From: Paul
Date: Sat, 18 May 2002 15:20:26 +0100
Subject: Re: [TSSP] Racing arcs
Bart, John, Intriguing that you can turn the racing arcs on/off by quite modest adjustments to topload height, primary coupling, and tuning. And your coils each demonstrate a different type of racing arcs - the full length from Bart's, and the ones that gather round the top and bottom of the coil from John's. I think we've quite a lot of work to do with your coils. We'll have to go quite slowly otherwise we risk drowning in too many numbers. John wrote: > ... perhaps the pri-sec voltage stress might not actually cause > an arc-over, but it may cause a sort of corona effect which may > promote racing sparks on the secondary. I've noticed that any > sharp point on the primary, can promote racing sparks on the > secondary where it faces the sharp point. Just general overall > pri-sec voltage stress may have the same effect perhaps. I think that's a very good idea. Because, a) it explains the extreme sensitivity to the height of the primary. b) it accounts for the coming and going of racing arcs as the tuning as altered. Best of all, it is something we can test for in the models, at least to a first approximation. I'm modeling the primary as a lumped inductor so I don't have the surface charge distribution. Nor do I have the surface charge distribution of the outer face of the secondary - I only have the distribution of the sum of the inner and outer charges. This needs more thought, but if the primary is exerting a radial field on part of the secondary, then that will alter the balance of charge between inner and outer faces of the coil. Suppose the secondary voltage was negative, so there is negative charge on both outer and inner surfaces of the winding. Then bring up a positively charged primary and more of the inner surface -ve charge shifts over to the outer surface. The *potential* on the secondary surface remains fixed, but the charge on the outer surface (and hence the E-field at the outer surface) is higher when the primary is at the opposite potential to the secondary, than when it has the same sign of potential. We shouldn't be looking at the longitudinal potential gradients, instead we should be looking at the total magnitude of the E-field, ie including the normal (radial) component as well, and including in this an accounting for charge drawn to the outside of the winding. I think we can get this by calculating the Cexternal for the secondary with the primary in/not in place, and taking the difference to get the pri-sec cap per unit area, and thus the extra charge per unit area, and from that to the surface field radial to the coil. Hmm, some stuff to think about there. The 'hottest' spot may not be directly between the primary and secondary, but may be a little way above. If the best tuning for a coil's streamer performance is a few percent -ve, and the primary is wound in such a way that in -ve tuning the primary volts is the opposite sign to the secondary volts, then whenever the unfortunate coiler approaches a good tuning, the racing arcs start to break out. Lowering the primary sharply reduces the radial field and we wrongly blame overcoupling? Other coils, identical but for an opposite primary sense, would be free of racing arcs. Is the cure simply to flip the primary? That'll take a bit of careful thought over the relative signs, phases, etc, hmmm... > I should mention too that the various cases I saw in my > coils where the notches didn't look well defined were all > cases where the coils were actually tuned reasonably well. That makes sense if by 'tuned reasonably well' you mean that Fpri was close to Fsec. Because then the secondary would be detuned by the streamers and the primary would be unable to transfer all its energy. But then I've a nasty feeling that you meant the coil was tuned reasonably well for streamers, in which case that's hard to understand. -- Paul Nicholson, --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.