Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Power output of a simple generator

  1. Jan 9, 2013 #1
    How do I convert the RPMs of a simple turbine (e. g. a windmill) into the power it generates? Let's say that I want to know if a single windmill of a certain size can power a refrigerator. Assume that I already know how long the blades have to be to produce the RPMs, and I know the energy requirements of the refrigerator. I am only looking for a basic equation of RPMs to power (watts or joules). Thank you.
  2. jcsd
  3. Jan 10, 2013 #2
    I don't know the answer, but I at least know that its not that simple.
    RPM alone don't tell the full story. You could get a small baby-toy scale turbine to rotate at the same RPM as a big power turbine.
    What you can get is - for a given wind speed, and for a given turbine type of certain size you can find out the mechanical power output. The mechanical to electrical conversion is often >90% efficient.
    The theoretical limit to the maximum power that can be harnessed is given by
    Power_max = 0.5 x Swept Area x Air Density x Win_Velocity^3
    But to get anywhere near the maximum power you need to have a state-of-the art turbine design.
  4. Jan 10, 2013 #3
    Hi, I_am_learning - Thanks for your input. I was afraid it might not be as simple as I had hoped. Apparently I will have to keep digging.
  5. Jan 10, 2013 #4


    User Avatar

    Staff: Mentor

  6. Jan 10, 2013 #5
    Thanks, Russ!
  7. Jan 10, 2013 #6

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Thanks Russ !

    i've been looking for an easy to remember number for that conversion, and 5252 is it .
    52 twice makes me think of Tex Ritter's "Deck of Cards"

    So at 5252 RPM torque and horsepower have the same number. That's right in operating band of small outboard motors i tinker with.

    old jim
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook