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Re: eddy current with secondary coil



Original poster: Dave Lewis <hvdave-at-earthlink-dot-net> 


I would have to agree with Bart on these points.   A secondary with a no
top load looks alot like a 1/4 wave resonator.  As the top load gets
bigger the situation approaches the simple LC model.  The current
distrubution along the secondary changes as well.

The calcuations presented by Antonio shows a very small change is
inductance when a conductive sheet is placed at the end of the secondary
winding.  As I understand, this calcuation is for low frequency or DC.
In the case of DC, the flux lines will penetrate and pass though the
torroid very easily, hence one would not expect an inductance change as
was found.  This makes sense.

For high frequency conditions, the conductive sheet will immediately try
to repel the flux lines by virture of eddy current flow.  This is a time
dependant process that becomes more significant as the frequency and
thickness of the conductive sheet is increased.  Eddy currents cause
heating so I would expect some kind of loading term to appear in the
coil's impedance as frequency is increased.

Maybe this not of much importance.  However, I am inspired to make some
measurements of my own with an HP impedance analyzer we have in our lab
at work.   I'm thinking of measuring the impedance (magnitude and angle)
of a small coil with and without a conductive sheet placed right up on
the end of the coil verses frequency.  This will do nothing to address
Bart's points about current distribution effects under resonance with a
top load but would be interesting none the less to see the results... at
least for me.

I'll report back with what I find.

Dave Lewis




Tesla list wrote:
 >
 > Original poster: Bart Anderson <classi6-at-classictesla-dot-com>
 >
 > Ed,
 >
 > Were these measurements taken at resonance? Adding a topload will not
 > change the low frequency inductance, however, it definitely will change the
 > inductance at resonance. It has to. The induction is caused by the current
 > flowing through the secondary. At resonance, the current is not distributed
 > evenly, but occurs in uneven portions along the length of the secondary.
 > These circulating currents cause non-uniform induction in portions of the
 > winding. The induction can be very significant. On my small 19mH secondary,
 > the inductance at resonance drops to 14.3mH, however, when I add the
 > topload, the inductance at resonance increases up to 18.3mH. As far as I'm
 > concerned, that is significant.
 >
 > The topload definitely affects current distribution and must therefore
 > affect both inductance and capacitance. Because your measurements assume
 > otherwise, were there any low frequency measurements used? any? You said
 > that you and others have taken careful measurements with and without
 > topload. Will you explain the measurement method used?
 >
 > Take care,
 > Bart
 >
 > Tesla list wrote:
 >
 > >Original poster: Ed Phillips <evp-at-pacbell-dot-net>
 > >Tesla list wrote:
 > > >
 > > > Original poster: Dave Lewis <hvdave-at-earthlink-dot-net>
 > > >
 > > > In my view, eddy current effects are a good reason to place your torroid
 > > > a few inches above the last turn on the secondary.   A conductive sheet
 > > > right over the top of the secondary looks like a shorted turn that is
 > > > magnetically coupled.  Putting some air gap between the last secondary
 > > > turn and the torroid loosens the coupling up so that its not an issue.
 > > >
 > > > One way to visualize how much space you need is to imagine the magnetic
 > > > flux density within the center of the secodary as more or less evenly
 > > > distributed along the cross section.   Given that the area is pi*D where
 > > > D is the secondary diameter, you'd like at least that much area for the
 > > > flux lines to exit the top of your secondary and bypass the torroid
 > > > without compressing.  That would be a vertical spacing of at lease D/4.
 > > > Thats derived by equating the area of the secondary pi/4*D^2 to the area
 > > > below the torroid and last turn pi*D*X.
 > > >
 > > > Dave Lewis
 > >
 > >     I and others here have made careful measurements of the inductance 
of a
 > >secondary with and without a toroid slapped up against the end.  For a
 > >coil of 3" x 15" I find the change in inductance is less than 0.1%
 > >which leads me to conclude it can't have any effect on secondary
 > >losses.
 > >Ed
 > >
 > >
 > >
 > >