Electronic – Finding number of pole pairs in a BLDC motor

brushless-dc-motormotorrotor

I have a 3phase CDROM BLDC motor for which I don't have any datasheet. How can I find out the number of pole pair in the motor?

Hint:-With a magnet, I could check that there are some N and S poles in the rotor inner layer but how can I know the exact number of pole pairs.?outrunner type rotor

Startor

Thanks, Charles Cowie. But I have another bldc motor which has 3 windings at 120 degrees apart. But its datasheet says is a 4 pole pair motor. Can you explain how it is possible.?3 windings at 120 degrees

Best Answer

With a magnet, I could check that there are some N and S poles in the rotor inner layer

Best not to use a magnet (a Neodym magnet is strong enough to demagnetize a Ferrite magnet). Just use a screwdriver or other object made from ferrous metal. It will be attracted to each magnet pole in the rotor, so mark the first attraction point and move it around the circumference of the rotor while counting poles until you arrive back at the start.

I have another bldc motor which has 3 windings at 120 degrees apart. But its datasheet says is a 4 pole pair motor.

Depending on magnet configuration and winding pattern, The number of stator arms or slots may be higher or lower than the number of magnet poles. The chart below shows some example combinations (blue boxes are known good combinations, orange may work but were not tested). You can see that a 3 slot motor may have 2 or 4 magnet poles.

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