PWM inverters and their influence on motor overvoltage

Abstract

Modern voltage source pulse width modulation (PWM) inverters have elevated adjustable speed AC drives to a performance level comparable to DC drives. To accomplish this, voltage source inverters (VSI) now employ third generation insulated gate bipolar transistors (IGBT); however, the fast rise times associated with these devices produce unintended consequences. This paper examines motor overvoltages resulting from IGBT PWM VSI. Previous investigations have shown a critical power cable distance exists beyond which twice bus voltage (2 pu) terminal voltages are possible. This is determined primarily by the rise time of the devices and cable characteristics. This paper investigates motor voltages >2 pu, their cause and solution. In contrast to motor voltages /spl ges/2 pu, the paper shows motor voltages >2 pu primarily are determined by the PWM carrier frequency and modulation strategy. Furthermore, the paper demonstrates motor voltages up to 3-4 pu are possible. Software modifications to the PWM modulator and modulating signal are presented to reduce >2 pu motor overvoltage. Additionally, a hardware correction, easily implemented within a field programmable gate array (FPGA), is disclosed and experimentally demonstrated.