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  • Slow RPM Alternator?

    Discussion in 'The main mechanical design forum' started by jawnn, Jul 20, 2012.

    1. jawnn

      jawnn Well-Known Member

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      I want to generate electricity for small batteries (maybe 16 amp hours needed) with pedal power, from a bicycle. And I need advice about what kind of generator to use.

      Is it possible to get more electricity from a wide generator? Like something made for wind power?
      I watched a video about how to turn a ceiling fan into an alternator….but have nothing to compare it to.
       
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    3. jawnn

      jawnn Well-Known Member

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      If I spin my cranks at 60 rpm with 52 sprockets on the front and 12 to 14 on the rear with a 559mm rim with belt down to a 3" pulley, how fast is that?

      More magnets will fit on a wider alternator? And does that make a difference? Or is it that the amount or the size of the magnets not really matter? How about the size of the coils?

      I know the tip of a large fan blade travels much faster than the axle even though the rpm is the same.
       
    4. jawnn

      jawnn Well-Known Member

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      An then there is the load....can any one tell me how much physical resistance I would get from actually charging the batteries?​
       
    5. jawnn

      jawnn Well-Known Member

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      OK I am getting info else where. Like I need a dc generator, 50% of my energy is wasted when charging batterys So why botther?
       
    6. Dana

      Dana Well-Known Member

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      Need more information. 16 amp-hours at what voltage? And how long can you take to charge them?

      For your second post, with a 13 tooth driven sprocket (you said 12-14) the output would spin at 1761 rpm. This is a 29:1 ratio, so the rider would have to exert 29x the torque required by the generator.

      As for why bother, only you can determine if it makes sense. Your losses shouldn't be as bad as 50%, though, depending on the drive design.
       
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    7. Michael Ross

      Michael Ross Well-Known Member

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      Some of the highest quality bike generators are not large in diameter.
      http://www.bumm.de/produkte/mehr/fahrrad-dynamo.html
      That are not typically alternators, but are instead dynamos/generators.

      Get a good overview of the state of the art here:
      http://www.peterwhitecycles.com/lightingsystems.htm

      If you are building your own it will be very difficult to build a small diameter unit.

      If you charge a battery at 10Watt/hours, yoo can be sure that you lost a good amount more than that to inefficiencies. Some are in the generation (I^2R heating), quite a lot is lost in the chemical reactions in the battery itself.

      It is common to charge a capacitor and light an LED with that for some light when stopped.

      If you go to a prop in the wind the inefficiency is much greater. The best wind turbines are around 40% efficient, you won't come close to that on your own. Much better to take the power directly from the hub or wheels.
       
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    8. jawnn

      jawnn Well-Known Member

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      charging

      I saw a video where the chain went directly to the generator from the crank, not through the rear wheel. So it was a slow charge?

      Which maybe better for Lead acid? Apparently I would need Lithium for fast charging. But would that put a larger load on the generator?
       
    9. BlightonJMW

      BlightonJMW New Member

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      As part of our senior project in school, we wanted to add a generator to a human powered vehicle. We had our generator positioned on the main chain in between the crankset and the rear cassette. We also had a voltage amplifier to help us out at slower cruising speeds. I'm not the one who did the design work on it (my main task was the roll cage and frame) but we were only powering large capacitors and LED lights.
       
    10. jawnn

      jawnn Well-Known Member

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      what can actually be maintained:

      I set up a motor with a load and found that I can generate only 30 to 40 watts continuously.

      David Butcher says he can maintain only about 60 watts, and that's after he warms up. maybe a young healthy man can get up to 100 watts, but that's incredibly hard.
       

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