Over-voltages computation algorithm for motors fed from long cable PWM drives

This work presents a computation technique to analyze the high frequency over-voltage problem in long cable PWM drives. Differential mode (DM) analysis for voltage and its resulting current along the cable is first developed in the frequency domain. The cable length, its electrical characteristics (R,L,G,C), the motor impedance, the inverter impedance as well as voltage rise and fall times are all considered. Then, by applying the boundary conditions at the cable ends and assuming the cable without losses, delay equations, describing the voltage at the inverter side as well as the voltage at the motor side, are developed in time domain. From these equations, an appropriate algorithm has been deduced to model the voltage at the motor terminals. This paper includes computation results, carried out using Matlab software, and important experimental outcomes on an industrial 5 kVA ASD prototype.