Let's unravel HVACR complexities with Tim R. Skell!
Our expert, Tim R. Skell, spent almost 3 years sharing his expertise in the #FunFactFriday series on LinkedIn. Now, we are giving him a forum to exchange news, knowledge, and passion for HVACR technologies in the new ABB column.
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Perhaps you need an Analog Input (AI) or an Analog Output (AO), or a Digital Input (DI), or a Relay Output (RO). Perhaps you are wondering who quoted this job with this particular controller, and why didn’t they upsize to the next larger unit?
Here is a workaround for some of those cases. Most of these installations will have BACnet to variable speed drives (VSDs). You can then use BACnet to command the VSD’s AOs and ROs or monitor the VSD’s AIs and DIs.
The VSD can easily be programmed to “pass through” AO/RO control or AI/DI status. Those VSD inputs and outputs could have absolutely nothing to do with pump/fan motor control.
Let’s take as an example an Air Handling Unit where you need an extra AO to control the chilled water valve position. You could control the Supply Fan VSD’s AO and wire that AO to the chilled water valve actuator. Now you are using BACnet to the VSD, and then the VSD’s AO to the actuator, to control the actuator/valve position.
A quick nugget personally learned from the school of hard knocks:
Make sure you choose a VSD that is closely tied to that Input/Output’s (I/O’s) application.
Assuring this connection is essential, as you must consider scenarios where a VSD is powered down (locked-out tagged-out). Once power is lost, you'll no longer be able to use the VSD's pass-through I/O. In the AHU application example above, this wouldn’t be an issue, because if the supply fan isn’t running, then you wouldn’t be actively modulating chilled water.
The ABB ACH580 drive supports pass-through I/O and has 2 AI, 2 AO, 6 DI, 3 RO plus option cards for more. HVACR applications rarely require all of those I/O, so there is a good chance you have some free I/O available to “borrow” from the VSD, to get yourself out of jail.
