Many are not aware of the full potential of an ABB VFD. Most of the time, the VFD is just a follower device—hanging on the wall waiting to be told what to do. Start. Stop. Speed up. Slow down. This is because a higher power—a building automation system (BAS)—is involved. Regardless of brand, automation systems all seem to run on the same concepts; computing input data and making decisions. But, what happens if you don’t have a BAS or it is being utilized to its limits? This provides an opportunity to use some of the potential, inherent (AKA free) control capabilities of ABB VFDs.
For example, let’s say you have a BAS and recently added a VFD to a hot water pump in a mechanical room on an end of the building being updated. Previously, the pump was on a constant velocity system. The pump motor was operated by a contactor that pulled in during a call for heat. Thus, the pump runs flat out all the time. Another update to the system was swapping old three-way valves for two-way valves on all end devices. The idea is to run this pump as slowly as you can while distributing an effective amount of hot water. To accomplish this, you install a differential pressure sensor on the end of the system piping allowing you to determine whether to speed up or slow down the pump.
What if your BAS has been installed in that portion of the building? You are short the physical control points needed for operating the drive and the differential pressure sensor, but you’re in luck! Your BAS controllers are capable of BACnet MS/TP (RS485) communications. So are ABB VFDs! With something as simple as a two-wire BACnet comm link wired back to the BAS, the BAS can utilize many different capabilities of the VFD.
Primarily, the start, stop, speed, feedback and fault functions can all be used over a communications system. Secondly, the differential pressure sensor (you thought I forgot about it, didn’t you?) can be wired to one of the VFD’s analog inputs. As long as there’s a suitable input signal, the BAS can read the value of our analog or digital inputs through the BACnet communications. The BAS can take that data, compute and decide what to do from there. During this, the ABB VFD’s control board basically became a universal expansion panel.
The VFD doesn’t need to do PID regulation to be handy. It is also possible to use the VFD’s inherent (FREE) control capabilities as a signal relay device of sorts. Imagine a rooftop unit with the supply fan being driven by an ABB ACH 580 VFD. The BAS could feed the control signals (speed, start, stop, etc.) via the BACnet interface. Multiple sensors (outside air AHU supply temperature, etc.) could be wired into the VFD with the BAS reading their values of the same BACnet system. All this data is conveyed over a simple pair of wires vs. a gang of costly, longer wires going to a junction box.
Hopefully these simple, real-world examples generate other possible applications for the standard control capabilities of an ABB ACH 580 VFD.