Re: Overcoupling?

To: tesla-at-pupman-dot-com
Subject: Re: Overcoupling?
From: "Robert W. Stephens" <rwstephens-at-headwaters-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Wed, 05 May 1999 22:57:14 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme
Tesla List wrote:
> 
> Original Poster: Gary Lau  03-May-1999 1507 <lau-at-hdecad.ENET.dec-dot-com>
> 
> >Original Poster: Terry Fritz <twf-at-verinet-dot-com>
> 
> >>I was tuning up my coil again a couple of days back (I now have my other
> >>NST back from repair and have stretched by alu duct toroid to 36" dia - 2kW
> >>in) and at full power (2kVA) it now tunes at around 8.3 turns (WinTesla
> >>predicted 7.6). However when I hit this tune I think that I identified
> >>symptoms of 'double-humping' - I saw a ring of heavy inter-turn corona and
> >>mild sparking about 3/4 of the way up the secondary, and the occasional
> >>spark all the way down the secondary coil.
> >>
> >>I raised the secondary up about 1/2" and this was reduced, although not
> >>absent. The sec base is now about 1/4" above the level of the primary, and
> >>the gap around the sec is about 1.5". My toroid's centre disc is temporary,
> >>not covered completely with foil, so maybe this is causing probs too.
> >>
> >>Anyone else have any experience with coupling adjustments? If you have any
> >>advice I'd be happy - I don't want to reduce the coupling too much...
> >>
> >>PS Spark length so far is 60", although since I am in a confined space, if
> >>I try for beyond this, the coil seems to favour the roof beams and walls as
> >>targets. I hope I can achieve more - do you think I can get that 6 feet??
> >>
> >>Alex Crow
> 
> >Hi Alex,
> >
> >   When in doubt, I would reduce the coupling.  Too little coupling and your
> >sparks are short. Too much and your coil may burnup!!
> >
> >       The corona area you mention is probably a natural high field
stress area
> >that linear wound Tesla coils have about 2/3 up the secondary.  Raising or
> >lowering the top terminal may help this if it looks like it is going to
> >arc.  I would fill in the center disk covering as this may help the high
> >field stress things you are seeing.  I think your coupling is really too
> >high and the output sparks may easily grow longer if you reduce it.
> >Overcoupling is really bad and can cause many problems.  It is always a bit
> >odd to move the secondary away from the primary to get longer sparks but
> >this is often the case...
> >
> >       Terry
> 
> If the main consequence to overcoupling is damage to one's secondary from
> the sparks that race along the length of the secondary, could this be
> helped by adding several disks of an insulator, say LDPE, along the
> length of the secondary, disk ID tightly hot-melt glued to the secondary,
> to effectively increase the top-bottom surface length of the secondary?
> I seem to recall seeing this done on someone's wep page, no idea who that
> was.  Same idea as on the ribbed HV insulators we see on pigs, NST
> bushings, etc.  In clear acrylic, it had that kind of futuristic, Jetsons
> look to it too!
> 
> Gary Lau
> Waltham, MA USA

Gary,

The 'Jetson's style coil' you saw is a lab prototype experimental model
of mine in a constant process of development which I call MTC-1.  The
white PE disks are ends from a hydraulic hose manufacturer's reels.  A
jig saw was used to make a ~15-3/4 inch hole to clear the HV resonator
winding in a snug fit and then as you say, hot melt glue (LDPE) seals
the disk to the winding surface.  I think it is somewhat effective, but
when the racing sparks eventually break through between the winding and
the disk the damage is extra hard to repair.

This coil is visible at: 
http://www.geocities-dot-com/capecanaveral/hangar/3108/rwsmap3.html
It is maintained there courtesy of fellow List coiler Bart Anderson.

Robert W. Stephens