Download Typical HVAC System Design Instructions for Tucson Medical Center

Variable frequency drive appendix
Applications
Fans
Pumps
Chillers
Vacuum Pumps
Air Compressors
Typical Design and Operating Sequences
Operate multiple variable speed devices in parallel rather than one at a
time.
Do not use circuit setters in variable speed pumping systems unless
across the line 60 Hz operation would damage the system or building.
Discussion:
 The pump selection should not be able to damage building systems in across
the line 60 Hz operation.

A pump or fan designer can select just the right device so it operates in “the
sweet spot” BUT often designers add “fudge factors” so the installed device is
too strong and requires permanent flow restriction devices (think of these
manually adjusted flow restrictions as a 24/7 energy parasites). Circuit
setters are not to be used for variable speed systems unless 60 Hz operation
will cause damage.

Some systems require “N+1” redundant fans and pumps for important
systems. When appropriate, provide redundant fans and pumps with variable
speed drives operating in parallel at equal speed rather than using the
traditional “lead/lag” control sequence. If one device fails the other device(s)
simply speed up to maintain setpoint.

A variable speed 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
HVAC Design Recommendations by Don Rainey – 21 FEB 2011
common for devices to fail at start-up so the lag device may be at greatest
risk when it is needed most.
Minimize the wiring length between VFD and motor with a maximum length of
60 feet at 480 VAC. If wire length must be over 60 feet then use an inverter
rated motor or reduce motor voltage to less than 300.
Avoiding bearing failure:
One of the quirks of IGBT VFDs is that they can induce voltage to build up on 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 easiest way to identify potential bearing
problems is to measure voltage from the operating motor shaft to ground using an
electronic meter. Go through the entire range of anticipated speeds. If that voltage
exceeds about 3.5 VAC, 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 vendor may have some other
recommendations.
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:
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
HVAC Design Recommendations by Don Rainey – 21 FEB 2011
an exhaust fan) consider using a lower voltage (<300 VAC) 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
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.
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.
HVAC Design Recommendations by Don Rainey – 21 FEB 2011
Performance Specifications to be included in Bid and
Purchasing documents.



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 motor 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
problems. Verify this performance as part of the equipment start-up.
HVAC Design Recommendations by Don Rainey – 21 FEB 2011