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Solution

Fan Array Integration

Fan arrays are becoming more preva- ferent from other applications, although some being a single point-of-failure. If the primary
lent in HVACR applications in many parts care must be taken such that the data is accu- VSD fails, the secondary VSD would then power
of the world, replacing traditional set- rately presented on the operator workstation the array. Redundant VSDs are popular for VAV
ups. Fan arrays have multiple small fans (OWS) graphics. A couple items to remember: (variable air volume) and for fan array designs
instead of one larger fan. The result? ƒ pdate the graphical image to show an array that are designed to run above nominal utility
U
Improved redundancy, easier maintenance, instead of a single fan – easy to miss this frequency (i.e. 50 or 60 Hz). The BACnet inte-
and enhanced performance. But what simple task when doing graphics work offsite gration to a redundant package should result in
about their integration over BACnet? Let’s and one is thinking about the single VSD both drives shown and their status/data prop-
explore how fan arrays work and integrate ƒ erify the text is accurate for all variables erly identified on the OWS graphics. Depending
V
into building management systems (BMS). displayed on the graphics – we are dealing on the application and owner’s preference, the
with “total” power and “total” amps instead redundant drive package could be configured
of individual motor power/amps to either manually transfer to the back-up VSD,
Before diving into integration, we first must automatically transfer, or allow the BMS to con-
understand the different fan array designs – as A package, with one-large-VSD powering multi- trol which drive is powering the array. While ide-
each design impacts how to integrate and what ple small motors, will include some form of indi- ally these items would be clear in the sequence
data is available. Fan arrays may be powered vidual overload protection per motor, such as of operation and appropriate control wire pulled
by one large variable speed drive (VSD), one manual motor protectors (MMP). The most com- upfront, one beauty of BACnet is that it allows
VSD per motor/fan, or by electronically commu- mon integration “miss” with these array designs flexibility to have these types of controls changed
tated motors (ECMs). For example, a 2x2 array is to forget to monitor the MMP status. A sin- quickly on the fly with minimal wiring changes.
designed with 5,5 kW motors/fans, may be con- gle motor that becomes gradually overloaded in
trolled by a single 22 kW VSD, (4) 5,5 kW VSDs, the array (i.e. bad bearing) is disconnected from One VSD per motor
or (4) 5,5 kW ECMs. the VSD by the MMP. These MMPs can come
with an auxiliary contact for status which can From an integration viewpoint, the one-VSD-per-
The controls contractor’s integration to the fan be monitored by the local controller. Ideally, any motor solution (Figure 3: One VSD per motor fan
array will differ based on which of the (3) above open MMP would throw an alarm in the BMS array) has some similarities to the above, but
designs are used. Let’s look at some integration and alert the facility personnel. Pro tip: Wir- there are a few key differences:
scenarios for each of those designs. ing the aux contacts in series back to a VSD’s ƒ Data pulled from the VSDs is for each fan/
unused digital input and then monitoring that DI motor – thus both individual values/status for
One large VSD, multiple small motors over BACnet – saves on wiring costs and a DI on each VSD, along with the total values (i.e.
the local controller. Consulting Engineers should power), should be available on the graphics
T
VSDs designed for HVACR applications usually take note to clearly document the need for aux ƒ here are no MMPs to monitor – traditional
have BACnet MS/TP included as standard and contacts on the MMP and for the controls con- BACnet integration makes it easy to see the
they may offer BACnet/IP as an option. This one- tractor to monitor their status. status of each VSD and verify proof-of-flow
large-VSD (Figure 1: Single VSD fan array) is the
easiest to integrate of the (3) fan array designs. There is a sub-set of the one-large-VSD solu- Are you also responsible for implementing equip-
Controls contractors have been integrating VSDs tion, and that is a redundant VSD package. This ment safeties, such as a high static safety or
into the system for years, whether fan, pump, package (Figure 2: Redundant VSD fan array) smoke alarm? If so, make sure the safeties are
or cooling tower applications. The initial BACnet has a primary VSD and a secondary/back-up wired back to all the VSDs. Due to the temptation
integration to a VSD in a fan array isn’t very dif- VSD, thus removing the VSD component from of simplicity, one pitfall is to run the safeties into

Figure 1: Single VSD fan array Figure 2: Redundant VSD fan array

12 12 BACnet Middle East Journal 12 11/23