Monitoring of partially broken rotor bars in induction machine drives

Detection of rotor bar asymmetries in induction machines operated by an inverter can be difficult due to disturbances introduced by the fast switching power devices. Furthermore interference of the fault indicator with control dynamics of the load level lower the accuracy of most detection methods proposed. These disturbances can be avoided if the detection can be realized at standstill of the drive using the inverter as a measurement device. In this paper the sensitivity of a method is analyzed that enables the detection of rotor bar defects at zero load and around standstill. The method is based on the excitation of the machine with voltage pulses generated by the switching of the inverter. Measuring the resulting response of the machine phase currents, the spatial distribution of the transient inductance can be identified with high accuracy. An asymmetry in the rotor bars leads to a distinct change in the spatial distribution of the transient flux linkage and thus also of the transient inductance. The accuracy is such that even partial broken bars can be detected. Measurements on a machine with a specially manufactured copper rotor cage are presented. The resistance of a single bar is stepwise increased by changing the material of the bar from copper to aluminum, brass, and finally steel. It is shown that even the stepwise increase of a single bar resistance before the bar is broken can be clearly detected. It is thus possible to monitor the machine even for a developing rotor defect.