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  • Off grid garden pump burnouts

    Can't figure out how to post to off grid discussions but here goes:

    At my community garden we have a seasonably adjustable solar array:
    8 panels
    each 20 watt 17.2 volt 1.17 amps wired in parallel.

    Wired directly to a Johnson bilge pump
    DC 12v 7.5 amps to create aeration and a waterfall in our koi and turtle pond.

    no battery storage, converter, regulator, etc involved, just a switch.

    The trouble is for which we would fully appreciate someone's advice is:
    the pump only lasts at most a year. I'm told the motor gets burned with over voltage i full sunlight
    the windings get burned and with under amperage when the irradiance lessens.

    So, should we add a small battery that would charge up and then feed the pump? With appropriate regulators etc? Over voltage and under amp? Trouble is we want the pump pumping when the sun is out, not charging the battery. And not in position to add panels.

    Any ideas anyone?

    Jonathan Keller
    for La Plaza Cultural Community Garden
    www.laplazacultural.com
    Last edited by LaPlazaGarden; 08-08-2018, 12:27 PM.

  • #2
    I think your setup is a decent matchup of parts. My suggestion is check the motor voltage under best sun
    and see if it exceeds the motor rating, perhaps 15V. If not, you may be just wearing out motor brushes with
    long hours, might be replaceable. If voltage is too high, I would be inclined to throw a shut regulator in
    parallel with the motor, clamping the voltage to around 15V. It should have a 200W capability to avoid
    damage in case a motor disconnects. A zener diode teamed with a big transistor(s) and heat sink should
    do it. Bruce Roe

    Comment


    • #3
      Doing the math tells me you are way under powered to even run the pump. 8 x 20 watt panels in parallel is 9 amps on a motor requiring 12 amps. You system is very poorly designed and nothing is matched up to work.

      No battery is going to help because it will be dead in a matter of a couple of days. You did not mention how many hours the pump runs each day, but if I assume 12 hours per day the pump uses 140 watts x 12 hours = 1680 watt hours. Even on the longest day of the year your 8 panels can only generate 500 watt hours, not even 1/3 of what it would require.

      So there is nothing anyone can really do to help you out other than start over with a working design. The only thing I can think of is to use a pump made to work directly from solar and just accept whatever water it can muster to pump.
      MSEE, PE

      Comment


      • #4
        Originally posted by bcroe View Post
        I think your setup is a decent matchup of parts.
        Bruce read what he said. He does not have enough panel wattage to even run the pump. Pump pulls 12 amps, and panels can only generate 9 amps on June 21 and June 22, less the remaining 363 days of the year.There is no over voltage issue, chronic undervoltage.
        MSEE, PE

        Comment


        • #5
          So I mispoke the pump only draws 7.5 amps and actually works great pumping a strong flow when the sun is out. Until it burns out! So I have to find an appropriately sized battery that will feed the load as well as charge the battery???

          Comment


          • #6
            Originally posted by LaPlazaGarden View Post
            So I mispoke the pump only draws 7.5 amps and actually works great pumping a strong flow when the sun is out. Until it burns out! So I have to find an appropriately sized battery that will feed the load as well as charge the battery???
            I would not use a battery.

            Two things you can try:

            1) Voltage limiter. This is just a device that will burn off the excess voltage in the form of heat. 3 50W 5.1V zeners in series would do this. Mount them on a heatsink.
            2) LCB. This is a device that matches the impedance of the motor at low speeds to the panels at low light levels. It will also limit max voltage in some cases; check the data sheet.

            Comment


            • #7
              The pump will not run full speed much of the time, but apparently its running enough. That could be boosted some
              with more panel current. I don't see under voltage hurting a DC brush motor, although this could be an exception,
              check it out. If that is a problem, one of those linear current motor booster (what was that?) things should cure it.

              I certainly would avoid a battery and all its complications. Bruce Roe

              Comment


              • #8
                Originally posted by LaPlazaGarden View Post
                So I have to find an appropriately sized battery that will feed the load as well as charge the battery???
                No a battery is not going to work. With 80 watts of panel power if in direct sun light from sunrise to to sunset can only charge and maintain a 12 volt 70 to 80 AH battery. Put a brand new battery fully charged and things will great for the entire day. Come night the pump still runs great. Next morning battery is a dead and your brand new boat anchor.

                Point here is you do not have enough panel wattage to even run the pump for a day, and there is no excess power to recharge a battery. Think of it like this. It cost you $100 per day to live and you only make $20 a day. Your bankrupt standing in in soup lines, and living in a cardboard box.

                Your best bet like I said in my last post. Buy a pump made to work directly from a solar panel and accept whatever pump rate you get. That is what you are doing now except using a motor not designed to do what you want. It is not rocket science.

                You are looking for Surface Solar Water Pumps.

                Like this one....
                Last edited by Sunking; 08-08-2018, 02:03 PM.
                MSEE, PE

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