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Rebuilding my 18 inch TC

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LAN_403

Tesladownunder

Mon Apr 09 2007, 08:16AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I currently don't have a TC of longer than about 4 foot arcs from my 4 foot coil. This is a bit small compared to my previous coils with up to 8 foot sparks from a 6 inch coil.
As the Discovery Channel in Cananda was showing some interest in my stuff, I thought I would revisit my 18 inch coil.
Plans are detailed here with the original coil completed 2 years ago giving 5 foot sparks with a rather thin wire primary and minimal tuning.
Secondary former. The former is made of 3 mm (0.12 inch) polypropylene sheet welded around end caps and one centre cap. This is light (weighs 9 Kg, 20 lbs) and the polypropylene is a suitable plastic in regard to dielectric losses. It measures 45 x 150 cm (18 by 59 inches) and cost AUD$250 to be made up by my local plastics shop.
Secondary windings: 1330 turns of 0.9 mm(19 AWG 0.037 inch) wire on 45 x 137 cm (18 x 54 inches) elevated 15 cm (6 inches). I have obtained 11 kg (24 lbs) of C 220 (220 deg C) polyimide coated. Due to some miscalculation I ran out 4 inches short.
Toroid: I have used a truck inner tube of 9 x 33 inches covered with aluminium tape. Of course in keeping with Murphy's Law (above) I found a puncture half way through taping the tube requiring hasty patching. Cross fingers and hope.
Capacitor: 0.09 uF (4 rows of 15 times 0.33 uF 1600 V Type CD 942C 16P33K).
ARSG: the ARSG (asynchronous rotary spark gap) uses an angle grinder (nominal 10,000 RPM) which with a 1/2 inch Tufnol wheel with 8 x 1/4 in bolts giving up to 9000 rpm =1200 bps. Unlike the 4 inch coil which works best with the ARSG flat out, this one works comfortably at about 120 bps. The stationary electrodes are 0.2 inch (5 mm) 2% thoriated tungsten. The spark gap doesn't start to trigger until the variac is putting out about 90 V AC when gap is minimised, increasing to 180 V AC when the electrodes are worn down.

And now the new stuff:
For power I have a dedicated 250V 32A line. I have two HV transformers (?potential transformers) rated at 10kV each. These are fairly solid epoxy coated types and have 100V and 50V inputs each. Put in series they give 300V in for 20kV out. So perhaps 16.6kV at 250V. I don't know the ratings which is a worry particularly when one rating is only 30VA. Hopefully this is the load that still gives accurate voltage division only.
The variac is a 250V 20A one from a old motor driven voltage regulator ex physics dept. All this goes into a trolley that was originally the base for a 8kVA Xenon arc photocoagulator. It has lots of 3 phase stuff in it much of which I have removed.
I have run it on my 4 inch coil and it all works VERY nicely. It should do since I can dial up twice the voltage as I can on my 4 MOT setup. Still lots to do cosmetically and internally with wiring and monitoring.
Next is planning the primary then firing up.

TDU

1176106465 10 FT0 Tesla18supplyconstruction2

sparky

Mon Apr 09 2007, 08:51AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

...go with a 18" 3 coil TC magnifier system??? That might be fun.... sounds like everything is solid!

Steve Ward

Mon Apr 09 2007, 04:25PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I don't know the ratings which is a worry particularly when one rating is only 30VA. Hopefully this is the load that still gives accurate voltage division only.

Well, what are the winding resistances? That should let you know if its gonna cook itself or not at 4kVA or whatever. But by the looks of it, they surely better be good for more than 30VA!

Tesladownunder

Tue Apr 10 2007, 01:08PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Steve Ward wrote ...

[Well, what are the winding resistances? That should let you know if its gonna cook itself or not at 4kVA or whatever.

My meter was giving some megohms for the secondary, but the figure wasn't stable and I don't trust it. It ran the 4 inch coil hard until it sounded like the ARSG was arcing over so I am sure it is OK. (I also did a 20A DC run through the primary which only dropped 3.5V and didn't die)

sparky wrote ...

...go with a 18" 3 coil TC magnifier system??? That might be fun.... sounds like everything is solid!

