Motor pole pair selection

1

Hello!

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!

2

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

3

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

4

Thanks, this is good to know!

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