One of the problems that can affect BL AC Servo Motors is demagnetization. There are a couple of checks you can do in your shop to see if the magnetization is good or not.
To do this you will first have to uninstall your motor from your machine. Once uninstalled you should be able to rotate the shaft freely with no resistance to the rotation you put on it. Next step is to jumper U to V, U to W, and V to W separately. To do this you can use an insulated screwdriver or insulated wire to make contact between the two wires. Once this is done on each of the connections, when you rotate the shaft of the motor you should feel a sort of a “bump, bump” resistance in the shaft as you rotate it. The resistance has a tiny bit of space between each “bump”. Once this check is done the last check to do is to jumper all three leads together (U, V, and W) and rotate the shaft. You should feel a constant resistance as you rotate it. If at any point the motor, you are checking does not pass these tests then your motor is demagnetized. At this point, you can send it to us, and we can re-magnetize the motor for you.
For further assistance, you can refer to this video on our YouTube channel: