Speed & Rotating Magnetic Field in 3 Phase Motors

We recently posted an article demonstrating how generators work and an overview of a Kaplan Turbine.

Today, let’s take a look at the rotating magnetic field and synchronous speed in a 3 phase electric motor:

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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.

fig01 simple wind Speed & Rotating Magnetic Field in 3 Phase Motors

Figure 1: A simplified 3 phase winding

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.

fig2 simple winding magnetic field around conductor Speed & Rotating Magnetic Field in 3 Phase Motors

Figure 2: Magnetic field produced by simplified winding and a single wire

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.

fig3 Rotating magnetic field Speed & Rotating Magnetic Field in 3 Phase Motors

Figure 3: Variation of magnetic field around the winding with time

Figure 4 depicts the corresponding current values at three instances in each winding.

fig04 ac current Speed & Rotating Magnetic Field in 3 Phase Motors

Figure 4: Variation of 3 phase AC current with time

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.

fig05 4 pole winding Speed & Rotating Magnetic Field in 3 Phase Motors

Figure 5: A winding that will produce 4 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.

eqn1 Speed & Rotating Magnetic Field in 3 Phase Motors

Variations in Number of Poles

In an actual motor, there can be two to 14 poles. Figure 6 shows a motor with ten poles.

fig06 10 pole motor winding Speed & Rotating Magnetic Field in 3 Phase Motors

Figure 6: A motor winding that will produce 10 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.

 

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