Reluctance Motors for Electric Cars – How to Work?

Reluctance Motors for Electric Cars – How to Work? -Before continuing to read the problems that I discussed in this article, in relation to the previous material, please read the previous articles in sequence as follows;

What is Forces Acting on a Rolling Vehicle?

Lorentz Force Law for EV Electric Motor

Coulomb’s Law for EVs Electric Motor

Biot-Savart’s Law for EVs Electric Motor

Brushed DC Motors for EV and Hybrid Cars

Brushless Motors for Electric Cars

Reluctance Motors for Electric Cars – How to Work?

While most synchronous motors use a permanent magnet rotor, there’s another intriguing option that Tesla has started to use in some of its vehicles: the reluctance motor.

Consider what happens when you place a magnet near a piece of magnetic material: not something that has already been magnetized, just something that magnets are able to stick to, like iron. Why do magnets stick to this material? When a magnet gets close to it, the magnetic poles of that material align themselves to the magnetic field of the magnet, creating a mutual attraction. This is the fundamental principle underlying reluctance motors.

Consider the rotor and stator illustrated in Fig. 1. Starting in the position illustrated on the left side of the figure, we energize PP1, creating a magnetic field between the poles at the bottom right and top left. These energized stator poles attract the closest poles of the iron core, in this case, A and C, ending up in the position illustrated on the right. From here, we energize PP2, which attracts the nearest poles of the iron core, in this case, B and D. From there, we energize PP3, attracting poles A and C, then PP1, attracting poles B and D, etc.

Reluctance Motors for Electric Cars
FIGURE 1 Operation of a reluctance motor.

Note that this reluctance motor only works because the number of stator poles and rotor poles is mismatched. In this case, there are six stator poles and four rotor poles. This ensures that at every moment, there are rotor poles and stator poles out of alignment, allowing them to be attracted to each other to create rotation. If the numbers of poles were equal, and the poles were aligned, firing any of the stator poles would simply pull outward on the rotor poles directly in front of them, which isn’t useful for generating rotation. Even though the number of stator and rotor poles is mismatched, the rotor still turns at the same speed as the rotating magnetic field, so reluctance motors are also considered synchronous motors.

While reluctance motors rely primarily on electromagnetic stator poles and a rotor that is magnetic, but not a permanent magnet, the reluctance motors that Tesla is using are said to be permanent magnet reluctance motors. This potentially confusing name simply refers to the fact that they use a small amount of permanent magnet in the stator poles to smooth out the rotation of the motor, which can otherwise be choppier than we might like for use in an electric vehicle.

Most electric vehicles on the market today use some kind of synchronous motor, with a rotating magnetic field in the stator that pulls a magnetic rotor around with it. But there’s another important type of motor in use today that uses a similar stator, but a different kind of rotor, one which doesn’t use magnetic material. Instead, these motors work by inducing a current in the rotor that responds to the rotating magnetic field created by AC current applied to the stator. For this reason, they’re called AC induction motors. To appreciate how AC induction motors work, we must first study Faraday’s Law.

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  1. Pingback: Faraday’s Law in Electric Vehicle Motors | mech4cars

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