The correct answer is “Never.” A variable frequency drive (VFD) is generally described as a “motor control device that controls the speed of AC induction motors.” A VFD has never pumped a drop of water.
Despite this, I hear many people in our industry state that their VFD “pumps 800 gpm”. A VFD is an electrical device that, when connected correctly with a pump, can make that pump behave like a variable speed pump. A pump with a VFD is not a VFD. It is a variable speed pump (VSP).
I realize this is a minor point, and most people understand that a VFD is not a pump, but it can be misleading (or at least confusing). Imagine what will happen if somebody tries to hook a VFD to a 12-inch water main. As a matter of clarity, we should not refer to a pump with a VFD as a VFD. I can’t help but get mildly annoyed when someone says, “We have two constant speed pumps and a VFD,” when they should say, “We have two constant speed pumps and a VSP.”
We tend to use a lot of incorrect terminology in the water industry, such as saying we have “60 pounds” of pressure when we really should say 60 psi (pounds per square inch). Pounds are a unit of weight, not pressure, but we’ve been saying this wrong forever, so people are used to it. Let’s not do the same thing with VFDs.
In our OpenFlows software, we try to avoid saying VFD when we mean VSP, but I can’t guarantee that it hasn’t snuck in somewhere.
What are the consequences of calling a VFD a pump? Other than a scowl from me, not much. But it would be nice if we would use the correct terminology.
If you want to contact me (Tom), you can email tom.walski@bentley.com.
