Re: eddy current with secondary coil

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Bart,

On 5 Oct 2003, at 9:29, 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?

I checked the effect of the terminal on a range of coils by first
measuring the inductance and resonant frequency, then cutting a slit
in the terminal to prevent it being a shorted turn. The difference in
resonant frequency for the resonators that I measured was in the
vicinity of 1%. Low frequency inductance checks with a Sencore
inductance analyser yielded much the same change in inductance. A lot
would have to depend on the size of the terminal vs the coil and the
proximity of one to the other. The coils in question were about 6"
dia and the terminal was about 15" dia and plonked pretty much on
top, maybe an inch or so above the top of the winding.

Malcolm