What do we mean by “enclosure”? The term denotes the external assembly that surrounds a motor’s working parts (stator, rotor, and winding). It serves three purposes. The first (we won’t discuss it any further here) is to support and tie together the overall structure.
The second purpose of a motor enclosure is to safely isolate those internal parts from exposure to meddling humans or a potentially damaging environment. Enclosures are designed to prevent prying fingers or tools from either incurring or causing electrical or mechanical damage involving contact with live conductors or spinning fans. They are also a means to minimize entry of moisture, dirt, or other contaminants that might harm working parts.
A motor enclosure’s third purpose is heat dissipation. Radiating surfaces or cooling air flow paths must be built into any enclosure so that the heat from internal losses can be carried away, allowing the winding to operate within its rated temperature limit. In U.S. practice, NEMA standards divide all motor enclosures into two families — “open” and “enclosed.” The former allows free interchange of air between motor interior and exterior. That air is circulated by fan action of the rotor itself, often augmented by separate fans built onto the rotor or mounted on the shaft. The most common “open” design is what NEMA calls “dripproof”. As the name implies, its air openings are so arranged that anything falling directly down from overhead will kept out of the motor. Wind-driven rain, blowing snow, clouds of dust, or material splashing up from the floor won’t necessarily be blocked, however. Hence, dripproof motors aren’t for outdoor use. In larger sizes, “weatherprotected” construction suits that purpose.
The “enclosed” design is defined as having no direct air interchange between inside and outside. Most common version: the TEFC (totally enclosed fan-cooled) type. Tight fits, gaskets, and seals exclude most contaminants. But no TEFC machine can ever be considered either “air tight,” “dust tight,” or “water tight.” Cooling is by radiation of heat off the enclosure surface (usually ribbed or finned to increase its surface area) into an air stream driven by an external shaft-mounted fan. Proper maintenance must keep the surface area unobstructed by dirt buildup, which would prevent efficient heat transfer.
Because of today’s industry trends toward reduced maintenance, and cleaning of premises by hosing down with pressurized water, the TEFC design is becoming the enclosure of choice. However, the TEFC machine is subject to internal condensation of moisture from the internal air, which inevitably “breathes” in and out of the enclosure during temperature changes. Therefore, special drains, or internal heaters to prevent condensation when the motor is idle, are usually necessary. The difficulty of providing adequate enclosure surface requires large enclosed machines either to use separate heat exchangers (coolers) or be fed with clean air through ducts from some remote location. That’s impractical for the small sizes.
Before choosing the enclosure for any motor, consider the surroundings carefully. The less expensive open versions are suited only to clean, dry locations.
Richard L. Nailen, P. E.