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SMART IDEAS FOR YOUR BUSINESS
®
VARIABLE SPEED DRIVES
FACT SHEET
Variable speed drives (VSDs) allow motor-driven devices
such as fans, pumps and automation equipment to vary
the rate of speed at which they operate based on the
actual needs of the equipment, rather than operating at a
constant full speed.
Many motors operate at a constant speed all the time,
regardless of load demand, but constant speed means
constant energy consumption. When constant speed isn’t
necessary, VSDs allow a motor’s operation to be
continually tuned to match the varying demand. Adding
VSDs to the motors that run fans and pumps can reduce
energy use significantly, and incentives from ComEd’s
Smart Ideas for Your Business® program can help get
you going.
HOW VSDS SAVE ENERGY
The most common type of VSD is a variable frequency
drive (VFD). The most efficient VFD systems include
three-phase induction motors combined with a VFD
controller, a solid state electronic power conversion device
that changes the motor input frequency to control the
speed of the motor.
Electric motors that drive equipment like pumps and
fans normally operate at a constant speed. Fluctuations
in demand are usually adjusted by some form of
mechanical throttling, such as a valve or louvers, to
control water or air flow speed and volume. A VSD
handles these fluctuations by varying the current and
motor speed–thereby making effective use of the motor
without wasting energy.1
To quantify the savings that can be achieved with the
use of VSDs, consider that when a shaft speed is reduced
by 10 percent, power consumption is reduced by 27
percent. If speed is reduced by 20 percent, power is
U.S. Department of Energy Energy Efficiency and Renewable Energy, Improving Motor and
1
Drive System Performance: A Sourcebook for Industry (Golden, Colorado: National
Renewable Energy Laboratory, 2008), 30.
2
3
Ibid., 89-90.
Phil Gregory, “Take Control of Your Processes with Variable Speed Drives.”
(Paper presented at ComEd’s Energy Efficiency Expo, Rosemont, IL, 19 May 2010).
4
Ibid.
ComEd.com/BizIncentives
reduced by 49 percent. Understanding those ratios
requires a little math: the fluid or air flow varies directly
with the pump or fan rotational speed. However, the fan’s
input power requirement varies as the cube or third
power of the speed ratio. Thus if 100 percent flow
requires full power, 75 percent flow theoretically requires
(0.75)3 = 42 percent of full power.2
Energy use is also optimized as the motor starts up.
VSDs are often equipped with “soft starting” features
that decrease motor starting current to about 1.5 to 2
times the operating current, dramatically reducing the
impact of fan starts on an electrical system. VSDs also
reduce the “voltage sag” that can occur when a large
motor starts quickly. Voltage sags can dim lights and
cause other equipment to shut down or restart.3
Soft starts typically allow a motor to develop 150 percent
of its rated torque while the VSD is drawing less than 50
percent of its rated current. The controlled start-up also
helps extend motor life and reduce maintenance costs.4
APPLICATIONS FOR VSDS
The most common application of VSDs is on HVAC pumps
and fans. In a typical building, maximum capacity is
required only about 10 percent of the time; most of the
time, buildings require 40 to 70 percent of maximum
energy. The VSD delivers its savings by controlling motor
speed to provide only the precise airflow and pressure
required to maintain desired heating or cooling levels.
Another common application of VSDs is on air
compressors. A VSDis ideal for “trimming”—matching
pressure to demand. A compressor equipped with a VSD
varies compressed air flow in response to detected
changes in air system pressure to maintain an exact and
constant pressure level. As demand falls and more air is
delivered into the air system, the system pressure will
rise and the VSD will reduce the compressor’s speed and
output. An increase in compressed air demand causes
system pressure to fall and the VSD to increase output.
Demand control ventilation is a growing application for
VSDs. DCV systems regulate exhaust fan speed based on
need, reducing the fan’s energy consumption by 30 to 50
percent. Because these systems keep heated and airconditioned air from being lost, HVAC system energy
consumption can also drop by 20 to 30 percent as well.5
DCV systems for office space, parking garages and
commercial kitchens may be eligible for standard
incentives based on exhaust fan HP or square feet of
conditioned space.
Other applications for VSDs include:
» Cooling tower fans
» Industrial or process equipment
» Pumps
» Supply fans
» Compressed air systems
» Exhaust fans
5
U.S. Environmental Protection Agency, ENERGY STAR ® Guide for Restaurants: Putting
Energy into Profit (www.energystar.gov, 2009), 6.
According to the U.S. Department of Energy, motordriven equipment accounts for 64 percent of the electricity
consumed in the U.S. industrial sector. Industrial plants
can often reduce their current electricity use and costs by
about 5 to 15 percent or more by improving the efficiency
of their motor-driven systems.
CONTACT US
For more information about Smart Ideas for Your
Business, including applications and incentive amounts,
visit www.ComEd.com/BizIncentives. You can also
reach us by phone at 888-806-2273 or email us at
[email protected].
Smart Ideas® is funded by ComEd customers in compliance with Illinois Public act 95-0481.
© Commonwealth Edison Company, 2012