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Generator Design Questions (Electromagnetics)

  1. Jan 2, 2012 #1
    I am trying to use Faraday’s law to calculate the number of turns a coil requires to produce a certain voltage. This is what I have so far.
    V = -N * change in (( tesla * area meters squared)/ seconds)
    N = -1*(-V/ change in (( tesla x area meters squared)/second))

    N = number turns
    V = volts
    T = strength of the magnet in Tesla
    A = area of the magnet in meters squared
    S = times the magnet passes the coil per second

    The voltage is V=115.

    For T(tesla) I’ve taken 1/3 the strength of the magnet to account for the distance of the coil from the magnet.(if anyone knows how to accurately calculate the strength of the magnet at “x” distance from the magnet I’d like to know that formula as well) The magnet I have is rated at 5,233, Tesla is 10,000 so I am calculating 5,233/10,000/3 = 0.174433.
    The Tesla is T = 0.174433…

    My magnet is cylindrical, 3” wide x 1.5” tall. Using the formula to calculate for square inches of a cylinder, 3.14*r2*H, you get 3.14*1.5*1.5*1.5 = 10.5975. A square meter in inches is 1,550.0031 inches. So the square meter is 10.5975/1,550.0031 = 0.0068370831000273
    The area in meters square is A = 0.006837

    Seconds, S, is calculated as the number of times the magnet passes the coil each second. So if I have 1 magnet and 1 coil with a rotational speed of 60 RPM, with 60 seconds in a minute S is equal to 1*1*60/60 = 1.

    With all my values solved for I can calculate the number of turns needed in my coil.
    N = (-1) * ( -V / ( ( T * A ) / S )
    N = (-1) * ( -115 / ( ( 0.174433 * 0.006837 ) / 1 )
    N = (-1) * ( -115 / ( 0.001192598421) / 1)
    N = (-1) * ( -115 / 0.001192598421 )
    N = (-1) * ( -96,428)
    N = 96,428

    I have 3 questions, and 3 sub-questions, pertaining to this whole scenario.
    1. Are my calculations correct? That seems like a lot of windings to produce a standard 115V.
    2. Using this formula, the resulting values, and, I assume, the gauge of wire used in the coil, how do you calculate for A(current)? I’d like to use 16 ga AWG
    3. The third is more complicated and leave me with more questions. 96.4k turns is a lot of turns no matter what gauge wire you use. From what I gather the more magnets and the more coils the fewer turns per coil. What I am not 100% clear on is if they are directly proportional. For instance, let’s say I have 25 magnets and 25 coils. Right now I input the extra coils and magnets in the calculation for seconds like so, 25*25*60/60=625, and I replace for S. I won’t go through the formula again in detail but what I end up with is 155 turns per coil. Is this the correct way to account for the extra coils and magnets? Is there a law of diminishing returns splitting the V(voltage) across multiple coils? And what affect does it have on the current produced?
    I have more questions but being my first thread I’ll leave off now.
  2. jcsd
  3. Jan 5, 2012 #2
    First of all, a drawing or schematic of your machine would make it easier to understand.

    As for calculating the magnetic flux density ( B not T ) and the magnetic flux ( phi ) a easy approach is to use a magnetic circuit equivalent, see here:

    As for how large magnetic field the magnet produce you need to consider the whole circuit and the hysteresis for your magnet.

    What do you mean by 115V ? RMS, peak ? Using one coil would give an odd shaped output voltage.

    For wire size, what is the rating of 16 AWG? What is your maximum output current? How is the heat dissipation for the inner wires?

    I would advice you to look at someone others projects to get at feel for the design and get the conceptual knowledge of how a generator works. I believe there are many blogs and project sites out there describing homemade generators ( windmill applications and so forth)

    Good luck!
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