Re: eddy current with secondary coil

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

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


Antonio,

My simple measurement confirmed what you've been saying.   The metal
plate caused a very small change in impedance when placed right at the
end.  The effect was very small, like 1% or less.

The coil I tested is about 2mH and consists of some uncounted number of
turns of #34 wound on a 3/4" piece of PVC schedule 40 water pipe.  The
wind is about 6 inches long.   Its a small coil as coils go for sure.

I measured the impedance of just the coil, connecting the analyzer to
the wire lead at each end both with and without a metal plate slapped up
against the end.  I tried a thin sheet of copper (5mil) and a thicker
piece of aluminum (60mil).  Both made about the same *very* small
decrease in the impedance up to about 1MHz.  Beyond that you could tell
much since there was a resonance at 1.4MHz.

I left the plots at work so I'll have to report the hard data later.

Anyways, this result was counter intuitive and very enlightening.   I
was expecting a bigger change, especially at the higher frequencies.

Thanks
Dave Lewis




 >  > 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