Totally Enclosed Motors

The choice for severe applications below 250 hp is the totally enclosed fan-cooled (TEFC) motor (see Figure 7-1). TEFC motors separate the internal and external ventilating air. Breathing is the only way external air ever gets inside. At some size above 250 HP, manufacturers presently switch from a rib-cooled to an exchanger-cooled motor for totally

Figure 7-1. A direct-connected, TEFC two-pole motor driven centrifugal single stage compressor. (Courtesy ofA-C Compressor Corporation}

enclosed fan-cooled (TEFC) applications. These exchanger-cooled motors are often referred to as totally enclosed air-to-air cooled (TEAAC) motors. Fin-cooled TEFC motors are becoming available in ever increasing horsepower, and are presently available from some manufacturers at sizes up to 2000 hp. These appear to be the preference of many users for the most severe applications. Above 500 hp (approxi mately), totally enclosed motors with water-to-air coolers (TEWAC) cost much less than TEFC motors and, in large ratings of synchronous motors, may cost even less than WPII. Large synchronous machines are frequently enclosed and supplied with coolers for mounting in the motor foundation at lower cost than integral-mounted coolers.

Totally enclosed motors offer the highest degree of protection against moisture, corrosive vapors, dust, and dirt. TEWAC motors also have the advantage of reduced noise level.

Division 1 Enclosures

Explosion-proof motors can withstand an internal explosion without igniting a flammable mixture outside the motor. These motors are totally enclosed, fan-cooled, and specially machined to meet Underwriter's Laboratory Standards. Explosion-proof squirrel-cage motors are available up to 3,000 hp at 3,600 rpm, but the larger sizes are usually not practical or economical.

Force-ventilated (F-V) motors are suitable for hazardous locations. Safe air is brought in through a duct system, passed through the motor, and then discharged preferably through another duct system to the limits of the hazardous location. The ventilating ducts should be pressurized to prevent entrance of contaminated air. This indoor construction has been largely replaced by outdoor motors such as weather-protected or totally enclosed types. It might be noted that while the F-V is normally used indoors, there are exceptions where it has been put in outdoor service.

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