Abstract: A Variable Frequency Drive (VFD) supplies power to rotate a motor, controlling both the speed and direction. The method normally used for this power conversion by the VFD results in energy losses and line harmonics. When the motor is driven by the VFD to be rotating at the same speed and direction as if the motor was straight across the incoming AC power, a transfer in power supply to the motor can be made using contactors to bypass the VFD. When in this bypass mode, the energy losses of the VFD are greatly reduced, and the line harmonics due to the VFD are greatly reduced. When it is subsequently recognized that the speed reference has deviated from being the same speed and direction as if the motor was straight across the incoming AC power, a transfer in power supply can be made from using contactors to once again drive the motor from the VFD.

Abstract: A Variable Frequency Drive (VFD) supplies power to rotate a motor, controlling both the speed and direction. The method normally used for this power conversion by the VFD results in energy losses and line harmonics. When the motor is driven by the VFD to be rotating at the same speed and direction as if the motor was straight across the incoming AC power, a transfer in power supply to the motor can be made using contactors to bypass the VFD. When in this bypass mode, the energy losses of the VFD are greatly reduced, and the line harmonics due to the VFD are greatly reduced. When it is subsequently recognized that the speed reference has deviated from being the same speed and direction as if the motor was straight across the incoming AC power, a transfer in power supply can be made from using contactors to once again drive the motor from the VFD.