Cogging in permanent magnet motor

1086 Views 7 Replies 4 Participants Last post by  remy_martian
This is not an EV question, but I hope the Motor forum is the right place for it.

I have a battery powered lawn mower with a 3-phase pm motor of inrunner design. When I turn the mower blade by hand (power off) there are 18 distinct magnetic detents per revolution. I have read that the cogging frequency of this type of motor is the least common multiple of stator slots and rotor poles. I think this means I have a stator with 9 slots and a rotor with 6 poles. Right or totally full of it?

Thanks,
Dan
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Clarification, I think I mean 3 slots per phase?

Dan
Another solution is 18 stator slots would produce cogging at 18 cogs per rev. The magnets are attracted to the stator pole teeth and create a detent position.

But i don't think you can determine the number of rotor poles by counting cogs.

If you measured the frequency of the inverter and the RPM of the motor then you could determine the number of poles.

The number of stator slots can vary depending upon the winding pitch and amount of overlap between phase windings.
Another solution is 18 stator slots would produce cogging at 18 cogs per rev. The magnets are attracted to the stator pole teeth and create a detent position.

But i don't think you can determine the number of rotor poles by counting cogs.

If you measured the frequency of the inverter and the RPM of the motor then you could determine the number of poles.

The number of stator slots can vary depending upon the winding pitch and amount of overlap between phase windings.
Thanks, KennyBobby. I'll try energizing one phase and count the number of strong attractions to that position. Should work, right?

Dan
I'll try energizing one phase and count the number of strong attractions to that position. Should work, right?
Yes, I would expect so... that count will be the number of rotor pole pairs (not poles).
How do you energize just one phase--is it a wye-winding with the center tap exposed?

If you can access the 3 leads for the motor windings, then you can build a 3 resistor wye circuit and connect it in parallel to the 3 motor leads. use 100k or larger resistors and solder three ends together. This will create a center tap reference point for measuring the back emf as you manually spin the rotor. Put an oscilloscope lead on a phase wire with ground on the center tap. As you spin the rotor thru one revolution, you will see a cosine wave on the scope. Count the number of number of cycles to determine your pole count. e.g. a 6 pole motor will generate 3 sine waves per revolution. N-S-N-S-N-S
How do you energize just one phase--is it a wye-winding with the center tap exposed?

If you can access the 3 leads for the motor windings, then you can build a 3 resistor wye circuit and connect it in parallel to the 3 motor leads. use 100k or larger resistors and solder three ends together. This will create a center tap reference point for measuring the back emf as you manually spin the rotor. Put an oscilloscope lead on a phase wire with ground on the center tap. As you spin the rotor thru one revolution, you will see a cosine wave on the scope. Count the number of number of cycles to determine your pole count. e.g. a 6 pole motor will generate 3 sine waves per revolution. N-S-N-S-N-S
Right, kennybobby, I was blinkered and only thinking in delta. I'm not going to open the motor up. I do have an oscilloscope, and so your suggestion is probably the way I will go. Am I correct in thinking this will work whether the motor is wound wye or delta?

Thanks.

Dan
You have an oscilloscope...look at the frequency of a winding excitation and also measure the RPM.
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