Re: eddy current with secondary coil

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

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

Hi Malcolm,

I don't believe the test you described is looking at the inductance at 
resonance, but of the low frequency inductance and the resonant frequency 
with the resonator/topload changes. Was the inductance measured while the 
coil was being driven at it's resonant frequency? The test I did in my 
reply to Antonio maybe isn't the best method to check this even if it did 
come out so well. I'll perform some more this week, maybe on a larger coil. 
I know I performed a number of tests on dc measurements, both C and L, and 
Fres was off. When you consider the affects of current distribution, it 
should be off to some degree. When we measure Fres, we are measuring 
everything all thrown into the pot. How can we use a Cdc measurement and 
Fres measurement, and expect to get a correct L (or vice-versa)? L would be 
something between Ldc and the real L seen at resonance. This error would be 
put on the shoulders of C (which would end up just as wrong). We seem to 
have only one truely known value which makes the other two difficult to find.

Take care,
Bart

Tesla list wrote:

>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