Just started reading through some of these posts, and while I am not a pump expert, I am more a water treatment expert, I have done my fair amount of work with VFD's on everything from small pumps to 400 HP electric motors, and I have used them in air balancing systems, as soft starts, multiple input air duct systems for pharmaceutical dust control systems in 600,000 square foot manufacturing facilities and even on little ultra pure water system distribution systems, and I have not seen a massive failure rate, or any failure rate for that matter. What causes the VFD's to perform so badly in a well application?
There are lots of good applications for VFD’s. I am not sure fans are one of them because fans act so much like centrifugal pumps. But with positive displacement pumps, conveyor belts, escalators, elevators, closed loop systems, or even hybrid cars, variable speed drives can be very beneficial.
It is only in one particular niche where a VFD is misused and more trouble and expense than it is worth. That niche is with pumps that have centrifugal impellers working with a static head to overcome, which is pretty much all water supply system applications.
There are several reasons for this, and most have to do with the limits in the laws of physics. Centrifugal impellers lose head by the square of the RPM. When lifting water from a well and/or producing a steady pressure, this loss of head greatly limits the minimum speed the pump can run, which greatly limits any possible reduction in energy consumption.
When you understand the almost magical properties of a centrifugal impeller, you will know that simply restricting the flow of a full speed pump with a valve reduces the energy consumption almost exactly the same as when varying the speed. So the real question is, why would anyone want a complicated, expensive, and problematic device when a simple, inexpensive little valve will do the same thing?
The reason the little VFD’s for house pump systems are so unreliable is that they are trying to make a “plug and play”, easy to install, less expensive version to compete with a simple little valve. Then these cheaper little VFD’s are installed in well houses and crawl spaces where the ambient temperature can get higher or lower than the VFD can tolerate. They are also susceptible to lint, dirt, insects, and other stuff being drawn into the electronics by the cooling fan. The lack of qualified maintenance from home owners leads to an even quicker demise of residential size VFD’s.
In commercial applications VFD’s are larger, more robust, expensive, and installed in climate-controlled rooms with air purifiers, surge protection, harmonic filters, and other protection. They are maintained by qualified personal, which change filters and replace cooling fans on a regular basis. The environment in these mechanical rooms must be kept cool and clean, even when the pumps are not running.
I have heard from many a facility, especially in warmer climates, that the air conditioning for the VFD’s in mechanical rooms uses more energy than the pumps themselves. This along with the fact that a VFD actually increases energy consumption per gallon produced makes a liar out of anyone who claims a VFD saves energy.
I was schooled in Electrical Engineering. In the late 70’s and early 80’s I was building computers and radio transmitters, which is basically what a VFD is. I like to say I am an expert on VFD’s, which is why I would never use one for a centrifugal pump application that must produce a static head.
I can get much more technical about it. I have lots of articles, pump curves, calculations, and references on my web page if you are interested. I have several letters to editors and have written articles correcting statements from pump manufactures, VFD manufactures, and even the DOE on my web page.
It all relates to a story about a writing instrument NASA developed to work in space. The pen had to work in extremely hot as well as sub zero temperatures. It had to work in zero gravity as well as multiple G’s. It needed to work upside down, right side up, sideways, in any condition, and on any surface. NASA spent millions developing the pen, and each one that went into space cost millions to produce. Russia had the same requirements for a writing instrument that would work in space. They saved millions of dollars and actually had a more reliable device by simply using a #2 pencil. In other words, when there are two or more ways to accomplish the same task, the simplest way is usually the best.
