Abstract
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. Cronje