in the "over quench" thing I definitely have to disagree. I think, your
performance problems when using 4 gaps instead of 2 must have had some other
reasons. In my opinion, you never can have enough single gaps in a spark
gap. Have a look at the outstanding rotary gap, Greg Leyh has built many
years ago for the Electrum coil:
This amazing beast uses EIGHT gaps in series! With this, I also want to
bring another interesting fact into discussion: The "opening speed" of the
gaps! So it seems, we have 2 variables to play with:
1. mechanic dwell time, which seems to be the same, no matter of the number
of gaps used. Only disc speed is important and
2. "opening time", here comes the number of gaps into play. The more gaps,
the higher the opening speed, which is by the way not intuitive for me. Greg
Leyh states, that one single gap has an opening speed of 430feet/sec or Mach
0,4. His design uses 8 gaps, so 8 x 0,4 = Mach 3,2 total opening speed. By
the way, I can not find the reason, why he used 4 discs with only 2 gaps
each.
Woulnd't it have been much easier, to use only 2 discs, with each 4 gaps?
Both would give a total of 8 gaps, the same dwell time and opening speed.
Big advantage: You have to mess arround with only 2 instead of 4 discs on
the shaft.
Regards,
Stefan
----- Original Message -----
From: "David Rieben" <drieben@xxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Monday, October 20, 2014 8:20 PM
Subject: [TCML] Number of stationary electrodes & gap spacing in RSG
Hi Phil and Stefan,
First of all, congratulations to both of you on a pristine design for your
coils. Both systems would still be downright pretty if they never made the
first spark :-))
Now as far as rotary gap spacing and such, please allow me to share my
personal experience in this matter. My coil is the Green Monster and is of
the generic AC driven from a typical 14.4 kV pole pig with an ASYNC rotary
gap (about the simplest rotary gap driven coil system- I like keeping it
as simple as possible). Now I had originally ran with a single 10.5" dia.
X 1/2" thick G-10 disc with 8 flying tungsten electrodes and just 2
mounted (1/2" dia) stationary tungsten electrodes. Now my rotary gap isn't
nearly as pristine and precision made as either of yours but I was still
able to get sufficiently close spacing between the flying and stationary
electrodes to get steady and reliable firing without stationary/flying
electrodes crash. I then decided to try out 4 stationary electrodes as
opposed to 2. After modifying for this setup, I was still able to get
reasonably reliable firing across 4 gap points as opposed to 2, but there
was a new issue that I had not had with the 2 gap points. Since my flying
electrodes were now each seeing 2 firings per revolution, as opposed to
one, they were now getting hot enough during operation that they were
actually blistering the G-10 material around them on the disc, in spite of
the generous airflow generated by whirling at ~3000 rpm! Also, it actually
seemed as if the output sparks were ever so slightly weaker at a giving
power input setting than they were with just 2 gap points. I ended up
installing 3/8" shaft collar sleeves on each (3/8" dia) tungsten flying
electrode to help add a bit of thermal mass as well as increased surface
area. This did seem to keep them a "wee-bit" cooler, but there still
seemed to be a slight decrease in the coil's output.
Long story short, I ended up going back to the original 2-gap point design
and the flying electrodes are now running notably cooler (I left the added
shaft collars in place since they also help anchor the flying electrodes
in place). The gap spacing tolerance is also less critical and the output
sparks are now back to like they originally were ;-)
I recall reading somewhere in one of the old TCBA newsletters that it was
possible to "over quench" a spark gap to the point that the coil's output
deteriorates, since every gap in a series gap system wastes some of the
energy that ultimately ends up in the output sparks (maybe Richard Hull?)
I know it seems strange that there would be a noticeable difference in the
output of a multi-kilowatt pole pig driven coil system with just the
addition of 2 extra seriesed gaps and maybe it was just my imagination,
but it sure seemed that the output was not quite as bright with 4 gaps as
opposed to 2. May work a little different for a DC res. charging system
though, as I have no personal experience with this type of setup.