Re: eddy current with secondary coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br> 

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

 > 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.

It's not DC, but any frequency low enough so the DC inductances and
mutual inductances are still valid.

 > For high frequency conditions, the conductive sheet will immediately try
 > to repel the flux lines by virture of eddy current flow.

This happens too at low frequency. Inductances and mutual inductances
are independent of frequency, at low frequency.

 > 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.

Eddy currents are effect of mutual inductance. The heating is
consequence of the resistance of the "wires" (my model transformed
the plate in a series of concentrical rings that have currents that
simulate correctly the eddy currents, circular too). The effect
can be more intense at high frequencies just when skin effect comes
into play, increasing the resistances.

 > 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.

This would be interesting. The main difference from the case of a
Tesla coil is that in the later there is no wire connection to the
top end of the coil, and the current distribution along the coil
is not exactly uniform. The effect of the plate above the coil would
then be even smaller.

Antonio Carlos M. de Queiroz