Failure modes and field testing of medium-voltage motor windings

Reliable electrical motor performance is vital to profitable plant operation. Although motor maintenance spending constitutes the largest component (50%-70%) of electrical maintenance budgets, its cost is relatively small compared to the lost opportunity costs of most driven equipment in pulp and paper processes. Based upon a 1983 study by the Electric Power Research Institute, Palo Alto, CA, 37% of electrical motor failures can be attributed to stator problems. The other 63% (rotating assemblies, including bearings) are susceptible to imbalance, misalignment, and other mechanical problems that can be diagnosed and mitigated through the proper application of vibration monitoring, tribology, and other well-known practices. This paper focuses on evaluating the condition of stator windings. To minimize the cost of motor ownership and optimize opportunity, it is essential to have a reasonable understanding of a motor's condition and its residual life. This can be accomplished using several different testing methods, ranging from online monitoring to offline testing. One word of warning is in order: although many popular test methods are valid, some are misleading and of little practical value. What follows is an explanation of common winding failure modes, and some results of common tests and technologies.