Rotating Magnetic Field & Synchronous Speed | 3 Phase Motors

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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
For this special arrangement magnetic field produced by 3 phase A.C current flowing through it will be as shown at a particular instant. These currents are at 120 degree phase difference with each other. You can compare it with magnetic field produced by a single wire carrying current.
Fig.2 Magnetic field produced by simplified winding and a single wire
Since the AC current in coil varies with time, corresponding magnetic field produced by them will also vary. Following figure shows how magnetic field varies, as current in coil varies.
Fig.3 Variation of magnetic field around the winding with time
Corresponding current values at 3 instances in each winding can be noted from following figure.
Fig.4 Variation of 3 phase AC current with time
You can note from these 3 instances that orientation of magnetic field changes with current, but its magnitude remains same. Or it is a magnetic field of uniform strength rotating. Speed of rotation of magnetic field is known as synchronous speed.

How to Quantify Synchronous Speed

The 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.
Here is a case of winding that will produce 4 poles.
Fig.5 A winding that will produce 4 poles
For the 4 pole system magnetic field will again rotate through 2 poles for full revolution of current. As a result in this case magnetic flux will rotate only half the revolution. So you can note that speed of magnetic field has become half this time. So for general P pole system, synchronous speed is given by following equation.

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
So for a 60 Hertz power supply magnetic field will rotate at 75.36 rad/sec. Rotating magnetic field can make the rotor rotate for induction as well as synchronous motor. Speed of rotation of rotor is a strong function of synchronous speed. So it is quite important to select a motor with suitable number of pole, which will match rotational speed requirement.

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