Working of Single Phase Induction Motors

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Single phase induction motors require just one power phase for their operation. They are commonly used in low power rating applications, in domestic as well as industrial use. This article is aimed at giving you a conceptual overview of the working of single phase motors.

The following article gives a detailed description of the video lecture.

Parts of a Single Phase Induction Motor

The main components of a single phase motor are the rotor and stator winding. The rotor is the rotating part, the stator winding helps in rotating the rotor. In Fig.1 the iron layer lamina which is fitted inside rotor is not shown, for better viewing of the rotor bars.

Fig.1 Mains parts of single phase induction motor : Rotor and Stator
The winding has got 2 parts; One main winding and an auxiliary winding. The auxiliary winding is placed perpendicular to the main winding. A capacitor is connected to the auxiliary winding.
Fig.2 Single phase motor has got 2 perpendicularly placed wingdings

Working of Single Phase Motor

To understand its working better, let’s assume there is only one coil in the main and auxiliary winding.

Fig.3 We will analyse a case with both the wingdings are having one coil
Assume no current is flowing through the auxiliary winding. AC current passing through the main winding will produce a fluctuating magnetic field.
Fig.3 AC current passing through main winding will produce a fluctuating magnetic field
The working of single phase induction motors is simple. Just put one rotor which is already rotating, inside such a magnetic field. You can note one interesting thing; the rotor will keep on rotating in the same direction.
Fig.4 A rotor with initial rotation will keep on rotating in a fluctuating magnetic field
The reason behind this phenomenon is explained in coming sections.

The Reason – Double Revolving Field Theory

The fluctuating field is equivalent to the sum of two oppositely rotating magnetic fields. This concept is known as double revolving field theory. By looking at the figure below, you can easily understand the theory. Here one fluctuating quantity is represented as a vector sum of 2 oppositely rotating quantities, for 3 time instances.

Fig.5 Fluctuating field at RHS of each figure is equivalent to sum of 2 oppositely rotating magnetic fields
The effect of the rotating magnetic field on the rotor is interesting. Since the magnetic field is varying, electricity is induced in the rotor bars due to electromagnetic induction. In Fig.6 blue arrows on the bars represent current induced. So here is a situation of current carrying bars are which are immersed in a magnetic field. This will produce a force according to Lawrence law, so the rotor will start to rotate.
Fig.6 Effect of RMF on rotor : It will induce a starting torque
But here we have got 2 such oppositely rotating magnetic fields, so the torques produced by them will be equal and opposite. The net effect will be zero torque on the rotor. So the rotor won’t start, it will simply buzz.
Fig.7 Motor will not be able to start here, since there 2 torques which are equal and opposite
But if we can somehow give this rotor an initial rotation, one torque will be greater than other. There will be a net torque in the same direction of initial rotation. As a result the loop will keep on rotating in the same direction. This is the way a single phase induction motor works.
Fig.8 An initial rotation of rotor will produce one torque greater than other

How to Provide Initial Rotation ?

But one big problem remains; how to provide such an initial rotation to the rotor ?. Nikola Tesla, a famous Yugoslav inventor suggested one ingenious solution to this problem.

If we can cancel any of the rotating fields, we will be able to start the motor. The auxiliary winding cum capacitor arrangement is used exactly for this purpose. Auxiliary winding also produces 2 oppositely revolving magnetic fields. One of them will cancel the RMF of the main winding and the other will get added up. The result will be a single magnetic field, which revolves under specific speed. This phenomenon is shown diagrammatically in the following picture.

Fig.9 Effect of auxiliary winding cum capacitor arrangement : One RMF gets cancelled other gets added up
Such a magnetic field can give starting torque to the rotor. Or the motor will self start. After the rotor has attained a specific speed, even if you cut the auxiliary winding, it will keep on rotating, as explained earlier. This cutting action is done through a centrifugal switch.

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