Updated: Feb 4
In nearly every mechanical room for both comfort and process cooling, the variable frequency drive (VFD) has a place. Due to the cost of VFDs going down significantly over the years and the majority of motors having a baseline of being qualified as premium efficiency and able to lower frequencies without overheating, the VFD is the go-to in lieu of the common starter. The ability to provide soft starts and vary the flow when two-way valves are present in the system is fantastic. Also, on many occasions, you can get rid of or shut off the bypass valve in your loop. You would be amazed at how many places we go to find the client has no clue where their bypass valve is nor do they have a clue if it works, if it's wide open, cracked open, or closed off. Just fixing the bypass valve, which is usually pretty cheap to fix, has saved clients thousands of dollars that they were wasting.
So, VFDs can save you money but they aren’t always the right fit. If your motor is a standard efficiency in lieu of a premium efficiency, you could see trouble with overheating and failure when the drive tries to lower the frequency below ~80%. Also, is your system designed for constant volume and everything was installed and running properly? Trying to introduce a variable flow scenario when talking pump VFDs might create a system effect that you didn’t want.
I recently had a big client that thought all motors should have VFDs regardless of what the factory designed or recommended. This caused big problems on their condenser fan motors for their air-cooled machine because there was no canned-subroutine or algorithm in the Product Integrated Controller (PIC) panel to understand how the system was to operate with this newfound ability to unload and load up the fans. As a result, their fan curve was askew. This resulted in numerous issues and parts of their warranty were void.
We here at Hoffman Mechanical learned a while back to have subject matter experts instead of a jack-of-all-trades. Consequently, we have expert VFD technicians on staff who can assist you and service your VFDs. If you already have VFDs, keep them maintained. We have a multipoint inspection and cleaning that we perform for some very big names that have hundreds and even thousands of massive VFDs on their campus. Engineers for one client performed an internal audit, showing they were saving hundreds of thousands of dollars by maintaining rather than waiting until the equipment failed and replacing it. Not to mention the downtime that might result if it failed.
To conclude, if you want to explore VFDs, we are happy to discuss that with you but understand that we may steer you away if it doesn't make sense for your system or application. Our desire is to help our clients save money with our solutions and avoid pitfalls.