An analytical approach for calculating critical voltage-sag removal time of induction motors using manufacturer technical data

A critical voltage-sag removal time (CVSRT), reflecting a duration from the beginning of voltage sags to the instant when speed of an induction motor reduces to the critical value, is important for maintaining the motor’s stable running. The CVSRT can be usually determined by time-domain dynamic simulation that always requires a full set of induction motor parameters. This approach is quite tedious since the simulation must be repeated until the correct CVSRT is found. Also, the motor equivalent-circuit parameter is often not readily available, particularly with the on-site motors. Thus, this paper presents an alternative analytical approach to determine the CVSRT under balanced and unbalanced sag conditions. A motor slip-torque curve mainly required for CVSRT calculation is analytically computed using manufacturer technical data instead of the equivalent-circuit parameters. The slip-torque curve is modified to take effects of voltage unbalance and feeder cable impedance into account. The obtained slip-torque curves are validated against the published data. The CVSRTs straightforwardly computed from the proposed analytical approach are compared against those from the time-domain simulation. A good agreement between them is observed in satisfactory manner.