Solar power, DC pump for pond

Discussion in 'The Projects Forum' started by nbw, Jun 28, 2011.

  1. nbw

    Thread Starter Member

    May 8, 2011
    36
    10
    Hi all,
    I would like to run a very small 12V DC pond pump, Imax = 300mA from 2 x 5W solar panels. Given the low impedance of the motor, connecting it directly to the panels doesn't result in the motor starting up. With a small bank of 3 x 3.7V Li-ion batteries in series as plan B, I wondered if there might be a simpler way (Plan A) of 'soft-starting' the motor or similar. Something along the lines of perhaps 5,000 - 10,000uF of capacitors to act as interim store, charge up, to provide the motor with enough 'grunt' to start. If I have batteries in the circuit, of course, there are aspects of monitoring under-charge, over-charge, etc.

    Any thoughts? I do have about 20 x 1000uF caps lying around which might be ideal. They are rated at 16V, so I'd employ something along the lines of a 13V zener mechanism to ensure they didn't get cooked :)

    kind regards
    Barno
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    I don't think Plan A will work. Inertia sucks (power), and sucking slowly doesn't seem like an improvement. I say grab your alligator clips and start bodging it with the caps you have laying around. A reality test is actually better than a page of math to eliminate doubts, and it won't cost anything except your time.
     
  3. nbw

    Thread Starter Member

    May 8, 2011
    36
    10
    List:
    1. 20 caps, check
    2. Solar panels, check
    3. DC motor, check
    4. Bucket of water, check
    5. Bottle of beer, check :)

    just need some sun now! Winter over (down) here
     
  4. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    Ohhh...I didn't think about your hemisphere. You have like, Nebraska in December. Well, at least it's easy to keep your beer cold. :)
     
  5. nbw

    Thread Starter Member

    May 8, 2011
    36
    10
    Yeah it's crazy alright. At Christmas it's stinking hot, everyone's in T-shirts and jandals (or thongs LOL as the Australians call them :) (aka flip-flops)
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    I'm impressed to have a post not asking to power a huge pump from a tiny solar panel, like we usually see here. Kudos for at least having a reasonable starting point.

    Are you sure the pump won't start (at 300mA) under full illumination? Those panels should have no problem supplying that max current. Using caps won't really help unless you have a way to keep the pump off while they charge up. I suppose you could use a comparator to watch the cap voltage, and only attempt to turn the pump on when there's a good charge. Batteries are probably the best way to go, but as you note you'll need some circuitry to protect the battery.
     
  7. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    I think you would benefit from finding some equipment to measure the actual startup current and voltage traces of the motor. This means a 2-channel scope and a shunt. While I agree with the experimental approach of, say, trying the capacitors, the actual measurements will let you calculate pretty well how much transient charge and at what voltages those caps will have to deliver. Better to know the enemy than just throw stones in the dark...
     
  8. russpatterson

    Member

    Feb 1, 2010
    351
    16
    I went through this whole thing a couple of years ago. You can start the motor by charging up the caps but you need to disconnect the motor during charge up (like was said above). I wound up in a 2 year exploration of microcontrollers and power electronics. I don't want to rob you of that fun but if you want to buy a controller (or a kit) that does just what you want let me know. http://www.flickr.com/photos/20841173@N08/5571419267/in/set-72157626256254069/

    More info here: http://www.backyardsolar.blogspot.com/
     
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    I'd consider using batteries to run the pump and perhaps having a timer turn the pump off for 2hrs/day at solar noon to charge the batteries. Or have two batteries that you can rotate between pump and charger every x/days.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,094
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    Good idea. I have a very cheap digital lap timer, and the clock function is powered by a small internal battery. I'm not sure it can switch the outlet without AC power, but if it can, it would be perfect for shutting off the pump for a while so the capacitors can charge. Maybe at 10AM or so, when the panel might run the motor if it was started, you turn the pump off for 15 minutes. Then attempt a restart.
     
  11. nbw

    Thread Starter Member

    May 8, 2011
    36
    10
    Yeah, I've seen lots of people say 'I have a physically large solar panel (a square foot)... bet that can run a horsepower or two pump!'

    Some good ideas there guys - I too was wondering about something simple like a 555 timer one-shot to ensure the motor was off for say 2 mins while the caps charged, then let the charge through.

    If I used a 556 I could possibly set up a second timer to allow the pump to run for say 30 mins (a large R-C combo) then let it go to sleep while the caps charged again for 2 mins.

    Or, have the caps charge - and as soon as an LDR detects a good level of light, let that charge through so the pump starts up.

    So many ideas!
     
  12. WellGrounded

    Member

    Jun 19, 2011
    32
    2
    You need a recording digital meter to see the surge current and voltage drop.

    Get an 0.01Ω resistor by using 18.75 inches of 16 AWG gauge wire and put it in series with the motor. This little resistance won't affect the circuit. Use a strong battery to start the motor and record the voltage across your resistor and multiply the peak millivolt voltage by 100 to get your peak surge current draw.

    Now repeat the same experiment with your solar panel and see how much you fall short on the peak current that you need, so you can calculate if you want to get additional solar panels and put them in parallel.

    This would have the effect of making the internal source resistance of the power supply less and allow more current to flow on start up since the internal resistance and the motor form a series circuit and there will be a higher voltage on the motor because of the voltage divider effect with less internal resistance in series.

    Also, you may want to record the solar panel voltage drop at start up to see how much your voltage is dropping, like when an AC motor starts and we see lights dim for a fraction of a second because they may share the same power supply.

    Oh, and by the way, down under do you position your solar panels upside down?

    Danny
     
  13. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    I suspect Solar Noon is North for them, yes...
    I do know that their toulets flush correctly, although backwards from a northern perspective!


    Chances are that if the pump is connected to the PV alone, the pump would be off during the evening and would have difficulty starting in the morning and staying on in late afternoon as the PV's efficiency would be far deminished without even basic solar tracking.
     
  14. WellGrounded

    Member

    Jun 19, 2011
    32
    2
    The other alternative is simply to see what the peak output voltage of your solar panel is. If it is less than 13.6 Volts then simple Plan B is OK.

    If your solar panel can run your motor after it is started then keep your solar panel permanately connected in parallel with a battery. When you go to start the motor the battery will provide the necessary additional current for a few seconds. When the motor is running then the excess available current of the solar panel will trickle charge your battery back to peak voltage. When the motor is off then the charging cycle is faster.

    Place a 1.0Ω 5 Watt resistor between the + of the solar panel and the + of your battery. When you start the motor most of the power will come from the battery and the resistor is there only to restrict the charge current when the motor isn't running and make sure that there isn't a direct path.

    If the solar panel puts out more than 13.6 volts then put a 13.6 Zener Diode across the battery.

    I don't think that you will need more than a 12.0 Volt 4.0 Ampere Hour battery to start the motor. That battery can provide 10 Amperes short term if needed since it can light up two 12.0 Volt, 55 Watt car headlights.

    Danny
     
  15. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    It would really help if we knew the open ckt. V & short circuit current of pannels. How long should pump operate each day as a minimum?
    There was a circuit published here to do exactly what you requine- if I can find it.
     
  16. Bernard

    AAC Fanatic!

    Aug 7, 2008
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  17. Bernard

    AAC Fanatic!

    Aug 7, 2008
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