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If a contactor is used to disconnect the VFD from the motor it should be interlocked to only switch when the VFD is off. The VFD should not be allowed to operate with the output from the drive to the motor as an open circuit. There should be a contactor on each side of the drive, ie, between the drive and the supply and between the drive and the compressor motor. They should be interlocked to break the supply side first.When switching on, the motor side contactor should be made first. When using a VFD bypass, care should be taken to ensure there can be no voltage feedback to the VFD. Therefore when the bypass contactor is closed and the bypass is in operation, the contactors on either side of the VFD must be open.