Single Phase Induction Motor

SINGLE PHASE INDUCTION MOTOR

Introduction

For general lighting purpose in shops, offices, houses, schools etc., Single phase supply is commonly used. There are numerous domestic applications like mixer, automatic washing machine etc. Which work on single phase supply, consisting of single phase motors. The power rating of such motors is very small. Some of them are even traditional horse power motors, hence also called fractional kW motors.

Construction of Single Phase Induction Motors

Single Phase Induction Motor has basically two main parts, one rotating and other stationary. The stationary part in single phase induction motors is called stator while the rotating part is called rotor.

The stator has laminated construction, made up of stampings are slotted on its periphery to carry the winding called stator winding or main winding.

The number of poles for which stator winding is wound, decides the synchronous speed of the motor. The synchronous speed is denoted as N_S and it has a fixed relation with supply frequency F and number of poles P. The relation is given by:

N_S = 120 F / P

The induction motor never rotates with the synchronous speed but rotates at a speed which is slightly less than the synchronous speed. The rotor construction is of squirrel cage type. In this type, rotor consists of uninsulated copper or aluminum bars, placed in the slots. The bars are permanently shorted at both the ends with the help of conducting rings called end rings. The entire structure looks like cage hence called squirrel cage rotor. The construction and symbol is shown in the Figure 3.29.

As the bars are permanently shorted to each other, the resistance of the entire rotor is very very small. The air gap between stator and rotor is kept uniform and as small as possible. The main feature of this rotor is that it automatically adjusts itself for same number of pole as that of the stator winding.

Working Principle of Single Phase Motors

In the single phase induction motors, single phase a.c supply is given to the stator winding. The stator winding carries an alternating current which produces the flux which is also alternating in nature. This flux is called main flux. This flux links with the rotor conductors and due to transformer action e.m.f gets induced in the rotor. The induced e.m.f. drives current through the rotor as rotor circuit is closed circuit. This rotor current produces another flux called rotor flux required for the motoring action. Thus second flux is produced according to induction principle due to induced e.m.f. hence the motor is called induction motor.

Important difference between the two is that the d.c. motors are self starting while single-phase induction motors are not self starting. Let us see why single phase induction motors are not self starting with the help of a theory called double revolving field theory.

Double Field Revolving Theory

According to this theory, any alternating quantity can be resolved into two rotating components which rotate in opposite directions and each having magnitude as half of the maximum magnitude of the alternating quantity.

In case of single phase induction motors, the stator winding produces an alternating magnetic field having maximum magnitude of φ_1m.

According to double revolving field theory, consider the two components of the stator flux, each having magnitude half of maximum magnitude of stator flux i.e., (φ_m/2). Both these components are rotating are in opposite directions at the synchronous speed N_S which is dependent on frequency and stator poles.

φ_f = φ_b = φ_1m/2

Let φ_f is forward component rotating in anticlockwise direction while φ_b is the backward component rotating in clockwise direction. The resultant of these two components at any instant gives the instantaneous value of the stator flux at that instant. So resultant of these two is the original stator flux.

At start both the components are opposite to each other. Thus the resultant φ_R = 0. This is nothing but the instantaneous value of stator flux at start. After 90°, the two components are rotated in such a way that both are pointing in the same direction.

Hence the resultant φ_R is the algebraic sum of the magnitude of the two components so

φ_R = (φ_1m/2) + (φ_1m/2) = φ_1m. This is nothing but the instantaneous value of the stator flux θ = 90°

Both the components are rotating hence get cut by the rotor conductors. Due to cutting of flux, e.m.f. gets induced in rotor which circulates rotor current. The rotor current produces rotor flux. This flux interacts with forward component φ_f to produce a torque in one particular direction say anticlockwise direction. While rotor flux interacts with backward component φ_b to produce a torque in the clockwise direction. So if anticlockwise torque is positive the clockwise torque is negative.

At start these two torques are equal in magnitude but opposite in direction. Each torque tries to rotate the rotor in its own direction. Thus net torque experienced by the rotor is zero at start and hence the single phase induction motor are not self starting.