28-03-2014, 03:05 PM
BUYING AN ENERGY-EFFICIENT ELECTRIC MOTOR
ENERGY-EFFICIENT ELECTRIC.pdf (Size: 105.69 KB / Downloads: 21)
Why is improving motor efficiency important?
Over half of all electrical energy consumed in the United States is used by electric motors. Improving
the efficiency of electric motors and the equipment they drive can save energy, reduce operating
costs, and improve our nation’s productivity.
Energy efficiency should be a major consideration when you purchase or rewind a motor. The annual
energy cost of running a motor is usually many times greater than its initial purchase price. For example,
even at the relatively low energy rate of $0.04/kWh, a typical 20-horsepower (hp) continuously running
motor uses almost $6,000 worth of electricity annually, about six times its initial purchase price.
What is an energy-efficient motor?
Motor efficiency is the ratio of mechanical power output to the electrical power input, usually ex-
pressed as a percentage. Considerable variation exists between the performance of standard and
energy-efficient motors (see Figure 1). Improved design, materials, and manufacturing techniques
enable energy-efficient motors to accomplish more work per unit of electricity consumed.
Energy-efficient motors offer other benefits. Because they are constructed with improved manufac-
turing techniques and superior materials, energy-efficient motors usually have higher service fac-
tors, longer insulation and bearing lives, lower waste heat output, and less vibration, all of which
increase reliability. Most motor manufacturers offer longer warranties for their most efficient models.
What efficiency values should I use when comparing motors?
When comparing motor efficiencies, be sure to use a consistent measure of efficiency. Nominal efficiency is
best. Nominal efficiency is an average value obtained through standardized testing of a population of motors.
Minimum guaranteed efficiency, which is based on nominal efficiency, is slightly lower to take into account
typical population variations. Minimum guaranteed efficiency is also less accurate, because the value is
rounded. Other efficiency ratings, including apparent and calculated, should not be used.
In the United States, the recognized motor efficiency testing protocol is the Institute of Electrical and Elec-
tronics Engineers (IEEE ) 112 Method B, which uses a dynamometer to measure motor output under load.
Different testing methods yielding significantly different results are used in other countries. The NEMA name-
plate labeling system for design A and B motors in the 1- to 500-hp range uses bands of efficiency values
based on IEEE 112 testing.
When is an energy-efficient motor cost effective?
The extra cost of an energy-efficient motor is often quickly repaid in energy
savings. As illustrated in Table 2, each point of improved motor efficiency
can save significant amounts of money each year. In typical industrial appli-
cations, energy-efficient motors are cost effective when they operate more
than 4000 hours a year, given a 2-year simple payback criterion. For ex-
ample, with an energy cost of $0.04/kWh, a single point of efficiency gain
for a continuously operating 50-hp motor with a 75% load factor saves 4079
kWh, or $163 annually. Thus, an energy-efficient motor that offers four points
of efficiency gain can cost up to $1,304 more than a standard model and
still meet a 2-year simple payback criterion. A utility rebate program would
further enhance the benefits of an energy-efficient motor.