And what do you think the odds were that that professional still works for the company eight years later, and what are the odds that the required cable that fit the computer and fits the PLC is still around somewhere when you have trouble?
I think 8 years is a little optimistic about how long a VFD will last. But that is telling and kind of the point. A regular dumb pump with a non-VFD control should last 30 years. In that same length of time they will get to sell you at least 4 VFD's, and replace the VFD damaged pump/motor at least once or twice. Like I said, as long as people fall for the VFD hype, they get to sell you at least 4 times as much equipment to keep your water flowing.
A VFD is a microprocessor based pulse width modulator. It is a computer, similar to the one on your desk top. Finding cables that fit, or any replacement parts for a computer just a few years old is impossible. There is no repair on a VFD. They simply want you to throw it away and purchase a new one when they fail. They no longer even make parts to repair a 4 year old computer, as newer VFD's were introduced, trying to solve some of the problems inherent in the older VFD models.
I don't bash VFD's because I sell a competing product, I bash VFD's because I know exactly how they work, or more precisely how they don't work. I studied electrical engineering, and started using VFD's before the 90's. Every 18 moths to two years since then, they have come up with a new model VFD to try and solve the problems with the older models, making the 2 year old VFD's obsolete. This is planned obsolescence at its finest, and the average person is falling for it hook, line, and sinker. The main problem is, mother nature or the laws of physics will never let them solve the problems of VFD control. They are getting better at Band Aiding the problems, but they can't be solved. There are inherent problems with changing the frequency and varying the pumps speed that will never be solved.
That is why I make a competing product that mimics the control of a VFD system, without varying the pumps speed. When you understand that the horsepower or energy use of a pump decreases when restricting the flow with a valve the same way the energy use drops when varying the pumps speed, it makes perfect sense. This is just the natural way all centrifugal type pumps work, but is so counter intuitive that most engineers don't understand it, especially electrical engineers. The anger I see from these people when explaining how pumps really work is incredible. Their anger doesn't intimidate me. I want to see a pump curve that proves me wrong, or they are just wasting my time being angry.
Running a pump larger than needed for the demand is wasting energy anyway you control it. The best way to save energy on a large system is to use multiple pumps of different sizes. That way you can turn on the pump closet to the size of the demand required. A VFD or a CSV can round the corners between the different demand rates and allow the system to switch on or off the right size pump for the load demanded. But it is having multiple size pumps to meet multiple different demands that will save energy, not the VFD.