Motor pole pair selection


I’m selecting a motor for a new project and I’m curious about pole pair limitations.

From what I’ve read, a lower pole pair count will suffer from higher cogging at low speed, while too high of pole pair count will be limit the motor’s maximum speed. I don’t need to go very fast, 2000 RPM max. I know there is an anticogging feature in RazorsEdge branch but I have not tried it yet. Just want to get an idea of the motor selection process.

This thread gives some info about it with the equation: Max RPM = 600 Hz / # Pole Pairs * 60, where 60 is a conversion factor from seconds to minutes. So to find the highest number of pole pairs I can use: 60 * 600 / Required Max RPM.

My questions are:

  1. Where does the 600 Hz limitation come from?
  2. Are there any other constraints I should look out for?
  3. How do people typically select pole pair count? Should I just try to find a motor with as many pole pairs as possible while still being able to reach my desired speed, so in my case 18 pole pairs?

Thanks for the help!

@madcowswe @Wetmelon do you guys have any insight on this?

The controller will usually become unstable around this point, given the current hardware & software design. Lower electrical frequencies are easier to control. You’ll want to balance your requirement for low cogging and the stability of the controller

Thanks, this is good to know!

Out of curiosity, is the current limiting factor the MCU speed or is other hardware limiting it?