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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
- 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
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.