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Re: Rotary Gap Electrodes - A first-hand experience

>>From bturner-at-apc-dot-netWed Oct 23 21:20:39 1996
>Date: Wed, 23 Oct 1996 01:27:25 -0700
>From: open_minded <bturner-at-apc-dot-net>
>To: tesla-at-poodle.pupman-dot-com
>Subject: Rotary Gap Electrodes - A first-hand experience

>Hi all...

>  Well, I had a very successful, and eye-opening experience tonight. Lit
>up my larger coil system with the new, 3600 RPM sync. motor. The coil
>sure came to life...I had frantic, white, energetic discharges waving
>into the air. Physically measured strikes came out to 67" to a metal
>ladder near the coil. (Hey, whatever works.) Not shabby for a secondary
>with only 22" worth of windings.
>
>  One interesting thing though - I sure vaporized the tips off the
>stationary electrodes in my rotary gap! Whoooeeeee. I had 1/8" tungsten
>rods fixed into brass holders. At full poop, the stationary rods
>achieved a nice, brilliant yellow after about 20-30 seconds. The rotating
>electrodes (same stuff) stayed nice and cool.
>
>  Pre-ionization was quite evident as the discharges initially started
>out very long, then after about 5-7 seconds, shrank back as the electrode
>tips got exceedingly hot. At least I think it was pre-ionization. After
>the gap cooled for a few seconds, it would again light up with long
>discharges that again would shrink back a bit as the tips heated up.
>Anybody experience this phenomenon? (BTW - this same coil *didn't* have
>this behavior with it's old 1800-RPM gap with other electrodes, so I am
>rather sure that everything else hasn't changed.)
>
>  Looks like a quick retro-fit is in order here. I will replace the 1/8"
>rod in the stationary holders with 1/4", and cut some rather massive
>and heavily finned brass heatsinks. Hopefully, this will help to keep
>the (fixed) electrode temperature down below the 'nasty' point.
>
>  Aside from dwell times and such, one must be careful to match the
>electrode size and shape to the operating power levels! Either that, or
>I will need to figure out a way to liquid-cool them....!!!!
>
>- Brent

Brent,

Congrats on your spark length vs coil length.  Not shabby at all!
I saw the pictures you posted on your new rotary gap.  Nice work!  I 
see you mounted your rotating 1/8th inch tungsten rods the same way I 
did with an allen setscrew into the side of a bolt head.  Damned good 
idea! : )

I don't think heavy heatsinks on your stationary electrodes is going to 
solve your problem.  Tungsten does not conduct the heat away from the 
source fast enough so a heatsink at some distance from the arc won't 
do the job.  A larger 1/4 inch diameter stationary contact will 
reduce the evaporization rate at the tips and also improve the heat transfer away 
to the heatsink, but I think you should investigate blowing high 
volume, high speed air jets past the ends of your stationary 
electrodes for forced cooling using a vacuum cleaner blower.  This 
also helps in quenching.  See pictures of my rotary with blown gaps 
on the funet.fi rws picture files.  I'm running a 0.125 mfd cap at 7 
kVa with this gap at 400 PPS in my MTC system without any trouble
whatsoever.  Since those pictures were taken the stationary electrodes have been 
replaced with tungsten carbide pads brazed onto 1/4 inch steel mounting 
shafts.  The blower effectively cools the pads below the melting 
temperature of the brazing.  There is therefore obviously no 
electrode incandescence.  The first time this happened my electrodes 
would fall off.

I agree with your diagnossis that the incandescent gap electrodes 
were causing pre-ionization (and power arcing or impaired quenching, 
same thing) which ultimately lengthens your dwell to far too long and 
system performance falls on its face.

As a point of interest, prior to going tungsten carbide on the 
stationary electrodes they were grade 8 steel bolts.  The heads often 
went incandescent and then the blower would cause then to emit a 
shower of tiny globules of burning metal, just like from a cutting 
torch going through steel.  This happened even with the same blower 
running.  I wonder if, with the steel electrodes, once a certain tip 
surface temperature was reached steel started to shed a cloud of 
metal ions, which in turn started the power arc phenomenon, which in 
turn caused more heating at the surface, which caused more 
evaporation, etc., resulting in a runaway condition.  On the other 
hand, at the same tip surface temperature that broke down the steel, the 
tungsten carbide's surface remains stable and does not emit anything 
to feed a runaway chain reaction?  Anyone wish to comment on this 
hypothesis, or share a similar experience?

regards,
rwstephens