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Help understanding genreator spec sheet?

  1. Sep 5, 2013 #1

    If somebody could help me interpret the attached spec for this drive-shaft powered AC generator I'd be very grateful. The application is to power a 2500 square foot home in WA state in a remote location. This is a stand by system because the power is not reliable. The house has a 100amp panel, and no unusual power requirements save for central a/c.

    From reading about typical home power consumption, it seems like a 20kw model should handle peak load. Does that sound right?

    The real question is though is how powerful a motor (in HP) I'm likely to need to run this generator.
    The chart below talks about "Code H" and "Code F" and talks about "motor starting", so I don't know if that only means "on startup" if that the continuous HP/torque required to power it to 90%.

    It seems like it's either 1/2 or 2/3 hp per kw produced, but that's a meanigful difference. Also this thing will be running nearly continuously so I don't think I will starting and stopping it frequently, so wondering what the "continuos" power requirement might be, or does tha tnot matte.r

    Attached Files:

  2. jcsd
  3. Sep 5, 2013 #2


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    Science Advisor

    I quote from;

    When started with full line voltage, AC motors draw line currents substantially greater than their full load running current rating. The actual magnitude of this current is called "inrush current." It is a function of the motor horsepower and design characteristics. It is also called Locked Rotor Current.
    A letter used to indicate the "Design Code Rating" on the nameplate is referred to as the "Code Letter." The Code letter of the motor is an indication of the locked rotor KVA per horsepower. It is a function of the motor's design. Code letter ratings indicate the starting current motor will draw. Code letters below F indicates a low starting current; beyond F indicates a high starting current.
    The motor's Code Letter is helpful in determining the maximum rating of the motor's electrical circuit protection. A replacement motor should have the same rating as its predecessor.
  4. Sep 5, 2013 #3


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    Staff: Mentor

    Could you be more specific about the power requirements of the house? Is the heat from fuel/electric resistance/heat pump? Is the stove electric? The dryer? Water heater?
  5. Sep 5, 2013 #4


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    Gold Member

    The motor that is required is an internal combustion motor, which will need to be about 10 horse power, just as the spec sheet suggests.
    These motors are pretty cheap if gasoline is acceptable and fuel storage is not an issue. If they are diesel, the price goes up but the engine is more efficient and robust, plus the fuel is less volatile, also big plus.
    The buyer has to decide whether it is worth installing an automatic backup, where the engine kicks in by itself when the power drops out, or whether a manual start is acceptable. Also, the duration of the backup power needed is a big factor.
    Note that these setups need to have code approval. For one, it is essential to disconnect the backup from the grid, lest it electrocute the repair crews.
    That said, a 20kW backup is a good size. Washington state is not the tropics, there is no need for a gigawatt A/C, pumps usually need about 5kW for the starting surge, fridge needs are in the kilowatt range and lighting etc is just rounding errors.
    Last edited: Sep 5, 2013
  6. Sep 5, 2013 #5


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    A 20kW output alternator with an efficiency of about 80% will need 25kW drive.
    There are 746W per HP, so 25kW is equivalent to 33.5 HP.

    A contactor should isolate the mains and connect the alternator to the house only once the engine is up to speed. Starting the IC engine should not therefore be a problem.

    The 1800 RPM requirement suggests a direct coupled diesel would be appropriate.
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