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Thread: Well Pumps - AC or DC

  1. #1
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    Cool Well Pumps - AC or DC

    Quote Originally Posted by Mike90250 View Post
    Grunfos makes some DC pumps, but the pump and it's controller are as expensive as a PV, battery, inverter setup to run a generic AC pump.

    The most important thing is to properly size the pump, the combo of Motor & pump stages is critical to get the best performance per watt. And a 240V, 3 wire pump is easier for an inverter to start than the same pump in a 2 wire configuraton.

    Mike "been there, done that" B.
    It looks like you know lots about this subject. Can you advise us. We want to power our well pump with solar. Based on what you have said it looks like we will stick with our AC pump and not go with DC. This is the pump that we have now:

    Wayne Reliant One
    3/4 HP - I don't understand this - does this mean it is less than 1 HP?
    V: 115/230
    A: 12.2/6.1
    RPM: 3450
    HZ: 60

    It looks like this pump doesn't meet the 240v that you mentioned in your post. Do we need to buy a new one?
    What kind of inverter do you recommend?
    What kind and how many solar panels do you recommend?

    Thanks for your help
    Last edited by Mike90250; 09-29-2011 at 05:53 PM. Reason: subject line, spl;it thead

  2. #2
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    Maybe I know enough to be dangerous

    I had to do a lot of research for my own situation 3 years ago. I concluded that a conventional, 240v AC 1/2 hp pump was going to meet my needs.
    I was also installing a whole house off grid system, and was going to have a XW6048 inverter (6,000w 48v battery bank) to power the house, and I may as well use the the 240v AC to run the pump, or invest in a lot of new (back then) and expensive tech for a DC direct solar pump. So I went with a system that any well / pump tech could understand without going to the big city for help.

    3/4 hp is a really good sized pump, and I would expect it to draw 1500w when running.
    But do you need that large of a pump ?? How deep is the well ? Pressure tank or elevated storage ? Farm irrigation, or 1 house ?
    3/4 hp is eithr a really deep well, or a lot of water usage.
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  3. #3
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    Save yourself a lot of heartburn and bite the bullet now.

    Use a DC pump ran during the day to fill a cistern, pressurize the tank for distribution.
    MSEE, PE

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    A 3/4 horsepower pump will take a 4 to 5 kW source to make startup surge.

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    Quote Originally Posted by RCinFLA View Post
    A 3/4 horsepower pump will take a 4 to 5 kW source to make startup surge.
    Can that be right? Let me see if I have this right ......If I buy panels that are 220 watts that means 5kw (5000 watts) divided by 220 watts per panel = 23 panels - yikes! Is that right.

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    [QUOTE=Mike90250;33008]Maybe I know enough to be dangerous

    I had to do a lot of research for my own situation 3 years ago. I concluded that a conventional, 240v AC 1/2 hp pump was going to meet my needs.
    I was also installing a whole house off grid system, and was going to have a XW6048 inverter (6,000w 48v battery bank) to power the house, and I may as well use the the 240v AC to run the pump, or invest in a lot of new (back then) and expensive tech for a DC direct solar pump. So I went with a system that any well / pump tech could understand without going to the big city for help.

    3/4 hp is a really good sized pump, and I would expect it to draw 1500w when running.
    But do you need that large of a pump ?? How deep is the well ? Pressure tank or elevated storage ? Farm irrigation, or 1 house ?
    3/4 hp is eithr a really deep well, or a lot of water usage.[/QUOTE


    Our well is 15 feet deep. We have a small organic garden which is growing -was one acre - will be 3 acres soon, but we live in TN and get lots of rain. We have a yoga center where there are about currently 8 people taking showers, doing laundry, cooking and eating. We expect that number to double in a few years. As it turns out, we now have one pump (the one described) that pumps the water out of the well and pushes the water closer to the main house (50 feet). There is a holding tank at the main house and another pump (need to get the data on this one). From this holding tank, the pump is located in our main building and distributes the water upstairs and to a few close by cabins. For right now we just want to solar power the one pump that brings the water up from the ground.

