[TCML] Advice needed: DRSSTC Primary winding

[Finn Hammer]

Grant,

I will advise you against using an internal primary winding, although 
*I* do it all the time....
The reason is, that the voltage gradient between primary coil and 
secondary is going to be of a character that will invariably lead to 
corona on the primary coil.
You cannot solve a corona problem by adding insulation material, it will 
make it worse.
The only solution to a corona problem is either field control or 
potting, ie. removing all air in the intervening space.

A couple of years ago I wrote a .lua script for FEMM, which draw the 
geometry of an internal primary, for subsequent finite element treatment 
to solve the complex electrical field between windings.
It was later rewritten by Russian coiler, Father Dest, and it became 
very good.

Using this script it was possible to produce a series of FEMM models of 
internal primary´s, and it was shown that if you want to control the 
corona, substantial amounts of potting material had to be used.
I never pursued the problem, although there still is a bucket of potting 
material in my store purchased at that time.
In coils designed to run at low duty cycles, internal primary's are a 
possibility, because it takes some time before the corona *get's at* the 
plastic, but for ordinary coils, running flat out for minutes on end, It 
is not practical.

Cheers, Finn Hammer

Grant Visser skrev:
> Hi all,
>
> I am building a small DRSSTC (3.5" x 15.8" Secondary - 1600) and was
> wondering if I could put the Primary winding "inside" the Secondary, and if
> so, how the design parameters should be adjusted?
>
> The former for the 2ndary is plexiglass as is the former I'd like to use for
> the primary (1.9"). I do have commercial nomex transformer insulation which
> I could use for additional insulation between the two coils. ie  secondary
> winding/plexiglass/nomex/airgap/primary winding/plexi/... etc.
>
> IF this "could" work ... how would I need to change the design parameters of
> the primary? I was going to use 6 turns of 3mm copper tubing ...
>
> How would this affect the desired coupling co-efficient and overall energy
> transfer?
>
> Any words of wisdom greatly appreciated :-)
>
> Grant
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