That might not be a bad suggestion if things don't go well. For some reason I only planned a H:D ratio of 3 when I designed it originally a couple of years ago so it seems to be a bit short to me. Still if all runs well I should get 10 ft due to the higher power and more efficient secondary. Still need to build the base and primary, hopefully tomorrow and have sparks within 24h. I've planned an 18 turn primary and have room to double layer it if I need more. So I am designing by guesswork although my temporary lashup was resonant at 11 turns and 60kHz, if I recall. I will need some margin to add extra stuff to the topload.

I can't get JavaTC or FanTC to recognise my toroid details.
I seem to recall this was a JAVA and the latest version of Windows problem. Any other programs I can use?

TDU

Michael W.

Tue Apr 10 2007, 02:25PM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

JavaTC didn't seem to work in my Firefox Browser, I had to use IE. Did you try a different browser?

Coronafix

Wed Apr 11 2007, 12:19AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Are you going to make a larger toroid for it?

Tesladownunder

Wed Apr 11 2007, 12:33PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I made up the primary today of 17 turns which was about 40m of 1/4 inch tubing along with the strike rail, and support for everything on castors.
The truck tyre toroid deflated with an obvoius leak when I went to pump it up a bit tighter. I tried a can of auto inflate latex foam but with little success so it is a pretty limp toroid. I really need a bigger one and I have the tuning range for it.
I went to run it and it was a real fizzer. Best tuning was at 11 turns as I recalled, but the spark gap was firing but then power arcing at low voltages. I think I must have some overheated or carbonised areas on my ARSG disc or something else awful. Best was 2 foot sparks for about 1 second before ? power arcing or something in my gap. Possibly in the caps as I haven't used the full bank for a couple of years.

So more work yet.
TDU

1176294675 10 FT23416 Tesla18newperspective

Tesladownunder

Sat Apr 14 2007, 12:22PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

OK, I have decreased the secondary height from about 4 inches to 2 inches to increase the coupling.
I have left the ARSG out of the box so I can see the gap. It is not doing anything wrong, I think, but after 1/2 second, the streamers appear to fade and the gap becomes quieter and more continuous sounding. The stationary tungsten rods become red hot.
My interpretation is that they are not quenching properly particularly as they heat up. I am running at about 1/2 speed ie perhaps 4000RPM with 8 electrodes = 500BPS.
Running at full speed doesn't really help and just makes it fade faster.
My question:
Should I remove every second contact so that there are only 4 contacts but double the speed to allow faster electrode presentation and quenching?

My deflated toroid is a shocker now. I will have a MUCH bigger double toroid tomorrow which should help further

Nevertheless performance has increased. Here is a 7 foot ground arc.

TDU

1176553307 10 FT23416 Tesla187ftgroundstrike1000

Marko

Sat Apr 14 2007, 01:03PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Wow, this looks pretty promising now.

How far are your gap electrodes spaced?

Some 'proffesional' high-power gaps use really tight spacing, to a fraction of mm, down to 0,2mm or so, and they still don't come in contact! that seems to greatly reduce losses in the gap.

I wonder if you could mount some sort of fins over the motor to blow some air over the stationary electrodes...

Tesladownunder

Sat Apr 14 2007, 01:36PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I space my electrodes so they just dont contact the flying electrodes when I apply a lateral force and I check all 8 flying electrodes. I assume powerful vibrations and resonances so don't try to push the gap too close. The Tufnol disc was professionally drilled and is very smooth running even at 9,000 RPM although I do have some neoprene damping in the ARSG box. The gap is about 2mm each side. The stationary electrodes are only 1/4 inch tungsten with a notch so they will snap off if struck.
I did try a leaf blower to improve quenching but it was not ideally placed and appeared to make no difference.
I guess I should try a series gap first which is simpler, because I can't really just replace the 4 electrodes once I have taken them out and this gap is also used on my other coil where it needs to go fast.

And fins. The flying electrodes are quite chunky and move a lot of air as is and already slow things from 11,000 to 9,000 RPM.

Another problem is some voltage which I can feel appearing on my variac handle. It still happens when I am standing on an insulated plastic sheet with the variac handle having a separate earth so probably just because I am 12 feet or so from the running TC. I will need to recheck my earth integrities. The potential transformer HV earth can be a 1/2 inch spark away from mains earth and seems to be well isolated on these transformers.

TDU

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