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Diesel generator and capacitor question

  1. Apr 2, 2009 #1
    I am currently designing a water distribution system to be installed in Honduras and have run into a small problem. We plan on using a 1.5hp pump which runs at 115 v and 17 amps but the startup requires 106 amps. So to run the pump we need a 2kW diesel generator but to start we need at least a 12.5 kW generator and were told a solution to our problem might be using a capacitor and a smaller sized generator. The problem is that we don't really understand much about capacitors and were looking for any help you might be able to give such as what size and type of capacitors we might need. Thank you for your time.
  2. jcsd
  3. Apr 2, 2009 #2
    You need to use a capacitor-start single phase motor. Their locked-rotor amps for a 1 1/2 HP motor is about 90 amps (1800 RPM) or 80 amps (3600 RPM). At one time, there were repulsion start electric motors available, which used less starting amps, but they are no longer available. A flywheel on your generator will help. You should base the generator size on surge current rating, as well as continuous current rating.

    Usually water pumps require less starting torque (and amps) than water pumps, so you may not need the locked-rotor amp rating on the generator..
    Last edited: Apr 2, 2009
  4. Apr 2, 2009 #3


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    Do a search on soft-start devices for single phase motors.
  5. Apr 2, 2009 #4
    Bob S: Did you mean to say something else here: "Usually water pumps require less starting torque (and amps) than water pumps, so you may not need the locked-rotor amp rating on the generator.."? This is double talk.
  6. Apr 3, 2009 #5
    Sorry. I meant to say
    "Usually water pumps require less starting torque than air compressors, so you may not need the motor locked rotor rating on the generator."

    I am basing my estimates on the Emerson motor catalog: http://www.emersonmotors.com/products/ecatalog.htm [Broken]
    Last edited by a moderator: May 4, 2017
  7. Apr 3, 2009 #6


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    The actual generator part of the "diesel generator" may be able to handle a significant short term over-current. Some it may even be possible to get away with just using a big enough flywheel on the diesel.

    Other than that I second the soft-start idea. Really that would be the best approach IMHO
  8. Apr 3, 2009 #7
    That is entirely believable. For the water pump, you don't have any losses during starting except flow losses through the valves. For the compressor, you have to begin compressing gas, which while it is not "lost energy" it is "stored energy" so it is temporarily lost at a time when you really can't afford to lose any energy from the system, i.e. during starting.
    Last edited by a moderator: May 4, 2017
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