Download Varialble frequency drives (VFD) design guide

VFD design guide
Purchasing and applying variable frequency drives (VFD) in
HVAC applications
Introduction
Modulating airflow and water flow in HVAC applications is a primary way of minimizing
energy consumption while providing desired temperature and ventilation control.
Modulating flow can be accomplished by varying resistance to flow with valves and
dampers. Another more efficient way to modulate flow is by adjusting pump or fan
speed to provide the minimum pressure needed to satisfy the prevailing needs.
Although many ways of modulating pump and fan speed have been developed, VFD
motor control has become the dominant method for HVAC equipment.
VFDs got off to a rocky start in the HVAC industry. First, they were quite expensive.
Next, a new output device called an IGBT was used to reduce the cost but early IGBT
VFDs damaged some motors. More recently, VFD manufacturers have learned to
compensate for the damaging characteristics of IGBTs, motor manufacturers have
made standard motors more tolerant and we have learned some installation techniques
to prevent motor damage.
Today, VFDs are reliable and cost effective. When properly applied, they provide
substantial energy savings for HVAC and other motor driven equipment including: Fans,
Pumps, Chillers, Vacuum Pumps and Air Compressors
Technical Tips from the Field
Operate multiple variable speed devices in parallel rather than one at a time? This
concept makes people nervous but it can save energy and improve reliability. If you
find this information confusing or intimidating, seek out the services of a qualified
consultant with plenty of VFD retrofit experience. Consider the following:

A pump or fan designer can select just the right device so it operates in your building
in “the sweet spot” but often designers add “fudge factors” so the installed device is
a bit too strong and requires permanent flow restriction (think of this as a 24/7
energy parasite). Perhaps you have VFDs installed already and the need for
permanent flow restriction has been eliminated. The bottom line is that many
existing pumps and fans can satisfy your needs most of the time at significantly
reduced speed.

Hospitals tend to have redundant fans and pumps for important systems. There
may be a “lead/lag” control sequence that runs one device at a time and balances
run hours so neither device remains inactive too long (an inactive bearing or seal is
unreliable).

The following statement uses ball park numbers and is only true if the device has
excessive pressure capability (remember the fudge factors) and if the system flow
resistance is dominated by variable flow devices (control valves & dampers) rather
than long pipe or duct runs. Please note that this may not work well for constant
volume fan systems unless the fans are way over designed (too much fudge factor
or a change in system).
A device running at 50% flow might use 25% of the energy used at 100% flow. Two
devices running at 50% flow each can potentially provide a total of 100% flow with a
total energy consumption of 50% compared to one device providing 100% flow.

Studies have shown that a device running at half speed may last 4 times as long as
at full speed. If two 5 year lifetime devices operate lead/lag at full speed, they may
provide a total of 10 years of service. If the same two devices both operate full time
at half speed, they may provide a total of 20 years of service.

