RE: OT: Effect of Compression and Expansionon the Inductance of Toroids?

[Cortland Richmond-2]

 
Ron AC7AC   wrote:
> ... I'm confident that the added capacitance by squeezing the turns
together
> is not what is causing my L-meter to show increased inductance. If
anything,
> the capacitance would tend to cause the L-meter to show lower inductance.

Inductance is a result of the magnetic field intercepted by each turn.  An
ideal inductor has each turn intercepting the same (entire) field.  In a
solenoid much longer than it is wide, however, this does not happen, and
the inductance realized is lower than the ideal case, sometimes much lower.
While a ferrite core concentrates the field, and a toroid insures almost
all of it remains within the core, the winding is after all halfway
surrounded by air and some of the field is still not shared by all turns.
Compressing it reduces this effect.   A Bug Catcher coil comes closer to L
changing as the square of turns than a Hamstick(r)!

That's my take, anyway.

One way to test this would be to cover the winding with ferrite. A
babushkoroid! Or try a pot core. If my surmise from High School physics is
correct, the more the field is in ferrite, rather than air, the less effect
compressing the turns will have.  Indirectly, we might also try surrounding
a toroid with a solenoid to see if coupling to that winding decreases as
toroid turns are compressed, but leakage coupling will depend on the angle
each part of the toroid winding has to the solenoid -- and the sum of all
the fields on a completely occupied cores at a surrounding solenoid should
be zero regardless of leakage, I think. Perhaps instead of a solenoid, an
external toroid wider than the inductor under test might be used.   But I
like the babushkaroid.

Remember, you saw it here first!


Cortland
KA5S


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[Jim Brown-10]

Ron said:

>>I'm confident that the added capacitance by squeezing the turns
>>together is not what is causing my L-meter to show increased
>>inductance. If anything, the capacitance would tend to cause
>>the L-meter to show lower inductance.

On Tue, 30 Aug 2005 07:00:35 -0700, Cortland Richmond wrote:

>Inductance is a result of the magnetic field intercepted by each
>turn.

Having recently completed a LOT of research on how coils wound on
ferrites behave, I think I have a handle on what is going on.

Several basic principles are at play.

1. When the mu of a ferrite toroid is much much larger than air, it
contains virtually all of the flux. This is the case at frequencies
where LOSSES in the ferrite are low.

2. Ferrites chosen as cores for resonant coils and transformers are
usually chosen to have low losses (high Q) in the frequency range
where they are used (for example, in the K2).

3. Conversely, ferrite cores used for RFI suppression should be chosen
to have HIGH losses (low Q) in the spectrum where suppression is
needed.

4. Many ferrites are semiconductors (that is, between conductors and
insulators), so they also have permittivity, and they will act as a
dielectric. So even if all we do is pass a wire through a long ferrite
core (like a bead or a clamp-on), there will be capacitance through
the core between the opposite ends of the wire. This capacitance will
be in addition to the capacitance between turns.

5. Ferrite parts also can exhibit a DIMENSIONAL resonance, whereby
standing waves are set up in their cross sectional dimension at the
half-wave frequency. This mostly happens with LOW frequency ferrite
materials (MnZn).

6. The equivalent circuit of a ferrite choke or coil is two parallel
resonant circuits in series. One resonance is the DIMENSIONAL
resonance, the other is the CIRCUIT resonance between the coil and the
stray capacitances (of #4). Both of these resonances have significant
R components as well.

So the apparent change in L as the turns are expanded or compressed is
simply the CIRCUIT resonance moving as the stray capacitance changes.
For all practical purposes, L does NOT change. C changes.

There's a lot more about this in an applications note on my website,
that also includes some references to the literature.

http://audiosystemsgroup.com/SAC0305Ferrites.pdf

Jim Brown  K9YC


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[Ron D'Eau Claire-2]

Jim wrote:
So the apparent change in L as the turns are expanded or compressed is
simply the CIRCUIT resonance moving as the stray capacitance changes.
For all practical purposes, L does NOT change. C changes.

--------------------------------

The inductance measurement method I used does not rely upon a resonant
frequency of any sort. It measures the inductance by differentiating a
square wave.

The frequencies involved do span the RF spectrum, of course, since the
waveform used to test the inductance is a square wave. A square wave is made
up of the fundamental and a very large number of harmonics, all added
together.

The validity of the inductance measurement using this method is borne out by
noting that the inductance value reported is that at which the inductor can
be used to form a resonant circuit at the desired operating frequency within
the normal range for the core material. That is, for a core designed for HF,
a measured 3.1 uH resonates properly at HF (e.g. 3.1 uh in parallel with 47
pf resonates at about 13 MHz).

Ron AC7AC

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[Jim Brown-10]

On Tue, 30 Aug 2005 09:18:57 -0700, Ron D'Eau Claire wrote:

>The validity of the inductance measurement using this method is borne
>out by noting that the inductance value reported

Yes, if the measurement is made at a sufficiently low frequency, Xc is
insignificantly low.

Jim



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[Ron D'Eau Claire-2]

Jim wrote:

Yes, if the measurement is made at a sufficiently low frequency, Xc is
insignificantly low.

---------------------------

The measurement is made at H.F. I'm sorry, I apparently confused the issue
by saying the applied square wave was at 60 kHz. That's NOT the measurement
frequency. The edges of the square wave include components extending through
the H.F. range, at least. The inductance is measured directly by noting the
effect it has on the edges of the square wave.

The advantage of that system is that it measures the inductance directly.
And a shift in the inductance of about 9% was observed by scrunching the
turns together.

Ron AC7AC

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[Jim Brown-10]

I disagree. I think you are probably observing the change in capacitance. I
agree that the higher frequency components associated with the square wave
rise time is the measurement excitation.

Jim

On Tue, 30 Aug 2005 10:00:23 -0700, Ron D'Eau Claire wrote:

>Jim wrote:

>Yes, if the measurement is made at a sufficiently low frequency, Xc is
>insignificantly low.

>---------------------------

>The measurement is made at H.F. I'm sorry, I apparently confused the issue
>by saying the applied square wave was at 60 kHz. That's NOT the measurement
>frequency. The edges of the square wave include components extending through
>the H.F. range, at least. The inductance is measured directly by noting the
>effect it has on the edges of the square wave.

>The advantage of that system is that it measures the inductance directly.
>And a shift in the inductance of about 9% was observed by scrunching the
>turns together.

>Ron AC7AC



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