Simulation and observer based detection of airgap asymmetries caused by rotor eccentricity in inverter fed AC machines

A new method to detect fault conditions in inverter fed AC machines caused by asymmetries in the airgap is presented in this contribution. In the first part the influence of these asymmetries on the transient electrical behavior of the machine is investigated by simulation results obtained from a magnetic equivalent circuit model. Starting from these results the proposed method is derived. It is based on the reaction of the machine current to voltage pulses caused by the switching of the inverter. The continuous switching of the inverter is necessary for the current control loop during normal operation of the drive. Using a special observer structure any changes in the reaction of the machine caused by airgap asymmetries can be detected and isolated. The only inputs necessary for the realization of the method are the DC link values of the voltage and the current. The observer estimates the transient parameters of the three phases and by comparing the three phase values an error signal can be calculated. The method is insensitive to changes of the machine parameters due to temperature or saturation as long as these changes occur symmetrically in all three phases. The method is applicable in both steady state and transient operation. The practical operation of the method is verified by measurement results at different points of operation and with different fault positions/magnitudes.