HVAC VARIABLE FREQUENCY
DRIVES (VFDS)
POWER FORWARD TIP SHEET
WHAT IS A VFD?
A Variable Frequency Drive (VFD) is like a throttle on a car:
• Based on demand, a VFD adjusts the speed of an HVAC fan or pump motor, saving energy and prolonging equipment life.
• Without a VFD, an HVAC fan or pump motor is either 100% on or 100% off. There is no middle ground.
• VFDs provide a soft start by eliminating the initial power surge and mechanical shock of switching the motor from
off to on.
• VFDs have the potential to save significant energy and reduce
operating costs for a variety of motor-driven applications.
HOW DO VFDS WORK?
• A VFD responds to a signal from a controller, allowing the
equipment to run at the power needed to meet heating and
cooling demand.
• The typical frequency adjustment range in HVAC applications
is 10 - 60Hz AC.
• A VFD is wired in series between the main power source and
the motor.
• VFDs have a bypass switch so that HVAC operation is not
interrupted if the VFD needs maintenance.
CONTINUES ON BACK
PSO OFFERS REBATES
FOR INSTALLING VFDS
ON THE FOLLOWING
EQUIPMENT:
• HVAC FAN MOTORS
• CHILLED WATER PUMPS
• DOMESTIC WATER PUMPS
• VENTILATION FAN MOTORS
HOW DO VFDS SAVE ENERGY?
FAN CONTROLS vs VFD
air of an HVAC is system is controlled by throttling an inlet vane or braking
an outlet damper. A better way to control the airflow is using a VFD that
continuously regulates motor speed. When air needs are low, the VFD will
use less energy. For example, when an HVAC system requires 50% airflow,
the VFD uses 21% of full airflow energy, whereas an inlet vane control will
use 65% of full airflow energy and an outlet damper control will use 87%
% ENERGY CONSUMED
instantaneous load of the heating or cooling system. Without a VFD, the
60
An HVAC VFD provides the amount of energy needed to handle the
0
100
80
40
20
of full airflow energy.
20
40
60
80
100
% ENERGY CONSUMED
VFDs reduce maintenance costs and extend operating life, allowing motors
OUTLET DAMPER
to run at less than their fully rated speed. VFDs also allow more precise
INLET VANE
control of airflow, which helps to increase the comfort level in buildings.
VFD
ARE VFDS EXPENSIVE?
Paybacks for VFD installations on HVAC motors can be quite attractive. The relative cost of a VFD often decreases as
the size of the motor increases. The larger the motor, the lower the cost per horsepower. As a rule of thumb, adding
a VFD to a motor costs approximately $200 to $500 per horsepower. Customers taking advantage of PSO rebates
for VFDs can save even more.
VFD SIZE AND REBATE LEVELS
HORSEPOWER
REBATE
2
$150
HORSEPOWER
REBATE
7.5
$500
HORSEPOWER
25
REBATE
REBATE
HORSEPOWER
$1,200
60
$2,750
3
$200
10
$650
30
$1,400
75
$3,500
4
$250
15
$700
40
$1,850
Over 100 HP
$45/HP
5
$325
20
$925
50
$2,300
Over 200 HP
Requires
Custom
Application
ARE VFDS RIGHT FOR YOU?
If you’re thinking about integrating VFDs into your HVAC system,
TOTAL OWNERSHIP COST
OF HVAC MOTORS
consider the total cost of ownership. The initial purchase cost of motors
and VFDs is small in comparison to the energy cost, which can exceed
75% of the total life cycle cost. VFDs also increase equipment life, making
your investment go even further.
ENERGY COST 75%
MAINTENANCE COST 15%
PURCHASE COST 10%