SEDAC TECH NOTE
Variable Frequency Drives
TN 11-01: Revised November 2011 A variable frequency drive (VFD) is a type of adjustable speed drive used to control the rotational speed of an alternating current
electric motor by adjusting the frequency and voltage applied to the motor. Electric motors drive many types of equipment,
including fans, pumps, and air compressors. Motor-driven equipment accounts for 64% of the electricity consumed by U.S. industry,
according to the U.S. Department of Energy1.
Although equipment can generally operate at velocities less than the maximum design speed, motors typically drive equipment at a
constant rate. Flow and pressure are regulated through the use of a throttling device, such as a valve, damper, or bypass. A variable
frequency drive provides a more efficient way to control varying flow rates and pressures. VFDs are also often called variable speed
drives, variable frequency inverters, or frequency converters.
VFDs are used for motor control in diverse applications. Before
investing in VFDs, consider the type of load and potential
benefits. Motor loads fall into three categories: constant torque
loads, constant horsepower loads, and variable torque loads.
Constant torque loads have torque that remains constant
regardless of the speed at which the system is running. This
category of load includes conveyors, extruders, positive
displacement pumps, and compressors. The advantages of VFDs
for this application are precise speed control, soft starts and
stops, and energy savings from low speed operation.
Constant horsepower loads are loads in which power does
not vary, regardless of speed. The torque here varies inversely
with the speed of the motor. Examples of constant horsepower
loads include lathes, drilling, and milling. Since power remains
constant for this application, a VFD would not achieve any energy
savings, though qualitative benefits could still be realized. Here,
qualitative benefits refer to non-quantifiable factors, such as
better process control.
Ideally, variable torque loads maintain torque that varies directly
with the speed squared, and power that varies directly with
the speed cubed. Since fan and pump efficiencies decrease at
reduced speeds, the Commercial Energy Auditing Reference
Handbook suggests it is more accurate to assume the power
varies with the speed squared.4 As a result, a reduction in
speed significantly decreases motor energy use and demand.
For example, decreasing motor speed by 10% reduces required
power by 19%. Due to the energy savings potential, VFDs are
most commonly used for this load type. Typical applications
are centrifugal fans, blowers, and pumps. Additional benefits
are more precise process control and maintenance savings from
reduced stress on the system.
“ITP BestPractices: Motors, Pumps and Fans.” EERE: EERE Server Maintenance; 20 Dec
2010. <http://www1.eere.energy.gov/industry/bestpractices/motors.html>
2
www.oregon.gov/ENERGY/CONS/Industry/docs/AuditGuide.pdf?ga=t
3
“Variable Frequency Drives Introduction.” Office of Energy Efficiency. Web. 20 Dec. 2010.
<http://oee.nrcan.gc.ca/industrial/equipment/vfd-ref/index.cfm?attr=24>
1
BENEFITS OF VFDS
The use of variable frequency drive control offers several
advantages. The most significant benefit is its potential to
reduce electrical energy consumption and demand from
motor-driven processes.
Figure 1 below compares the relative power requirements
of a fan at different flow rates, using three types of throttling
control: outlet damper control, variable inlet vane control,
and VFD control. Although VFDs save far more energy
than throttling, the technology has not yet achieved
widespread adoption. According to the Bonneville Power
Administration, throttling continues as “one of the most
common and inefficient methods to control a fan or pump.” 2
120%
% of Design Power
A P P L I C AT I O N S
100%
80%
60%
40%
20%
0%
50
Outlet Damper
75
% Flow
Inlet Vanes
100
VFD
Figure 1. Comparison of relationship between power and
flow for different fan control types.3
Variable frequency drives also have the potential to reduce
system maintenance and related costs. Control with a VFD
affords the capability to “soft start” a motor, which means
the motor can be brought up to its running speed slowly
rather than abruptly starting and stopping. Soft starting a
motor results in less mechanical stress on equipment and,
over time, less maintenance is required. Similarly, running
the motor at lower speeds extends the lifetime of other
equipment components, including shafts and bearings.
Note that variable frequency drives have the potential to
fail, and a manual bypass should be included so that the
motor can operate in case the VFD fails.
