Terry, Steve,
When I figured out my equation for spark length, I used my own Tesla coils for the lower power calculations, and I used other people's results with larger coils to calculate the higher power levels. I had to do that since I never built any very large coils. The longest sparks I got were about 82". My work at the low powers seemed to suggest that coils ran a lot more "efficiently" at a low break rate. However for large coils people were reporting good efficiencies even at high break rates. It's possible that a low break rate might give a better efficiency even for large coils. I realize of course that efficiency is not always the coiler's goal. The high break rates can give brighter streamers and interesting sound effects as it ramps up, etc. By the way, for my equation I considered only the very longest sparks such as the rare 14' sparks you mentioned, not the average spark lengths. I'm thinking also that in many cases a very powerful coil might tend to give constant strikes to the ground. This will mean that the sparks tend to be quite curved and this will reduce the point to point length compared to if the sparks went straight outwards and hit something in the distance such as a metal ladder or something. If a large coil was taller or if the toroidwas raised such that the sparks would never hit the ground, then some longer sparks might be obtained? Also in a coil that produces multiple simultaneous streamers, they will be shorter than if the coil producesjust one streamer, or maybe 2 alternating side to side streamers. If raising the power starts formingmultiple streamers, that will limit the spark length. Regarding transformer losses, I supposed that just cranking up the power higher (assuming more power is available) would compensate in the sense that longer sparks would be produced, with the penalty of reduced efficiency. And of course in the case of anyone who is turning up their variac to 280 volts, their transformer will be saturating and wasting power that way. Another thing that I've always thought about is that if a person is running with their ballast/transformer combo in resonance, it seems their transformer primary voltage will go much higher than normal and that could cause transformer saturation and wasted power also. But it won't be apparent just looking at the variac setting.
John
-----Original Message-----
From: David Rieben <drieben@xxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sat, Apr 18, 2020 10:00 pm
Subject: Re: [TCML] John Freau's streamer length equation
Hey Terry, John, Steve,
I seem to have hit the ceiling on the output spark length from my ARSG pole pig driven classic coil system as well. Available power is not really an issue for either as I can handily feed 240 to 280 volts at ~100 amps all day from my 4-stacked 1256-D powerstat and homebuilt ~110 lbs worth of inductive ballast control panel assembly. When I set my variac dial at about 85 (which is about equal to the 240 volt mains), my coil draws about 90 amps and produces ground striking sparks mainly in the 10 to 12 linear point/point feet range. I may get the occasional rogue streamer that will reach out to 13 or possibly 14 feet, but 10-12 feet is pretty much my limit.
My primary cap is a Maxwell 31885 0.1 uFd, 75 kV rated unit and has proven to be a very robust and reliable capacitor. I tried swapping the cap and retuning with (2) GE 9L18CCL101 protective capacitors in series, each rated at 0.25 uFd, 13.8 kVAC for a 0.125 uFd primary cap. This did work well up to around 70 or so on the variac dial but seemed to almost ‘choke down’ beyond that power level with no apparent gain in spark length, brightness or ground stroke frequency. Theses caps also have the internal bleeder resistors and their external metal casing would begin to noticeably warm when running the variac up past 70 or so. I’ve since changed back to the original Maxwell cap and the output continues to increase in brightness and ground strike frequency pretty much throughout the variac range, although there does seem to be some resonance rise issues when I run past about 90 on the variac dial!
I have also recently changed out the old 10 kVa rated, 14.4 kV pig with a 25 kVa rated, 14.4 kV pig that I have had sitting idle for the past several years. I have not gotten a chance to run my coil yet with the transformer upgrade, but John’s mention of transformer losses got me to wondering if this may give me a notable increase in spark length output. Btw, my secondary coil is ~1260 of #19 AWG magnet wire wound about 49 linear inches on a 12” schedule 20 grey PVC sewer pipe form and my topload is a homemade 12” x 56” donut form.
David Rieben
PS: Thank you for that allowing me to give your orphaned 10 kva single eared piggy a new home, Terry. 😉
Sent from my iPhone
> On Apr 17, 2020, at 7:40 PM, Terry Oxandale <toxandale@xxxxxxxxxxxxx> wrote:
>
> Thanks John. Got a few more caps (.030 @ 35K) available, so making plans to add a couple more in another row to increase cap size from .045 to .060, then retuning. Ballast stays cool (#10 wire that was a large circuit-bkr closing solenoid coil). Rotary is variable speed at max of about 340 bbs. Got a good piece of 8" PVC in the garage...thinking of a rewind. Arc performance seems to be pretty linear with increased power at this point.
> Ox
> On Friday, April 17, 2020, 05:19:48 PM CDT, Futuret via Tesla <tesla@xxxxxxxxxx> wrote:
>
> Terry,The toroid might be 30" x 10"? In any case it's large enough. Do you find that as you increase the input power from let's say, 3600 watts to 4500 watts that the spark length doesn't increase much? If so that could be just a tuning issue, and you may need to tune outwards on the primary, although you probably did that already. I assume you're using a variable speed rotary gap? And if the capacitor is somewhat small that might limit the efficiency too. If you're using a static gap, it might be power arcing at the higher power levels. Also the transformer might be showing high losses at the higher power levels, or the ballast you're using could be wasting a lot of power (if so it will run very hot over time). If you're using a resistive ballast that will burn up some power. Then there's the possible power factor issue if you're not measuring true power but rather VA. Those are just some things that come to mind. All the best... John
>
> -----Original Message-----
> From: Terry Oxandale <toxandale@xxxxxxxxxxxxx>
> To: tesla <tesla@xxxxxxxxxx>; Futuret <futuret@xxxxxxx>
> Sent: Tue, Apr 14, 2020 7:46 pm
> Subject: Re: [TCML] John Freau's streamer length equation
>
> Thanks John. The toroid is an OLD group buy from here years ago...thinking maybe 34" x 10" (stored in a crate right now). It looks like a stick with a large donut on top (6" diameter secondary). All the same, I'm still tweaking with caps and primaries to obtain a little more.
> Ox
>
>
> On Tuesday, April 14, 2020, 05:07:59 PM CDT, Futuret via Tesla <tesla@xxxxxxxxxx> wrote:
>
> Terry,
> A large enough toroid diameter is needed to get the arcs to reach the full length. Arcs tend to not be able to reach beyond 4 times the toroid diameter no matterhow much power you use. (unless you use huge power such as in Bob Golka'sproject Tesla... something like that). Also sometimes the VA input is a lot higherthan the wattage depending on the power factor.
> John Freau
>
>
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