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Re: Secondary Coil design



Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Gregory,

On 15 Nov 2001, at 11:54, Tesla list wrote:

> Original poster: "G by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<bog-at-cinci.rr-dot-com>
> 
> Hello List,
> 
> I have noticed a failure pattern in all of my secondary coils and am 
> searching for a solution. Each one has had a failure within the 
> bottom 25 turns (shorts between turns).During the most recent 
> failure, I was watching the operation very carefully in a dark room 
> and I am sure there wasn't any arc struck between the secondary and 
> the primary, or even corona.

The highest voltage gradient per turn occurs in the bottom portion of 
the coil. Polythermaleze wire should fail in that fashion so it 
suggests to me that the wire insulation is not the best for this 
application. I have tested polyesterimide wire to 8kVDC between two 
adjacent wires (standard insulation thickness for 0.8mm wire). It is 
rare that 1kV/turn will be exceeded in the base region except on high 
energy coils. 

 
> I have not noticed any other report of this problem. Has anyone had a 
> similar occurrence?
> I may try using a larger gauge wire toward the bottom to help deal 
> with the higher current in this region. Is current even the problem?

Even lots of wire heating won't damage the insulation types I've 
mentioned. I don't think current will be the problem. 

> Would using a larger gauge wire cause a damaging voltage gradient?

No. The opposite in fact. And it will have more robust insulation per 
turn (insulation thickness increases with wire gauge.

> Is space winding the bottom turns a possible solution? (also- 
> negative effect on coupling?)

If you want a coil with near identical parameters, spacewinding with 
smaller gauge wire is an excellent choice. It costs less and is 
lighter to boot. And you can fill the space between turns with 
electrical varnish if you really need to.
 
> 
> Thanks!
> Gregory
> 
> PS- I came across some construction trash dwv pipe labeled 'triple 
> wall hdpe'. It is lighter than a thinwall pvc pipe and is a 
> lower-loss plastic. Would the air pockets that exist between the 
> layers cause a secondary coil to burn through, or last longer?

Sounds like a good choice to me. 

Regards,
Malcolm