Motor Reacceleration to Improve Process Uptime

Abstract

Reacceleration is a method of automatically restarting motors after an unexpected deacceleration caused by system voltage events such as dips, outages, or bus transfers. Reacceleration schemes are designed to minimize process disruptions by rapid detection of supply loss, recovery/ monitoring of acceptable transient torque limits, and then automatic reclosure of the motor contactors. Depending on the connected load and minimum available fault current, the reacceleration may be “instantaneous” or a staged event designed to assure that the bus voltage is maintained at an acceptable level during the process restart. Poor coordination of the reacceleration process may cause further system outages if the nominal bus voltage drops below tolerable levels. Special consideration is required to prioritize loads, to maintain production, to prevent lifting of safety valves, and to avert equipment damage. Various processes differ in their ability to withstand temporary outages, from milliseconds (ms) to several seconds. Today's protection and control intelligent electronic devices (IEDs) provide settings, dedicated measurement, and timing circuits to allow these variances to be preprogrammed and activated based on outage duration and magnitude. Short-time outages within millisecond range combined with electrically held contactors provide the least complex situations for enabling a reacceleration system. More complex situations arise when motors are still decelerating and when the supply is restored, especially if the contactors were maintained closed during the outage. Certain more advanced IEDs also provide multiple reacceleration schemes that are automatically deployed based on longer outages—typically up to 30 min.