In a lead/lag control sequence, if the operating device fails, flow will stop until the
control sequence senses the failure and starts the lag device. It is also common for
devices to fail at start-up so the lag device may be at greatest risk when it is needed
most.
Is this way too much information? Consider this approach: If you have redundant
equipment with VFDs, give it a try manually on a medium load day… meter the energy
consumption going into the VFD to account for the energy wasted by the VFD. Be sure
and run both devices at an equal speed. Open circuit setters, inlet vanes and other flow
restricting devices (remember previous positions and settings). Pressure independent
flow control devices may not be field adjustable and can mess up your results. Once
you have opened up your system, check energy consumption for “one device”
automatic operation.
Now go to manual operation of both devices. Keep gradually reducing device speed
until your total energy consumption drops below the “one device” energy consumption
or until you notice some temperature or pressure control limitations. If this test
produces some energy benefits, then consider permanent implementation with the
necessary control sequence changes. Of course you’ll get more savings if you remove
permanent restrictions such as circuit setters and pressure independent flow control
devices.
Minimize the wiring length between VFD and motor with a maximum length of 60
feet at 480 VAC. Reduce motor voltage to less than 300 if wire length is over 60 feet.
Avoiding bearing failure:
One of the quirks of IGBT VFDs is that they can induce voltage to build up in
motor shafts. If that voltage gets very high, electric arcs will pass through and
eventually destroy bearings. The conditions that lead to bearing failure are
difficult to anticipate but can be easy to identify and resolve. The first step is to
measure voltage from the operating motor shaft to ground using an electronic
meter. Be very careful measuring voltage on a rotating shaft. The author uses a
short piece of solid 12 ga. wire on the end of the electronic meter probe and
applied at the dimple in the center of most motor shafts. Go through the entire
range of anticipated speeds. If that voltage exceeds about 3.5 VAC to motor
base, you may have a problem.
A simple solution may be to adjust the VFD “carrier frequency”. If that does not
work, consider attaching a grounding brush to the shaft. Your VFD vendor may
have some other tricks up their sleeve.
Consider requiring that shaft voltage be checked and excess voltage conditions
be resolved if discovered during VFD start-up.
Reset temperature and pressure setpoints to maximize value of VFDs.
Increase temperature differential across system to maximize value of VFDs.
Don’t use VFDs for motors that constantly run at full speed:
Requirements for more air changes, ductwork leaks, partially closed dampers,
plugged water systems and poorly implemented renovations are often addressed
in hospitals by speeding up fans and pumps. Eventually, VFDs simply run at full
speed 24/7. (See Reducing Opposition to Flow).
VFD electronics waste about 3% of the energy that passes through the VFD at
full load and they waste a higher % at lower loads. Fortunately, energy savings
start building rapidly as motor speed is reduced so the speed where energy
savings compensate for VFD losses is about 98%. If you have VFDs that must
run full speed 24/7 and the opposition to flow or leakage cannot be reduced, put
the VFD into bypass or eliminate the VFD until the flow problem is resolved.
Don’t keep spare VFDs on the shelf an expect them to work immediately after
installation.
VFD capacitors lose their ability to immediately be charged if left discharged for
several months. It can take 3 or 4 days to get them back in working condition.
One medical center is keeping a 200 HP VFD on the shelf under a constant
trickle charge. This VFD is the backup for 125 HP, 150 HP and 200 HP VFDs.
Discuss backup issues with the vendors to determine your best approach.
Maximizing the use of VFDs at your facility. (Overcoming Stagnation)
Some of your consultants and some of your staff have had bad experiences with VFDs
or they have heard bad stories. They may still be hesitant to use VFDs. If they are
hesitant to use VFDs, they may not be very skillful at making the best use of VFDs.
Design Concepts and Clarifications
A. Sheaves and Impellers
Motor Speed should be used as the adjustment mechanism for balancing critical
paths in air and water systems. After testing and balancing is complete, adjust
sheaves, impellers and motor sizes as necessary so that the motor operates at
55 to 60 Hz and motor amperage should be between 70% and 95% of full load
amperage when the maximum desired system pressures and flows are
produced.
When the motor operates in VFD bypass at 60 Hz, system pressures and flows
shall not cause problems and the motor current shall not exceed full load
amperage. It may be necessary to install pressure protection switches and/or
duct blowout panels to protect variable air volume systems from over-pressure.
Coordinate these requirements with the Testing and Balancing requirements.
B. Line Reactance
Provide between 3% and 5% of input line reactance. This may be provided in
the form of separate line reactors at the input of the VFD, reactors included as
part of the DC bus or a combination of the two totaling 3% to 5%.
C. Output Rate of Rise, Peak Output Voltage and Wire Length
A primary purpose of the specification is to purchase and install VFDs that will
not damage typical premium efficiency motors. Implementing the following
requirements should eliminate motor insulation and bearing failures associated
with VFD use. 1) Control the output rate of rise or use output circuitry, which
prevents the peak output voltage from reaching 1,000 volts to ground at the
motor. 2) Limit 480 VAC wire length to less than 60 feet between the motor and
VFD (shorter is better). 3) If a small motor must be mounted on the roof (typically
an exhaust fan) consider using a lower voltage (240 VAC or less) motor so an
unlimited wire length can be used.
D. Mounting VFD
Mount the VFD close enough to the 480 VAC motor to keep the wire length
below 60 feet (shorter is better). Coordinate with Division 16 designer to insure
that this requirement is met.
It is also necessary that the VFD be solidly mounted to structural members.
Unistrut type structures can be used in most mounting circumstances. Do not
mount VFDs directly to the flexible sides of air handling units, plenums or
ductwork.
E. By-Pass Starter or Redundant VFD
A manual by-pass starter is typically required. Critical need applications may
require an automatic bypass feature. In some critical applications, a backup fan
or pump with VFD is provided, in which case, by-pass starters may not be
necessary. Motors larger than 75 HP may require a soft-start feature in the bypass starter.
If motors are typically driven faster than 60 Hz or multiple motors are driven by a
single VFD such as in a FanWall, bypassing to another VFD may be most
appropriate and manufactures are beginning to offer these products.
F. Amperage Interrupt Capacity
Requirements can vary depending on the electrical system design. The nominal
requirement is a 65,000 RMS symmetrical ampere interrupting capacity. Some
electric services require less capacity so the Division 15 designer should
coordinate with the Division 16 designer to determine the appropriate
specification.
F. Radio Frequency Sensitive Applications
A VFD may be installed in the vicinity of highly sensitive electronic equipment.
An appropriate FCC rating may be necessary in these applications and this
requirement may result in the use of older 6-step technology VFDs. Some of the
control and interface requirements in the guide specification may not be possible
with 6-step VFDs so it may be more practical to heavily filter an IGBT VFD if all of
the modern control features are needed.
G. Interface with HVAC Controls
Most VFDs now have low cost/no cost ways of interfacing digitally with the HVAC
control system. The cost is usually lowest if the interface is included in the bid
documents rather than added later so it is usually a good idea to include this at
the time of purchase.
Some critical equipment may need to continue running even if the HVAC control
system is having problems. Consider hardwiring ON/OFF and speed controls
directly to an appropriate stand-alone control panel so that it can operate VFDs in
an adequate manner even if communications to the rest of the system are
inoperable.
H. Interface with the Fire/Lifesafety Systems
A VFD specification should require all features that might ever be needed for
interface with Fire/Lifesafety systems. Coordinate with Division 16 designer to
insure compliance with all prevailing requirements.
Performance Specification
VFDs are a mature product that is well suited to HVAC applications. The application
and sequences of operation will have the most dramatic impact on the value of the
product. These performance specifications may help insure long motor life and allow
you to use standard off-the-shelf premium efficiency motors instead of waiting for an
inverter rated motor.



