Motor, generator standards

Electrical Apparatus, Nov 2004 by Nailen, Richard L

Motor, generator standards

NEMA Standards MG 1-2003, Revision 1-2004: Motors and Generators. National Electrical Manufacturers Association, 1300 N. 17th St., Suite 1847, Rosslyn, Va. 22209; (703) 841-3200. $79 (looseleaf binder). 524 pages.

Although many national and international standards are involved in motor technology, this set of NEMA standards has for decades been considered the bible of the motor industry in the U.S. Note the plural "standards"-each paragraph in MG 1 constitutes a separate standard, rather than a component of a larger whole.

"Standard" usually implies mandatory compliance. Hence, to assure impartiality, many standards-writing groups (such as UL, NFPA, or ANSI) must include representatives of all affected points of view. Inasmuch as NEMA is a manufacturers' trade association, it need not do so. Much of the language of MG 1 is therefore less restrictive. Although "shall" and "shall not" appear throughout the document, many provisions are recommendations only, with such wording as "should," "may," or "can be expected."

Applicable to both a-c and d-c machines, MG 1 is divided into four Sections. First is "General Standards Applying to all Machines," with basic definitions of machine terms and types, terminal markings, dimensions, enclosures, balancing and vibration, and noise. Section II deals with ratings and performance (torque, efficiency, frame assignments, and application data) for "small" (fractional hp) and "medium" (integral hp) motors. Section III extends such information to "large" motors, generally those above 500 hp. In Section IV, "Application Considerations" appear for inverter-duty motors and others subjected to harmonics, and synchronous generators. Each Section is divided into "Parts" covering specific topics.

This latest revision includes a rewrite of sect. I Part 5 on enclosure requirements, for closer agreement with IEC 60034-5. It also adds a new table of allowable overhung shaft loading in Sect. I Part 14 and omits the remaining references to the defunct "Design E" motors from Sect. IV Part 30.

Most of the text is clear and unambiguous, although information on some topics sometimes appears in different form in another place, without cross-reference. Probably the most troublesome feature of this revision (as in the past) is incompleteness in the index, such that anyone not fully familiar with the document cannot readily locate pertinent passages, such as Para. 12.45 cautioning against motor operation at more than 1 percent voltage unbalance.

Nevertheless, no one with responsibilities in the electric motor industry should be without a copy of NEMA MG 1. It can be ordered from Global Engineering Documents, 15 Inverness Way East, Englewood, Colo. 80112-5776; (800) 854-7179.-RLN