Diagnostics of rotor asymmetries in inverter fed, variable speed induction machines

Electrical machines spread over production lines in the industry and can cause the shutdown of whole equipments if they collapse. In the past, safe and reliable operation was basically ensured by manufacturers and users of electrical machines with the help of simple protections such as overcurrent and overvoltage devices. Nowadays, many operators use condition based maintenance strategies in parallel with conventional corrective maintenance so that not only reliability and safety are increased, but also economical losses can be reduced. In the case of induction machines, common faults are asymmetries in the squirrel cage as for example broken bars or fractures in the end rings. This type of fault causes a characteristic harmonic in the different variables of the machine that can be used for the purpose of diagnostics. Although several condition monitoring techniques have been proposed for machines supplied from the mains, the issue is still open in the case of inverter fed variable speed induction machines. In this work, the influence of field oriented control in the fault signature of an induction machine with a single broken rotor bar is investigated. A comparison of different indicators based on the Fourier's Series is presented. Besides, several variants of the field oriented control have been addressed, especially the sensorless schemes. This analysis is validated both through simulations and experimental data obtained in a test bench with a faulty squirrel cage induction machine.