PUBLISHED ON December 16, 2020   Electrical Machines

Basics of synchronous reluctance motor

Synchronous reluctance motors are the electrical machines, used to convert electric energy into mechanical energy. It is a new giant of the electrical world. It is fascinating how these motors are superior to Nicola Tesla’s induction motors! the one being used for more than a century now.Although SynRM’s were invented in the 1800's, they found their light of the day with the digital revolution in the 1960's, the modern day advanced electronics, controls SynRM’s. You will be amazed to know that Tesla’s model 3 car uses an advanced version of these SynRM motors. Although SynRM’s were invented in the 1800's, they found their light of the day with the digital revolution in the 1960's, the modern day advanced electronics, controls SynRM’s. You will be amazed to know that Tesla’s model 3 car uses an advanced version of these SynRM motors(Fig 1a).

In this article I will take you on a journey through understanding SynRM motors, what is reluctance? about the domain structure, also we will learn why SynRM's rotor is shaped as it is and their applications( Fig 1b). Now first I will explain to you the basic physics of SynRM.

The basic Physics of synchronous reluctance motor

To understand the physics behind the torque production (on just an iron rotor!) in SynRMs let me first explain to you two things, the reluctance phenomenon and the attraction of iron to the magnetic fields.

1. What is reluctance?

Reluctance for magnetic flux is a property similar to resistance for current. I can simply say reluctance is opposition to the magnetic flux lines. Magnetic flux always has a tendency to flow through the path of least reluctance. Therefore I can say maximum flux passes through the iron nail, that is because its reluctance is less(Fig 2). Whereas air has more opposition or high reluctance than iron. This states it all.

2. How does an iron attract magnets?

Now that you know maximum flux passes through iron than through surrounding air, how does this magnet generate a force on the nail? This is the common phenomenon of magnetism we all observe in day to day life. Do you know why that is so? Let’s see. Iron has a domain structure, as shown in Fig 3a, I can say that domains are small magnets inside the iron. But Naturally these domains are arranged randomly and thus cancels out the total field. But as permanent magnet’s magnetic flux flows through the iron nail, the same domains are aligned in a single direction. Obviously the entire iron nail will then have a resultant magnetic field, and it acts like a temporary permanent magnet (Fig 3b). Rest you can guess, an attractive force will be generated between the nail and the permanent magnet.

What is reluctance torque?

Now i will explain to you about reluctance torque with the help of experiment, Here i have considered a solid iron bar, which is free to rotate, as shown in Fig:4. To generate torque, I will keep an electromagnet, offset to the iron bar. We all know that the iron bar will definitely be attracted to the electromagnet due to the reluctance force, and it will rotate. That's exactly what we call the reluctance torque. However, after being aligned with the magnetic field, the torque on the iron bar becomes zero. This is a crucial concept you have to note. When the iron bar and the magnetic field are perfectly aligned, the torque on the rotor will be zero, this is illustrated in the Fig 4.

You might have observed, the iron bar here rotated in alignment to the magnetic flux lines. Summarizing it in a simple way the iron rotor of SynRM will align itself with the natural shape of the magnetic field. If I keep this rotor and introduce a magnetic field that is rotating in nature! You guessed it right, a rotating magnetic field produced by 3 phase coils. Our rotor will rotate behind the magnetic field to catch it up. Unfortunately it can never catch it up, so we’ve achieved our rotational torque(Fig 5). Let’s design our practical rotor in the next bit.

The rotor of SynRM

You now know, our basic iron rotor aligns itself to the natural path of the magnetic field. And if we rotate this magnetic field; the rotor will rotate along with it, to be in a minimum reluctance state. It is quite logical to design the rotor as per the path of RMF. How is the natural path of RMF? Using FEA I have deduced that. Please note that the RMF is for a 4 pole type (Fig 6).

Alternate layers of magnetic material as iron and non-magnetic materials as inconel is arranged along the magnetic field lines.The non-magnetic materials will oppose the RMF passing through it as shown in Fig 7. This ensures the rotor tries to rotate to achieve minimum reluctance state.

The modern SynRMs use a slightly different design, the rotor design based on slotted cuts and thin laminations. These laminations reduce the iron losses of the motor. There are no magnets or short circuited windings. There are types of rotor of SynRM Axially laminated rotor, and Transversally laminated rotor as shown in Fig 8.

Applications of SynRM

Nowadays SynRM has more scope in the electrical world, it's commonly used in

1. Clock

2. Some washing machines

3. Recording instruments

4. Electric vehicles

That’s all about synchronous reluctance motors, We hope you now have a good understanding of synchronous reluctance motors. If you want to know more about synchronous reluctance motors and their working you can check this link, How SynRM work? it will be more helpful for you.