Today, let’s take a look at the rotating magnetic field and synchronous speed in a 3 phase electric motor:
Whether synchronous or induction, 3 phase electric machines use rotating magnetic fields for its operation.
Here’s a detailed look at the video.
A Simplified 3 Phase Winding
To understand the concept of a rotating magnetic field for 3 phase machines, view Figure 1. It has just three coils arranged 120° apart.
For this arrangement, a magnetic field is produced by 3 phase AC flowing through it, refer to Figure 2. These currents are at a 120° phase difference with each other. Compare it with magnetic fields produced by a single wire carrying current.
Since the AC in coil varies with time, corresponding magnetic fields produced by them also vary. Figure 3 shows how magnetic field varies, as current in coil varies.
Figure 4 depicts the corresponding current values at three instances in each winding.
Note from these three instances that the orientation of the magnetic field changes with current, but its magnitude remains same. The speed of rotation in the magnetic field is known as synchronous speed.
Quantifying Synchronous Speed
The big question, how to quantify synchronous speed? The simplified winding case had just two magnetic poles. Magnetic flux is able to rotate through two poles when current take one complete revolution. For a two pole case, synchronous speed is same as the frequency of AC current.
Figure 5 depicts a winding producing four poles.
For the four pole system,the magnetic field rotates through two poles for a full revolution of current. As a result in this case, magnetic flux rotates only half of a revolution. The speed of the magnetic field has reduced to half its size 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 two to 14 poles. Figure 6 shows a motor with ten poles.
For a 60 Hertz power supply, the magnetic field rotates at 75.36 rad/sec. Rotating magnetic fields can make the rotor rotate for induction as well asynchronous motors. The speed of rotation for a rotor is a strong function of synchronous speed. So it is important to select a motor with a suitable number of pole, which matches rotational speed requirement.