Q: A CNC lathe retrofitted with GSK 980TD, but the drive is not in use, the AC servo motor in free mode and the wiring should be all right.

A: Connect the SON signal of AC servo drive to the ground, it can be used as amplifier.

AC servo motor & drive:

http://www.cncmakers.com/cnc/controllers/Servo_Motor_Driver/AC_Servo_Motor_Driver.html