Any piece of electrical apparatus is designed for use in a certain way. Nameplates tell you a lot, such as power limits, proper voltage, or operating temperature. But they can’t tell you everything. For motors, many operating conditions are defined by NEMA standards.

Such standards become important in deciding how to match motor construction to a possibly hostile environment. Open motors, needing constant passage of cooling air, can’t be sealed off from their surroundings. Even a totally enclosed machine must allow for a rotating shaft passing through its enclosures. So motors can’t have the “rain tight” or “dust tight” enclosures possible for non-rotating apparatus.

However, many motor suppliers do offer lines of “severe duty” or “chemical duty” motors. What is “severe duty”? How are such motors built?

The answers begin with an understanding of NEMA Standard MG1 14.3. That document defines “unusual service conditions” — an operating environment to which standard motors are not necessarily suited. If a standard motor is subjected to those conditions, the warranty may be void. Says NEMA, “The manufacturer should be consulted” if any of these conditions will exist:

  • Exposure to abrasive or conducting dust
  • “Very dirty” surroundings, such as to interfere with normal ventilation
  • Chemical fumes, steam, or oil vapor
  • Damp or very dry location
  • Abnormal shock or vibration from external sources

That’s only a partial list. (And what is “abnormal”? How dry is “very dry”?) Remember that these “unusual” conditions may be quite normal and customary for the motor user — in a paper mill, for example. Nevertheless, they are outside the scope of normal operation as far as the motor design is concerned.

To deal with such conditions, the typical “severe duty” motor includes these features: highly corrosion-resistant nameplate and fastenings; extra bearing seals to keep out dirt and moisture; special internal and external paint finishes; and epoxy coated windings. But each supplier takes their own approach; no industry standards apply (except that IEEE No. 841 covers certain motors specifically for the petrochemical industry).

When moisture, corrosives, or dirt are present in the motor’s environment, assume that “unusual service conditions” apply and be sure the motor supplier knows what those conditions are. Otherwise, that motor is likely to die young.

What motor service conditions does NEMA consider to be “usual”? Only these: ambient temperature between -15 and +40 degrees C; altitude not over 3300 feet; rigid mounting; unimpaired ventilation; sinusoidal, balanced voltages — these are the only conditions termed “usual.”

Richard L. Nailen, P.E.