Patent ID: 7332885

Claim:
A method for controlling a variable speed drive to ride-through a voltage sag, comprising the steps of: providing a motor and a compressor coupled to a mechanical load; providing a variable speed drive to power the motor, the variable speed drive including an active converter stage and an inverter stage electrically coupled by a DC link stage; monitoring a DC voltage of the DC link stage; monitoring an input parameter of the active converter stage; regulating the DC voltage of the DC link stage with the active converter in response to a change in the monitored DC link voltage; in response to the DC voltage being less than a predetermined first threshold voltage, executing the steps of: transferring the regulation of the DC voltage of the DC link stage to the inverter stage; removing the mechanical load from the compressor; and controlling the DC voltage of the DC link stage with the inverter stage by reversing power flow from the motor to the DC link stage.