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inductor question

Discussion in 'Electronic Basics' started by R.Spinks, Sep 19, 2004.

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  1. R.Spinks

    R.Spinks Guest

    If you have a length (l) of wire coiled to make an inductor and you stretch
    the wire (ie. space the coils further) but do not change the diameter (see
    www.wowway.com/~rspinks1/question.bmp) is the inductance the same? I
    understand inductance to be L = (mu N^2 A) / l + 0.45d. So if I'm stretching
    the coils but not changing l
    (length) will the inductance change? I guess that technically the cross
    sectional area (A) would be changing slightly due to stretching to coils ...
    but I'm not sure if that minor variation is negligible - since I'm not
    actually changing the diameter. I thought that you compress/decompress
    coils (which is what I think this example is) to tune inductance... is this
    what's happening? How does a variable inductor work? I have no equipment so
    I can't just coil some wire and stretch it myself to see. Insight is
    appreciated. Thanks.
     
  2. Where does your formula come from? The closest reference I could find
    was this page:
    http://www.qsl.net/in3otd/indcalc.html

    A common formula for an air core cylinder coil is Wheeler's formula,

    L (microhenries) =(0.8 * N^2 * R^2)/(6*R + 9*L + 10*B).
    N = total number of turns
    R = average radius = (inner radius + outer radius)/2
    L = coil length (along the axis)
    B = thickness of the winding = outer radius - inner radius
    (all dimensions in inches, coil immersed in air)

    The L in that equation does not refer to the length of the wire, but
    to the length of the cylinder of the coil's form. So stretching the
    coil out into a longer cylinder certainly changes L and thus the
    inductance.
     
  3. R.Spinks

    R.Spinks Guest

    I got it from "Introduction to Electronics" by Dorf. 2nd Ed. pg. 247 (old
    college book)
     
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