From: FutureT@aol.com
Date: Thu, 31 May 2001 13:33:22 EDT
Subject: Re: [TSSP] Racing arc clues
In a message dated 5/30/01 7:38:11 PM Eastern Daylight Time, paul@abelian.demon.co.uk writes: > Lots of informative comments from all, and I don't see any that > I disagree with. > > John's comments are thought provoking... > > As you increase the toroid size, a higher topvolts is required > for breakout, and eventually a toroid size is reached at which > the secondary will break down before the topload breaks out. Hi Paul and Everyone, I'll introduce myself here on the list. Some of you know me from the TCML. I've been coiling for 10 years, and doing a few experiments along the way, I think it's just great to gradually chip away at TC mysteries. I think it's a fantastic project that Paul and you all have been working on. I wish I had been able to contribute more with measurements and all. I forgot to mention that if a pointed rod is installed onto the large toroid, it will tend to prevent racing sparks by letting the sparks break out at a lower voltage. Some folks say that large toroids actually prevent racing sparks, although I've never seen this happen on my coils. Maybe they used a breakout rod on their toroid, thereby changing the results? Paul, you mentioned the BPS, and I thought I'd say a few things about that. I wanted to say that from a practical standpoint, another factor that determines whether racing sparks occur with a given secondary, coupling, and spark length, is the breakrate. High breakrates cause the spark length to result relatively more from spark growth over successive bangs, so the bang size and toroid can be smaller for a given spark length, making racing sparks less likely. I guess that folks using 120 bps (or 100 bps), tend to be plagued with racing sparks more often than folks using higher breakrates. The low breakrate coils need a larger secondary for a given sparklength to prevent racing sparks. I think that the toroid can be smaller for a given spark length too, at high breakrate. I seem to remember that sparks can break out from a larger toroid, for a given bang size, at high breakrate also. If they can't, then the breakrate won't really affect the allowable toroid size versus the secondary and bang size. Unfortunately in my work, I've seen better "efficiency" at low breakrates, which has made racing sparks common in my work. > > This implies that for a given secondary there is a maximum > tolerable toroid size. Below this size, the system will always > break out from the top (assuming correct tuning). Above this > size, you risk damage to the secondary. In some cases, the allowable toroid may be so large relative to the secondary, that it may be undesireable to use a larger toroid anyway. For instance Richard Hull's Nemesis TC used a 14" by 45" secondary, and a 15" by 60" toroid. His sparks were up to 15 feet long, and tended to hit the primary at times, so it would probably have been unwise to add a larger toroid and even more power to his coil, because the sparks would hit the primary and the ground too much. I've never seen any racing sparks on Richard's coil, even though he said his coupling was set at 0.25. I often get racing sparks at only k = 0.12. Maybe larger coils can tolerate a tighter coupling without breakdown? More likely it may have been because Richard used a higher break rate, making his bang size relatively small for the spark length and secondary size. Somehow, I suspect that racing sparks are more of a problem on small coils, perhaps due to them being overdriven. Small coils may tend to be more stressed voltage-wise for their size. I wonder if folks encounter racing sparks often simply because they are using a too-small secondary. The height of the secondary seems to be a big factor, since this determines the voltage per unit length. I often try to push maximum performance from a small coil, which is probably why I have so much trouble with racing sparks, luckily, just raising the secondary by 1/2" often eliminates the racing sparks. > Associated with this > would be a maximum voltage capability, and consequently a > maximum wallplug power for any given BPS. Would be nice to know > how to calculate these limits for a given coil. To do so, we > would need to know at what voltage gradient the secondary > breaks down. Is this a function of the wire size, insulation, > smoothness of surface coating, and a host of things, or is > there is more simple recipe? I think that just making the coil a little taller helps a lot by reducing voltage stresses. > > Following from this is the question: Can we do anything to > maximise the 'tolerable toroid size' by suitable choice of > primary design, by way of ensuring that the secondary voltage > gradient remains as smooth as possible at all times when the > secondary voltage is high. In my old coil, I used to use a 4" dia secondary with a primary that was designed for a 6" secondary, so there was about a 2" space between primary and secondary, instead of the usual 1" space. This didn't seem to help to reduce racing sparks, which makes me think that the coupling itself causes at least as much of the problem as the primary stresses, but it was all somewhat subjective. More careful tests are needed. > > I wonder if optimising for max tolerable toroid size as defined > above is the same as optimising for maximum voltage .. hmm thinking > aloud .. no its not, the former implies you're willing to crank up > the input power as much as is necessary to achieve breakout, > whereas the latter is all about getting the max voltage at the > top for some fixed bang energy. > > The notion of a max tolerable toroid, and its associated > max power and max volts, is quite appealing - these seem to define > a firm set of upper limits for a given coil. Is something like this > in common use amongst coilers? It sounds like one of the first > things you'd want to know about a coil. My guess is that most coilers don't really consider the voltages involved, etc. They just design based on practical rules of thumb such as, "use a big secondary for a powerful coil, etc." A larger toroid can probably always be tolerated though by just loosening the coupling a little (up to a point anyway). The benefits of the larger toroid will probably outweigh the possible lost energy caused by the looser coupling. > > As for sharp points on the primary and secondary, well I think > they have specific causes in weaknesses in construction and 'battle > damage' and they can surely be identified as such without too much > difficulty. I think we should focus our attention on the racing > arcs that do not have an obvious cause. It does seem that once racing sparks occur on a secondary, it compromises the wire insulation, and makes the racing sparks much easier to form in the future. Richard Hull had a magnifier resonator which was wound with a Teflon insulated wire. This resonator built up a lot of static, and also was plagued with racing sparks. I don't know if there was a connection there. When the secondary is too short, racing sparks sometimes run all the way down the secondary. It certainly would be nice to pin down the whole racing sparks issue. My comments above are mostly anecdotal in nature I suppose, but I figured I'd mention some things that came to mind :) Cheers, John Freau > > We will need to distinguish between racing arcs that form when a > properly adjusted coil is driven right to its limits, and those that > occur when the coil is badly tuned or over-coupled. The former > is surely our main concern, since the latter can be fixed by > adjustment and if the problem persists, it is one of the former > category. > To summarise, we can attempt to maximise the performance of a coil, > in terms of a largest tolerable toroid, by engineering its secondary > voltage gradient for maximum uniformity. To do this we do not need > to know just what the secondary breakdown limit actually is. > > If, in addition, we also knew the breakdown voltage gradient of > the secondary, we could in principle calculate this maximum toroid > size. > > -- > Paul Nicholson, > Manchester, UK.
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.