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Rotary vs Air Blast gap

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crashstudio

Mon Oct 05 2015, 11:25PM

Registered Member #54484 Joined: Sun Feb 15 2015, 06:18PM
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Posts: 46

I'm currently building an asynchronous rotary spark gap using a grinder and controlling the speed with a variac. I mounted 4 threaded brass rods to a grinding wheel, took it for a spin and it's pretty well balanced. Still have to sand down the edges though. Do I need to add some sort of air flow for quenching this?
Once this is completed I plan to build an air blast gap. After reading a lot on spark gaps I've found many people listing greater results. Some even swear an air blast is by far the best type for an SGTC.
Anyone here have experience with these 2 gaps?

Here's the link to my TeslaMap specs and rotary disk pics.

Benjamin

Tue Oct 06 2015, 12:07AM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

Hi crashstudio, I have built both types of gaps. I am assuming that you are using an nst. I find that an arsg is better when you are using a power supply with lots of current, like microwave oven transformers. An air blast or vacuum gap will work very well with your setup.A arsg will provide no advantage.

crashstudio

Tue Oct 06 2015, 02:35AM

Registered Member #54484 Joined: Sun Feb 15 2015, 06:18PM
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Benjamin; I'm using 2 NST's in parallel. 15kv 60ma, total 120ma. You said an async wont give me any advantage but it should at least do the job right? TeslaMap says my max spark length should be 72.6 in so I'm hoping my rotary will give me at least 48" to 60". Also, once I switch to the air blast do I use the capacitance value suggested for a static gap?
Check the drive link for pics...

Benjamin

Tue Oct 06 2015, 04:31AM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

For an async or air blast gap you will want to use the suggested value of 34nf. you will not need any air flow for quenching on a rotary. your rotary will work, but not nearly as well as an air blast or vacuum gap. what I have done is used a vacuum cleaner motor connected to a PVC T fitting that splits into two "horns" of 1/2 inch copper pipe ending in 2 brass fittings that face each other, if that makes sense. Check out

for a similar gap.

Graham Armitage

Tue Oct 06 2015, 02:23PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

For what it's worth, I have had much better results from an air gap than RSG, especially when using NSTs. One of the most successful (and easiest) gaps I built was with two thin walled 1" pipes aligned axially and separated by the desired gap. One pipe was close and attached to a handle to adjust it closer or further from the other pipe edge. The second pipe was attached to small vacuum cleaner. This pulled air from outside the pipes and back into the "exhaust" pipe. I enclosed the whole thing in a muffler box to reduce light and noise but allow air flow. Because of the high flow rate of the air the spark was quenched very quickly which is what you want. Hope this helps.

Results can be seen here -

crashstudio

Tue Oct 06 2015, 03:33PM

Registered Member #54484 Joined: Sun Feb 15 2015, 06:18PM
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Posts: 46

Benjamin wrote ...

For an async or air blast gap you will want to use the suggested value of 34nf.

Someone else told me to go with the 55nf suggested by teslamap for sync rotary. This won't work? I literally just finished rebuilding the mmc from 34 to 55. Now if I just rewire it each string will have a different length of wire connecting them causing a difference in resistance right?
My rotary will be adjustable up to 10,000 rpm. 55nf won't work within that range?

Benjamin

Tue Oct 06 2015, 04:32PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

55nf will only work with a sync rotary not async! Async gaps in general work best at a high break rate, above 240 bps. if you try to run them at a lower break rate you will have surging due to the gap speed going in and out of phase with the charging frequency. A sync gap allow the nst bank to charge a much larger cap bank. With such a large cap size your nst's wont be able to charge them quickly enough each cycle and the result will be sporadic firing and a much reduced spark length. My advice is to switch back to 34nf and build an air blast gap, they're not that hard to build.

crashstudio

Tue Oct 06 2015, 06:01PM

Registered Member #54484 Joined: Sun Feb 15 2015, 06:18PM
Location:
Posts: 46

If I just rewire the mmc with an equal length of wire per string that should eliminate the difference in resistance right? I really don't want to take it all apart and rearrange everything again.

Benjamin

Tue Oct 06 2015, 06:40PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

All of the strings should have the same number of capacitors but the length of the wires connecting them are not that important as the difference in resistance between them is very small. the only way you can use that configuration is with a sync rotary gap or a sync triggered gap. 3 strings of 13 caps each will give you 34.62nf at 26 kv which is more than enough voltage rating at the right capacitance. I'm sorry but It looks like you have to rearrange it, I know how much of a pain it is as I have had to do that several times before too :).

edit: I just realized that If you simply remove 2 of your strings that will give you 27.2nf which is about 1.3 times your resonant value of 21.2nf which is perfect!

crashstudio

Tue Oct 06 2015, 09:37PM

Registered Member #54484 Joined: Sun Feb 15 2015, 06:18PM
Location:
Posts: 46

Benjamin wrote ...

edit: I just realized that If you simply remove 2 of your strings that will give you 27.2nf which is about 1.3 times your resonant value of 21.2nf which is perfect!

I thought about that but wouldn't the mmc be happier with more caps? It would mean less of a load per cap right? I would prefer to stretch the life of these as long as possible. I might just remove 5 and rewire the whole thing to 3 strings of 13 instead of removing all of them and rearranging.

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