Energy-efficient Motors

Energy-efficient Motors

Of all the electricity used in the commercial sector. Most motors in the commercial sector are used to power fans and
pumps in the heating, ventilation and air conditioning (HVAC) systems. It’s not unusual for a motor to consume many
times its capital cost in electricity every year. For all these reasons, it’s clear that the motors in your building probably
offer significant opportunities for savings.
Types of Motors
“Induction” motors are by far the most common type found in commercial buildings. They are used in approximately 90
percent of applications, so chances are you have quite a few of them in your facility. They are reliable and inexpensive
and operate on single-phase or three-phase power, although most motors larger than one horsepower are three-phase.
When comparing motors, the most common characteristics to be aware of include: enclosure type, speed, efficiency and
service factor. When replacing a motor, be sure to compare these specifications on the replacement motor with those
recommended by the equipment manufacturer.
Enclosure type – The motor enclosure refers to the casing of the motor and is designed to match the motor to its
operating conditions. The two most common types are Open Drip-Proof (ODP) and Totally Enclosed Fan-Cooled (TEFC).
ODP motors are made with ventilation openings in the casing that are positioned to keep liquid or particles from falling
into the motor from above. They are a very common motor for HVAC fans and pumps. TEFC motors have enclosed
casings, as the name implies, to keep moisture and particles out. They are equipped with an integral fan for cooling and
typically are used in dirtier or wetter environments.
Speed – Induction motors are available in a wide range of speeds although 1,800 revolutions per minute (RPM) is the
most common, accounting for more than 50 percent of the motor population. Motors of 1,200 and 3,600 RPM also are
popular enough to be stocked by distributors and manufacturers.
Efficiency – Standard-efficiency and energy-efficient three-phase motors are available from most manufacturers of ODP
and TEFC motors in speeds of 1,200, 1,800 and 3,600 RPM. Some manufacturers also offer a line of premium efficiency
motors. Higher efficiency motors are built with higher quality materials and other design features that improve their
performance over standard models.
Service factor – The service factor specifies the capacity of the motor to withstand prolonged overload conditions. A
service factor of 1.0 indicates that prolonged operation above full load can damage the motor. A service factor of 1.15 is
typical for motors one horsepower and above, and indicates that the motor can work at 1.15 times its rated horsepower
without failing.
Ways to Save on Motor Energy Costs
Turn it off – The simplest and most obvious method of saving motor energy is simply to turn it off when its not needed.
Motors often run unnoticed when they are not needed, increasing energy costs. Motors can be switched manually and this
is a fine solution for many applications, but there are also timers and sensors available that will turn them off
automatically. Examples of motors that could be turned off at night include those for service hot water circulation, air
compressors and ventilation fans.
2
Reduce the speed on a fan motor – Another simple method of reducing motor energy costs is to reduce the speed of an
HVAC fan. Energy consumption of fans and pumps varies according to the speed raised to the third power, so small
changes in speed can make big changes in energy consumption. The chart below shows the relationship of fan speed in
revolutions per minute (RPM) to power
consumption. Most HVAC equipment
comes from the factory with fan motor
speeds preset, although they can often be
reset on-site by an HVAC technician if a
slower speed will still deliver the
necessary airflow. Be sure to check with
your technician or building engineer
before reducing fan speeds to make sure
that doing so won’t adversely affect
indoor conditions. Most air conditioning
equipment is designed to deliver about
400 cubic feet per minute of airflow per
ton of cooling capacity in order to
function properly.
Use variable speed drives (VSD) for
variable loads – Some loads driven by
motors don’t need to operate at the same
speed all the time. For example, pumps
and fans often don’t need to produce the
same flow all the time. These types of
loads offer big opportunities for savings by moderating their speed according to their load. For example, reducing a fan’s
average speed by 20 percent with a VSD can reduce energy consumption by more than 40 percent. Actual savings will be
slightly less than those shown on the power consumption graph since variable speed drives themselves aren’t 100 percent
efficient. Some examples of potential VSD savings are provided in the table below.
Potential Savings from Variable Speed Drives for Fans and Pumps
Average Percent
Speed Reduction
Potential Energy
Savings
Annual Energy Cost
Savings for a 5
Horsepower Motor
Annual Energy Cost
Savings for a 10
Horsepower Motor
10% 22% $272 $543
20% 44% $543 $1,087
30% 61% $753 $1,506
40% 73% $901 $1,803
50% 83% $1,025 $2,050
60% 89% $1,099 $2,198
Note: The annual cost savings estimate assumes a 5 or 10 horsepower motor operating
3500 hours per year at the average speed reduction shown in the chart. The potential
energy savings assume approximately five percent energy losses due to the VSD.
