[Elecraft] OT: Effect of Compression and ExpansionontheInductanceof Toroids?

[W3FPR - Don Wilhelm]

You are quite correct Ron, the variability will be small.

Flux is dependent only on Ampere-Turns, so the total flux will not change
unless the number of turns is changed. There will be some flux leakage, and
I figure that is the parameter that changes more than anything else because
that would modify the effective perneability of the core.

The inductance of an ideal toroidal core depends only on the radius of the
toroidal core, the number of turns and the radius of each turn - there are
no other terms in the equations (other than constants - core permeability is
one of those 'constants'), so any variability must depend on just how far
from the ideal a particular toroidal coil is - and the only thing that can
change once the toroid is wound (with tight turns) is the effective
permeability (due to flux leakage).

One very nice thing about toroids is that they do have very little flux
leakage (that is why toroids are 'self-shielding'), but we do know that some
flux leakage does exist since one can de-tune a resonant circuit by placing
a finger next to the toroid - if it were an ideal toroid with no flux
leakage, adjacent objects would have no effect.  Even an air-wound toroid
will display this self-shielding effect if it is wound with perfectly even
spacing between the turns, but if the spacing is altered, more flux leakage
will exist and the inductance will change because the effective permeability
will be modified.

73,
Don W3FPR

> -----Original Message-----
>
> Yes, I think you may be absolutely right, Don. I measured a maximum effect
> of 9% in the one example I tested - a random core lying on my
> bench that had
> about a dozen turns on it. That agrees with your experience of
> finding up to
> a 10% change available.
>
> I saw your suggestion before, and it certainly sounds plausible.
> I wonder if
> something else might be at work here too. Or perhaps I'm looking the same
> effect you are in a different way. Here's my thought. The increased
> inductance/turn of the toroid compared to ordinary air-wound
> coils is caused
> by the very high permeability of the core compared to air. That makes the
> inductance fairly independent of the spacing between turns since
> spacing the
> turns hardly reduces the magnetic flux in the core that is
> available to each
> turn. But I wonder if crowding the turns together doesn't
> slightly increase
> the inductance by providing a lower permeability path for the
> magnetic flux,
> since the distance the flux must travel along the toroid before it passes
> through all the turns of the coil is shorter. In other words, the same
> effect one sees by using closer spacing in an air wound coil, only much,
> much less due to the efficient magnetic path provided by the core.
>
> Does the formula you are using account for a reduction of flux density
> around the distance of the torus due to the losses in the core, or does it
> assume a constant flux at all points?
>
> It's always interesting when experiment fails to support
> predictions. True,
> it most often turns out to be an invalid experiment that causes
> that result,
> but I can't see the problem here, especially considering that the
> effect of
> added distributed capacitance has an inverse effect on the reading on this
> type of 'meter'.
>
> Ron AC7AC
>
>
> -----Original Message-----
> From: W3FPR - Don Wilhelm [mailto:w3fpr at earthlink.net]
> Sent: Monday, August 29, 2005 9:22 PM
> To: Ron D'Eau Claire; elecraft at mailman.qth.net
> Subject: RE: [Elecraft] OT: Effect of Compression and
> ExpansionontheInductanceof Toroids?
>
>
> Ron,
>
> I have the same inductance meter, and it does a good job because it does
> measure using the LR time constant rather than frequency, but 'physics is
> physics', and with a true toroidal inductor, there is still no
> dependency on
> the turns spacing (the math says so).
>
> Now, for the practical side of things, I do believe that your results (and
> others with similar findings) deviate from the 'classic physics' treatment
> of the ideal (ignoring practical behavior) - and when the toroid turns are
> irregular (not evenly spaced around the circumference of the
> core), there is
> some part of that coil behaving as a solenoidal inductor where
> turns spacing
> IS a factor.
>
> Remember that a toroid is simply a solenoidal coil formed into a
> circle with
> the ends of the coil meeting.  This ideal toroid has equal turns
> spacing all
> the way around.
>
> So my current conclusion is -- IF the toroid is equally spaced around the
> core, the inductance is not dependent on the spacing of the turns, BUT, if
> the turns are compressed anywhere around the core, additional factors rear
> their ugly heads because the inductor is a combination of a
> solenoidal coil
> and a toroid coil, and the math becomes quite complex - just how much of
> each effect depends on just how much deviation from an ideal toroid shape
> exists in the configuration at hand.
>
> In a frequency dependent tuned circuit, how much of the frequency shift is
> due to the inter-turn capacitance or a change in inductance is
> (to me) only
> a matter of curiosity - the plain fact is that some change in the resonant
> frequency (or inductance) can be achieved by changing the turns
> spacing, but
> we all know that the adjustment range is small - the really big
> determining
> factor is simply the number of turns.
>
> As an example, I often improve the 10/12 meter Low Pass Filter
> characteristics of a KPA100 by squeezing the toroid turns "just the right
> way", but I determine what is the 'right way' by monitoring the impedance
> with my MFJ259B as I do it.  Sometimes it is 'this way', while other times
> it is 'that way' - the batch to batch change in the permeability of the
> cores likely accounts for more variation than the turns spacing
> itself. BTW,
> this change does not really help the KPA100 output, but it
> improves the base
> K2 10 meter efficiency at 10 watts or lower when the KPA100 is installed.
>
> The overall inductance change that I have experienced is about
> 10%, so that
> is within the normal design tolerances using 10% resistors and
> capacitors -
> so except for satisfaction of the curiosity factor, I would say
> just to wind
> the toroids with the proper number of turns, and 'tweak' them
> in-circuit as
> required and as close as is permitted by your measurement capability.
>
> 73,
> Don W3FPR
>
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