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Electric generator coil design

  1. Nov 2, 2011 #1

    i am developing a home-made generator but i am having some trouble with coil design.

    Going on Faraday's induction formula :

    V = N A (2 pi RPM/60) B

    a 50 turn coil with a radius of 20mm going at 20,000 RPM should give me 80 volts or so, in practice i am getting much less than this.

    Does wire gauge have no bearing on induction, i would have thought that a finer wire would give me a higher voltage ?

    Any help greatly appreciated,

  2. jcsd
  3. Dec 22, 2011 #2
    (I'm a new member) Can someone help me with a windmill generator design? I'm working on a 9.9 kw machine (45 kw max) prototype and need some assistance in induction generator design, both ac and dc coils.
  4. Dec 25, 2011 #3
    You know how many windings on you're pole piece, and have a fair idea what your rotational velocity is, that leaves your flux density, B, as you most likely culprit. To get sufficient field strength, you need to keep your flux flowing either through the magnets or iron. An air gap is necessary for mechanical reasons, but you don't want it to be large.

    For a fun, demonstrator generator, it's hard to be a step motor with a crank. They can put out a few volts at hand cranking speeds, which makes them great for science fair demonstrations.

    For slightly larger designs, DC motors are sometimes good. I've hear that the ones used in tread mills can be used in small wind generators.

    For something slightly larger, a DC brush-less motor is great. More and more, they use these in appliances - especially air conditioner air handlers. They also use small ones for RC models.

    If you wish to get larger, the AC induction motor is often the way to go. When driven beyond it's synchronous speed, it behaves as a current source. However, it's not a good option for stand alone power and should have some support components for start up and to ensure that it's current is mostly in phase with it's voltage.

    I've also heard of using car alternators, though I understand that this can be a bit difficult due to the required excitation current, output voltage, etc. Anyway, supposedly a Kid in Africa set up a number of DC systems using this technology.
  5. Dec 25, 2011 #4
    So many people are building these things, you might as well learn from their experiences. If you Google "diy wind power coils", there are many sources of plans, videos, etc.
  6. Dec 25, 2011 #5

    jim hardy

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    Gold Member

    wow - you're talking 60 horsepower here..

    hard part is to find something suitable for low speed.

    i'd see if local metal scrap yard has electric forklift with salvageable motor.
  7. Jan 15, 2013 #6
    Thanks for the answers, sorry i never replied before. Massive life changes took me away for over a year !

  8. Jan 16, 2013 #7
    Can you please explain more about this formula? what are N, A and how to choose B?
  9. Jan 16, 2013 #8
    N is the number of turns in the coil, A is the area of the magnet face and B is the magnet strength in Tesla.

    I think i've got that right.
  10. Jan 16, 2013 #9
    I am thinking about the problem and i think that rotating the magnetic field at speed 20000 rpm is very high, it will not let the coil to sense the change in flux to generate the required voltage.
  11. Jan 16, 2013 #10
    No, higher RPM results in more voltage. The problem is solved now, just move the magnet nearer, because the B field drops off in strength in a big way as you move further from the coils.

    Thank you,

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