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SGTC underperforming, suspecting sparkgap

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Pcnerd

Wed Apr 09 2014, 12:55PM

Registered Member #88 Joined: Thu Feb 09 2006, 02:37PM
Location: The Netherlands
Posts: 29

Hi,

Together with a friend we are currently finishing our SGTC with a 80mm secondary. Picture is here: http://berg-vanden.nl/dump/4hv/total.jpg. Power supply is 7.5kv NST.

We are not seeing the expected performance and were are suspecting our sparkgap, here is the story:

At first we suffered from issues which we determined to be in our filter. The issue was that the TC would not start quit good, or it would stop out of itself. A video of our best run is found here: https://www.youtube.com/watch?v=49POf1jr1yo

After this we created a new filter: http://berg-vanden.nl/dump/4hv/filter.jpg
And then we re-ran the tesla coil and stumbled into more problems, in particular with our sparkgap. A picture of our sparkgap consisting of two brass threaded rods with 2 acorn nuts fitted inside a PVC T-joint with a hairdryer blower in the leg of the T. The top is cut open to allow for the air to get out. Picture: http://berg-vanden.nl/dump/4hv/sparkgap.jpg.

At first we started the TC by first starting the blower and then the NST. Setting the width for the sparkgap here resulted in a very small width giving very poor performance with bursts of long streamers every now and then, similar to the streamers of the firstrun, together with a "sparky" sound from the sparkgap. See this video:http://youtu.be/yRgCDiD0Cvk.

Then we figured that it might be smarter to start the TC first and then turn on the blower, this resulted in better performance, but not yet near that of the first run with the old filter. Video here:https://www.youtube.com/watch?v=MnIbSPDN2bc

Since then we expected our sparkgap to be not well designed, and we would like your expertise on this subject. Is the sparkgap indeed at fault and what would be a good (though simple) design to outfit our TC with?

For references, here is our javatc: http://tomzooi.com/dump/4hv/javatc.txt

cheers

Pcnerd

Thu Apr 17 2014, 01:24PM

Registered Member #88 Joined: Thu Feb 09 2006, 02:37PM
Location: The Netherlands
Posts: 29

Found myself the materials for a synchronous rotary spark gap(see attachement, isn't it cute?), going to test it soon, will let you know soon.

In the meanwhile I'm wondering the following:
-Where would be the "tipping point" where one would use a RQ gap instead of a normal sparkgap,
-and where would it be to change from RQ sparkgap to rotary SG?

Sulaiman

Thu Apr 17 2014, 05:31PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

your javaTC output indicates a 0.35cm static spark gap,
it looks like more than 3.5 mm in the photo'

are you sure that the motor for your rsg is synchronous?
it looks to me like an induction (squirrel cage) motor,
so you would have to grind two flats in the rotor to make it synchronous
an arsg can put a lot of stress on an NST
... make sure that your safety spark gap is adjusted properly

the two critical things to adjust are the spark gap
and the primary resonant frequency / primary tap point.
The spark gap should make a very sharp snapping sound
and the arc should be thin and blue/white
if the spark is wide/fuzzy/non-white then more air blast is needed.

Hazmatt_(The Underdog)

Fri Apr 18 2014, 12:56AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

1. Get rid of Aligator clips! If you haven't done this, do it now. Get some really beefy wire for your interconnects, like 10 AWG silver teflon, works great. Remember that every inch of wire connecting to the primary side IS A LOSS!!! Treat it as such. Anything inbetween the gap, cap, and primary IS.. I repeat IS the primary! So make those leads very short.

2. SINGLE BREAKOUT POINT. You want a single long streamer?... SINGLE BREAKOUT POINT. It's not a big secret, but it tends to make your coil seem like a really hot performer.

3. It is a lovely gap. I like the advance/retard feature, very cute.

Pcnerd

Mon Apr 28 2014, 08:51PM

Registered Member #88 Joined: Thu Feb 09 2006, 02:37PM
Location: The Netherlands
Posts: 29

haha, no worries guys, it's synchronous allright (2 sides milled off indeed). Filter is there with MOV's to protect even against a failing safety gap.
The alligators clip were for testing and are the 230v supply for the motor, the coil runs on 10 awg you can see in the earlier pictures of the filter etc.

We're working on making the leads as short as possible, but we'd like to find the proper tap point first.

Thanks for the tips at least :).

redruM69

Tue Apr 29 2014, 07:50AM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

Have you verified with a sync stobe that the RSG is in correct phase? Sounds like its close to 90 degrees out.

Is there a reason you need it synchronous? I could never get my sync coil even close to the performance I gained by switching to async. It went from being a mediocre coil to an absolute beast. I understand that theoretically a sync coil should be the most efficient, but it was difficult for me to get it dialed in.

Steve Conner

Tue Apr 29 2014, 09:09AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

A synchronous gap will only give a performance improvement if your tank capacitor is big enough to fully load the power supply at a low break rate, like 100 or 120bps.

If you have a smaller tank capacitor, then a higher break rate will give best performance, and beyond about 240bps, the difference between sync and async is hardly worth worrying about.

TLDR: It's actually the big tank capacitor that improves performance, the sync gap is just required to charge a big capacitor at a low break rate in a consistent way without massive voltage surges.

Pcnerd

Fri May 16 2014, 04:09PM

Registered Member #88 Joined: Thu Feb 09 2006, 02:37PM
Location: The Netherlands
Posts: 29

Today some testing, the tesla coil performed way better then in previous test with the normal spark gap (videos coming soon), however the motor appeared not to be very apt for the job. It's a 20-30watt microwave oven fan motor with two sides milled flat. Without any load I manages to get in sync and stay there, but run for some time it will get really hot and since it wouldn't get back ing sync very well. Next step is to try the vacuum motor synchronous trick and replace the old motor with that one (have to adapt the SRSG slightly for that). Thanks for the tips so far btw. We calculated the javaTC with a rotary spark gap and caps are loaded up to 93% which should give 70+cm spark in perfect circumstances, which isn't bad).

Graham Armitage

Fri May 16 2014, 05:47PM

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

Can I ask why you would not use a simple static spark gap, perhaps with some good air quenching? With a small NST like that, a rotary gap seems like overkill. I have also killed many NSTs with a rotary gap - never quite understood why, but just stopped doing it :( Nice looking rotary gap though - great workmanship!

GrantX

Sat May 17 2014, 06:39AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Just had a look at the photos of your setup and filter and in both photos it appears as if the strings of MOVs (blue disks, 6 in series) are only connected at one end, at the NST outputs. The other end appears to be floating, instead of connecting to ground. This won't be the cause of your performance woes, but currently the MOVs aren't going to do their job of suppressing voltage spikes that make it into the filter. Just noticed it and thought I should point it out in case you weren't already aware.

I don't have much experience with rotary gaps yet, but I use a static gap in a small SGTC with a 12.6nF tank cap, which is almost the same size as yours. I found that large electrode mass/surface area combined with a very high airflow helped immensely with quenching problems. I also noticed that increasing the coupling of the coils had a MUCH larger effect on performance than fiddling with the spark gap distances.

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