Sometimes it Would Have Been Better to Have Left it in the Box
When you’ve been inside as many industrial and commercial buildings as we have, you encounter many variable frequency drive (VFD) installations, and begin noticing where they can go wrong. Sometimes VFDs are unwisely installed where motor speed can’t change. In one project, VFDs were retrofitted on water-tower “lift” pumps. Implementing speed control for these pumps is like bench pressing 200# and thinking that if you just “pump” slower it will feel like 100#. It won’t; which is why VFDs aren’t usually as beneficial for lift applications. Nor are they appropriate for base load equipment that runs continuously at full load. In these two cases, the design was flawed. More commonly, the design if fine, but incomplete commissioning (Cx) prevents proper operation.
Retro-Commissioning Drives is Essential
Driven by energy code requirements and decreasing costs, thousands of VFDs have been deployed in HVAC systems and processes. In most installations, VFDs adapt flexibly to changing load conditions; frugally supplying the minimum pumping or fan capacity needed. But often, stemming from the way complex building mechanical systems are delivered, VFD Cx is forgotten.
Several professionals interact in the design and construction of medium to large sized buildings. A designer selects motors and drives and states control intent. A controls engineer implements the controls; filling in the blanks where control sequences leave off. A mechanical contractor installs the hardware, including fans, pumps and pressure transducers-and perhaps mounts the drives. An electrician hooks them up.
The VFD will typically be installed with some default, but not optimal, level of function: a high pressure/speed setting that will ensure that comfort will be maintained at design conditions. After terminal equipment has been balanced, a controls technician programs the myriad schedules, setpoints and logic. If the project is racing toward completion, some programming might occur before systems can be fully tested. Rarely, a Cx engineer (someone experienced with proper operation of the system as a whole) will test that all the pieces are functioning harmoniously and as intended.
Along the way, the system gradually moves from lifeless hardware to a state where it is capable of providing its intended service-but perhaps not optimally. Has it ever happened that, in the fast paced world of new building construction, no one circles back to optimizing the performance of the VFDs? We have encountered many cases where drives were left unconnected or were programmed with fixed speed setpoints of 100% or greater! Energy-wise, this is worse than having no VFD at all.
If Unfamiliar with the Technology, Just Disconnect the Controls
If all goes well with drive Cx, there can still be problems later. The manager of a large retailer reported having trouble with an air-cooled condenser-but had “fixed” it. Accustomed to fan cycling, he was frustrated that some condenser fans were running a lot (but, unknown to him, at reduced speed and power). Amp logger data revealed that the problem was “corrected” by disconnecting VFD controls and restoring the familiar cycling operation.
Properly matching VFDs with their most viable applications, being familiar with how equipment will behave differently than with standard controls, and being clear up-front about how the system will be operated are all crucial to a successful project. Nevertheless, in most cases, some kind of post installation commissioning is necessary to verify that your VFD project will have the impact you had hoped for.