There is Err2 alarm on the AC servo driver, or sometimes it is black screen of the driver, use universal meter to measure the input voltage by AC mode, but it is too high to measure.

Cut off the power of the AC servo driver, and measure the output power is 220V from the isolating transformer, it is normal. But after connect to the AC servo driver and CNC controller, the voltage is high again.

Then we found the insulation resistance to earth of the AC servo motor is infinity by universal meter, use the magneto-ohmmeter to check out the insulation resistance was break-down, this is the reason of the problem, change the AC servo motor it is solved.