Measures to cope with AC motor insulation stress due to IGBT-inverter supply

The fast and hard switching IGBT-inverters made it possible to use standard AC motors without any derating as a variable speed drive, but posed in certain cases, especially with long cable connections, high line voltage levels and low motor partial discharge inception voltages, new danger to motor life by increased voltage stress. By studying the nature of this stress and by close examination of the resulting motor failures, it was possible to find measures to cope with this problem. Different solutions were developed for mass fabrication and are now at hand. Their advantages and limits are discussed and compared, proving that it is possible to benefit from the advantages of IGBT-inverter operation without any risk to motor life.