Diagnosis of induction machines in time-varying conditions

The diagnosis of induction machine faults is commonly realized through motor current signature analysis (MCSA), i.e. by the spectrum analysis of the input currents. Specifically in case of broken bars the amplitude of the left sideband component of a phase current is monitored in order to sense its variation. Nevertheless MCSA has some drawbacks that are still under investigation. The main concern is that under time-varying conditions an efficient frequency transformation cannot be made, since slip and speed vary and so the left sideband frequency. A short time Fourier transformation cannot be made as for large machine the slip is very low and a large time acquisition period is required to achive a suitable frequency resolution. Time-varying conditions occur in case of variable speed for railways applications, or for wind generators. Therefore recently the diagnosis of machine under time varying conditions is being extensively investigated.