Electric Motors - Service factors:
A change in NEMA standards for electric motor service factors and temperature rise
has been brought about because of better insulation used on electric
motors. For instance, a 1.15 service factor -- once standard for all
open electric motors -- is no longer standard for electric motors above 200 hp.
Increases in electric motor temperature are measured by the resistance
method in the temperature rise table. Electric motors feature a
nameplate temperature rise, which is always expressed for the maximum
allowable load. That is, if the electric motor has a service factor
greater than unity, the nameplate temperature rise is expressed
for the overload. Two Class-B insulated electric motors having 1.15
and 1.25 service factors will, therefore, each be rated for a 90°C
rise. But the second electric motor will have to be larger than
the first in order to dissipate the additional heat it generates
at 125% load.
Electric motors feature a service factor, which indicates how much
over the nameplate rating any given electric motor can be driven
without overheating. NEMA Standard MGI-143 defines service factor
of an ac motor as "...a multiplier which, when applied to the rated
horsepower, indicates a permissible horsepower loading which may
be carried under the conditions specified for the service factor..."
In other words, multiplying the electric motor's nameplate horsepower
by the service factor tells how much electric motors can be overloaded
without overheating. Generally, electric motor service factors:
- Handle a known overload, which is occasional.
- Provide a factor of safety where the environment or service
condition is not well defined, especially for general-purpose
electric motors.
- Obtain cooler-than-normal electric motor operation at rated
load, thus lengthening insulation life.
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Electric Motors - Efficiency:
Small universal electric motors have an efficiency of about 30%, while
95% efficiencies are common for three-phase machines. In less-efficient
electric motors, the amount of power wasted can be reduced by more
careful application and improved electric motor design.
Electric motor's feature an efficiency level, which also depends
on actual electric motor load versus rated load, being greatest
near rated load and falling off rapidly for under and overload conditions.
Electric Motors: Introduction to Electric Motor Design Engineering
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