Determining motor pole count

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

Method 1

Needed items:

1. 3 phase AC/BLDC motors
2. A working servo drive

Procedure:

1. Disconnect feedback device cable from drive, also turn of Hall sensor setting if it's on
2. Set drive in torque control mode and enable it
3. Manually set torque setpoint (with Granity Testing page TSP1 value and Set Abs button) to produce some noticeable torque on motor (motor not rotate but will start holding position when current is increased). Typically 10-30% of motor rated torque is sufficient.
4. Rotate motor shaft by hand or tool and count the distinct snapping positions per one revolution.
5. 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.

Method 2

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

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 10-30% 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. Check motor specifications first, or use lower starting voltage if unknown.

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