Let's start with the picture above. In the middle a kind of star with four elevations. There are six coils around. At the moment, the elevations are exactly adapted to the two horizontal coils. If you now power these two off and the upper right and the lower left on, the rotor in the middle would continue to rotate exactly 30°. That's the effect of the reluctance force.

Reluctance is magnetic resistance. This was significantly larger after switching to the other two coils. It could be brought back to its minimum value by turning the rotor.

The stator is roughly equivalent to that of an asynchronous motor, but the material in the rotor needs only normal iron material. Special value should here, such as at transformer plates, placed on the mobility of the dipoles. Permanent magnets, rare earths or copper cages are not required.

The magnetic field of the stator protrudes into the rotor and penetrates into all areas where it can flow well. Therefore, the cross section of the rotor is particularly pronounced by cavities. They are also called barriers to the magnetic flux.

Basically, the reluctance motor is not quite as smooth running as the others. It is a pulsating torque, which can lead to unwanted vibrations of the engine. This obviously comes from the magnetic resistance that changes even with small rotation angles.

It needs a pretty good controllers of the current and its switching. These are called 'frequency converters'. Without such, e.g. with normal three-phase current from the household grid, the engine would not even start.

The complex control makes the manufacturers of such engines act with caution. Obviously, only Tesla has taken the step to a powerful engine as a vehicle drive in Model 3. There it was probably given up the alleged energy inefficiency, which can be seen in the consumption values compared to the models S and X.

So it is possible to get that under control. Also, the smaller number of coils compared to the other motors would have to be increased, of course, the factor 3 must be taken into account. It is also easier to cool there, because in the reluctance motor the heat is more concentrated there.

Allegedly, the frequency converter does not even have to be informed about the current state of the rotor. Presumably, it can easily increase with a cycle from a slow to the desired speed and the rotor is taken with, similar to a synchronous motor.