Should I put a variable frequency or constant pressure pump system in?
Just as the car industry has seen radical changes with self driving cars and the use of alternative fuels, the water pumping industry has experienced significant changes too. One of the most significant changes is the use of advanced pump controllers that monitor the water pressure and adjust the pump speed to exactly match the water demand. While the pumps have changed very little, the pump controllers and control methodology has changed significantly. These new pump controllers are referred to as VFD’s (variable frequency drives) or constant pressure controllers. Before discussing the advantages/disadvantages of this newer tech, let’s discuss the traditional technology that has been in use since the 1940’s.
In the traditional method of operation, pumps worked in conjunction with a large pressure tank and a pressure switch. The pressure tank held a significant volume of captive air and when the pump is turned on, water is forced water into the air filled pressure tank. As air and water fill the tank, the pressure of both the air and water increase. When the pressure is high enough, the submersible well pump is turned off. As water is used the pressure decreases and the water is exhausted from the pressure tank with the compressed air forcing it out, when the pressure drops to a preset level, the pressure switch turns on power to the pump and it runs at full speed until the pressure tank is refilled and then the pump is turned off.
The advantages of a constant pressure system are subtle, but should not be overlooked! We’ve compiled a list to help as you consider updating your pumping equipment. If you are interested in where to purchase pump equipment, check out this recent article.
VFD/Constant Pressure Advantages
Pumps that operate in the traditional method require large pressure tanks or expensive valves to keep the pump from starting/running full speed and then shutting off in quick succession. Starting/stopping rapidly generates a lot of heat in the motor and too much of this can cause pump motor failure. This rapid cycling can also cause damage to piping and other equipment as pressures and water flow change very quickly. Because VFD’s strategically increase or decrease the pump speed to match your demand, rapid cycling is a thing of the past with a VFD controller. Because it is unlikely that you need all the water your submersible well pump can produce, using a VFD controller means that your well pump will NOT be running at full speed when you are using water. These slower speeds translate into less wear and tear on the pump and motor. Less wear and tear means longer pump/motor life!
VFD controllers are constantly monitoring the pressure and adjusting pump speed and only a small pressure tank to act as a buffer is needed. This means that the expense and space required for large pressure tanks is no longer required!
Using a VFD designed for a 3 phase pump motor means that smaller wire sizes are suitable for running the motor than an equivalent single phase pump motor. High quality copper wire is expensive and the savings in wire can be realized by switching to a 3 phase VFD based system. 3 phase motors are typically considered by most industries to be simpler and more reliable than their single phase counterparts. At OPS we love solutions where installation cost savings and increases in reliability are both possible!
VFD’s designed for use with water pumps integrate advanced features. Automatic shutdown/feedback on dry well conditions, problems with the pressure tank, or pressure sensor are just a few of the standard features in today’s advanced pump controller. With the traditional pumping method, protection from rapid cycling or dry wells requires additional components and careful calibration.
One interesting advantage of constant pressure controllers/VFD’s is that they are solid state electrical devices. This means that they don’t have any open electrical contacts or switches. In the traditional system, the pressure switches/relays are a key failure point. The electrical contacts corrode with moisture or are subject to insects crawling in them. Because the VFD systems don’t have this weak link, they are not prone to these types of failures.
Many customers in rural areas struggle with poor power quality with low voltages and other issues. Variable frequency drives/constant pressure controllers are essentially taking the AC power from the utility, converting it to high voltage DC power and then re-inverting the power at the correct voltage/frequency the pump needs. What this means in layman’s terms is that, when using a VFD, the expensive pump deep in your well is going to see clean power, even if the utility is giving poor quality power. Keep in mind that VFD’s can correct minor power issues, but major power issues should be addressed by working with your utility.
VFD/constant pressure controllers drawbacks
VFD’s should be installed by a technician with a high level of competency and experience with pumps, electricity and wells! Key information and adjustments should be made to ensure optimal operation and long life. Improper setup can actually lead to premature pump failure in submersible pumps! Submersible pump motors should never be run at lower than 30 Hz/half speed as damage to the motor’s hydrodynamic thrust bearing can occur at speeds lower than this. A technician will understand the limitations of the motor/pump and VFD they are working with and properly program the VFD to account for these parameters.
