Rotating Magnetic Field and Synchronous Speed | Single Phase Motors

August 15, 2013

Single phase AC motors use rotating magnetic field for its starting. The way rotating magnetic field produced in a single phase machine is quite different from that of a 3 phase machine. In this article, we will see how rotating magnetic field is produced in a single phase motor.

Rotating magnetic field from single phase current

Single phase current passing through a coil will produce a fluctuating magnetic field.

Fig:1 Fluctuating magnetic field produced by a single phase current

But, how to make such a field rotating? To make so, Nikola Tesla a famous Yugoslav inventor came up with one ingenious solution. Put one more winding which is perpendicular to first winding. Bypassed current from same power source is passed through second winding. So in second winding magnetic flux will fluctuate perpendicular to magnetic field of first winding.

Fig:2 In secondary winding magnetic flux will fluctuate perpendicular to first winding flux

Now let’s include a capacitor in second winding circuit. Presence of capacitor will make sure that this current is at 90 degree phase difference from first winding current.

Fig:3 A capacitor in secondary winding will make sure that the fluctuating fluxes are in 90 degree phase difference

Variation of resultant magnetic field with time

Resultant magnetic field is sum of these two fields. Now note the variation of resultant magnetic field with time.

Fig:4 Resultant is sum of 2 fluctuating fields, this is same as rotating magnetic field

You can note that with time magnitude of resultant magnetic field remains same, but orientation changes. This is same a rotating magnetic field.This is the way rotating magnetic field is produce in single phase motors. The same concept can be explained through double revolving field theory. Speed of rotation of magnetic field is known as synchronous speed.

Fig:5 Magnetic field rotates in the direction of arrow

Different number of poles

Winding in Fig.5 produced 2 poles. In actual machines there could be many poles, so that synchronous speed can be varied. Following figure shows a 4 pole magnetic field.

Fig:6 A 4 pole rotating magnetic field

Such a magnetic field can start rotor of single phase motor.


This article is written by Sabin Mathew, an IIT Delhi postgraduate in mechanical engineering. Sabin is passionate about understanding the physics behind complex technologies and explaining them in simple words. He is the founder of Learn Engineering educational platform. To know more about the author check this link