Detection of rotor eccentricity caused air gap asymmetries in inverter fed AC machines based on current step response

With the application of fast switching power electronics and high dynamic control to ac machines the occurrences of fault condition have increased in the past years. The degradation of the bearings is the most frequent fault condition on modern inverter fed drive applications. In this contribution a new reliable method is proposed to detect fault conditions that result in air gap asymmetry. It is based on the step response of the machine phase currents to an excitation with voltage pulses applied by the switching of the inverter. Simulation results of a magnetic equivalent circuit model of an induction machine are presented to show the influence of air gap asymmetry on the transient electrical current slopes of the machine. Based on these results the necessary transient excitation and the evaluation algorithm are derived to detect and isolate these asymmetries. The only inputs necessary for the realization of the method are the phase currents which are also necessary for the current control loop during normal operation of the drive. Thus no additional sensors are needed to implement the proposed method. Finally, measurement results are given for a special machine equipped with eccentric sleeves in the bearing housings. The applicability of the method is verified for stationary as well as transient operation.