One of the drawbacks with a VFD to consider is if you have frequent power outages. As we mentioned before, VFD/constant pressure installations don’t require or have large pressure tanks installed. This means that there will be very little reserve water for use while the electricity is off. If you suspect this could cause problems, you can request that your installer put in a larger pressure tank.
Repair and replacement expenses associated with a VFD system can be much higher. In the event of a pressure switch and control box failure with a traditional 2 HP pump system, the cost may be in $200-$300 range. If you were to have a failure in the vfd/constant pressure controller for your well pump, a 2 HP controller will cost ~$1200!
Does a VFD/Constant pressure pump system cost more than a traditional pumping system?
If you are in need of a complete new system or are doing a major overhaul of your pumping equipment there is good news! The most cost effective time to upgrade to a VFD/constant pressure pumping system is when your well pump has failed and the pump has to be removed from the well for the major overhaul. The cost savings resulting from the smaller pressure tanks and decreased wire size mean that the costs for the traditional type of system and VFD systems are almost equivalent. Upgrading pumping equipment to a VFD is not recommended from a cost perspective if you just have a simple problem with your traditional pumping system that involves the pressure switch, pressure tank, or control box.
Does using a VFD/Constant pressure pumping system save electricity?
One item that comes up is that VFD’s can save electricity on pumping water. The initial thoughts in the industry were that there were some energy savings that could be realized by the installation of VFD technology. This was based on the premise that the pumps would be running at lower speeds and using less energy. Factors such as losses in efficiency from the VFD power conversion losses and the pump running at low/inefficient speeds offset the energy saved from running at lower speeds. OPS did real world comparison a number of years ago to understand this dynamic better. We designed the experiment and ran it over a period of several days, apples to apples, with identical flow and pressure characteristics for both. Our findings showed that the use of a constant pressure/VFD controller doesn’t result in a significant increase or decrease in energy usage over the traditional method of operating pumping equipment. Now, if the VFD controller is installed to replace valves that have been installed on your pump system to reduce flow or keep the pump equipment from cycling rapidly, the VFD will save you electricity! That valve essentially burns off pressure/flow by restricting the water whereas the VFD simply slows the pump down to match the lower flow requirement and thus uses far less electricity to do the same work the valve was doing. If you’re interested in more information about how to save on electricity and your power bills, check out our recent blog on this topic!.
What VFD pump controllers are good options?
Out technicians at Oakville Pump Service are well versed in the many different manufacturers that make vfd’s for the water pump industry. Over 20 years ago we started using the ABB ACS550 Series VFD’s with software we developed ourselves as this was the best performance and most cost effective solution for our customers. (We still use these and have a programming guide available that makes setting these units up a snap.) Since that time Franklin Water, Goulds and Pentair have all come up with fantastic off the shelf units at very decent price points. Many of these units are simply repackaged from manufacturers such as ABB or Danfoss with proprietary software. The best price point for a VFD system will come when the equipment is purchased as a package deal with pump, controller and pressure transducer included. Goulds offers some fantastic booster pump systems that come with all the basics included.
If we are replacing or retrofitting a VFD to a residential system, we’ll typically use a Pentair Intellidrive. These come at a great price point, are based on proven Danfoss technology and can be used on single or 3 phase submersible well pumps, as well as 3 phase centrifugal pumps. Keep in mind that if you want to use this unit on your existing single phase submersible pump, you’ll need to OVERSIZE the Intellidrive as per page 4 of the manual. We also use the Goulds Aquavar SPD and Aquavar IPC Pump Controllers. One of the nice things about the Goulds Aquavar SPD controllers is that they are very simple to setup with just a few dipswitches. The Franklin Water Subdrive and Subdrive Connect are also great VFD’s that we use frequently with great connectivity options and the Danfoss VLT Aquadrives are another high quality industry standard VFD. Many times a manufacturer will give an extended warranty if the pump, motor and pump controller are all by the same manufacturer, so make sure to check this aspect when looking at options. If you plan to install a VFD controller yourself, make sure that you are reasonable comfortable with single and 3 phase electricity, read the manual completely as well as the Franklin AIM manual.
