Ride-through issues for DC motor drives during voltage sags

Voltage sags are a common occurrence in power systems. Although a typical sag is only 5 to 20 cycles, electronic equipment may fail or "trip". A trip of industrial process equipment, such as a motor drive or programmable logic controller, may be extremely costly. Methods to improve the "ride-through" capability of the equipment usually is a system solution: uninterruptable power supplies, motor-generator sets, etc. Often, only a few of the devices in an industrial process are very sensitive to sags. Therefore, a better solution is to desensitize the components which are affected by sags. Compared to AC variable frequency drives, DC motor drives are particularly susceptible to sags because they inherently lack electrical energy storage devices. The addition of capacitance on the DC-side of the drive will not improve ride-through and can be detrimental to controllability. Examples of DC drive response during sags are shown and methods for de-sensitizing DC motor drives for sag-related problems are discussed.