That's interesting David. Funnily enough I was talking with the other UK coiler I run with, over that very issue the other day - one pair or two pairs of stationary gaps.
Without any empirical knowledge of two parallel pairs (4 electrodes) I felt that if the stationary gaps were in parallel (with the aim of sharing the current / heat burden) then maybe the stationeries would run cooler.
I reasoned that only one pair will ever fire at one time, as the gaps would need the machining tolerances of NASA, to both coincide together for a discharge that is measured in mere millionths of a second. So assuming the workload evens out between them, they would each do half the work.
(If the gaps were in series for quenching assistance with a conductive band around the rotor then obviously it's a different matter - tried that once, but had other issues.)
If my parallel thinking was correct the flying electrodes would still only be seeing the same amount of firings though, be it with one stationary pair or the other pair.
Your experiences now make me think my reasoning is wrong though, as only both pairs of stationeries firing together could cause the overheating, by the reasoning you mention - they are firing twice as much.
The decreased performance of yours could of course have been down to quenching issues from the (then) overheated electrodes?
David wrote>>" in spite of the generous airflow generated by whirling at ~3000 rpm!" Don't you run at 3600 rpm, or is it a Dc motor?
Phil
-----Original Message-----
From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of David Rieben
Sent: 20 October 2014 19:20
To: Tesla Coil Mailing List
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.
David Rieben
Teslalabor <teslalabor@xxxxxxxxxxx> wrote:
>Hi Phil,
>
>thanks! You are right, close gap clearances on rotary gaps seem to be
>important, I observed this some years ago on a SRSG, running 200bps @
>3000rpm. But always was wondering about this, because the arc ignites long
>before the electrodes are coming together.
>The firing voltage on my new rotary will be arround 26kV so maybe close gap
>clearance will not be so important. We will see!
>Yes, the experiments will show, if the brass holders for the stationary
>tungsten electrodes are able to take the heat away properly. If not, it's
>not a big deal to change them for massive copper pieces oder aluminum heat
>sinks.
>Your idea with the air movement control is pretty cool! Will have to keep
>this in mind. But the air movement the disc produces @ 4000rpm is incredible
>:-)
>
>Regards,
>Stefan
>
>----- Original Message -----
>From: "Phil" <pip@xxxxxxxxxxx>
>To: "'Tesla Coil Mailing List'" <tesla@xxxxxxxxxx>
>Sent: Monday, October 20, 2014 11:07 AM
>Subject: Re: [TCML] Finished: New 600bps Rotary,Power Controller + Variac
>for DC resonant charging TC
>
>
>Stefan,
>That's a very nice, sturdy RSG you have built, especially as it is never
>easy getting an outrigger bearing to align easily, but with the revs you are
>using it is a good idea! A setup like that should allow some close gap
>clearances as well which I always thinks helps. (I run 10 thou clearance @ 6
>- 7 Kw 3000 rpm).
>If you do get overheating problems you will find a switch to copper
>electrodes will have a vast difference, as the thermal conductivity of brass
>is not very good (copper being about 10 times higher I think). The rotary
>ones rarely get any heat to them, but the stationeries tend to ablate
>mostly, presumably made worse because they are not cooled as much.
>I use approx' 8 inch tall one inch diam' copper bars as terminal posts, and
>recently did some experiments with smoke to find the air movement paths on
>mine, and you may find a carefully sited piece of Lexan to act as a wind
>shield will direct the rotor draft towards the stationary terminal posts.
>(https://www.flickr.com/photos/33962508@N03/15318168107/ The black Lexan
>piece slides across in front of the stationeries and directs the substantial
>draft sideways)
>
>Nice to see a motor driven variac in use, but the thought of "overkill"
>comes to mind :-))
>
>Regards
>Phil Tuck
>
>www.hvtesla.com
>
>
>
>
>
>-----Original Message-----
>From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of Teslalabor
>Sent: 19 October 2014 18:53
>To: tesla@xxxxxxxxxx
>Subject: [TCML] Finished: New 600bps Rotary, Power Controller + Variac for
>DC resonant charging TC
>
>Hello all,
>
>as announced some weeks ago, I now want to show you my current proceedings
>with my DC resonant charging teslacoil. I just finished the rotary spark
>gap, motor driven 3-phase variac which feeds the high voltage transformer
>and also the power control box. Here is a video of it:
>
>https://www.youtube.com/watch?v=wVpRE38UXqE
>
>The rotary is made of a 450W DC Motor with a maximum speed of 12500rpm. The
>speed is controlled by a variac. The disc-diameter is 25cm, equipped with 6
>tungsten rods, each 6mm in diameter. The motor has a tacho generator on its
>shaft, which feeds a speedometer in the control box. So this gives me an
>absolutely precise measurement of both, disc speed AND bps! The meter
>reading, which has a scale from 0 - 10, is just this simple:
>
>x1000 = rpm's
>x100 = bps
>
>The rotary consists of 4 stationary electrodes, mounted to massive brass
>columns, 2 on each side of the disc,
>which results in a total of 4 spark gaps. I think, this in combination with
>the high speed / low dwell times will have some positive effects on the
>quenching properties of the gap and very low speeds will be possible without
>
>igniting a firewheel.
>In the video the rotary is accelerated up to 4000rpm (400bps) which is the
>target bps for my system, where the maximum power will be processed, but
>also up to 6000rpm are possible I think. Dwell time at 4000rpm is 252µs, at
>6000rpm it's 168µs. But I should be aware of the following: Each electrode
>weights 42,736g, so at 4000rpm it's weight rises to 86,7kg, at 6000rpm
>already
>195,1kg! At the moment I'm happy with 400bps, maybe I shouldn't try more
>rpm's with that calculations in mind :-)))
>
>The 3-phase motor variac can also be controlled with the control-box,
>voltage can be cranked up and down automatically and stopped at every
>voltage.
>
>Regards,
>Stefan
>
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