Information Sheet
Colorado | Minnesota | New Mexico
Motor Variable Frequency Drives (VFDs)
ASDs/VFDs Control of your energy cost and improve system
performance. Save money with our rebates too!
Why fans and pumps are expensive When drives don’t generally save to operate Xcel Energy offers Custom Efficiency rebates
Most equipment fans and pumps were
selected to meet a maximum system demand,
or potential future demands. This means that
most are oversized and rarely operating at
their full design capacity. Variable frequency
drives (VFDs) or adjustable speed drives (ASDs)
provide an efficient flow control alternative
by varying the motors’ speed. Controlling the
speed/flow can significantly reduce energy use.
For example, reducing the speed (flow) by
20% can reduce input power requirements
up to 50%.*
Evaluate your drives—
the greatest cost savings
opportunity and potential
improvement exist when you: • Start with your largest Horsepower motors
followed by the longest hourly running
motors, then determine where mechanical
control devices are located.
• Evaluate whether you are over producing or
supplying, and determine potential turn down
opportunities.
• Use fundamental engineering rules like the
Affinity Laws to determine airflow (CFM),
pressure (P) and horsepower (hp) which are
all affected by most fan and pump speed
changes.
*Source: Department of Energy, 2007, and the
Affinity Laws of basic engineering
for drives installed in the following scenarios.
You should contact us during your evaluation
and before you order/purchase equipment.
• Installing a VFD and then operating it at
a constant speed/torque—When a motor
is set to run at full speed, then installing a
VFD will not save energy and can result in
higher energy usage and cost. If the drive is
set to run slightly less than 100%, it still may
be more expensive to operate because of the
drive efficiency losses and more expensive
than re-sheaving (changing the pulley size)
the existing motor to reduce speed.
• High Static Pressure installations—
Drives on submersible pumps, or any above
ground pumps that operate systems with
a high static dominated pressure level like
boiler feed water pumps, water and fire
protection systems in high-rise buildings,
and those that move water upward to fill a
reservoir offer varying results. Installing a
VFD system with high static pressure is a
good control method for this situation and
will often save energy compared to other
control methods. However, if you overstate
your systems pressure set-point, then the
VFD provides higher static head pressure
than necessary and does not turn down as
far as it should, which reduces the savings
potential.
Prescriptive rebates
(up to $8,000) and
custom rebates
See rebate applications for details
Up to 98% of the equipment
lifetime costs are for
operating, while 2% is from
your equipment purchase.
What should you focus on?
information SHEET
Motor Variable frequency drives (VFDs)
Colorado | Minnesota | New Mexico
Energy Efficiency Specialists
1-800-481-4700 or e-mail energyefficiency@xcelenergy.com
• Poor Sequencing—The best sequencing
for VFD systems depends on the end use
application. For example, cooling towers or
evaporator fans are often set up in lead-lag
fashion where each fan immediately turns on
and off based upon demand, which is good
practice. Adding a VFD to the existing leadlag configuration may consume more energy
because the drive’s programming algorithm
could activate multiple fans under its control
to start earlier, operate longer and at a higher
energy consumption level; furthermore, drive
loss factors compound the inefficiencies.
What should you do?
When choosing or installing a VFD, first make
sure that your are saving as much energy as
possible by choosing the solution that best fits
the end use application. Make sure that your
programmer knows your system and how to
maximize the benefits of the drive for your
end use application.
Act Now for Prescriptive and
Custom Rebates that offer fast
paybacks, improved comfort,
lower operating costs.
Need more information or help filling
out the application?
Contact us at 1-800-481-4700 or
energyefficiency@xcelenergy.com
• Installing a VFD to soft start motors to
reduce in-rush current or demand—
A VFD used for the purpose of eliminating
equipment failure at start up, or to reduce
demand charged by soft starting motors don’t
save significant energy. Starting a motor does
require a significant in-rush current, for a very
short period. However, demand charges are
calculated by some utilities as the average
energy usage over a period, typically 15
minutes. The energy required is the same
regardless of whether the VFD is installed;
therefore, there are no utility demand
cost savings.
Take control of your
energy costs today
Call your account manager or our
Energy Efficiency Specialists
at 1-800-481-4700 for more information
or visit xcelenergy.com/rebates
FPO
Cert no. SW-COC-1865
1-800-481-4700 | xcelenergy.com
© 2010 Xcel Energy Inc. | Xcel Energy is a registered trademark of Xcel Energy Inc. | Northern States Power
Company-Minnesota, Public Service Company of Colorado, Southwestern Public Service Company,
Xcel Energy Companies | 10-09-231 | 10/2010 | CRS # 3074