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Thread: Generator watts required for 1.5HP 230V submersible pump?

  1. #1
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    Generator watts required for 1.5HP 230V submersible pump?

    A 230v 1.5 HP submersible pump is metering about 11.4 amps per leg. The motor nameplate states running current to be 10.6A. Metered starting amperage only increases about 1A above running amperage. Should a portable 120/240 generator rated 3100 continuous watts and 3750 watts surge be sufficient? If not, please tell me how to correctly calculate. Thanks for help.

  2. #2
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    It will be close, but it should work. For running conditions, 3100 watts divided by 240 volts is 12.9 amps, and that exceeds the amount you are reading. For starting, 3750 watts divided by 240 volts is 15.6 amps, and that also exceeds the amount you are reading.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    Quote Originally Posted by oldchap View Post
    A 230v 1.5 HP submersible pump is metering about 11.4 amps per leg. The motor nameplate states running current to be 10.6A. Metered starting amperage only increases about 1A above running amperage. Should a portable 120/240 generator rated 3100 continuous watts and 3750 watts surge be sufficient? If not, please tell me how to correctly calculate. Thanks for help.
    11.4A at 230V is about 3000VA. I think the chances of the generator in question starting this motor is dubious, but possible.

    Interesting that it is already running in overload.
    Bob

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    This thread is back

  5. #5
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    Quote Originally Posted by oldchap View Post
    A 230v 1.5 HP submersible pump is metering about 11.4 amps per leg. The motor nameplate states running current to be 10.6A. Metered starting amperage only increases about 1A above running amperage. Should a portable 120/240 generator rated 3100 continuous watts and 3750 watts surge be sufficient? If not, please tell me how to correctly calculate. Thanks for help.
    Sumpins up with this pump.
    What is this pump being used for?

    Is it dual voltage pump?

    What is the voltage at the pump?
    If Billy Idol or John Denver is on your Ipod go and re-evaluate your life.

  6. #6
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    110817-0920 EDT

    If this pump is only a standard induction motor there is no way the initial current is just 1 A above steady state. I might guess at a ratio 5 times, thus maybe 50 to 70 A.

    This does not mean the generator has to have a peak rating of 50 to 70, but it does mean more information is need on the characteristics of the pump motor, and whatever generator might be used.

    .

  7. #7
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    Quote Originally Posted by ActionDave View Post
    Sumpins up with this pump.
    What is this pump being used for?

    Is it dual voltage pump?

    What is the voltage at the pump?
    This is a 230v deep-well submersible water pump (not dual voltage) that has been in regular and satisfactory use for 3-4 years. The voltage at the pump disconnect housed some distance away from the pump is 238.5 to 241.5 by my meter at different times of the day. Power company voltage varies during this extremely hot summer. The pump itself isn't accessible for a voltage test without pulling it from the well.

    I can say to all who have replied that the generator described in the inquiry will not start the pump. My measure of the starting current is likely inaccurate because I can't be sure with my digital meter in the split-second of starting. I stated in my initial post the highest reading I could detect, which apparently was inaccurate.

    Now I have tried the generator. The line amperage jumps to over 18 when the load is applied, while the volt meter on the generator drops from 240 to under 100 and the overload trips. Won't work! Any way, my sincere thanks to each of you who responded to my request. Thank you very much! Bob in TN

  8. #8
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    Quote Originally Posted by oldchap View Post
    I can say to all who have replied that the generator described in the inquiry will not start the pump. My measure of the starting current is likely inaccurate because I can't be sure with my digital meter in the split-second of starting.
    Several years ago I did a similar test with a string of incandescent bulbs and found this similar issue. After comparing the results to a scope I found me and my DMM was not fast enough to get an accurate reading of the surge. It was a good lesson for me.
    Jack

  9. #9
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    oldchap:

    The motor inrush or starting current is overloading the generator.

    I have a DeWalt radial arm saw with a motor in the HP range you are describing. At the end of over 100 ft of #12 and at 120 V the motor draws about 70 A for 5 seconds or so to start. This is a large inertia load and the voltage drops considerably, thus the long startup time. If changed over to 240 this might be in the range of 35 A. This is a sustained startup current to supply energy to the rotating mass. Separate from this there will be a shorter and larger peak for initial magnetizing current. My above 70 A was to supply energy to get the blade and motor rotor up to speed at low voltage due to line drop,

    If my Honda 5000 W generator was where the saw is I would run an experiment, but that is not feasible.

    Saws and pumps are different, but in the first fractional second they will be similar.

    Rent a larger generator and experiment.

    .

  10. #10
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    Just FYI for future reference:

    A "quick and dirty" rule for generator sizing on 3 phase motors that has never failed me is if the motor is starting across-the-line, use 3X the HP as the minimum kW rating of the generator , 2X if it is a soft starter and 1.5X if it is a VFD. This is based on the reserve (starting) capacity of the average portable generator. So in your case if that is a 3 phase motor, I would use AT LEAST a 4.5kW rated generator.

    If the motor is single phase, 4X for across-the-line, so if your motor is single phase, a 6kW would be the smallest that I would suggest (or whatever standard size is closest). But the soft starter and VFD issues are more complicated for single phase, not worth going in to here unless someone wants to know.
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