Alternatives for assessing the electrical integrity of induction motors

The authors present alternatives appropriate for detecting incipient failures in three-phase induction motors of the sizes commonly found in the process industries, with an emphasis on techniques resulting from a recently completed project. The techniques discussed are divided into three categories: periodic tests, sequence tests, and the decision-function method. The use of the effective negative-sequence impedance test is recommended for in-plant predictive maintenance applications. This test is sensitive to a wide range of degradation mechanisms that will ultimately result in motor failure, and its use can allow for timely repair of the motor before costly downtime or a catastrophic failure is incurred. The technique can be applied while the motor is in operation. It is recommended that this predictor be monitored continuously, rather than being applied on a periodic basis, because an appropriate interval between tests cannot yet be defined. The use of the decision-function method will allow a more detailed analysis of motor performance and condition. However, the price for achieving this increased information is high and may not be warranted in many situations.