When replacing brushless servo motors, or when disconnecting them as part of routine maintenance, it's important to make a note of the exact motor phase connections at the drive and motor.

It's easy to make the assumption that the 3 phases on a brushless servo motor, typically labelled U, V, and W are the same as on an asynchronous motor, but they're not. The practice, for example, of simply swapping two phases over to change direction can not be transposed on to a servo system.

This assumption will cause problems, as the feed back device on a servo motor provides the drive with positional information necessary to determine the switching points for the commutation of the motor. This is different from the asynchronous motor whereby only the phase sequence for direction is critical.

If the connection information for the motor phase connection is lost then, after removing the load, all six permutations for UVW  should be connected and tested until a successful run in both directions is achieved. It's easier and better to make a note of the connections before disconnecting the motor!