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Stationary Bicycle Generator

  1. Apr 17, 2013 #1
    Hey all, I'm looking to build a stationary bicycle generator as a fun weekend project but have a couple of questions I can't figure out the answer to.

    It will be a regular bicycle with the rear wheel elevated and fixed to a stand. A belt will be attached to the wheel and the shaft of an electric motor. Now if I had, for example, an electric motor rated at 2000 rpm and 12V DC, then if I spin the motor at 2000 rpm it will generate close to 12V. If I spin the motor at a higher rpm, will it generate more electricity or will it just damage the motor?

    I'm looking to use this to charge a 12V car battery to use to power other things. If I spun the motor faster would it charge the battery faster? (I'm pretty sure the answer to that one is no). And if the motor was rated at say 24V, would I still be able to charge the car battery or would it damage it, or possibly charge it faster?

    I'm new here and apologize if I've posted in the wrong area.
    Thank you guys for all your help.
  2. jcsd
  3. Apr 17, 2013 #2


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    This looks a bit more suitable for the Electrical Engineering forum, if you want the best chance of a good answer.
    I have some comments about this, though. If you really want to generate some useful energy then you could do better than using some random motor that you have found. Many motors are fine as motors but not much use as generators. There are many sizes of alternator available as scrap from cars and motorcycles which would do the job and produce a far better output for a given effort.
    It might be worth while pointing out, if you haven't already done the sums, that the energy available from cycling could really disappoint you. To produce a few Amps into a 12V battery, taking account of all the inefficiencies, will require at least 100W of your Power and even that is quite hard to sustain for any length of time. For a significant charge (say 40Ahr) that would involve hours of pedalling. Not a problem if you're super fit Tour de Freance standard.
    Incidentally, there is no possibility of do any real harm to a 12V motor (assuming it is robust enough take the actual physical abuse) if you feed it to a 12V battery. Remember that you will need a diode to prevent the battery discharging back through the motor when you slow down
  4. Apr 18, 2013 #3
    Sophiecentaur's analysis seems correct.

    Elite (olympic etc) cyclists generate about 20W per kilogram of body mass when sprinting (several seconds only) and around 7W per kilogram for endurance events (1 hour or so).

    This isn't going to give you very much usable power.
  5. Apr 19, 2013 #4
    What tends to damage these electrical machines is the power dissipated within them (I assume an old style brushed motor rather than any form of electronic commutation). In this case that would be the current drawn times the internal resistance. So spinning it faster will up the output voltage, the limit being the breakdown voltage in the windings, normally not an issue in low voltage motors, but you are most likely to cause damage only if you try draw too much current. The windings may cook. There are magnetic limits on the behaviour of generators but you can usually ignore this. Quickest way for unknown motor is to plot winding temperature versus load current and see when it hits the softening point of the wire insulation.

    Charging a battery directly is a bad idea, you need a current limiter as the current rises dramatically with every increase in voltage over about 13v. However the input load also rises dramatically so your personal physical energy output limits (what you are physically capable of) will possibly protect the motor.

    But he's right, you will get pretty fit doing this, a 100W continuous is about 1/7th horsepower.
    Last edited: Apr 19, 2013
  6. Apr 19, 2013 #5


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    I definitely agree with Sophie that an alternator and voltage regulator are the best way to go with this.
    From an "I like to build stuff" mechanical aspect, I have a suggestion that might not be welcome but must, for the sake of my sanity, be mentioned. Forget the belt. Cobble together a generator mounting system that replaces the rear wheel and connects directly to the chain. It can slot into the rear of the frame in the normal fashion if you want to remove it to switch back to the wheel and take a ride.
  7. Apr 19, 2013 #6


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    Aside: Many years ago (1980) I looked into man powered aircraft. They need all the power they can get. From what I could tell they worked on 250W as the best a fit human can sustain for meaningful but short periods. A quote from the time was "It's easier to teach a racing cyclist to fly than a pilot to pedal".
  8. Apr 19, 2013 #7


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    I've never heard that before, but it's too right. Every viable human-powered machine, from the Gossamer Albatross to the hellofacopter that they're working on in an Ontario university (can't remember which one) has recruited world-class cyclists as their hamsters. They don't have to think about things like wind vector triangles or weather or load distribution; just pump your little legs as hard as you can and follow the cheese. :biggrin:

    That last sentence was purely facetious, of course. I'm hesitant to use the word "pilot" in reference to those people simply because most don't have the training necessary to hold a pilot's license, but they are the only hope of getting those things airborne. The extent of a regular pilot's leg action is pushing the rudder pedals, or tilting his/her foot to apply the toe brakes. Fighter pilots are as physically fit as cyclists, but not in the same way. Someone light in weight and stature, but with incredible leg muscles, is the requirement. (Come to think of it, a speed skater might have a chance, although they tend to have more upper body mass...)
  9. Apr 19, 2013 #8
    That makes it easy. Use a 250W generator. At 12v that is a 20A diode on a heatsink and a battery that can take charge at up to 20A. Then provided you don't let the battery discharge below about 11V before you start to charge it you will be fine.
  10. Apr 19, 2013 #9


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    It would be cheaper and more realistic to come to terms with the fact that you're never going to supply 250W and aim at something like 50W for five times as long. You'll never be able to persuade yourself to get on the machine if it's too much agony. It'll be a matter of flattery of the battery.
  11. Apr 19, 2013 #10


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    What a coincidence! That's the term that I used for my ex- sweet-talking her vibrator...
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