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Ac variac problem

As i made sensorless PMSM application with high-voltage digital motor control kit  to build 2 process when I increase the voltage with AC variac or activated Ac variac, connection with the card of the computer was disconnected and gave me the FTDI error. Reduce voltage and then I tried to connect the card again but LED on M3 section is not lit and the card is not connected to the computer. What could be the problem. Why was such a thing. I would appreciate it if you could help.