Identification of false rotor fault indications produced by on-line MCSA for medium voltage induction machines

Motor current signature analysis (MCSA) has become an essential part of the preventive maintenance program for monitoring the condition of the rotor cage in medium voltage induction motors in the pulp and paper industry. However, many cases of false indications due to interference from the motor or load have been reported. False indications can result in unnecessary inspection and outage costs (false positives) or major repair/replacement costs and loss of production (false negatives). The objective of this paper is to present the potential root causes of false indications, and provide guidelines on how commercially available off-line and on-line tests can be applied for identifying false indications from a field engineers´ perspective. Case studies of false MCSA indications and results of alternative commercial tests for improving the reliability of the diagnosis are provided through measurements on 6.6 kV and laboratory motor samples. Finally, new test methods under research and development for reliable rotor fault detection are summarized and unresolved problems are listed. This paper is expected to help field maintenance engineers prevent unnecessary motor inspection and forced outages, and guide researchers target future research towards industrial needs.