Specify energy-efficient motors – When replacing an existing motor or when specifying new equipment, consider using
a high-efficiency motor. High-efficiency motors use better quality materials and are manufactured to higher quality
specifications than standard-efficiency motors. They are five to 10 percent more efficient on average than standard motors
in the smaller sizes (25 horsepower or less). Federal efficiency standards now require minimum efficiencies for electric
Motor Power Consumption Vs. Speed
for HVAC Fans and Pumps
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Rated RPM (%)
Power Consumption (%)
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motors so the older motors in your facility are likely to be less efficient than the lowest efficiency motors of that size you
can buy now.1
Be sure to check with the manufacturer or your building engineer when installing energy-efficient motors on fan and
pump applications because some energy-efficient motors have higher speeds than standard motors. As discussed above,
increasing fan or pump speed can actually result in an increase in energy use. So, it’s important to specify that the new
motor has a full load speed no greater than that of the motor it’s replacing. The following table provides a comparison of
the efficiencies of standard- and high-efficiency motors, and examples of potential savings with high-efficiency motors.
Annual Energy Cost Savings With High-Efficiency ODP Motors
Motor Motor Efficiency (%)
Size
(Hp)
Standard-Efficiency
Motor
High-Efficiency
Motor
Annual Energy
Cost Savings
($/yr)
1 76.5 83.8 17.07
1.5 77.4 85.2 26.59
2 79.7 85.8 26.74
3 82.6 88.4 35.71
5 84.1 89.3 51.89
7.5 85.9 91.0 73.34
10 86.9 91.3 83.12
Source: Efficiencies: MotorMaster. Assumes 3500 hours per year of operation.
Properly size motors – Many motor systems are oversized, and a significantly oversized motor will run at low efficiency
increasing energy costs. An oversized motor also costs more to buy. The efficiency of most motors peaks around 75 to 80
percent of full load and drops off sharply below 40 to 50 percent of full load, although these ranges vary by design and
manufacturer.2 High-efficiency motors tend to maintain their efficiency over a wider range of loads than standard motors.
Motors loaded below 50 percent are almost
always attractive candidates for replacement.
However, because the relationship between
efficiency and load varies among different
types and sizes of motors, be sure to check
with the manufacturer or building engineer
before replacing an oversized motor.
Reduce the load – Often it’s possible to
reduce the load on a motor and save energy
by reducing pressure losses in pipe and duct
runs with low-pressure loss elbows and
fittings. Duct and pipe systems with lower
pressure losses (usually expressed as “static
pressure”) can often use a slower speed fan or
pump to deliver the same amount of flow. As
shown above, this can result in big savings.
Other ways to reduce the load on a motor
system include aligning the motor drive, and
replacing inefficient drivetrains such as belts, chains, and gears with direct drive systems.
1 The Energy Policy Act of 1992 went in to effect October 24, 1997 establishing minimum efficiency standards for motors of NEMA
design A and B, from 900 to 3600 rpm, between 1 and 200 horsepower.
2 Energy-Efficient Motor Systems, ACEEE, 1991.
Motor Efficiency vs. Load
74%
76%
78%
80%
82%
84%
86%
88%
90%
0 10 20 30 40 50 60 70 80 90 100110120130140150
Percent Full Load
Efficiency
Standard efficiency 10 HP
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Perform regular maintenance – For maximum performance and greatest energy efficiency, lubricate drivetrains
(bearings, chains and gears), keep drive belts at their proper tension, clean fan blades, check pump impeller blades for
wear, and replace air filters regularly. Most maintenance actions pay for themselves with longer lasting equipment and
less downtime even without the energy savings.
Should You Buy New or Rewind?
When you have a motor failure you’ll need to decide if you should buy a new motor or fix the old one. A common cause
of motor failure is problems with the motor windings, and the solution often is to rewind the old motor. Because it is
economical in terms of initial cost, rewinding of motors is very common particularly for motors of more than 10
horsepower. However, the motor rewinding process often results in a loss of motor efficiency. It is generally costeffective
to replace motors under 10 horsepower with new high-efficiency motors rather than rewind them. When
deciding whether to buy a new motor or rewind the old one, consider the cost difference between the rewind and a new
high-efficiency motor, and the potential increase in energy costs of a rewound motor that is less efficient than the original.
For More Information on Energy-efficient Motors
Contact the Web sites of the Association of Energy Engineers, the U.S. Department of Energy’s Energy Efficiency and
Renewable Energy Network (EREN), and the U.S. Environmental Protection Agency’s Energy Star Buildings Program.
The MotorMaster software package is a comprehensive, user-friendly motor selection and evaluation tool that is available
for download from the EREN website.