4
Doty, Steve. Commercial Energy Auditing Reference Handbook. Lilburn, GA: Fairmont, 2008.
SMART ENERGY DESIGN ASSISTANCE CENTER
PROVIDING EFFECTIVE ENERGY STRATEGIES FOR PUBLIC AND PRIVATE BUILDINGS IN ILLINOIS
COSTS
Costs for a variable frequency drive vary significantly, dependent
upon the application and features required, such as horsepower
(HP). According to the RS Means Mechanical Cost Data 2011,
installation costs start at approximately $642 per HP for a 3 HP
drive and level off at roughly $101 per HP for a 200 HP drive.5 In
addition to the drive itself, installation cost may include cable and
conduit, foundations, and labor expenses.6 Due to these costs,
SEDAC recommends investigating the application of VFDs for
motors that: (1) are greater than 1HP (2) run 2000 hours or more
per year and (3) can have their speed reduced.
E XA M P L E
Consider a 20-horsepower motor that drives a centrifugal pump. The
pump operates at full speed for 365 days annually, 24 hours each day.
The operational cost is calculated with the following formula:
ADDITIONAL RESOURCES
Bonneville Power Administration
Offers calculator to estimate savings from the
installation of VFDs on pumps and fans.
www.bpa.gov/Energy/N/industrial/xls/
ASDCalculators.xls
U.S. Department of Energy Industrial
Technologies Program: BestPractices
Find tip sheets, case studies, software, and other
resources for motors and drives.
www1.eere.energy.gov/industry/bestpractices/
motors.html
So, when constantly running at 100% speed (and assuming $0.10/kWh),
the cost is:
Natural Resources Canada
Provides an introduction to VFDs as well as a
guidebook and case studies.
Since this particular pump accommodates a varying load, the pump
does not need to be run at full speed throughout the day and therefore,
a variable frequency drive can be employed to reduce the pump motor
speed. The pump load schedule is: 20% of the time at 50% full speed;
50% of the time at 80% full speed; and 30% of the time at 100% full
speed. Very often, the savings from the installation of a VFD to control a
motor are estimated using the pump affinity laws, which estimate that
the power required by a motor is proportional to the cube of the speed.
To account for a decrease in fan and pump efficiencies when run at
reduced speeds, the Commercial Energy Auditing Reference Handbook
suggests using a modified affinity law to estimate savings, as follows: 4
http://oee.nrcan.gc.ca/industrial/equipment/
vfd/vfd.cfm?attr=24
At 50% speed for 20% of the time:
At 80% speed for 50% of the time:
FIND ENERGY INCENTIVES
Public bodies:
DCEO - ILLINOIS ENERGY NOW
$92 per motor HP for chillers, pumps, fans.
Customers of:
COMED - SMART IDEAS®
$25 per motor HP for chiller, $60 per motor HP
for fans and pumps, or $100 per motor HP for
air compressors.
AMEREN IL - ActOnEnergy®
$90 per motor HP for pumps and fans
SEDAC
Who We Are
The
Finally, at 100% speed for 30% of the time:
@%  20   0.746 ⁄  30%  8760   $0.10/  $3,921
Annual cost savings from installing a VFD on this motor are:
Based on an estimated installation cost of $4,025 from RS Means, this
application would have a simple payback of approximately 0.93 years.
Mossman, Melville. RSMeans Mechanical Cost Data 2011. Kingston, MA: R.S. Means, 2010.
www.oregon.gov/ENERGY/CONS/Industry/docs/AuditGuide.pdf?ga=t
5
6
www.SEDAC.org | 1-800-214-7954 | info@SEDAC.org
Smart Energy Design Assistance Center
1 Saint Mary’s Road, Champaign, IL 61820
Smart Energy Design Assistance Center
provides no-cost advice and analyses to
Illinois private and public facilities to increase
economic viability through the efficient use of
energy resources. SEDAC is sponsored by the
Illinois Department of Commerce and Economic
Opportunity in partnership with ComEd and
Ameren Illinois Utilities. SEDAC is an applied
research unit of the School of Architecture at
the University of Illinois at Urbana Champaign
and is supported by the 360 Energy Group.