Spectral analyses of directly measured stator and rotor currents for induction motor bar breakages characterization by M.C.S.A.

By exploiting an appositely-made cage induction motor, same measurements of stator and bar currents were done with progressive rotor damage (increasing number of broken bars), for a complete characterization of motor current spectra under fault conditions. We used a three-phase wound-rotor induction machine that has been converted in a squirrel-cage machine with current-measuring capability on two bars. A multiphase symmetrical components-based model was carried out, for a better theoretical understanding of sidebands genesis, and for experimental results matching. Lower (LSB) and upper (USB) sidebands of fundamental current were detected, that characterize bar breakages; moreover, bar current spectra confirm the presence of direct and reverse slip-frequency multiphase symmetrical components (which produce rotor current unbalance), that are forecasted by mathematical theory. Some tests were performed by applying MCSA technique for fault-severity assessment; LSB and USB-based indicators have been tested and discussed