# Re: Racing Arcs Explained???

```Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Bart,

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Stay tuned (no pun intended) :o))) I'm building a spice deck to simulate racing arcs. I computed the initial voltage profile yesterday on my 8 inch coil and the greatest voltage gradient was near the bottom where the coupling is the greatest. I would think if a racing arc can be caused by this initial profile, it would arc down to the base and not up to the top (as observed). I believe wave action is a contributing ingredient and the spice model should simulate that.
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Gerry R.

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```Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Gerry, Steve,

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``` The key, I believe, is "the stress on the coil".
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Those words, in my opinion, is the racing arc situation in a nutshell. The relief of course is to find what causes stress. What type of stress? Well, a voltage stress in a part of the coil that is high enough to breakout. In the case of breaking out on the side of the coil, it finds it's path right down the surface of the coil (and sometimes may breaks out even after the surface arc has started and strikes the primary or strike ring = I've seen this). Most of the time, it's simply a path down to the bottom winding where RF ground impedance is low. If the stress is at the top (usually when the toroid is positioned too high above the top of the secondary), a racing arc can occur there, but most of the time, breakout from the top winding. Although there are frequency nodes that can aid or oppose the situation, there is 2 common situations.
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1) Tuning: Near resonance, but not perfect for the power. Power is involved here. A poorly tuned high power coil can produce racing sparks very easy. So can low power coils, but not as easily.
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2) Coupling: Plays right into tuning. Increase your coupling and you'll find out just how well your coil is tuned. It's important in my book to tune the coil and increase k to until you can no longer physically tune either any better - if you want to get max spark length. Tesla coil couplings can be somewhat high, but much depends it's tuning. I do something a little different than most. I like to set k relatively high and tune in the coupling vs. using a low coupling and tuning in the coil. When the latter is done first, it's hard to tell if k could have been higher. This is simply a view of my own to process as much secondary power as possible.
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Tuning and coupling are one and the same. Both should be "played with" for maximum performance. I realize there are multiple frequency nodes that can spell trouble (higher voltage stress in an unwanted part of the coil), but any coil can be tamed, and that is the key. If we shorten our coils, we decrease the creepage distance (not a good thing). If well make the coil too long, too much R that sacrifices power. We find ourselves in that 5:1 h/d ratio (or near) because of the voltage stress limitations we encounter. Geometry is important.
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Take care,
Bart

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