Variable speed well pump on solar
FarmerGates
| Posted in Energy Efficiency and Durability on
I have a project with an old existing constant well pump 3 phase, 240 volt system, pumping from 165′. It draws a lot of power.
Customer wants to hook up a solar generator to the well. HUGE power draw = HUGE solar capacity. The cost of a solar system to power the old pump is HUGE.
Can I install a variable speed drive (VSD) system and greatly reduce the power load and therefore reduce the size of solar generator I would need?
If VSD system will not reduce the HUGE power load, what are my other options ?
What are some of the best VSD systems?
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Replies
Smaller such units are usually called VFDs, for "Variable FREQUENCY Drives". For some reason they start to be called "VSDs" when they are on big things (like multi hundred horsepower chillers). I don't know why that is, but you'll have a lot better luck asking about a "VFD" for your <25HP or so motor.
A VFD *will* reduce the power drawn, BUT it's not magic. What a VFD does is to match the input power to the motor to the load on the motor, which helps to reduce how much power is wasted heating up the motor. If the motor needs to run at 100%, you're still going to be putting in about the same amount of power with or without a VFD. Where VFDs really shine is on lightly loaded motors, or on things like big air handlers (big blowers, basically), where you can vary the speed based on the demand for conditioned air.
If you have a constant pressure well system, a VFD might help cut down on average power. If you have a typical on/off cycling well system, I don't think you'll see much savings. You also have to make sure you don't run the pump too far out on the pump curve because you want to avoid cavitation. I'd check with the pump manufacturer before putting a VFD on your well pump for this reason.
Not all systems are a good fit for solar. What you might try as an alternative is running a relatively small well pump on solar when solar power is available, and run it ALL the time that solar power is available, having it fill a cistern. You can use a smaller pump to provide pressure when pulling water from that cistern. This might work, but you'll have to size the system to the water demand. If you have a huge water demand, you're going to need big pumps, and big pumps need big power. There is no way around the physics of this, unfortunately.
Bill
Here's my guess on the etymology. High-performance variable speed drives at high power were a thing back before semiconductors or inverters: usually by controlling the field winding on a dc brush motor. That terminology stuck. Eventually, for low power, you could get an SCR or triac control and use it on a single phase induction motor and get really bad performance, or on a universal motor and get sort-of-OK performance. So when real inverter drives became feasible at low power, it was important to distinguish them from the low-performance options.
I think the advantage a VFD might offer on a well pump is the slow start. I have VFDs on my 3 phase tools and they offer a nice ramped increase in speed. On those, I don't vary speed based on load. Basically, set it to run at 60hz (normal speed) with a 1 sec ramp on/off time. You don't want much here to cause delays at the faucet, I'm also not sure how this would interact with the pressure tank. The other advantage you may get is running a 3 phase well pump with a VFD, even without the speed ramping, your inrush currents would be reduced vs a single phase motor I think.
Outside the starting amperage portion, the energy required to pump the water is basically what it is. A few percent here or there depending on motor type is all.
Have you considered the Grundfos "SQ" line? This is the soft-start line of well pumps. For example, I purchased a 10SQE10-290 for my well specifically for solar.
Instead of trying to defy physics, just install a battery system. Even a small battery will suffice. It doesn’t take a lot of total energy to pump water, but the rate of energy draw lends itself to a battery as opposed to making the solar array big enough.
We use an off-grid solar powered well pump for pumping water to livestock on our farm. RPS Solar Pumps is the brand, and a 200 watt solar array has worked well when paired with two 12 volt lead-acid marine batteries. The system has provided 200 gallons of water a day at 100 feet of head with no problems, even in December.
To make a solar well system work for 24/7 water usage, you need a system that stores up water or electrons.
Seems to me you have little to lose by buying an VFD and connecting it to the 3 phase old pump in the well.
Most VFD can be programed to start slowly and gradually bring up the speed to a preprogramed number. Doing this will greatly lower the normally huge spike seen on startup.
If you need to run the pump at full speed to get the required flow their will no power saved.
Have you measured how many amps the pump uses when running? This number is likely to be much lower than the numbers call out on the pumps spec sheet.
Something like this $85 one may be worth trying.
https://www.amazon.com/Mollom-4KW-0-3000Hz-Variable-Frequency/dp/B0BM5RRX9M/ref=sr_1_6?crid=KBMB692A1FRS&keywords=3%2Bph%2Binput%2Bvfd%2B3hp&qid=1678918278&s=hi&sprefix=3%2Bph%2Binput%2Bvfd%2B3hp%2Ctools%2C138&sr=1-6&th=1
Walta