Condition monitoring of inverter fed induction machines by means of state variable observation

This paper suggests a method to monitor defects such as cracked rotor bars in induction machines. Rotor bar faults cause an asymmetric magnetic flux pattern in the air-gap. Thus, the current phasor (or voltage phasor at current controlled machines), the flux phasor and the air-gap torque differ from those of an ideal symmetric machine. The proposed condition monitoring method compares the outputs of a reference model that represents an ideal machine to a measurement model. Observing the deviations of these two models makes it possible to detect and even locate rotor faults. It can be applied to inverter fed machines as no frequency analysis is used. The method is verified by experimental results from a DSP-controlled IGBT-inverter drive.