Determining motor pole count

3 phase AC/BLDC motor winding schemes. A, B and C represent the phase wires coming out of motor.

If magnetic pole count of permanent magnet AC/BLDC motor is unknown, there is an easy way to determine it experimentally.

Needed items:

1. 3 phase AC/BLDC motors
2. Adjustable lab DC power supply

Procedure:

1. Wire DC power supply + and - wires between any of two leads of the motor. I.e. + to A and - to B while C is not connected.
2. Use the PSU to drive some DC current to the windings. Don't exceed motor's current rating. A good starting value may be 20-30% of motor continuous current. Now motor should start holding position as the rotor's magnets snap to the driven windings.
3. Rotate motor shaft by hand or tool and count the distinct snapping positions per one revolution.
4. Motor pole count is twice as much as the number of snapping positions found in the previous step. I.e. if motor snaps 2 times per revolution, it means that motor has 4 magnetic poles.

Tips:

• If motor is too stiff to rotate, reduce current.
• If snaps can't be felt, increase current to make them stronger.
• If motor has internal brake, it must be electrically opened before the test. Usually such brakes open with 24VDC voltage to brake wires.

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