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Definition: Thermal Considerations

Thermal considerations are involved in a large percentage of motor application decisions. The main adverse effect of overheated motors is the breakdown of insulation caused by excessive temperatures. Eventually, poor insulation leads to electrical shorts in the windings, necessitating a rewind of the motor. Under rare conditions, overheated motors can also cause fires. Many motors today, particularly the more expensive motors, are built with temperature sensors which allow monitoring of winding temperatures and allow for automatic shutdown if desired.

Thermal considerations during application of the motors depend largely on the enclosures involved.

TEFC - The most common, allows operation above 1/3rd of base speed. At slower speeds, the fan does not cool the motor sufficiently. (Note that good engineering would keep the motor operating above 50% of base speed. These figures assume an acceleration of 5 seconds or so from zero speed to operating speed.) Naturally, it is important to keep the air flow clear on this motor to prevent overheating.
TENV - The lack of fan means that these motors are designed to run hot. This also means that the motor frame size is almost always larger than the equivalently rated TEFC motor. TENV DC motors are almost always designed for variable speed operation, while TENV AC motors may be designed for slow operation -- or they may not be. Look for a designation in the catalog of "inverter duty" or "vector duty". Keep dirt or grime buildup off the external surfaces of the motor, since it is these surfaces which radiate the heat from the motor.
TEBC - The constant speed blower means that these motors are well-suited for operation at very slow speeds. Once again, keep the air flow clear.
TEWC - The rare water-cooled motor is very effective in slow/variable speed operations. It is also effective in normal operations, though expensive. A point in the TEWC's favor is that the motor exhausts almost it's entire heat production to the outside with the water flow -- typically about 4 to 8 liters per minute per hundred horsepower (75 kW). This can significantly reduce air conditioning load and improve worker comfort, particularly in warm climates. A flow switch alarm needs to be included to quickly shut down the motor if water flow drops below specified levels, because the motor will very quickly overheat without coolant.
ODP - The open motor is also somewhat effective in variable speed operations, as long as it operates above 1/3rd of base speed. A key point in dealing with these motors is to keep the motor clean of all lint, dust, and grime buildup, since this can quickly cause overheating if the vents become partially or fully clogged.
TEAO - This motor is designed solely for use in the air stream of a fan or blower, and is good for variable speed operations. Be careful to inspect regularly for lint and dust build-up on the motor, as this will shorten it's life.

Specialty motors, particularly servo motors and stepper motors, have additional thermal considerations involved. These motors have dual ratings:

Continuous torque - This is a rating based upon the ability of the motor to shed heat over time (typically given as the worst 10 seconds of operation). Some continous ratings are based upon a root-mean-square calculation; others are based upon a simple average.
Peak torque - This rating is based upon the motor's actual maximum short term torque output.