1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the relationship between generator output and RPM?

  1. May 9, 2013 #1
    First of all, if you have a generator with an open circuit, and you are pedal powering it, where does the energy go? I would say it is lost as heat, but not sure what could heat up. The coils in the generator itself?

    Next, what happens if you short the output of a generator? Does it make it very difficult to turn?

    In general, I'm just looking for a good web page on generator theory and design. I can't find anything on google besides just like the very very basic stuff. Ie, if I'm pedaling with x amount of torque, and y amount of rpm, what sort of voltage output should I expect for a given number of coil turns? What sort of design parameters are needed when designing a generator? (I imagine the important metric for output is voltage, and then the amps depend on the load of course, and power output can be calculated from knowing the generator efficency and knowing the power going in, which would be rpm and torque)

    Also, I would imagine that torque would change depending on electrical load, correct? My end goal is to design a power supply for a laptop that is pedal => generator powered, and I need to know how to spec out the correct generator. Also interested in powering other things, besides laptops.

    Last edited: May 9, 2013
  2. jcsd
  3. May 9, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    You mean the power delivered to the pedals? If the generator is on load, you'll find it harder to pedal. Should be a clue?

    Also consider: open-circuit, the terminal ends of the generator is a big capacitor.
    What is the action of the generator in that case? Where does the energy go?

    You could always try it and see?
    However, considering the previous answers should let you answer this one too.

    Generator design gets as complicated as you want it to be - though the questions you just asked can be answered from the basic sites which have frustrated you.

    The most basic description will involve a single loop, and let you compute the induced current.
    Each loop adds the same current again. The rest depends on geometry - which can be anything you like... which is why the kind of general website you are asking for does not exist.

    The torque is not so important to the power produced as the speed that the coils are rotating, the area of the coils, and the strength of the magnetic field. Under load you will need more torque to maintain the same speed since there will be more resistance to motion.

    Of course you are not the first person to want to run a laptop off a bicycle-generator - have you tried looking to see what other people have done?

  4. May 9, 2013 #3


    User Avatar

    Staff: Mentor

    What energy? If there is no load, the generator spins freely. If we exclude friction, there is no input energy. It's like trying to pedal a bike down a steep hill. You can't pedal fast enough to apply any torque to the pedals.
  5. May 12, 2013 #4
    I understand now that with no load, the generator would act like a flywheel. (ie, all energy spent pedaling would be stored in the angular momentum of the rotor)

    Not sure about the shorted case, and I have no generator to test with.

    I guess I'm wondering how much electrical power output a human can provide, but I think my main issue is that the difficulty involved in pedaling would be too subjective to make any sort of statement that would be of any use. :)

    I'm also wondering if there is a way to electronically control how difficult it would be to pedal. I'm assuming that a constant load would have a constant mechanical resistance. You could change that with gearing of course. But if you were charging a battery, I wonder if you could change the mechanical resistance by changing the charge rate of the battery.
  6. May 12, 2013 #5
    Since the current would obviously decrease from the lack of motion I would venture a guess the fault would counter the effort you exerted mechanically. Or in other words, the harder you pedal the more you increase the need for mechanical effort.
  7. May 12, 2013 #6
    Or am I thinking about that incorrectly? A short will still place a load. AC wouldn't behave that way since the definition of a short is somewhat more complicated.
  8. May 12, 2013 #7

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    I remember running a motor as a generator - when I took the magnet out of the motor, it would turn more easily.
    Been looking for references.

    Wouldn't the open circuit (air between the terminals) be modeled as a capacitor, and the closed circuit (wire between the terminals) would be modeled as a resister. In both cases, induced current is being driven back and forth through some internal resistance?

    To the latest questions: the bigger the load the harder it would be to pedal. It is usually easier to add the load to the wheel rather than to the electric circuit it drives.

    Determining the amount of work a human can do is subject to a lot of study.
    There are lots of methods ... to get you started, see:

    But you should be able to figure it out just by measuring your own output through pedaling.
    Which leaves the psychological aspect.
  9. May 12, 2013 #8


    User Avatar

    Staff: Mentor

    Depends on the human and the timeframe. A person in reasonable shape can probably output about 150 watts for an hour. A professional cyclist on a good drug program, 500 watts for a few hours.
    A typical stationary exercise bike does exactly that. All you need to do is vary the mechanical load on the generator.
    Yes, you could.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook