BLDC matching pole count on rotor and stator with FOC‌

[mishmish66]

Hi guys, I don't usually post on forums, but I could't find an answer to this anywhere. I am a complete amateur with bldc motors and I have been trying to learn. I know that motors don't ever have the same number of rotor and stator poles because then the cogging torque would make the motor practically useless. I was wondering though if you could break this rule if you're running a motor at low speeds using FOC‌. Does anyone on here know if that would be possible? or would the motor have almost no torque when the poles aren't lined up? Thanks for any help and I hope you're having a great day,

 

[BradtheRad]

Attraction/repulsion is strongest when a magnet passes near an energized coil. That produces torque. You can make a simple BLDC having 3 magnets and 9 coils. Your switching circuit energizes 3 coils at a time, pulling the magnets from one coil to the next.

If you want to get sophisticated, reverse coil polarity just as the magnet goes past the coil, to push the magnet away.

would the motor have almost no torque when the poles aren't lined up?

Right, torque is low when a magnet is far from an energized coil. To obtain high torque at slow speed then an obvious path is greater numbers of magnets and coils (to produce denser packing of attraction/repulsion). However it may also work if you spin it fast creating flywheel inertia, then gear down the rpm.

 

[mishmish66]

Right so mostly I was wondering, assuming that I was using field oriented control so that the magnetic field was lined up with the poles, would the misalignment make the motor useless even if the poles were small and the controller were smart. I'm curious if that could be used to directly drive a robotic arm or leg or something. I know the normal solution involves mismatchching the pole and rotor count, like on a standard motor with 12N14P or something, but I also know that some efficiency is lost that way.