IEEE guide enters the motor diagnostic field

Electrical Apparatus, Apr 2008 by Nailen, Richard L

Attempting to apply standards in a previously uncharted area

INSULATION SYSTEMS, DIMENSIONS, PERFORmance characteristics-many features and components of rotating electric machines are highly standardized. Despite design options and manufacturing variations, product interchangeability is readily available except in the larger sizes.

Once in operation, however, a motor is subject to servicing practices that have been much less fully documented. That picture has recently changed with issuance of the first IEEE document combining maintenance recommendations with diagnosis of trouble. Published in April 2007, IEEE 1415 is titled Guide for induction Machinery Maintenance Testing and Failure Analysis.

With the publication of this document, industry now has a complete set of standards (from several sources) covering the range from routine surveillance through major repair. Each of the documents has its strong points and weaknesses. They're not to be applied blindly.

To see how the latest addition fits in, we'll briefly review those publications here. IEEE 1415 is a companion to these four other standards:

* ANSI/NFPA 70B, Recommended Practice for Electrical Equipment Maintenance (launched in 1968).

* IEEE 1068, Recommended Practice for the Repair and Rewinding of Motors for the Petroleum and Chemical Industry (begun in 1989).

* ANSI/EASA AR 100, Recommended Practice for the Repair of Rotating Electrical Apparatus (first published 1998).

* ANSI/NETA MTS, Standard for Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems.

These five publications, and their relationship to one another, need to be viewed in light of their origins. Four different standards-writing agencies were involved, each with some viewpoints quite different from the others.

For example: Within the IEEE, motors and generators are the concern of two separate groups of engineers having sharply divergent interests. One is the Industry Applications Society, which produced the 1068 document (at 23 pages, the briefest of the standards examined here). The IAS comprises a number of committees drawn from specific user industries. Some (like the Petroleum & Chemical Industry Committee) are heavily involved in motor application. Members include motor designers, specifiers, operators, and maintenance personnel. Their interests tend to be those of the industry in which they work.

The second IEEE group (which produced the 1415 document) is the Power Engineering Society, under which is the Electric Machinery Committee. Although those titles imply broad interest, PES activities tend to emphasize the electric utility industry and the apparatus used in its power generation plants. Starting duty, environmental conditions, controls, maintenance practices, types of driven load-all these and many other operating circumstances can be worlds apart from what is everyday practice outside the utility plant.

Of course, the two groups are not totally isolated from each other. A broadly experienced engineer may be active in both organizations. IEEE 1068 was written as a joint project of both the PCIC and the EMC. However, the PCIC has dominated the Working Group membership (of the 80 members balloting the 1996 edition, three-fourths were PCIC representatives). And, as more than one rotating machinery specialist has declared, "If you're really involved with industrial motors, the IAS is where the action is."

No single document will answer every question a facilities operator, maintenance supervisor, or repair center may have. Each raises some questions that are left unanswered. And in even the simplest matters, they may offer differing answers to the same question. One example is megohmmeter test voltage. For a 4,000 volt motor, IEEE 1068 and NETA (the International Electrical Testing Association) recommend a 2,500 volt test; IEEE 1415 and AR 100 both call for either 500 or 1,000 volts; and NFPA 70B recommends either 1,000 or 2,500.

Wound-rotor machines have drawn little attention. For example, IEEE 1068 simply says "A squirrel-cage motor will be the type most often encountered" and includes no advice for testing or servicing a rotor winding. The NETA document bases insulation tests on the "nominal rating of equipment in volts," without reference to any voltage rating difference between stator and rotor. The same is true for NFPA 70B. Yet a rotor insulation system is normally rated for a lower voltage than the stator (for example, 750 or 1,500 volts in a rotor with stator operating at 2,300 or 4,000 volts).

Taking the publications in order . . .

NFPA 70B

In 1968, the National Fire Protection Association formed an Ad Hoc Committee on Electrical Equipment Maintenance to "develop suitable texts relating to preventive maintenance . . . with the view of reducing loss of life and property." As with all NFPA standards, the primary emphasis was on safety.

The first edition of the resulting standard, NFPA 70B, was issued in 1973. Subsequent revisions and expansions occurred in 1976, 1983, 1990, 1994, and most recently in 1998. Composition of the Technical Committee then was typical: the 20 members represented user industries, test equipment suppliers, protective device manufacturers, inspectors and regulators, contractors, electrical workers, NEMA, and the IEEE.