August 2009 Electrical Apparatus

August 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus August 2009 featured technical article,  by Richard L. Nailen, P.E.     

Only testing can assess the integrity of motor or generator winding insulation. Because of the variety of insulation materials and systems and their functions, the Institute of Electrical and Electronics Engineers (IEEE) has developed many different tests. No single one suffices to either diagnose insulation condition or support judgment of its future life.

Rotating machinery users and service providers must decide on the most useful test to include in maintenance and life extension programs. Most IEEE procedures fall into two categories: the Recommended Practice (RP), offering methods experts consider most suitable; or the Guide, suggesting various alternatives from which to choose. Of greatest value are two RP’s–IEEE 43 and 95–and two Guides, 56 and 432. Unfortunately, IEEE 432 has been withdrawn, to be superseded by an expanded version of 56.

Machinery manufacturers no longer devote much effort to standards development. Hence many pertinent documents (like IEEE 56) have lapsed or become outdated. Organization of the IEEE itself has led to emphasis on large generators rather than industrial motors. That’s because most insulation test standards originate with the IEEE’s Power & Energy Society rather than with the Industry Applications Society. An obvious consequence is that many standards omit material appropriate for low-voltage, random-wound machines.

Also, some widely used procedures do not appear in any published standard. An example is the common surge comparison test for evaluating inter-turn insulation. Although IEEE 522 deals with the subject, it is written for form-wound machines only and outlines methods unfamiliar to service centers.

Finally, two different and often conflicting standards have been written for insulation maintenance testing: IEEE 62.2 and 1415. Some passages describe procedures unknown in the repair industry and not compatible with rotating machinery practice. Users seeking guidance face some difficult choices.

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