How synchronous reluctance motor work?

December 16, 2020


Yogeshwari S Gaddam, B.E in Electrical Engineering Currently she is working at Imajey consulting engineers pvt. ltd. as a Team lead for Visual Education. She is passionate about understanding the physics behind complex technologies, such as motors: Induction motors, SynRM motors, and IPM-SynRM motors and explaining them in simple words. To know more about the author check this link

Introduction to working of SynRM:

In this article I will take you on a journey through the details of Synchronous reluctance motor. As the name suggests these motors work on the Reluctance torque. Put simply Reluctance for magnetic flux is similar to resistance for current. Magnetic fields always want to pass through the path where it Experiences least opposition more technically least reluctance. SynRMs have two major parts: stator and rotor. The rotor is simply made of iron.

Here iron gives less opposition to magnetic fields than air. Using this phenomenon SynRM motors generate rotation. To get this rotational motion, we cut slots in the rotor. when the magnetic field is non-aligned to these slots/cuts, the rotor rotates. This is because the magnetic field wants least reluctance. This rotation is definately by the virtue of temporary magnetic poles induced in the rotor. Because Iron has a domain-based structure which I can hypothetically say are small micro level magnets. Let me take you a bit deeper.

Fig:1 Two types rotor of SynRM

Working of SynRM

Now let me explain to you the working of SynRM in detail. You can see a two pole Rotating magnetic field here. As I have illustrated in the Fig below, the Interaction between the magnetic field and the rotor bar area is maximum at one angle as I have illustrated in the left image and very low at another angle seen in the right image below.

Fig:2 The rotor bar area is maximum at one angle and very low at another angle

With this analogy in mind, if i use a Rotating magnetic field(RMF) here. Amazingly, the rotor will try to rotate along with the magnetic field. The rotor tries to rotate with the RMF, as you can see in the Fig:3 below.

Fig:3 The rotor is try to rotate as well with it to align with RMF

We can conclude that the rotor always aligns itself to the flux path. You might be familiar with the donkey-carrot analogy; this case is the same. With the carrot tied to the donkey's head, as fast as he moves he won’t be able to eat it. So even though the rotor tries to align, since the RMF is rotating it will never catch up. Therefore, we need to design our rotor cuts/ barriers as per the shape of RMF.

So let's see how a most common 4 pole type RMF is shaped like?

To precisely know the shape of RMF I have used FEA simulation to produce a minimum flux path OR shape of RMF. Here I have used FEA software EM Work 2D by Solidworks that accurately simulates the results illustrated in Fig:4.

Fig:4 FEA illustration

By this FEA shape the Rotor barriers are designed as seen below.

Fig:5 FEA shape the Rotor barriers

Did you know why SynRm is not self started? I will discover this In the below topic.

Why is SynRM not self starting?

Do you have a question, why SynRM is not self started? your answer is here, There are two factors:

1.Speed of RMF

Consider Two pole rotating magnetic fields here,N pole approaches above the rotor, the iron bar’s domains start to align, and the opposite poles will have attractive force between them(Fig:6A). Now the rotor should rotate. The rotor does rotate, but this is because of the rotor's inertia, which causes it to achieve a very low speed compared to the RMF(Fig:6B).

By this time, the succeeding S pole will come upon the rotor, which causes a repulsive action. Thus, as the S pole approaches, the rotor poles are unable to change quickly, that’s why repulsive force will occur. This is the one reason behind it.

Fig:6A The iron bar’s domains start to align
Fig:6B Because of the rotor's inertia,it achieve a very low speed


Another one is, Domains take time to spin, this is one of the reasons why SynRMs are not self starting. This is a well-known phenomenon called hysteresis. Now let's move to the starting methods of SynRM.

Fig:7 Hysteresis

Starting method of SynRM:

As we know that, SynRM’s are not self started, there are some methods to start, SynRM can start using frequency controllers Or squirrel cage rotor bars, etc. In this section we will learn how SynRM starts using frequency controllers?

I will explain to you the most usable way to start this motor, by controlling the frequency of input currents. While starting the motor, RMF has to slowly speed up as shown in the Fig:8A. For your better understanding now I test this method. I can easily control the speed of RMF by varying the frequency of the input current. Initially RMF speed is almost zero, their opposite polarity magnetic pole induces in the rotor and it becomes attracted to RMF and slowly starts accelerating(Fig:8B).

Fig:8A The way to make this motor self start,is to reduce
the speed of RMF during start and then gradually speed it up
Fig:8B Initially the RMF speed is almost zero

Now lets understand, a smart controller device detects the position of the rotor. Depending upon this position, the controller will adjust the RMF speed, and there will always generate attractive force between the rotor and RMF. As the rotor speeds up, the controller increases the speed of RMF as well. Thus the rotor runs in synchronism. That’s it about the starting methods of SynRM. Now we will understand how SynRM can be controlled? Let's see in the next topic.

How are SynRM's controlled?

When adding a load on the motor, it results in the rotor rotating behind the field at an angle. If you remember from my last analogy of the donkey and carrot, there is always some distance between. Similarly, here we have some angle between rotor and RMF. This angle is known as “load angle.(Fig:9)

Fig:9 The load angle

In SynRM's normal operation, controllers play an important role. These include to manage the load on the rotor, selecting and regulating speed of the rotor. Now to simplify our explanation I take an example, suppose load on the motor were to increase suddenly, obviously the load angle will also increase, if the load angle crosses its critical limit the rotor will slip out of synchronism and come to a halt as shown in the Fig:10 below.

Fig:10 The rotor will slip out of synchronism and come to a halt

The controller comes to the rescue in such situations. It continuously measures the rotor’s position, efficiently adjusts the angle and magnitude of alternating current, and makes sure that the load angle is always below the critical limit. Clearly, SynRMs are software-powered. That’s it about how SynRM controlled?

Fig:11 If the load angle crosses their critical limit the rotor will slip out of synchronism and come to a halt

Advantages of SynRM over Induction motors!

SynRMs have started replacing induction motors in most of the industries due to its remarkable performance. In the electrical world SynRM has high demand. Now I will present some pros and cons of SynRM motors over induction motors.


No I squared R losses

SynRMs always run at synchronous speed, the speed of RMF.

SynRM’s rotor has simple construction, no magnet and short circuited winding.

SynRM has low manufacturing cost

Higher efficiency at the same power ratings

Induction motors

I squared R losses.

The induction motor always runs at speed less than its synchronous speed.

In an induction motor the rotor winding is of the squirrel-cage type.

Induction motor has High manufacturing cost.

Less efficiency at the same power ratings.

Disadvantages of SynRM

1. SynRM is Not self started.

2. Higher cost than induction motor.

3. poor power factor.

We hope you now have a good understanding of how synchronous reluctance motors work.

If you want to learn from the basics, Here our previous article will be helpful for you.

Other more articles:

Single phase induction motor

Dc motor, how it works?


imajey consulting engineers pvt ltd