Well pump current decreases as tank pressure increases

Discussion in 'Pumps and Tanks Well Forum & Blog' started by kk3, Apr 13, 2009.

  1. kk3

    kk3 New Member

    Messages:
    6
    Location:
    Illinois
    Hi all.

    House was built in the '70s and is still using the same pump and galvanized pressure tank. I don't know how deep the well is, but the pump is a 2hp Red Jacket submersible. Typically the pump draws around 12-1/2 amps when running normally. Recently, I've noticed the pump is not shutting off, because the tank pressure is not reaching the cutoff (50 PSI). If I adjust the pressure switch down so that the pump shuts off, and then readjust back up, I can monitor the pump current as the tank fills. The switch is set to cycle the pump on at 30 PSI. The tank fills normally from 30 PSI to just under 40 PSI, the then the pressure increase slows dramatically and stalls at around 45 PSI. At 30 PSI the pump is drawing around 11.7 Amps. As the tank pressure increases the pump current decreases to around 10.8 Amps at 45 PSI.

    This makes no sense to me. I'm thinking that I can't achieve cutoff because of a worn out pump, but I can't resolve this with the current readings. Thoughts?
    Last edited: Apr 13, 2009
  2. Waterwelldude

    Waterwelldude Well driller,pump repair. and septic installer

    Messages:
    303
    Location:
    Texas
    As the pressure increases on a submersible pump, the amps will decrease.This is normal.
    The symptoms you describe almost sounds like a hole in the drop pipe.
    A hole will make the pump work harder trying to fill the tank, and will take longer and longer to fill the tank.
    Are you getting more air than normal?

    If the is a relief valve or a plug in the top of the well that can be removed.
    Take it out after the pump shuts off, and if the water drops real fast, you more that likely have a hole in the drop pipe.


    Travis
  3. kk3

    kk3 New Member

    Messages:
    6
    Location:
    Illinois
    Please explain. As the pump builds head, I would expect GPM to decrease, but not amps. What am I missing?
  4. valveman

    valveman Moderator Staff Member

    Messages:
    4,382
    Location:
    Lubbock, Texas
    Amps follow GPM. This is counter intuitive but, it is just the way pumps work. I think you are right about the pump being worn. A hole in the pipe would keep the pressure from building in the tank but, the pump would still be pumping max flow, which means max amps. A hole in the pipe also usually means air blowing a glass out of your hand at the kitchen sink.
  5. kk3

    kk3 New Member

    Messages:
    6
    Location:
    Illinois
    Ahhh, Water Horse Power = (GPM * Dynamic Head) / 3960 so a pump curve that indicates 2 GPM at 800' = .404 Water Horse Power. The same curve shows 6 GPM at 600' = .909 Water Horse Power. Got it.

    No glasses flying around the house. :)

    Probable worn pump. :(

    Thanks guys!
  6. Gary Slusser

    Gary Slusser That's all folks!

    I would check for a leak in the drop pipe on a fitting under water. You do that by getting the pump to run, then shut off the faucet, when the pump shuts off shut off the main valve past the pressure tank and watch the gauge (unless you have a check valve on the tank inlet). If it falls you have a leak.
  7. kk3

    kk3 New Member

    Messages:
    6
    Location:
    Illinois
    BINGO! Thanks.
  8. Gary Slusser

    Gary Slusser That's all folks!

    Thanks for the feedback.

    I'll bet you never had any air in your water, right?
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