Calculation of Transient Torques on Motors During a Residual Voltage Motor Bus Transfer

To maintain a critical process upon loss of primary motor bus power, the petrochemical industry depends largely on residual voltage transfer, ignoring phase angle and closing the backup source when the motor bus residual voltage has decayed to 30%. To assess the consequences of out-of-phase residual voltage transfers, a transient simulation program models a bus with three motors of various sizes, inertias, and impedances. The three motors are loaded around 80% with fan, pump, and compressor loads to calculate the peak transient motor current and torque upon closure of the backup source breaker. Pre-transfer events and conditions produce an initial phase angle between the primary and backup sources, and the simulation reproduces the resultant residual voltage transfer closing angle and its effect on the peak transient current and torque. Individual motors exhibit positive and negative torques, oscillating from induction generator to motor, and the peak-to-peak torques are also recorded, as they can impact the motor shaft stress.