The most Common mistakes when setting up VFD’s for water pumps!
Submersible pumps must must never run at less than 30 Hz! NEVER! Why? Submersible pump motors have a specifically designed thrust bearing on the bottom of the motor that supports the weight of the impellers and the pressure of pushing the water up the pipe column. This thrust bearing is hydrodynamic in nature and relies on the viscosity and lubricity of water to function as a bearing. There is literally a thin surface of water these bearings are ‘floating’ on when they run. This hydrodynamic floating stops when the pump is below about 30HZ. It is important for submersible pumps to start quickly and get to 30 Hz, from that point they can be run at just about any speed you like so long as you don’t exceed the rated current/power of the motor. Running below 30 Hz damages the thrust bearing and significantly decreases the motor life of the pump.
The second most common mistake is running the pump backwards! Three phase motors can start and run in either direction. In a centrifugal water pump, the proper direction will have lots of flow and pressure, the improper direction will still have some flow and pressure. Never assume that a three phase well pump is running the correct direction, ALWAYS confirm. On smaller 3 phase pumps it is easy enough to open the discharge pipe at the well and run the pump at full speed, then turn the pump off and disconnect power. Any two of the Red, Yellow or Black wires going to the pump are switched and the pump is restarted. Is the water pressure and flow better the first time, or the second time? Which ever setup had the water flowing faster is the one to use as it will deliver more water, better pressure at much better pump efficiency! Many modern VFD’s designed for the submersible pump industry have output terminals color coded have mitigated the problem of connecting the pump wiring properly.
Another common setup problem with VFD’s is the correct sleep settings. “Sleeping” with a VFD on a well pump is important. When the pump is not actively moving water it should shut down to ensure that power is not wasted as it sits there and idles along. Equally important are the settings that tell the VFD to wake up and turn the pump on to supply water. If it “wakes up” slowly or only after too long a period of time, the water pressure will drop or stop all together before the VFD can get the pump up to speed. In some cases VFD’s have not been properly tuned to match the pressure to the flow demand. The VFD “hunts” and speeds the pump up and then slows the pump down overshooting the pressure required and then slowing down too much. This leads to pressure swings, which is NOT the purpose of a VFD-they should maintain a fairly CONSTANT pressure. these variations are typically caused by and installer that did not tune the VFD and the PID gain settings that are too high or derivation times that are too short.
A common mistake people might make when purchasing a VFD is not understanding what what type of incoming power is available (voltage and single/three phase) size their pump is, what the voltage of the pump is, if the pump uses 3 phase or single phase power. It is actually possible to have 3 phase 230 volt incoming power and, with the correct VFD connected in the right way, it can supply a single phase 230 volt pump. OPS has worked with specialized VFD manufacturers to help our clients take 208/230 single phase power and use it to run 3 phase 480 volt pumps! Why would anyone want to do this? Great question! There are specific cases where wells have been drilled and a 5” or 6” well casing is installed with a 230 volt single phase pump. A few years later the water needs increase and the well had more water available but which could not be tapped with the existing pump size. If you want more water out of the well, a higher horsepower pump is required and more than 7.5 horsepower in 230 volts are only available in 6” or 8” motors which will not physically fit in the well. 10 HP 4” motors are available, but only in a 480 volt 3 phase configuration. If you only have 230 volt power available you might think you are in trouble. Not with OPS! We have a great supplier of American made VFD’s that offer a voltage doubling vfd designed for well pump operation from single phase power!
The last common mistake is that some times the correct VFD is chosen, but it is not rated for outdoor exposure or rain! Make sure that, if the VFD is going to be used outdoor, it is rated as a NEMA 4 or IP66. It is also a good idea to get this rating even if they are installed indoors as it will ensure that any dust or water sprayed by accident will not damage the sensitive electronics inside the VFD!
Whether you use simple tech or integrate the latest and most advanced technology with your water system, Oakville Pump has experience technicians that can keep the water flowing! Give our team a call at (707)944-2471 today if we can help.