From: FutureT@aol.com
Date: Fri, 1 Jun 2001 17:09:18 EDT
Subject: Re: [TSSP] Racing arc clues
In a message dated 6/1/01 3:35:59 PM Eastern Daylight Time, paul@abelian.demon.co.uk writes: > Hi John and all, > > Probably most on this list are accustomed to reading your frequent and > considered postings on pupman, and I've certainly learned a lot from > reading them, so its great to see you pop up on here! > > Your comments have helped me to clarify things a little, and perhaps > you can check my reasoning below. Hi Paul, Thanks for the nice welcome and comments. I agree with your points (a) through (f), more below. > > First, I think we need to recognise that racing arcs are simply a case > of the secondary breaking down before anything else. Then the advice we > might give to cure a case could be one or more of the following, > > a) Reduce the drive power. (Not what you really want to hear). > > b) Use a smaller toroid. (Its lower breakout potential will ensure > the topload breaks out before the secondary.) > > c) Add a breakout point to the toroid. (Again, ensuring that the > topload breaks out first). > > d) Reduce the coupling. (The topvolts still reaches much the same > ultimate peak value, just takes a little longer to get there, and > we have a smoother voltage gradient on the secondary). > > e) Arrange a strike ring or some other earthed fitting to act as a > 'spark gap' limiter for voltage rise. (The toroid breaks out to > this object before the secondary breaks down.) > > f) Increase the BPS. (So that accumulation of streamer breakout from > the topload over successive bangs helps to keep the voltage down). > > Have I missed anything out? > > Item (f) is a little weak, because what happens on the first bang? > Initially there is no benefit from previous bangs so the secondary > gets the full punishment. Does (f) only work if you ramp up the power > with a variac? Good point, a variac may be needed for safest results. > > I suppose that racing arcs are the inevitable consequence of taking a > well built and tuned coil on a quest for spark length by fitting larger > and larger toroids. > > Referring to Richard Hull's coil, you could argue that he was using a > larger than 'allowable' toroid, in the sense that it was so big that > some other part of the system broke down first. We could revise our > definition of 'maximum tolerable topload', or whatever, to be the > largest toroid that the system can stand and still guarantee it will > break out before any other part of the system breaks down. I think I wasn't clear about Hull's coil. The secondary never broke down that I could see, and there were never any racing sparks, etc. The only thing that happened is some sparks from the toroid reached outward and arched around and struck the primary. I attributed that to the short length of the secondary, but come to think of it, very large coils sometimes do this also. So I'd have to say that Hull's coil tolerated that very large toroid very well. My point about Hull's coil is that it seems very rare if ever that a large coil has a too-large toroid, whereas small coils seem to encounter this more often. This is possibly because small coils see more voltage stress per unit length of the secondary. > > John wrote: > > > Some folks say that large toroids actually prevent racing sparks > > Wouldn't that be the case if the input bang size is not also raised? > ie if the larger top C was *not* matched by a corresponding increase > in primary C and instead the tuning was maintained by increasing > primary tap or turns. Then the final topvolts and the coil voltages > would be reduced by this change, making the racing arcs go away. Even though the theoretical top voltage would be lower, my thinking was that the voltage just before breakout may go higher with the larger toroid, even without increasing the bang size. I'm assuming that originally, with the smaller toroid, the voltage was capable of going higher than needed for breakout, but this higher voltage can never be realized because the small toroid lets the spark break out at a voltage dependent on the ROC. In the tests I did, I often did not increase the bang size, yet the racing sparks appeared only with the larger toroid. I retuned by changing the tap point, and I verified that coupling stayed the same. I could be wrong in my thinking though. I'll look at Stefan's charts tomorrow. Cheers, John Freau > > That would be really something. Surely the answer to this breakdown > threshold is sitting in a textbook on HV engineering somewhere? The > planet is covered in HV transformers, OK, most are not RF, but we can > surely find a starting figure. Do we simply use twice the insulation > breakdown value? I did this for my coil, but I've no idea whether the > answer represents the actual breakdown limit for the thing - it would > be nice and straightforward if it did. > > If we could collect statistics from coilers who can demonstrate racing > arcs - model each of their systems to estimate the likely V gradient, > it would give us a rough idea of the typical maximum stress that a > secondary can stand, and we might be able to see if there is any > correlation of this with coil radius, wire size, and insulation > break-down voltages. > > There's some well-gathered info on the web site of Stefan Kluge, > > http://www.stefan-kluge.de/ > > see especially the four charts at, > > http://hot-streamer.com/stk/tc/sub1.htm#eff > > It's hard to know what to make of these, but the reliance on spark > length doesn't help - we really need top volts instead. There are > obvious patterns beginning to form, but they have quite a large > deviation. Do they tell us anything about the limits and capabilities > of the coils themselves, or are the patterns simply a reflection of > current trends in coil building, I wonder? > > I think we need to take a closer look at Stefan's work - it's the best > performance survey I've seen so far, I think. > > Cheers All, > -- > Paul Nicholson,
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.