Under no operating conditions shall the line voltage to the motor exceed 1000
volts (to ground and from leg to leg) at any measurable frequency using an
electronic meter. This test may be performed at the disconnect or at the motor.
Verify this performance as part of the equipment start-up for 440+ VAC
applications.
Under no operating conditions shall the voltage from the motor shaft to ground
exceed 3.5 volts using an electronic meter. Verify this performance as part of the
equipment start-up.
All VFD bypass features shall work and start the driven equipment without
difficulty or slipping belts. Verify this performance as part of the equipment startup.
Calculating and Estimating Energy Consumption
A VFD vendor might tell you that in a given piping or ventilation system, changing flow
through speed modulation will change energy consumption by the cube of the flow
change. That is, if flow is reduced to 80% then the energy consumption will be reduced
to 80% of 80% of 80% or 51.2% of full flow energy consumption. Conversely,
increasing flow to 120% will increase energy consumption to 120% of 120% of 120% or
172.8%.
Alas, existing HVAC systems usually have a mixture of both frictional and control device
elements that can be difficult to characterize. What we find in the real world is that if
you retrofit an existing system to reduce flow during some of the operating hours, the
change in energy consumption frequently seems to work out closer to a “change
squared” result. This means that if we can reduce flow to 80%, the new energy
consumption will be 80% of 80% or 64%.
The utilities usually have standard calculators that estimate the energy savings
associated with replacing fan inlet vane control with VFDs.