Choosing diagnostic tests to monitor stator insulation in critical motors

The petrochemical industry has increasing interest in condition monitoring of stator winding insulation during service. Factory acceptance specifications developed from the American Petroleum Institute (API) standards often require a PD and/or power factor tip-up (PFTU) test for brand new windings as a factory acceptance test before installation machine is installed. Petrochemical industry users often employ partial discharge (PD) detection systems for condition assessment of critical motors, but the use of the PFTU test as a periodic evaluation technique is less popular. Specialized resources are needed to perform these tests at the factory and in the field, imposing different technical demands and requiring expert data interpretation to ensure value for the end user. PD and PFTU may serve as complementary techniques and are most useful when data are trended through the machine winding service life. Both types of measurement depend on stator winding geometry, insulating materials, insulation system construction and operating environment. This paper explains the PFTU and PD tests, describes considerations for set up and execution of measurements for factory acceptance tests and installed machines, and explores technical considerations to define an appropriate test regime to monitor insulation condition. Examples of PD and PFTU test data from factory acceptance of new stator windings and on-site measurements of installed machines are discussed.