Diagnostics of Induction Machines Operated from Inverters and Soft-Starters Using High Frequency Negative Sequence Currents

This paper studies the application of high frequency voltage excitation based stator winding diagnostic methods to three-phase ac machines operated from power converters that create the necessary high frequency excitation as part of their normal operation. The paper focuses on two specific operating modes: 1) machines operated from inverters in the overmodulation region, and 2) machines operated from soft-starters during startup. In both cases, high frequency voltage components at well-defined frequencies are created. The negative sequence currents induced from these high frequency voltages are shown to contain accurate information on the level of asymmetry (fault) in the machine. This information is significantly richer than exists in other modes of operation, i.e., inverters working In the linear modulation region or soft-starters in the steady-state, and provides interesting opportunities to complement other diagnostic methods.