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Typically,
the annual operating cost of a motor far outstrips its initial
purchase price. So saving energy in motors systems can be
a lucrative proposition –
A
typical 75 hp motor running at full load for 6,000 hours
per year would consume about $18,000 worth of electricity
at $0.05 per kilowatt-hour (kWh). A typical purchase price
for such a motor is about $4,000.
Three
factors determine motor energy use and savings potential:
- Operating
hours is perhaps the most important determinate of energy
use and savings. Some motors are operated on an intermittent
basis with only a few hundred total hours of annual operation.
However, many motors, such as those in HVAC systems, can
operate thousands of hours a year. Efficient motors in these
later applications can amass large savings.
- Load
can be either constant or variable, although the majority
of motor loads are constant or vary over a narrow range
of less than 25 percent. Most motors operate at 60 to 70
percent of their rated capacity. Determining actual motor
loading can be difficult, but some resources are available
to aid in estimating motor loading.
- Motor
efficiency represents the percent of input energy that
is converted into useful work. Efficiency varies among different
motors and, for each motor, also with load. A pre-EPACT standard-efficiency motor
has its maximum efficiency near its full load, while most
energy-efficient motors have their maximum efficiency at
approximately two-thirds of the full load, where most motors
typically operate. The efficiency of both types of motors
declines quickly at loads below 40 percent. For premium-efficiency
motors, look for NEMA
Premium labeled motors.
The relationship
between these variables and energy consumption is as follows:
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Energy
Consumption =
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Operating
Hours x Load
Motor Efficiency
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In
addition, the economics of motor selection decisions are affected
by electricity prices and operating speed. For the latter,
power consumption of centrifugal equipment, such as pumps
and fans, varies as the cube of the motor speed. Motor speed
varies significantly among motor designs, with energy-efficient
motors tending to have higher full-load speed than standard
motors. As a result, it is important to match motor speed
to the requirements of the load, as closely as possible. At
a minimum, the full-load speed of the replacement motor should
match that of the motor being replaced, so as not to increase
the load For more information, see ACEEE's Energy-Efficient Motor Systems
Handbook.
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