    Thanks for your help.

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    Quote Originally Posted by dbird108 View Post
    Can that be right? Let me see if I have this right ......If I buy panels that are 220 watts that means 5kw (5000 watts) divided by 220 watts per panel = 23 panels - yikes! Is that right.
    If it is an AC motor, yes it will take that much starting current. The current will drop down after pump is running to about 1400 watts for the 3/4 HP pump. My 5/8 HP 240vac house pump will not start with less then a 4kW generator or 4kW inverter. The startup surge will be close to locked rotor amps of motor. If inverter voltage slumps then the startup current will be less then locked rotor but will last longer. Many inverters have a short period surge capability of up to two times their rated capacity. Don't forget the yielded current load on batteries. For surge overload, inverter efficiency drops to maybe 50-60%. Too small a battery will have a collapse in DC voltage causing inverter to shut down.

    A DC motor will have less surge but still a surge current at startup.

    It is not the panels that supply the pump. It is the batteries and inverter. Panels charge the batteries. You need to figure the duty cycle of pump actual on time. My 5/8 hp pump averages about 0.5 kWH of power per day. My usage is about 250 gallons of water a day with 40 gallon pressure bladder tank running with 20 psi start, 40 psi stop. You don't get 40 gallons for each cycle of the pump, maybe something like 25 gallons, so to supply 250 gallons a day the pump must cycle 10 times per day. Depending on pump size and well depth the run time for each refill of pressure tank varies. Mine is less then three minutes run time per tank refill cycle.

    Startup surge last for about 0.5 seconds so there is 4 kW's for 0.5 seconds times 10 starts per day = 5.5 wH's per day. Converted to 48v battery AH's it would be 5.5 WH's / 0.90(inv eff) = 6.1 WH's => 0.12 AH's /day

    Run time is 3 mins at about 1000 watts times 10 cycles per day = 0.5 kWH's per day.
    Converted to 48v battery AH's it would be 500 WH's / 0.93(inv eff) = 538 WH's => 10.4 AH's/day

    So you can see the kWH impact on battery energy is not that great, but the 4 kW surge power to start the pump requires a fairly large inverter/battery system.

    Put in perspective, typical refrig will draw over 1kWH per day. A 30 watt inverter idle power consumption is 0.720 kWH's per day.

    Well depth head pressure is not necessarily the pipe depth of well. It depends on water table. My well is 70 feet down but water table is less then 10 feet down so pump is effectively pulling a 10 feet rise. This is added to tank pressure to get the net pumping pressure is avg. 30 psi /(0.434 psi/ ft head) + 10 ft head = 79 ft-head. You can look up pump gpm at 79 ft-head to get water volume pumped. Some pump charts are in psi, some in head pressure, some do head suction rise and psi output for given gallons per minute.

    As to original question of DC or AC pump, if you are going to be running a refrig, microwave and such, you will likely need a sizable inverter anyway so might as well use an AC pump which are much lower cost then a DC pump. AC pumps of less then 1 HP usually have a 120 vac or 240 vac wiring option so you can run off of a 120 vac inverter.

  8. #8
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    Quote Originally Posted by Sunking View Post
    Save yourself a lot of heartburn and bite the bullet now.

    Use a DC pump ran during the day to fill a cistern, pressurize the tank for distribution.
    Bump and repeat.
    MSEE, PE

  9. #9
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    Quote Originally Posted by Sunking View Post
    Bump and repeat.
    What do you use to pressurize the tank, another pump? I see no reason of cistern unless you are collecting rain water. Most of the power is going to be pressurizing a tank for useable house pressure of 20 to 40 psi. (unless you have a very deep well draw)

  10. #10
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    [QUOTE=dbird108;33046

    Our well is 15 feet deep.
    Thanks for your help.[/QUOTE]

    Here is the operative portion.
    very shallow well.

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