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Generators and Coils vs Output Volts/Amps

  1. Dec 13, 2012 #1
    I know this is a bit of a newbie question but I want to learn something about generators.
    I want to build a simple hand cranked generator. Just a hobby. Making coils, and using magnets is the easy part but I don't understand the specifics around this.

    Can anyone tell me how to determine how big the coil must be to generate a certain amount of volts and amps. Is there a formula or methodology for this or is it not possible to determine?

    For example. Do I need x length of wire at y thickness and N50 magnets to generate ABC Volts and D Amps?

    Is there a book or something I can read all about this topic. Something not too extensive but clear and easier than a medical journal to understand.

    Thanks in advance.
  2. jcsd
  3. Dec 13, 2012 #2


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    Any bookstore should have a Basic Electronics book or three where you can learn all about it.
    It isn't as simple as "do this, this, and this", you need to know how it all works and why.
    Of course a google search for "How to build a generator" should get you the specifics on how to physically build one, but knowing the electronics that make it work is important too in my opinion.
  4. Dec 13, 2012 #3


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    This is true and it is not a straightforward process.
    You can get the basics (which you will find in A Level courses and beyond) but to find the effective 'strength' of any magnet that you could buy, particularly when it is incorporated in a generator you might build, would be a bit more difficult and you would need to know that if you wanted to predict the output of your generator. It's when you actually want results in practical situations that you find how limited is the information that you get in preliminary courses.
    People seem to be reluctant to read books about technical matters and it is a shame as a book will have some structure to what it tells you. Self teaching by dipping in and out of a subject on the Internet can leave enormous holes in your knowledge.
  5. Dec 13, 2012 #4
    I agree, so this is exactly what I'm trying not to do. I'm trying to determine how to work out the output of a generator. Is it a factor which is uncontrollable and needs to be regulated once it leaves the coils? I know how to build a generator sophiecentaur but the specifics around working out the output is what I need.

    I do want a book.
    I do want to do the work..

    Though there are some key words here of which I will need to find the right book.
    Not just any book on electronics or its going to take me 10 years just to get onto the relevant topic..
  6. Dec 14, 2012 #5


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    Hi tomadom, welcome to Physics Forums. For something handcranked it will be low power, so you could choose the enamelled wire to be as thin as whatever is convenient to handle and make connections to without risk of stretching or breaking. I suggest that you dismantle the motor from a broken toy car or hairdryer to gauge their dimensions. Once constructed, the faster you can spin the rotor, the greater will be the voltage produced, so some form of gearing would be useful.

    A google search is certain to turn up lots of hobbyist information, including youtube videos. Good luck!
  7. Dec 15, 2012 #6


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    Good, practical advice there. Unfortunately it doesn't help with predicting the output power which depends equally on the magnet and 'core' characteristics, which are a bit imponderable. Efficiency is a big factor in this. You will know that you are producing significant power when there is an obvious difference between the effort needed when the load is connected and disconnected. There will be a limit to the benefit of just turning the thing faster when the Inductance of the coils becomes significant.
  8. Dec 16, 2012 #7

    jim hardy

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    I'm going out on a limb here, gonna suggest a book.


    I have the 1901 version of this book.
    Back then they were still figuring out how to teach the subject
    so some of the explanations are un-succinct, which i think is a good thing.
    It takes time for concepts to sink in. It was this book that for me made the light come on as to MMF's inside AC machinery.

    It's a fun book because the author speaks of Edison, Tesla and Steinmetz in present tense.

    But it's old and units seem strange

    Your question speaks to basic electromagnetics
    familiarize yourself with units of
    MMF (Amp-Turns, oersteds),
    Flux (Weber, Maxwell)
    Flux Density (Tesla, Gauss)

    and the formulas will start to look less mysterious.

    Another handy book is
    "Magnetics Measurements Handbook" by Jack M Janicke
    Last edited by a moderator: May 6, 2017
  9. Dec 18, 2012 #8
    Actually Jim,, that's exactly what I'm looking for. Thanks very much.
    I thought my thread was going to get way off track there for a minute.
    I'm looking that up right this minute and will purchase one soon.

    Thanks again.
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