Whether synchronous or induction, 3 phase electric machines use rotating magnetic field for its operation. In this article we will understand how rotating magnetic field is produced in a 3 phase motor. We will also come across concept of synchronous speed and number of poles on a motor.Detailed description of the video lecture is given following article.
A Simplified 3 Phase Winding
To understand concept of rotating magnetic field for 3 phase machines consider the simplified winding shown below. It has got just 3 coils. They are arranged 120 degree apart.
|Fig.1 A simplified 3 phase winding|
|Fig.2 Magnetic field produced by simplified winding and a single wire|
|Fig.3 Variation of magnetic field around the winding with time|
|Fig.4 Variation of 3 phase AC current with time|
How to Quantify Synchronous SpeedThe big question, how to quantify synchronous speed? The simplified winding case had just two magnetic poles. You can note that, magnetic flux is able to rotate through 2 poles when current take one complete revolution. So for 2 pole case synchronous speed is same as frequency of AC current.
|Fig.5 A winding that will produce 4 poles|
Variations in Number of Poles
In an actual motor there can be 2 to 14 poles. This motor is having 10 poles.
|Fig.6 A motor winding that will produce 10 poles|