Drive DC bus voltage control during power interruptions using kinetic energy recovery

Voltage dips and sags of short duration constitute a serious problem for electrical drives (frequency converters) as many are equipped with an under-voltage protection causing the system to trip within a few milliseconds after an input voltage sag. This tripping mechanism can be associated with a total loss of system control since the control electronics are usually powered by the DC-link capacitor, requiring a full restart. The proposed solution to this problem is to recover some of the mechanical energy stored in the rotating masses by kinetic buffering. During the interruption interval, the drive system continues to operate at almost zero electromagnetic torque, just regenerating a minor amount of power to cover the electrical losses in the inverter. The still controlled braking of the drive depends on the available load torque. Thus, the implemented regenerative braking scheme allows the inverter to keep its DC bus voltage at a predetermined minimum level, expanding the time during which the supply voltage can be reapplied without the time-consuming DC-link capacitor recharging cycle. Considering realistic conditions, advantages and drawbacks of different control approaches are discussed. Measured results are presented and evaluated to demonstrate the performance and the stability of the system.