VFD's are Great

VFD's are great. They save energy, they reduce stress during motor startup and at this point, they are relatively cheap. Almost any pump or fan that has services a varying load could benefit from one with a pretty reasonable return on investment (ROI).

There are all kinds of great resources on VFD's and their benefits. You can try here for a good primer: VFDs: six benefits

But during retro-commissioning I often find they are not configured for the purpose they serve.

All too often, we find them in hand. Putting a VFD in hand is when one removes control from the building automation system. Why? Lots of reasons. Just recently, we reviewed an air handler with two, relatively new VFDs. Neither was modulating. Why? They were in hand. We put an ammeter on the output of the BMS controller - it was sending 6V. So we put the meter on the VFD's control input screws, also 6V.

Hmmm, why is the drive not controlling to the BMS's control signal? It turns out, it had never been programmed to respond to the control signal. Someone just installed it in the cabinet, connected the electrical wiring and called it good.

This oversight caused a cascade of issues that eventually led the MTN team to standing on that roof (Thanks, we think?). The supply fan was stuck at 60 Hz, so full speed. It was over-pressuring the supply duct, pulling air too fast across the chilled water coil causing moisture carryover, and pretty much everyone downstream was not happy with the thermal environment.

The return fan was also in hand. Someone realized that 60 Hz was too much, so they had manually adjusted it to 50 Hz on the VFD's control panel. However, it never changed from that value. It certainly wasn't following the design engineer's intent, which was tracking the supply fan. Nor was it listening to the BMS telling it to modulate to building static pressure setpoint. Note - this was a separate issue that also needed to be addressed. When you have a return fan, it is probably best to control the exhaust damper to building pressure set point, not the return fan.

Fortunately, these issues were easily resolved - program the VFD for remote control. Energy savings were realized due to lower fan speeds for most of the year, the potential for mold growth was eliminated, there were no more ADA violations, and the occupants thermal comfort improved immediately.

Checking that your VFD's are modulating appropriately is probably a good thing to add to your Preaintenance Opportunities.ventative