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Question about bicycle generated power system

  1. Jul 22, 2010 #1
    I appologize in advance if my terminology is flawed...

    I'm interested in exploring how to produce power with a bicycle and have read a ton about it on the internet so far. However, I can't seem to find anyone running more than one motor from a single bike. I assume there's a good reason but can't find the asnwer anywhere.

    All of the systems I have seen have a single motor hooked up to a bicycle's rear tire by friction or directly to the crankset through a belt or chain. My question is, if you had the rear tire or the crankset hooked up to an axle (through friction or chain/belt) that had more than one motor hooked up to it, what would happen to the power you generate on the bike (lets say 100W)?

    I guess I'm really asking if it's possible to generate the same amount of power PER MOTOR (100W for 2 motors) or if the power you generate is split among the motors (50W to each)and if so, why? I understand there will be losses in efficiency (and power) by making the system more complex but don't where to start with quantifying.

    Thanks for any help!
  2. jcsd
  3. Jul 22, 2010 #2


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    Using a motor as generator, you would call the result a generator.

    As you draw more power from a generator, it gets harder to turn the shaft of the generator.
    So, if you are providing maximum turning power to deliver 100 watts, then adding another similar generator will not generate more power because you will not be able to produce the extra torque needed to turn both generators.
  4. Jul 22, 2010 #3
    Thanks for the reply!

    What provides the resistance you mention below in turning the shaft when another generator is added? Is it mechanically/physically more difficult? Is the torque you mention a constant thing to overcome or is it just to get it started? For reference 100W would not be the max, I can generate 500W for a few seconds all the way down to 175W for a full hour (although that is a strenous workout).

    So then I take it you would have to increase to 200W to power both generators at 100W? Or likewise, if input is not increased each generator would draw 50W each (or less?)?

    Does it matter what type of generator is used i.e. are there more efficient generators that would allow for more power to be produced or is 100% the max and anything less efficient gives you a loss?
  5. Jul 22, 2010 #4


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    Yes, you have to supply the power to turn the generator shafts and rotating two generators is more difficult than rotating one, assuming they both have 100 watt loads on them.

    It is hard to generalise, but if you did reduce the power from each generator to 50 watts, the effort required to rotate both generators would then be more comparable to the effort needed to roatate just one with a 100 watt load on it.
    In practice, the frictional losses in each generator would have to be considered.

    A 100% efficient generator would convert all the mechanical power used to rotate the shaft into electrical output. Real life generators are less than 100% efficient, so they produce less electrical power than the mechanical power supplied to them.
  6. Jul 22, 2010 #5


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    A doubling in the number of magnet/electromagnet interactions in the second generator.

    You don't really need to look at the details to know this. A generator is really just a way of turning mechanical energy into electrical energy. If you put 100W of mechanical energy in (via muscle power), you will not be able to generate more than 100W of electricity through any reconfiguration of generator(s).
  7. Jul 22, 2010 #6
    Got it. Thanks for clarifying for me fellas!
  8. Jul 22, 2010 #7


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    It's cool that you're able to generate power using a bicycle, and good work out to boot. I thought I'd point out a little calculation for you:

    You're able to generate a maximum of 175 watts for 1 hour, which equates to 0.175 kW*hr. Estimating electricity at $0.15/kW*hr, that equates to a savings of about $0.026 (2.6 cents) per hour. A good workout and quite healthy, but not exactly a great way to save money on your electric bill.
  9. Jul 22, 2010 #8
    Yeah, I figured that out at about step #2 :) More curious about the electronics of it all.

    Plus I'm doing the workout 5-6 times a week anyways. I figure I could put the power into batteries and charge the laptop and phone, but in reality I'd rather pay the pocket change.

    It is funny to hear people complain about soaring energy prices here, but when you tell them they spend about $2 a day on their per capita residential electricity they don't believe you.
  10. Dec 11, 2010 #9
    Agreed! but as u'll find in some of the developing country getting electricity supply is actually a problem. There it is more useful rather considering economic stuff.
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