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Solution

Figure 3: One VSD per motor fan array Figure 4: ECM fan array

to the shared start/stop relay logic, which results if so, what protocol) or if the integration point is cialized fire control logic available in Hand mode.
in an open safety removing the start command the intermediary controller. Assuming the inte- These are considerations that hopefully occurred
from all the VSDs. While this does stop the VSDs gration point is to the intermediary controller, up front in the design and are not something
(when the drive is in Auto mode), unfortunately now the controls contractor should determine being figured out during commissioning.
the safeties are no longer in the circuit for Hand which data points are offered from that controller
mode. Safeties must be functional in Auto and and which to show on the OWS. Too often that Summary
Hand mode. intermediary device has a very limited object list.
For example, an ECM may have 20 points of data There is an incredible amount of useful data in
ECMs available, but if the intermediary controller is only HVACR devices. That data is just waiting to be
requesting (3) status/data points over Modbus extracted via BACnet and displayed and trended
Over the past 10 years, ECMs have become from the ECM, then the BMS will only be able to on the owner’s OWS. For over 15 years con-
another popular solution for fan arrays. The ini- read those (3) data points from the intermediary trols contractors have been pulling that data via
tial popularity was due to high efficiency, thus device. Consulting Engineers should communi- BACnet from single VSD applications. With fan
energy savings, of an ECM compared to a tradi- cate clear expectations for the fan array’s inter- arrays and their multiple motors, we have the
tional AC induction motor. This healthy competi- mediary controller and the controls contractor’s added wrinkle of the possibility of multiple VSDs
tion of motor technologies resulted in additional responsible point list, otherwise the owner may or multiple ECMs being involved. Someone famil-
motors developed for fan arrays – ones like the have a very limited OWS graphics/data when it iar with a single VSD integration will typically
VSD powered ferrite assisted synchronous reluc- comes to an ECM fan array solution. Unlike when quickly figure out the nuances of integrating to
tance motor, that are more efficient than an ECM. integrating directly to the device (VSD or ECM), it a multiple VSD fan array. Proper integration to
While ECMs are no longer the most efficient solu- is much harder to adjust the points list after-the- an ECM based fan array is also possible but the
tion for fan arrays, some air handlers have stan- fact with intermediary controllers. available data may be limited if expectations were
dardized on them, and controls contractors should not communicated early on in the project. 
be prepared on how best to integrate them. Are you responsible for designing or implement-
ing smoke control/evacuation controls? If so,
One may expect that ECMs have similar integra- care must be taken and possible early coordina-
tion characteristics as the one-VSD-per-motor, tion with the authority having jurisdiction (AHJ)
but that’s not typically the case. The control and on what will be deemed acceptable functionality
integration with ECM based arrays are not as in Auto and Hand modes of operation. ECMs typ- ABOUT THE AUTHOR
standardized. Only some ECMs include BACnet, ically do not have specialized smoke control over-
with most (Figure 4: ECM fan array) using Mod- ride logic so a limited variation of smoke control Tim Skell, Global HVACR Application
bus to an intermediary controller or gateway. is handled via the intermediary controller. Criti- Engineering Manager at ABB, has been
That controller then talks BACnet to the BMS. cal fan array applications want to minimize sin- working with variable frequency drives
Even if the ECM has BACnet, there may still gle point-of-failure risks, which may result in a (VFDs) in the HVAC industry for over 20
be an intermediary controller involved for gen- Hand mode design that is hardwired (switches years. He is also active in the BACnet
eral control or to offer Hand functionality that and potentiometers). However, if that hardwired community and has received a past
is expected on air handlers. ECMs themselves variation of Hand mode is implemented to provide BACnet Member of the Year award.
do not offer a Hand function and the intermedi- the most reliable solution, then there is no spe-
ary controller/HMI can offer a Hand function via
touchscreen. However, that controller/HMI does
become a single point-of-failure risk, which can
take an entire array offline, thus defeating the
whole reason the Hand function was included.
Hand mode is intended for when the controls fail. Tim Skell
HVACR Application Engineering Manager | ABB Inc
tim.r.skell@us.abb.com | new.abb.com/drives
The first step in integrating to ECMs is to deter- www.linkedin.com/in/tim-skell
mine if the integration is directly to the ECMs (and

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BACnet Middle East Journal 12 11/23 13