Re: Copper vs. Brass in SG electrodes

To: tesla-at-pupman-dot-com
Subject: Re: Copper vs. Brass in SG electrodes
From: FutureT-at-aol-dot-com (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Sun, 25 Apr 1999 17:28:41 -0600
Approved: twf-at-verinet-dot-com
In a message dated 99-04-25 01:15:08 EDT, you write:

<< 
> Original Poster: Gary Lau  23-Apr-1999 1228 <lau-at-hdecad.ENET.dec-dot-com> 
 
> I'm slowly and carefully planning for my next undertaking, a sync RSG.
> I plan to use cylindrical electrodes, both stationary and rotating,
> either 1/4" or 3/8" diameter, so that the arc forms on the sides of the
> cylinders, rather than the ends, as appears to be more typical.  I see
> several advantages to this:

Gary,

Lou Balint of PA also built a gap of this type, but it was the series
quenching type.  It had two giant blowers forcing air into the unit to
assist the quenching, and to help spin the heavy rotor.  Unfortunately
at about 7000 rpm, the spinning electrodes bent from the centrifugal
force and struck the fixed electrodes, resulting in a hellish explosion
of flying electrodes.  Fortunately, damage was relatively minor; the
G10 rotor did not crack, so basically all that was needed was to 
replace the broken and bent elecrodes.  Lou will use thicker and 
shorter electrodes to solve the problem, but he has been busy with
other TC work.  The electrodes protruded 2.5" or so from each side
of the rotor, and were 1/4" thick or so, and were made of brass.
 
> 1) With the larger radius of curvature, premature breakdown will be
>    reduced, so the distance spanned by the arc will be less, with
>    correspondingly lower losses.

This was Lou's thinking also.  I often use acorn shaped nuts as
electrodes for similar reasons.
 
> 2) The arcing surface will be spread out over a larger area.  This should:
>    a) reduce electrode wear
>    b) reduce localized electrode heating

This was Lou's thinking also.  The cooler running may help the quenching
too.
 
> 3) Motor bearing play along the axis of the shaft won't matter.

True.
 
> Brass seems to be a much more common material than copper and I'm
> wondering if anyone can tell me why most references suggest that copper
> is the better electrode material?

My guess is that the copper was found to be more durable; less prone
to fast ablation.  I've seen from my experience that the brass ablates
badly, but I've never tried copper for comparison.  So I'm comparing
the rate of brass ablation with the rate of steel ablation.
 
> I'm also planning on using 1/2" lexan for the disk.  I've heard many
> times that plastics should not be used for the disk as electrode heating
> will loosen them and send them flying.  With an earlier attempt at a sync
> RSG I used 1/4" copper tubing cylinders, and the extreme turbulance on
> the rotating electrodes left them quite cool.  I'd be happy to hear from
> any others who have used lexan for disks.  FWIW, I'm using a 15KV/60mA NST.

I'm presently using lexan in all three of my rotary gaps.  In all cases, my
spinning electrodes run cool, so I haven't had any problems with heating
of the rotor.  I think that Ed Wingate said that his electrodes also run
cool even at 6kV+, so even at higher powers, the lexan may be OK.
I've run maybe 2600 watts max in my coil, so the 15kV, 60ma NST 
shouldn't present a problem.  Ed uses G10 or G11 for his
rotors BTW, he doesn't take any chances.  I also have a wooden guard
around my rotor, to catch any flying electrodes in case the unexpected
happens.  I also position the gap so if the electrodes fly out, they won't
hit me.  I've never had any rotor failures.  I've heard that certain 
chemicals can embrittle-ize lexan but I don't know any details.
Some coilers may be aghast at my use of lexan.  Another issue is
rotor rpm; since sync rotaries generally spin at 1800 or 3600 rpm,
the centrifugal forces are considerably less than at the higher rpms
often used in non-sync rotaries.

John Freau
 
> Regards, Gary Lau
> Waltham, MA USA
  >>