11cover15This is a summary of the Electrical Apparatus November 2015 featured technical article, by Richard L. Nailen, P.E.    Receive this article with your subscription to Electrical ApparatusSecond only to bearing failure, and usually much more costly, winding burnout is the most common cause of electric motor breakdown. In the absence of voltage transients, overheating is normally to blame. Standard test procedures exist to relate winding insulation temperature to life expectancy, but the results are often misunderstood. The basic tests subject controlled samples to cycles of heating, humidity exposure, and mechanical stress, intended to simulate actual motor operating conditions. Sample motors are also tested. But the intended function of such testing is to establish a basis for comparison between insulation systems rather than to arrive at the expected life of motors in service. Thus, if an existing system has been known to generally result in operating life of 20 or more years, for example, a new system exhibiting similar short-time test results should be expected to achieve the same result. Such expectations involve statistical probability and are necessarily based on user experience. Just as with anti-friction bearing life, some samples will fail quickly, others will last much longer, and many will cluster around some median point. And that user experience will necessarily involve a variety of operating conditions. Test samples are subjected to uniform heating. In contrast, winding temperature will vary widely throughout any actual motor. In all sizes, some open machines are cooled by external air drawn in either at both ends or at one end only. As size increases, radial vent ducts are provided to circulate cooling air through stator and rotor cores. Totally enclosed fan-cooled machines in the smaller sizes are cooled entirely by external air blown over frame surfaces in any of several ways. The usual assumption is that the hottest region within any winding will be found within stator slots near the center of the core. In some totally enclosed machines, however, heat dissipation from core to frame is more effective than from the end windings that are exposed only to internal air circulation and are therefore the hottest region. Such variations mean that the hottest point within any winding cannot normally be exactly located, nor its temperature measured. Assumptions are made; estimates are developed but that temperature, which will determine where thermal failure is most likely to develop, remains uncertain. The thermal classification of insulation systems, defining their allowable operating temperatures, is based on some allowance for this uncertainty. To order a back issue with the full article, “What do we mean by motor temperature?” call 312-321-9440 or visit our online webstore.
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