Well, what do you know, there actually exist hybrid motors! 🙂
Basically, when an electric motor has a stator with both electromagnets and permanent magnets, that’s a hybrid motor. It’s called hybrid because it is a cross-breed between a Permanent Magnet Motor and a Variable Reluctance Motor (which has a purely ferromagnetic rotor with salient teeth and without magnets or coils).
This type of motor is very important to me because it is said to have much better efficiency than a regular PM motor – and efficiency is becoming an obsession of mine… battery weight and size will be largely dictated by the efficiency of the system, and the motor plays a large influential part in this. And because of regenerative braking, the total losses are multiplied by 2: we lose energy when accelerating, then we lose some more when braking. So, a motor/generator with 90% efficiency actually has an effective efficiency of 90% × 90% = 81% when we consider the energy’s complete round-trip. Furthermore, it is also said to have better torque/weight ratio and, most importantly for a wheel-motor, a wider range of flat torque response (up to higher speeds).
The main differential aspect I was able to grasp from the papers and patents about this type of motor was that, because the main part of the air gap’s magnetic flux is actually generated by the permanent magnets, the inductance (or “electrical inertia”) of the stator coils is very low, which in turn allows the electronic drive to cut or establish the phase current a lot faster than in the other systems. This promotes better torque control and higher efficiency. In addition, the generated back-emf is lower than the usual, which allows for higher speeds with the same voltage sources.
Another positive aspect of this motor compared to my current PM model is that it uses a lot less magnets, and so it promises to be cheaper. 🙂
And the thermal management is also easier, because all the temperature-sensitive components (magnets & coils) are placed in the stator, where it is much easier to implement liquid cooling (if necessary).
So, here go some examples of this marvelous technology.
- Yuefeng Liao & Tom Lipo’s “Doubly Salient Permanent Magnet” motor (introduction and optimization strategy).
- Ming & Zhou’s “Split Winding DSPM” motor (analysis).
- Sawyer linear motor (concept and cool application).
- Flynn’s “Parallel Path Magnetic Technology” motor (concept and motor).
I’m definitely considering starting a new motor design based on this approach…