inductor question

[R.Spinks]

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.

 

[John Popelish]

"R.Spinks" wrote:

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.

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.

--
John Popelish

 

[Rich Grise]

On Saturday 18 September 2004 07:45 pm, Robert Monsen did deign to grace us
with the following:

R.Spinks wrote:
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 attached .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.




No, the 'l' in the formula isn't the length of wire, its the length of
the coil. Thus, stretching a coil increases the length, which decreases
the inductance.

Yabbut, it also changes the pitch, so it's not linear, or even second-

order, I think.

Good Luck!
Rich