Single Phase Induction Motor: Construction, Working, Torque-Speed characteristics, Types, Applications.

Introduction

In this post discuss about operating principle, working, types and application of single-phase induction motor.

Single Phase Induction Motor

Windings:

A single phase induction motor has two main parts namely stator (the one which is stationary) and rotor

The stator winding is connected across a single phase ac supply. The ac supply produces a rotating magnetic field in the air gap between the stator and the rotor.

The field rotates at a speed called synchronous and it is denoted speed by Ns. It is mathematically expressed as:

Ns = 120f/p R.P.M.

The induction motor actually slightly less rotates at a speed which is than the synchronous speed.

Construction

The rotor is said to be of squirrel cage type. As the construction of rotor appears to be like cage, the name the squirrel's squirrel cage rotor has been used.

Construction of a squirrel cage rotor

The rotor consists of copper or aluminum bars which are permanently short circuited at both ends using conducting rings called end rings.

Operation

A single phase ac supply is connected to the stator winding. This forces an alternating current through the stator winding.

This current produces an alternating between the stator and rotor.

The alternating flux passes over the stationary rotor conductors and induces an emf into it due to the transformer action.

Due to this induced voltage, a current starts flowing through the rotor conductors. This current will these produce its own flux called as the rotor flux.

The main flux produced by the stator winding interact with the rotor flux to produce a torque which will make the rotor to rotate in same direction as that of the RMF.

The rotor flux gets produced due to the principled induction hence the name of the motor is induction motor.

It is important to note that the single phase induction| motor are not self starting, like the dc motors.

Principle

Let us apply this theory to the single phase induction motors. Assume that the stator winding produces an alternating flux with the peak value of Φm and magnitude Φm/2

The resultant of these two components will be the resultant stator flux, at any given instant of time and the continuous rotation of the two components will result in the original stator flux.

Torque Speed Characteristics

The torques produced by the two oppositely rotating flux components, are exactly opposite to each other.

The resultant torque is equal to zero at N = 0 i.e.. at the time of starting. Hence the induction motor is not self starting.

But if we rotate the rotor in any direction by some external means then the motor will start rotating in that direction.

But practically it is not possible to rotate the motor by giving the initial thrust therefore it is necessary to modify the construction of the single phase induction motors, in order to make them self starting.

Torque Speed Characteristics

Types of Single Phase Induction Motors

There are various methods used for obtaining them phase shifted currents:

1. Resistance split phase motor.
2. Capacitor split phase motor.

The methods used to make an induction motor self starting are as follows. Based on these technique the single phase induction motor are classified as follows

1. Split phase induction motor.
2. Capacitor start induction motor.
3. Capacitor start, capacitor run induction motor.
4. Shaded pole induction motors.

Applications

1. Grinders
2. Refrigerators
3. Conveyers
4. Fans and air conditioners
5. Compressors