Rotating magnetic field(RMF) and its properties

February 17, 2020

Overview of Rotating magnetic field(RMF)

The rotating magnetic field is a resultant magnetic field, produced by supplied current passing via 2phase or 3phase windings, these windings and current produces RMF. We have understood the production of RMF in our previous article, I hope it is clear to you. But now one more important topic is here, which I want to discuss with you is the properties of the rotating magnetic field(RMF).

Before moving to the main topic of this article, we just spend only a few minutes to review what we have learned in the first part.

DC RMF, 2-phase RMF, 3-phase RMF

In our previous article Rotating magnetic field and its production I had mentioned DC RMF(Fig:1A), Walter Bailey has used DC currents and this arrangement had electromagnets and commutator rings, to generate RMF. But it was very tedious to operate that’s why It was replaced with 2-Phase RMF(Fig:1B) which used AC current supply invented by Nikola tesla. It was more popular but 2-Phase arrangement gives a dip in the rotating magnetic field and in the 3 phase design these oscillations are effectively reduced. That’s why it was replaced with 3-Phase RMF, invented by Mikhail dobrovolski. This 3phase RMF is generated using AC supply and windings that are displaced by 120º apart.

Fig:1A - Dc RMF
Fig:1B - Commutators has been removed in 2phase RMF

If you want to know who is the winner of RMF’s system? I have given FEA simulation of 2-phase RMF and 3-phase RMF below, where you can understand better.

Fig:2 - FEA simulation

I hope you understood this short summary. Now we are moving to our main topic: Properties of RMF.

Properties of Rotating Magnetic Field(RMF)

Magnetic field created by Permanent magnet and RMF. Do you know how it created? I will explain to you in simple manner.

1.Permanent magnet:

Permanent magnet is a type of magnet, made by ferromagnetic materials, such as iron, nickel and cobalt. It is able to create a strong magnetic field(Fig:3A). Now I can see here the magnetic field's direction is from the North pole and they loop around and then enter into the South pole, i have illustrated in the Fig:3B below. Now i want to discuss with you, how a magnetic field is create? What process would there be? To know the reason behind this, you have to learn about the inside structure of PM, You can see below, i have elaborate in simple words.

Fig:3A - A PM magnet is able to create a strong magnetic field
Fig:3B - Magnetic field lines comes from N pole and enter into the S pole

The inside structure of Permanent magnet

We have seen permanent magnets are material which is made of iron, nickel, cobalt. Please keep this in your mind there are numbers of domains inside the material. Domain is a small region, containing billions of atoms, where electrons are revolving in the same direction. All magnetic domains align with each other, and all domains are in different directions(Fig:4A). That’s why their magnetism is zero. When any other material comes around to the PM, domains line up in the same direction (Fig:4B). Then that material will become magnetized and create a strong magnetic field. I hope you have got your answer here. Now let's move to the RMF.

Fig:4A - Domains are in different directions
Fig:4B - Domains are in a single direction

2.Rotating magnetic field(RMF):

Magnetic fields created by the PM and RMF. Both magnetic field’s directions are opposite to each other as shown in the Fig:5A. PM’s Magnetic field direction is North to South and RMF’s direction is South To North. Here we will find out the reason behind this. When a 3 phase or 2phase AC is supplied to the windings. It produces a rotating magnetic field around it. This outside field of an RMF is never used to get any useful work. Because if you focus on this useful internal area of an RMF, there is a rotor inside it. Practically we can mark N & S pole according to the magnetic field direction as shown in the Fig:5B. It is quite logical to define the north and south poles as shown. Only if we mark the north and south poles as shown in the fig below, then we will get the magnetic field direction correct in the inside area you can see in the Fig:5B below.

Fig:5A - Both magnetic field’s directions are opposite to each other
Fig:5B - N & S pole marking according to the magnetic field direction

Now we have seen the properties of PM and RMF, what you have observed?

1. PM magnet's magnetism is always on, while RMF is temporarily magnetized until AC supply is there.

2. The Permanent magnets that you have affixed to your refrigerator, whereas RMF is used in AC machines, induction motors, and alternators.

3. The poles of a Permanent magnet can not be changed. The poles of an RMF can be altered.

4. Loss of Magnetic Properties:- Permanent magnet loses its magnetism, as it does by heating to a maximum temperature. RMF loses its magnetic power, when AC supply is removed.

Now i explain the last part of this article, is a synchronous speed.

What is Synchronous speed?

In AC machines, such as induction motor, synchronous motor, alternator, RMF is generated in the stator winding. The speed of rotation of the RMF, is called synchronous speed. It mainly depends upon the supply frequency and number of poles on the stator. We can measure synchronous speed by using this formula Ns=120*F/P.

Fig:6 - Synchronous speed

Ns= Synchronous speed

F= Frequency(50Hz in india)

P= No. of poles

Motors can be different no. of poles- 2, 4,6 poles.

Eg. if we take induction motor to calculate synchronous speed, then

Ns= 120*50Hz/4

Ns= 1500

You have to know, the actual speed of the motor is less. Because there is friction inside the motor, that is why their speed is a little less.

That’s all about the rotating magnetic field and its property. I hope you understood and enjoyed this explanation of RMF and its properties.

You may like these Articles:

Rotating magnetic field, how it is produce?

Tesla-model-3's IPM-SynRM electric motor


Yogeshwari S Gaddam, B.E in Electrical Engineering, Currently she is working at Lesics engineers Pvt. Ltd as a Team lead for Visual Education. Each day she encounters new challenges and loves the complexity that each project requires. Her area of interest in electrical engineering but also she is focusing on understanding the complex technology behind physics and explaining them in simple words. Yogeshwari has done projects such as Tesla model-3's motor(IPM-SynRM), RMF, SynRM motors, Etc.
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