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Well, here comes the big one. Time to give it my best shot. The big cheese...
Above shows the coil as rebuilt in 2007. See more sparks here on the sparks page.
First light
2005
Above shows sparks from a hasty lash up. The primary is thin 19G wire taped onto a flattened cardboard box on a wooden pallet. The topload is a toroid with a sphere, with a breakout point, sitting on top. The coupling was all wrong, the tuning very brief (11 turns) and everything was wet with dew. My ARSG gap was not optimised. The current draw was over 20 A (5KVA). Still the 5 foot sparks were gratifying and limited by the proximity to the basketball ring. Neither the Tesla coil or basketball ring were easily moved to get a peak spark length. There were some problems and the current draw did not 'feel' like a high inductance coil. This may simply be a reflection of the larger capacitor used. The power would tend to fade at intermediate power but not at high power. The spherical electrode makes this look like a scaled down Electrum.
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 coil
2005
Unfortunately, I have been beset by Murphy's Law. Winding was interrupted when only 1/3 complete, by rain and gale force winds. The following day the windings had loosened and overlapped, possibly due to compression of the rather thin polypropylene former. I unwound the few hundred meters and started again. This time the winding belt broke so family were called out to provide the rotation. Then the wire ran out about 4 inches short of the end. Either my calculations were out or the wire was less than the specified 11 kg. I am not happy about the smoothness of some of the turns as well but will have to wait and see. I gave several coats of polyurethane (total 1 litre) to fix the windings.
In 2007 a stray wire cause sparking to the secondary (upper left photo above) caused shorting in 3 places. The upper middle photo shows the lower short vaporized a short segment of wire. I removed 2 turns in 3 places, recoated with polyurethane. This time my motor setup in the upper right photo was more reliable with the motor rotating things for about 24 h and it runs fine despite looking a bit scrappy. The lower photo shows the coil running again. It is a 6 second single exposure. The flash goes off to highlight me, then in darkness I go and turn the TC on briefly. I am never anywhere near sparks in reality.
Primary flat spiral fairly broad of 6.35 mm (1/4 inch) copper pipe 12.5 mm (1/2 inch) spaced. The initial lashup was to determine the tuning range.
Toroid I initially 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. It stayed inflated for 2 years until I tried to inflate it a little more then the hole became permanent despite using tyre inflate and seal latex pressure pack. Later I used a double toroid covered with chicken wire. This seemed elegant, light and rugged. Also had good central support for a rotating breakout point. Alas after a full power run it appears that corona had eaten into the tube and created numerous holes (although the wire framework was intact).
The left photo shows the double toroid construction. The middle photo shows the toroid in situ. The right photo shows the holes from the corona.
Synchronous rotary spark gap (SRSG) 100 breaks per second (below). I have modified a 1/8 HP 2850 RPM 50 Hz motor for true synchronous operation at 3000 RPM = 100 bps for 2 electrodes. This was done by milling two flats in the rotor to a total of 30 % of the circumference with an angle grinder. For motors of half that speed you will need to grind 4 flats to a total of 40% of the circumference.
A synchronous motor is the most efficient way to discharge the tank capacitor as it discharges when it reaches full charge at each peak in the mains 50 Hz cycle which is 100 beats per second (BPS). The synchronous rotation is visible under fluorescent or incandescent light if there is a white mark on the disk. More accurately I use my high intensity Luxeon Star white LED run off half wave rectified DC adjusted so it only just turns on at the peak voltage. I will also use the ARSG as well to compare. This motor is a bit small but probably is OK for my current disc size but might not be for a larger one.
The middle photo above shows the spark gap connected to the MOT supply without a capacitor. The power arc extends almost around to the other contact. The last shot is with the normal Tesla coil setup with the capacitor discharge giving an intense spark. I planned to use a phase controller by John Freau. As it turned out though I never ended up using this system as a synchronous motor. I ended up using my ARSG driven by an angle grinder. Performance was poor probably due to poor quenching so I used a series static gap with a leaf blower which improved things a lot. Later (2007) I did use this motor running 8 electrodes at a relatively slow rate driven by 80 instead of 240 V. Still had to use the series quench gap though.
Tank cap 0.09 uF MMC (4 rows of 15 times 0.33 uF 1600 V Type CD 942C 16P33K).
Dual
Potential Transformer 2007
Still rather unfinished. Unfortunately this setup was like a tank to shift and there seemed no hope to be able to easily get it on to a trailer unaided. I therefore rebuilt it into a lighter shell.
So, here is the new one. This is much more transportable and ergonomic and with a smaller footprint. It is not too hard to get this up a trailer ramp with one person. Still needs a lot of work such as current and voltage metering, fans and panels. I have lots of things I could put in here like ballast coils and a 3 phase generator. There is a 50 A rated dual phi filter to help prevent mains interference.
I now have a 4.5 KVA generator which will give me some portability to my Tesla stuff.
Future plans.
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This page was last updated June 04, 2009
