Variable Frequency Drive (VFD)
Here at J.P. Anderson Well & Pump LLC, we install all types of pumps. In the old days it was very simple. The only differences were manufacturer brand, and size. Today it is Centrifugal, Submersible or some type of positive displacement pump. Mostly for larger applications such as farm or pond supply, it is submersible pumps.
When you start talking about water supply, with an owner, whether they be commercial or residential, you generally start with how much water does the demand need. This question isn’t always straightforward. If you are trying to keep a pond full, that can be calculated down to the GPM and pressure needed. Usually a low head, high GPM pump will be utilized. But what if they had a demand for 10 gpm@ 50 psi for wash down for horses and 15 gpm for geothermal supply? A person also would not know which would be needed at the same time or separately. This is where a VFD or variable frequency drive starts to make economic sense.
In the old days, just 15 years ago, we would have drilled two separate wells, and put in two different pumps. This was the only option. But today, we have so many more options and all with one well and one pump. This is how it works.
The largest volume needed should be calculated. The highest pressure needed should be calculated. Lets say this figure is 25 gpm at 50 psi. The well static is 40 ft, and pumping level @ 25 gpm for six hours is 80 ft (this figure should be calculated over a 6 hour pump test minimum). A 2HP motor (usually three phase for most VFD’s) and a 33 GPM matching pump end would be needed to supply this demand efficiently. We oversize slightly to compensate for short bursts of extra demand, or inefficiencies in the plumbing. This system would run the geothermal at the pressure needed and wash down the horses at the same time. When this occurs, motor would ramp up in speed to 50 or 60 hertz which would allow for higher RPM and supply more water. When the AC was satisfied and shut down the geothermal supply, motor would ramp down in RPM’s to 30 or 40 hertz, thereby saving electricity and wear and tear on pump. When no demand was needed, pump has a sensor that shuts it down to prevent dead head condition.
All VFD controllers have the added benefit of a “soft start”. In a normal constant speed motor, the pump goes for 0-60 hertz in a fraction of a second. This creates wear in the motor, and subsequently shortens the life. It also can damage the pump end over time. Proper check valve placement and tank sizing was what had to occur, in the past, to compensate for this, and in essence it was what some called “controlled water hammer”. Bladder tanks, and throttle valves (cycle stops or other devices) have all been utilized for this purpose. VFD eliminates all of that.
For example, a two HP @33 gpm traditional submersible needed a minimum of 66 gallons of drawdown in tanks to keep the motor from self destructing. Cycle Stop type valves claim to have eliminated a lot of the tank size with a traditional pump motor, but a lot of drillers have had poor success rates with these valves. I personally will only use one if the engineer or owner insist. I don’t like people pointing fingers when something fails! VFD controllers, although much more expensive than other types of systems, pays for itself when you have differing water demands. Our company only sells Franklin brand controllers. My experience has led me to never trust a pump company with electrical components! I won’t name the problems I have had, but one company has changed their design about a dozen times in a few short years. Franklin has evolved (for the good) too, but not that drastically. They had it figured out from the beginning. They also have a staff of field representatives standing by to help you. I like that.
Another cool design with a VFD, is if you need more water than the well can supply, but still have somewhat of a varying load, then you would install one well with a constant speed motor submersible sized for approximately 3/4 of the load, and install another well sufficiently away from the effect of the cone of depression from the first well. The second well would have a VFD controlled pump sized for the remainder of the load plus a little extra. These two systems would be tied into a relatively small bladder tank with a pressure switch control for the regular pump motor and a VFD sensor and panel for the other pump. When pressure drops the constant speed motor is set to turn on with a pressure somewhere (usually 10 psi) below the VFD. This allows the VFD to turn on and when it struggles to build pressure due to demand, the constant speed motor turns on and the VFD goes back to a lower speed. The constant speed motor then shuts off after demand had ceased, and will have run for the entire cycle without turning off. This design is good for the plumbing, the motor longevity and for the purpose of eliminating a lot of tanks. I have designed systems like this for geothermal use before, and they are very efficient.
My experience with VFD controllers dates back to the early 1990’s. I was working for a major steel company installing pumps in the wells my father was drilling for them. I received a call about a damaged motor on a 15 HP turbine pump we had installed. This particular pump was good for 150 GPM. We were only asked to install the pump in the new well and leave it for them to build a pump shed and they would have their people install the pumping accessories. They installed a Rhombus VFD controller, which had an operations manual that looked like my college physics book. Crazy! I went up and diagnosed the motor with my tester and determined the pump had to come out of the casing. We pulled the pump, and some, but not all of the wires were burnt in half just above the pump. The motor had also failed. We were accused, by the “professional electricians”, of damaging the drop wire as it was installed. We installed the new one and had one of their people watch us install it. That evening, that motor also failed. They finally asked our opinion (after the construction manager at the steel plant threatened to fire every last one of their arrogant rear ends!). After consulting with the motor manufacturer, Franklin Electric, we were able to tell this large corporation that their “fancy pump controller” wasn’t set properly. They were trying to go from 0-30 hertz in seconds instead of fractions of seconds. This controller had this setting as an option for the purpose of controlling above ground (air cooled) motors. After resetting the codes in their control box, the motor ran perfect.
We enjoyed teaching a large corporation of “professional electricians”, who couldn’t read a manual, how to make their pump motor survive, and we are so grateful to Franklin Electric that we have been loyal to them ever since. As a sidenote, Franklin Electric is one of the largest financial contributors to the SCGWA which can be found on the web at www.SCGWA.org. This non-profit is what looks out for the interests of well owners in the state of SC.
It is good to have friends in the industry in which you toil. These folks are the best. For more info on VFD controllers call us and we will give you an on-site assessment of what you need.
Thanks for reading
Jody Anderson/owner