Closed-form analysis of adjustable speed drive performance under input voltage unbalance and sag conditions

Voltage unbalance or sag conditions generated by the line excitation can cause the input rectifier stage of an adjustable-speed drive (ASD) to enter single-phase rectifier operation. This degradation of the input power quality can have a significant negative impact on the induction machine performance characteristics. This paper provides an approximate closed-form analysis of the impact of line voltage sags and unbalance on the induction machine phase voltages, currents, and torque pulsations for a general-purpose ASD consisting of a three-phase diode bridge rectifier, dc link, and PWM inverter delivering constant volts-per-Hertz excitation. Attention is focused on the impact of the dominant 2nd harmonic of the line frequency that appears in the DC link voltage during the sag/unbalance conditions, neglecting the impact of the other higher-order harmonics. In addition to the closed-form analytical results that assume constant rotor speed, both simulation and experimental results are presented that confirm the key analytical results, including the dominance of the 2nd harmonic in the resulting torque pulsations. The analytical results can be used as a valuable design tool to rapidly evaluate the approximate impact of unbalance/sag conditions on ASD machine performance.