Case stories of advanced rotor assessment in field motors operated with soft-starters and frequency converters

Advanced analysis of the motor starting currents enables very reliable diagnostics of the rotor condition, providing immunity against the eventual false alarms obtained with the conventional MCSA. However, the application of this methodology has been mainly restricted to the case of line-started machines. Its extension to the cases of soft-started or inverted-fed motors has been barely investigated, especially because the probability of rotor damage is claimed to be reduced under such conditions. Nonetheless, industrial cases related to rotor damages in induction motors operated via soft-starters and frequency-converters have been recently reported. This paper proposes a twofold transient-based approach for the reliable assessment of the rotor condition in induction motors operated with such elements. The method relies on the advanced analysis of the motor starting current by combining two types of time-frequency approaches (based on discrete and continuous transforms) in two subsequent stages: 1) fault pattern identification and 2) fault severity quantification. Field results obtained with large motors operating in a sewage treatment plant confirm the validity of the methodology for the rotor assessment in induction motors operated both via soft-starters and frequency converters.