

Q: How speed control is made in single phase motors?

It is usually controlled by applying a variable voltage from tapped transformers, variacs, potentio meters, and tapped reactors.

Q: Why shaded-pole single phase induction motor does not need any special starting technique like capacitors and auxiliary winding etc.

Because it is inherently self started motor. The construction of the poles is such that they give a sweep to the magnetic flux and motor starts rotating.

Q: Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?

Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

Q. After servicing a single phase fan it was found to run in reverse direction. What could be the reason?

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

Also Read about : Single Phase Induction Motor Principle

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