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pulley calculations

Discussion in 'The Projects Forum' started by Jsw123, Jul 4, 2009.

  1. Jsw123

    Thread Starter Active Member

    Jun 20, 2009
    I am trying to power a 1800 RPM generator with an engine running at 900 RPM thruogh a pulley system.

    My question is is half the available horsepower to the generator lost since the RPM's are doubled? In other words, do I need to double the horsepower of the engine to do this?
  2. Bernard


    Aug 7, 2008
    If your engine develops two hp. , the generator can develope 2 hp, minus losses, or about one kW. Pulley size just matches best engine speed to desired frequency.
  3. Jsw123

    Thread Starter Active Member

    Jun 20, 2009
    Ok but isn't there a power loss you need to calculate for when you have a larger pulley driving a smaller pulley.
  4. AlexR

    Well-Known Member

    Jan 16, 2008
    There is no power loss, the power output of your engine remains 2HP regardless of what you connect to it. What you will see though is a reduction of torque.
  5. SgtWookie


    Jul 17, 2007
    The total power transmitted remains about the same (less a few % for friction and slippage) - however, torque is reduced by roughly 50% when the speed is doubled.

    1 HP is approximately 746 Watts.
  6. bertus


    Apr 5, 2008
  7. mik3

    Senior Member

    Feb 4, 2008
    As SGT mentioned, torque is halved for a double in speed. Thus, find how much torque is needed by the generator at full load to see if the engine can provide it.
  8. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    If you are running a gasoline engine rated at 2HP, you need to know the RPM that the 2HP is developed at. It should be available on the engine manufacturers web site. It should say something like 2HP@XXXXRPM. If you are lucky, the site may have a HP curve chart showing the HP at various RPM. Remember, HP is the result of torque AND speed.
  9. GetDeviceInfo

    AAC Fanatic!

    Jun 7, 2009
    and to add some confusion, it may well turn out that you can at least double your power if you increase your engine speed to match your generator.
  10. Jsw123

    Thread Starter Active Member

    Jun 20, 2009
    Burtis ,do I the need to duoble the horsepower to make up for the lost torque?
    The reason I plan to use this pulley system is to save fuel and engine life.
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Then you should probably choose a ratio that runs the engine at rpms near its peak efficiency. That will be less than it's full power but will be more fuel efficient and give better engine life.

    Also expect to lose 10% power from a 2 pulley system, maybe as much as 15% since you are gearing it up which causes higher losses.

    And it may be a mongrel to start the engine with that type of load hanging off it, you may need a system to loosen the v-belt until the engine is going, then spin up the generator later.

    A far better option would be to get a 900rpm generator if you can find one. :)