Re: arc-overs!

Subject:  Re: arc-overs!
  Date:   Fri, 18 Apr 1997 00:29:24 -0400 (EDT)
  From:   richard hull <rhull-at-richmond.infi-dot-net>
    To:   Tesla List <tesla-at-pupman-dot-com>

At 08:51 AM 4/17/97 -0500, you wrote:
>Subject:  arc-overs!
>  Date:   Sat, 12 Apr 1997 17:32:42 -0400
>  From:   chuck seguin <seguinc-at-algonquinc.on.ca>
>    To:   tesla-at-poodle.pupman-dot-com
>Good afternoon everyone,
>Here is a frustrating event I have a challenge with.
>While I was tuning my last coil design, and pushing the envelope a
>little a witnessed a nasty arc-over from the bottom of my secondary to
>the ground of my primary.  Here is the set up.
>Cp=75.7 nF
>Lp=45 uH, flat spiral
>Ls=125.7 mH, 113mm x 667mm
>static spark gap
>DC supply 15kV, operated variably using a 120V variac
>The system is designed to operate at 75 kHz.  My primary starts at 1/4"
>away from the secondary coil form, 
>1/4" interwind spacing, 11.5 turns, 1/4" soft copper tubing.  The ground
>start nearest the secondary coil 
>form.  Since I operate at DC I have an actual ground for the power
>supply and a seperate RF ground to the same 
>AC mains ground.
>When I operated the system to its best, I had lifted the primary to
>roughly 1.5" away from the bottom of the 
>first winding on the secondary; in the hopes of improving the
>primary-secondary coupling.  This is where the 
>arc-over happened.  The arc started about 1.5 turns into the secondary
>and smacked into the ground connection 
>on the primary.  This persisted if I kept the voltage high enough.
>What surprises me is that there would even be enough potential 1.5 turns
>into the secondary to cause arcs to 
>ground on the primary.  This was a high current arc as well; very
>blue-green because of the copper vapor and 
>very thick and bright.
>What am I missing here?


You must have some awfully fine wire on that small coil to get all that
seconary inductance!  I think you problem is in raising the level of the
primary above the base of the secondary.  I would never recommend doing
this, regardless of the couple issue.  15KV RF can jump about 2.5 inches
an lightly ionized atmosphere.  Your voltage never built up on the
turn.  It just simply arced over to ground.  If your ground connection
the inner primary turn is not connected directly to the base of the
secondary then a voltage gradient is highly likely to be the culprit.

Richard Hull, TCBOR