An investigation into aspects of the online detection of broken rotor bars in induction motors

Recent failures of large induction machines due to rotor bar breaks have become a major industrial concern in South Africa. These failures have occurred while applying current condition monitoring methods. This would imply that current theories are either inadequate or badly implemented. This thesis investigates two currently used condition monitoring strategies which focus on monitoring the stator current and axial vibration to detect bar breaks as well as a third method which involves putting destructive shaft voltages to good use and using this signal to diagnose rotor defects. This document begins by familiarizing the reader with two conventional approaches which involve monitoring the stator current and the axial vibration to detect rotor bar breaks. Thereafter the origins of shaft voltage are discussed and its use as a condition monitoring tool is first theoretically derived and then validated by finite element simulations. A thorough discussion of the measurement equipment required is presented and ultimately the performance of these three methods is tested by means of a laboratory measurement as well as two on-site measurements. The results from these measurements suggest that when making proper use of both conventional condition monitoring methods, these methods have a 50% success rate in the detection of rotor bar breaks. The alternative method investigated, which involves monitoring the shaft voltage, has a 75% success rate in the detection of rotor bar breaks. This highlights the use of shaft voltage as a condition monitoring tool.Dr. S. R. Holm Prof. W. A. Cronj