Single phase PM hub motor?

Although I’m still very noob, I like researching and learning about different motor types and how they work...

For example this image I found which I believe is a single phase PM out-runner. But I have a few questions...



- Woundn’t this motor provide very high torque/power considering A: poles are both attracting and repelling at the same time (like all PM motors) B: all poles are active 100% of the time (just switching polarity), while always affecting 100% of the magnets?

- Aren’t these motors much simpler in almost every way to 3 phase PM motors? Number of phases/poles/wires/magnets, simpler to control (simply switch polarity on the single phase at the correct time) etc.

- I believe these motors can’t be gaurenteed to self start in the correct direction, but I don’t see how that matters on an ebike or similar (simply start rolling yourself, this is even a purposely added safety feature on some electric kick scooters).

- Am I incorrect in how I’m understanding these to work with my limited knowledge? Are there other downsides I’m missing, and why aren’t these used?

The motor in your diagram is typical for a small dc cooling fan.

In a typical 3 phase bike motor, the poles are alternately attracting and repelling the magnets also but having 3 phases ensures it will start in the right direction. It also maintains torque even when moving quite slowly. With a single phase motor like shown, you would have problems starting out on a hill, for example. Once it gets going above a certain speed, it would work reasonably well.

If I understand correctly, once manually started it would run poorly at low rpm, mainly due to high torque ripple? If so wouldn’t adding more poles help?

Assuming you could get it running smoothly/quietly enough, and didn’t mind no self starting, wouldn’t you end up with a much simpler and more powerful motor? (more powerful due to 100% winding uptime vs 1/3 uptime of 3 phase, if I’m correct on that)

Yes, the torque ripple would be worse, so you loose a little efficiency there and it tends to make the motor more noisy.

The power density of a motor is really determined by the heat dissipation, so having all the windings active just lets it overheat faster. I suppose the power density would be better until it got hot.