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I had a primary arc over into my secondary coil on my big tesla coil.
I thought I had insulated adequately, I had blackened the secondary with enamel, I was using insulated wire for a primary... and I wound it on cardboard on the outside of the secondary coilform.
There was a minor fire and minimal damage to the secondary, which can be used again after additional insulation.
I am, however, using a MOT 8-stack with this coil... that's 16kV at 500mA (though, I have it ballasted down to 40something amps (@120 volts) on the primary side for tuning purpouses)
It occurs to me that perhaps the peak voltage in this setup would be somewhere in the 20-30kV range, and that cardboard is really a bad choice for an insulator of high voltage (though cheap)
I was wanting to experiment with getting a higher output from a lower running wattage (though I wasn't planning on using a decreased running wattage- more bang for your buck) by winding the primary directly on the secondary coilform. This is an awefully large coil that I have attempted to build here (the secondary matches the over-the-top power supply- 36 inches of 26awg wound on a 8&3/4 inch diameter secondary PVC pipe).
Does anyone have faith in the inverse square rule? that if I move my primary closer to the secondary the magnetic field increases the inverse of the square of the distance away- something along those lines...
I SHOULD be getting a marked increase in efficiency here, when it's tuned no less... at least the difference of one foot in streamer length, at least.
And what then should I use as an insulator? I assume nothing that has air pockets- as wrapping saran wrap around the coilform would do... I've learned from my mistake with cardboard...
Registered Member #530
Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178
8 pack MOt stacks are very hard to insulate against RF feedback... I'm quite surprised that your transformers haven't flashed over already. Anyways your peak voltage is going to be 24 kV or so AC. That alone is grounds for concern - I'd use the flat primary design and keep a 2" space between your secondary and your primary ---this should cure your problems....
UPDATE
I just fired up my 6.25" X 30" secondary tonight and I got arcs from secondary to the primary. I was concerned! So I took off the inner turn of the primary and the dropped the primary down so that the last turn of the secondary was in line with the top edge of the primary....the racing sparks are now cured --- no problems at all! Too tight a coupling can cause severe problems with interarcing and racing sparks.
I used polyurethane varnish on my coil form. Works great!
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A close call with corona- insulation issue
