Re: one turn heating

Subject:  Re: one turn heating
  Date:   Fri, 6 Jun 1997 13:33:39 -0400
  From:  "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:  "Tesla List" <tesla-at-pupman-dot-com>

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-poodle.pupman-dot-com
> Subject: Re: one turn heating
> Date: Thursday, June 05, 1997 11:37 PM
> Subject:      Re: one turn heating
>       Date:   Thu, 05 Jun 1997 21:46:39 +1000
>       From:   Peter Electric <elekessy-at-macquarie.matra-dot-com.au>
>         To:   Tesla List <tesla-at-pupman-dot-com>
> References: 
>            1
> Tesla List wrote:
> > 
> > Subject:  one turn heating
> >   Date:   Tue, 3 Jun 97 08:38:40 EDT
> >   From:   pierson-at-gone.ENET.dec-dot-com
> > 
> > >> I suddenly saw some wisps of smoke or something coming from
> > >> secondary.  I shut it right down and found that 23" up from
> > >>bottom,  a turn or more (#15 wire) had gotten hot enough to
> > >>the Behr Super Build 50 over coating and had really gotten it
> > >>smoke.
> > 
> > >I don't have any idea why one turn or section of the secondary
> > >get real hot unless it is from overcoupling or not being right
on tune.
> >         Utter speculations follows
> > 
> How about some more speculation. As I understand it,  normal TC
> secondary will act like a 1/4 wave aerial - it will have a very
high V,
> low I at the top and the reverse at the bottom, a very high I and a
> low V.
> If you overdrive the coil, splitting occurs which will cause the
> secondary to resonate at double the frequency and the very high I
> appears somewhere near the middle, possibly causing it to overheat?
> Unfortunately, my single 60Ma Neon Xformer doesn't cause anything
> overheat!
> Cheers,
> Peter E.

Two other plausible causes:

1) A short between turns. The short can be *triggered* by ocassional
side sparks that erode the insulation. A true short will get real hot
real quick!

2) Not a short, but just sparks/corona issuing from this location. 

Both of the above can be *triggered* by scratches, direct hits from
discharges, and by splitting, which is a result of
over-coupling. If you have ever had sparks down the side of the coil,
then you may have had tiny punctures develop. These then become the
*prefered* points for future discharging. 

AS SOON as I see any side sparks on my coils, I apply another layer
or two of varnish. If caught early enough you can prevent major

The most important thing, of course, is to eliminate the CAUSE of the
sparking, which is usually overcoupling. Notice that as you pump
more power into your primary, you will usually experience more
symptoms of over-coupling. 

If a spark gap is opened too wide, you may *also* experience symptoms
very similar to over-coupling.

If you use space-winding then you get less of this over-coupled
arcing, but you *may* find degradation of discharge length.

If the damage is extensive, you may have to do some minor surgery.
Since you have a fairly large gauge wire, this will not be *too*
difficult. You can cut the wire right at the break and carefully
unwind 1/2 turn above and below the cut point. Cut and trim wires so
they meet in the middle. Carefully tin the ends of the wire, solder,
and then make an ohmmeter check to make sure they are actually
soldered. Then cover with Behr Build or whatever insulation you
prefer. Richard Hull has another method for repairing damaged
windings. Perhaps he will share it with us again.

Fr. Tom McGahee