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Re: rotary gap electrodes

Tesla List wrote:
> 
> >From chip-at-poodle.pupman-dot-comSun Aug 18 22:10:04 1996
> Date: Sun, 18 Aug 1996 21:43:19 -0600 (MDT)
> From: Chip Atkinson <chip-at-poodle.pupman-dot-com>
> To: Tesla List <tesla-at-poodle.pupman-dot-com>
> Subject: rotary gap electrodes
> 
> Greetings,
> 
> I'm asymptotically approaching the state of rotary gap construction known
> as completion.  I thought I would ask for others input before proceeding.
> 
> The design of my rotary has two 1" thick 14"x14" pieces of plexiglass to
> hold the bearings and act as side shields.  The stationary electrodes are
> going to be mounted to the plexiglass and poke into the box where the
> disc is rotating.  I am using 1/8" tungsten TIG welding rods for
> stationary electrodes.  The input power will be less than 10KVA.
> 
> My questions are: How much cooling of the stationary electrodes do I
> need to do?  Will putting the W (tungsten) rod into a piece of 5/8" brass
> rod 2" long be enough to prevent the rod assembly from getting so hot
> that it melts the plexiglass?  I have also considered some sort of
> non-thermoplastic insulator to surround the brass.  Another option is a
> 1" rod of Al with the W rod in it.  The Al could have cooling fins turned
> on it too if necessary.  I don't figure ohmic heating will be much of a
> factor, but the arc will be hot and so heat the electrodes.
> 
> Any input would be appreciated.
> 
> Chip

Chip:
	My rotary has certain features that are not too different to yours. 
However, I decided to use 1" diameter brass rod about 1 1/4" long as my
stationary electrodes.  These have been given a 3/8" diameter contact
area that then tapers back to the full 1" diameter at about a 30 degree
angle per side.  The other end has been drilled and tapped for a 3/8-16
threaded rod that then runs back into the Nylon 101 side walls.  I just
finished disassembling and then polishing these two stationary
electrodes tonight, after about a maximum of 18-20 minutes of run time. 
They are showing very little wear at this point--in fact they polished
up in about one minute using a powered buffing wheel so that only a very
small amount of metal erosion is still visible.  My meters showed an
average reading during the last run of 290 VAC at 32 amps to the pig
primary, which I'm not sure I believe yet.  They are brand new Simpson 4
1/2" panel meters, so they should read accurately if the RF wasn't
screwing things up too much!.  The point is I was running a solid load
through the spark gap system.  The moving electrodes are 3/8" stainless
Acorn nuts and they still look new except for some discoloration due to
heating.  
	I think spreading the heat over the large area in the stationary
electodes that I have has helped alot, however I don't have the shortest
possible dwell time during conduction.  No problem at all with heat
transfer causing my nylon to melt or even show any signs of wear.  My
present level of understanding makes me think that the Magnifier
rotaries need the real short on time in their rotary system, while maybe
what I currently have should be O.K. for a standard two coil system--or
is it too long of an on time??  Comments please from the group?  I
wonder why the tungsten 1/8" rod was choosen Chip--has there been an
earlier discussion that I missed covering this topic?  My run tomorrow
will use one less gap in the fixed gap system that's in series with the
rotary--I have a less than constant firing and the system doesn't even
start to fire until the powerstat is at about 75-80%.  I wonder what
others see with systerms that are debugged, at what percent of full
voltage output does one expect or want breakdown of the gap system to
first start?  I would guess darn close to the high end would be best,
but how close?

Chuck Curran

[ I chose 1/8" tungsten because of its high melting point, and because I have a
  1/8" drill -- chip ]