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

How much energy could I create with a given amount of rpm?

  1. Mar 24, 2013 #1
    I'm doing a personal project and need some help on things I could not find just on the Internet.
    If I have a device that can produce 65 rpm with 70 lbs of force on each revolution, and the drive shaft was connected to a generator...(here come the questions, and I know that the numbers will be incredibly small)

    - what generator out there would produce the most electricity per revolution given the strength of the motor per revolution? In other words, what is the biggest generator that could still be moved by that much force?

    - what is the average electrical output of this device per revolution or per minute?

    - (if you don't feel like doing the calculations for me) what is the average rpm of the given generator and what is ratio of revolutions of input to electrical output?

    Thanks for the help.
     
  2. jcsd
  3. Mar 24, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    None.
    Energy is not created nor destroyed.
    What does this mean? When and how is the force being applied? Is this force needed to overcome losses due to friction and so maintain a constant angular velocity?
    Oh OK - you appear to want a generator that provides no more than 70lbs in drag... I think the details are important.

    With no generator, the device spins up to 65rpm, and it takes 70lbs applied somewhere before it starts to slow down?

    With a generator, it will spin up to, probably, less than 65rpm - at which point the drag from the generator will keep it at a constant speed.

    What is happening, and this is important to your question, is that your whatsit is being supplied energy somehow. When the generator removes energy at the same rate, the thingy turns at a constant speed. If it draws energy faster, then the thingy will slow down. So the limiting factor is the rate that energy is being supplied to the thingy, and the efficiency of the generator.

    http://www.mpoweruk.com/energy_efficiency.htm
    ... that should get you thinking properly.
     
  4. Mar 24, 2013 #3
    Obviously energy cannot be created or destroyed. What I'm asking is what is a specific type of generator that does not take more then 70lbs (the equivalence of 70lbs) of force to revolve once, and with that specific engine how much energy is converted into useful electric energy per revolution (in wH/revolution).
     
  5. Mar 24, 2013 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    And I have answered that question.
    Enjoy.
     
  6. Mar 24, 2013 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Error - That doesn't compile.

    Force applied where ?

    go back to internet,
    look up definitions of force, torque and work.

    Then get your calculator

    horsepower = torque X rpm / 5252
     
  7. Mar 25, 2013 #6
    Maybe he means 70 ft-lbs of torque. In that case Torque X RPM is a unit of power, which is the most "electricity" you can get out of it at 100% efficiency.
     
  8. Mar 25, 2013 #7
    Thank you good sir, << Rest of comment deleted by Mentor >>
     
    Last edited by a moderator: Mar 26, 2013
  9. Mar 25, 2013 #8

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Nobody is trolling you.
    The questions and suggestions are sincere and based on, not just what you say, but years (sometimes decades) of experience answering this sort of question.
    Have you followed any of the suggestions or considered any of the questions?
    Perhaps even just letting us know if Curl's "maybe" is correct?

    If so then it remains only to pick the motor with the efficiency you want (see link post #2 for examples).
     
  10. Mar 25, 2013 #9

    Evo

    User Avatar

    Staff: Mentor

    Insults will not be tolerated here.
     
    Last edited by a moderator: Mar 26, 2013
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook