Re: RSG -- rotary spark gap

           Re: RSG -- rotary spark gap
           Mon, 31 Mar 1997 20:54:58 -0500
           "Edward J. Wingate" <ewing7-at-frontiernet-dot-net>
           Tesla List <tesla-at-pupman-dot-com>

Tesla List wrote:
> Subject:
>         RSG -- rotary spark gap
>   Date:
>         Fri, 28 Mar 1997 10:06:08 -0700
>   From:
>         "DR.RESONANCE" <DR.RESONANCE-at-next-wave-dot-net>
>     To:
>         "Tesla List" <tesla-at-pupman-dot-com>
> A 5/8 inch thick G-10 glass laminate makes a good rotor disc.  For 5-15
> systems use a 1725 RPM 3/4 HP motor equipped with 10 electrodes on a 12
> in.
> dia. disk.  3450 RPM seems to overquench.  We use 1/2 dia. brass thru
> holes
> that have been precision reamed .001 oversize and brass is press fit
> with a
> small press.  The brass is 1 1/2 inches long and each end fitted with a
> standard tungsten-molydenum screw in electrode.  They seem to last about
> 7
> years in rotary service.  We use a 1 inch dia. screw in electrode for
> the 4
> fixed electrodes and replace them once every 3 years (that assumes
> museum
> is doing 3 presentations a day x 6 days per week).  They are a little
> expensive but in experimenter service should last about 40 years or so!!
> Type LE and CE phenolic (carmel colored) seems to soften slightly so
> safety
> becomes an issue with these materials. Never try to build the rotor
> yourself --- always have a machine shop do it so absolute precision is
> maintained -- never mind expense either -- how much is an eye worth???
> A
> machine shop should balance the rotor to 5000 RPM.  Again cost enter the
> picture but these rotors can be dangerous so safety has to become a
> strong
> concern.  We have been making our rotors this way for over 20 years
> without
> ever having a single rotor explosion (knock on wood).
> DR.RESONANCE-at-next-wave-dot-net

Dr. Resonance,

G10, is indeed, a great material for spark gap rotors. I regularly run
the series rotary on my magnifier system between 7000 and 8000 RPMs with
no problems. G11 has a little more resistance to heat but is quite a bit
more expensive. Balancing is very important at these speeds and I do
both static and dynamic balancing on all of my rotors.

I disagree, however on the choice of a 1725 or 3450 RPM  induction motor
for the average Tesla experimenter's rotary gap. A better choice is a
1/2 or 1 hp AC/DC universal motor run either straight from a small
variac or powerstat on AC, or, for a little cooler and smoother running,
on DC by using a bridge rectifier with the variac. Fixed speed motors
are fine for museum displays, where the setup will never be changed, and
maximum performance is not all that important, but for Tesla coil
experimenters they do not allow the wide range of speed adjustment
sometimes needed to bring a coil into synergistic operation. If I were
limited to only one gap, it would be a variable speed without a doubt!
When experimenting, versatility is the name of the game.

Press fitted studs can work loose, as can screw in inserts and I feel
much safer with a rotary that uses setscrews installed in the outside
periphery of the rotor to positively hold the studs in place. Solid
tungsten studs are also a safer bet to prevent self- disassembly at high
speeds. 1/4" to 3/8" Lexan (polycarbonate) or 3/4" plywood covering the
gap is cheap insurance too! A supersonic piece of tungsten killing a
friend or an observer would not make for an enjoyable afternoon.

BTW, I must be missing something....
How do you press fit a stud into a .001 oversize hole? ;-)