General - Servo Motors: Checking Servo Motor Phases

A permanent magnet servo motor has 3phases and ground. Our standard servo motors also have a feedback device, such as incremental encoder (with hall sensors), absolute encoder or resolver. The feedback cable may be part or separate from the motor power cable.

If the motor is connected to a servo drive, power down the drive. Follow the manufacturers recommendations for time for drive to dissipate voltage. Disconnect the motor phase wires and ground wire from the drive. (Note the connections, to re-connect after testing.)  These are larger gauge wires than the feedback wires. There's no need to disconnect the feedback wires from the drive.

Use an ohm-meter to check the resistance between the 3 motor phases (1-2, 2-3, 1-3). Different manufacturers use different designations for their motor phases (U-V-W, X-Y-Z, 1-2-3, A-B-C) but for this test it is arbitrary. These should all measure the same resistance between the motor phases. Typical value is less than 20 ohms.

Also check the resistance between the motor phases and ground wires. These should measure infinite resistance.

If it's shorted (close to 0 resistance) motor phases or shorted to ground, the motor will need to be replaced or repaired. This condition would cause a fault on the servo drive. You can check the servo drive and if it's a newer digital drive, it'll have an error log. Older drives may have a fault LED that can indicate a Motor Fault, Overcurrent/Bridge /I2T Fault or other problem.

Is one motor phase shorted?

Typically if a servo motor is jammed through the machine, one particular motor phase will have more current running through it than the other two phases and will short before the other two phases. In the future, you could lower the current at the drive to prevent over-currenting the motor. Almost all of our motors have a thermal switch or thermistor that opens when the motor gets hot and also helps prevent over-currenting the motor. Check that this is connected to the drive; it is as standard for Parker drives & cables. Parker drives also continuously monitor the continuous current and peak current going to the motor and already limit it to the motor's rating. However if the control power is reset on the drive and the motor has cooled just enough for the thermal switch to close, this can defeat this protection; employ the drives control power or keep-alive to prevent over-currenting.   If relying on the motor thermal switch to protect the motor, limit the drive's peak current to twice the motor's continuous current rating.  The motor current rating is also based on an assumed ambient temperature. If the ambient temperature is high, the current would need to be decreased.

Are all 3 motor phases shorted?

It would be unusual for all 3 servo motor's phases to be shorted. In this situation the drive is sending too much current across all 3 phases while the motor is running. Check that the drive is configured correctly for the motor and that the current settings correct for the motor rating. Check the ambient temperature for the motor based on the motor's current rating. If the ambient temperature is high, the motor would need to be derated and the drive current settings adjusted. Or cooling would need to be employed on the motor (fan or blower, cold-plate or larger heatsink).