Re: eddy current with secondary coil

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

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
>
>
>
>