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What governs how hard it is to turn a dynamo?

  1. Mar 2, 2016 #1
    Say I wanted to attach a dynamo up to some pedals or something - what factors would affect how hard the shaft would be to turn?

    Is it governed by the physical configuration of the dynamo (windings, etc)? Or is it the (electrical) load attached that matters - and how does that work?

    If I wanted to be able to alter the resistance (as in physical difficulty) of the pedals up and down in steps, how could I do that? What kind of steps could I get? And how would that affect the power/voltage/current/physical-electrons-coming-out-of-the-dynamo?

    I feel like I know a lot of terms and 'theory', but I have very little instinct for how things go together in practice.
  2. jcsd
  3. Mar 2, 2016 #2


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    hi there
    welcome to PF :smile:

    to a very minor extent

    that's the one :wink:

    the heavier the load ie. the more current required by the load
    the more work you have to do to turn the dynamo shaft to generate that current

    power in = power out minus losses in the system

  4. Mar 2, 2016 #3


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    Welcome to PF!

    A dynamo or motor is basically a cluster of electromagnets and stationary magnets rolled into a cylinder. The magnets and electromagnets oppose each other and in dynamo mode, the magnets quite literally push the electrons through the wires.
  5. Mar 3, 2016 #4
    Okay, that's brilliant. So that sounds like I can 'step' the difficulty up and down just by adding in more and more resistors?

    I don't suppose either of you gentlemen would be able to recommend a good place to buy a dynamo so I can have a play with this at home? (Google just shows me lots of portable diesel generators or those little bike things, nothing in between.)
  6. Mar 3, 2016 #5


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    Last edited: Mar 3, 2016
  7. Mar 3, 2016 #6

    jim hardy

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  8. Mar 4, 2016 #7
    Okay, thanks.

    I think I need more voltage than a little bike generator can give me - but not as much as the car one would provide. It seems that dynamos are generally made for a specific purpose - you can't just come up with a rating and buy them off the shelf from somewhere?

    (Makes me wonder where those bike/car companies are sourcing them from.)
  9. Mar 4, 2016 #8


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  10. Mar 4, 2016 #9
    ...I've just realised, motors and generators are mechanically the same thing, aren't they? So if I know what wattage I want out, I can just buy and motor and run it backwards. Right?

    Nidum: thanks. It may sound silly, but when you're this new to a subject, you don't even know what words you should be Googling. That vocab (simple as it is) is often just as useful as the search results.
  11. Mar 4, 2016 #10

    jim hardy

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    dont run it backwards, just speed it up.

    So long as you don't get into brushless DC motors, yes they're the same .

    Get a small brushed motor and spin it with an electric drill to experiment.

    For the man on a budget scrap yards are handy. I buy electric motors for thirty cents a pound.
    Windshield wiper motors abound and are good for a few amps. Older Ford ones are three brush - an interesting variant to look up should you encounter one.
    Automobile radiator fan motors are popular among DIYers , good for perhaps ten amps. Get a single speed one with only two wires for starting out, they're simpler.
    Automobile heater fan motors are surprisingly powerful, some approach twenty amps.
    A permanent magnet dynamo makes voltage in direct proportion to speed, no regulator required.

    After you've got your feet wet you can advance to something with a field ?
  12. Mar 7, 2016 #11
    Thanks, Jim.

    The only think I've really played with are little stepper motors, which won't do at all. I shall take all this on board.
  13. Mar 7, 2016 #12


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