Re: arc-overs!

Subject:  Re: arc-overs!
  Date:   Thu, 17 Apr 1997 21:30:38 -0400
  From:   "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:   "Tesla List" <tesla-at-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?
> Chuck

The fact that you are getting a blue-green power arc at the base tells
that you are probably dealing with current flowing between the tank
and the RF ground. You mentioned that the primary begins only 1/4 inch
from the secondary. Try cutting back your inner primary to 1" away and
arcing should disappear. The arc you are experiencing has more to do
the inner primary trying to get to RF ground than any high voltage at
1.5 turn point on the secondary. 

Fr. Tom McGahee