Inverter/motor compatibility solutions

This paper reviews techniques for controlling the voltage waveform imposed on a motor by a voltage-source inverter. At standard supply voltages in the nominal 400 V region, the combined effects of voltage magnification in the motor cable and high rate of change of inverter output voltage (dv/dt) can cause unacceptable inter-turn voltage stress in the motor. This is usually only the case where the motor is of uncertain quality or has specific design aspects which render it vulnerable to inter-turn stress. Modern motors from reputable manufacturers are not significantly affected with supply voltages of 400 V nominal. However for supply voltages exceeding about 500 V the problem becomes rapidly more serious as the likely inception voltage for partial discharge is exceeded. The requirement of the remedial technique is to tailor the motor terminal voltage waveform in order to keep it within the safe operating boundary for the motor, which is typically a function of peak voltage and rise-time as shown. The main known techniques are reviewed here. For simplicity, only a single phase is considered. The waveforms are produced using SPICE models which have been verified against real systems, but are much simplified. In particular, loss-less line models have been used and loss has been introduced in lumped form at the cable terminals. This tends to under-emphasise losses at very high frequencies