When we talk about a “standard” a-c motor, what do we mean? How do we know which motor characteristics are truly standard, and which are specialties that must be spelled out to the supplier at extra cost? Knowing what standards documentation exists, and how it originates, can be helpful in dealing with such questions.
Why have standards at all? Without them, all manufacturers would build motors to their own preferred dimensions and horsepower ratings, using components meeting different criteria in each factory. No interchangeability would exist. We would lack any common basis for cost comparison. Finally, experience with one make of apparatus could give us no assurance of what to expect from any other.
In the United States, four agencies produce accepted motor standards. Foremost is NEMA (National Electrical Manufacturers Association). Basic motor enclosure definitions, horsepower ratings, minimum torques, nameplate temperature ratings, noise levels, dimensions (“frame sizes”) — these and many other features are detailed in the standards titled collectively “NEMA MG1” (the “MG”stands for “Motor and Generator”). This document covers both induction and synchronous a-c motors, singlephase and polyphase, as well as d-c motors and generators. These standards are reviewed completely at irregular intervals; the last complete re-issue occurred in 1998, but changes have taken place since.
Standard methods for rating winding insulation systems, and for testing motor performance, are developed by the IEEE (Institute of Electrical and Electronics Engineers), in such documents as test standard 112 for polyphase induction machines and standard 117 for low-voltage motor insulation. The IEEE doesn’t deal with motor performance levels or basic dimensions. IEEE standards are subject to revision, withdrawal, or reaffirmation at five-year intervals.
Both NEMA and IEEE standards are created by working groups or committees. Prescribed group makeup and voting procedures assure that all concerned interests can be heard. Of course, NEMA is a manufacturer trade association, so its viewpoint seems narrow to some. However, its members are sensitive to market forces; the motor user is not ignored.
Approval by an even more representative process, involving input from the public, government agencies, etc., can result in a standard’s adoption by ANSI (American National Standards Institute). NEMA MG1 is normally so adopted, for example.
A fourth source of motor standards, when hazardous area service is involved, is UL (Underwriters Laboratories). Their concern is solely with public safety. UL design and test procedures apply to all “explosion-proof” electrical apparatus, including motors.
Richard L. Nailen, P. E