From: Paul
Date: Sat, 12 Oct 2002 21:31:56 +0100
Subject: Re: [TSSP] Top Voltage
Boris wrote: > I think that reported values in order of 100 kV/cm and > such for earlier OLTC setting (longer rod + > sphere)were simply impossible to happen on 38 kHz coil > without breakout. I thought they were extraordinarily high. Maybe there was something wrong with the modeling. I'll have to go back over it and check (again!). > b)the rod itself gave away uniform corona sheet > earlier that decreased total field around sphere(i > doubt but can it be possible?) I was hoping for some such effect, but there was no sign of it by way of an early DC component to the base current. > there were quite impressive standard > deviations given for smaller electrode objects under > short lasting voltage pulses.However,never that large. In this case, the readings seem fairly repeatable, not a std dev thing but a real offset of the mean. > I have seen the picture of earlier setup.The probe > wasn't large enough to lower the field of the sphere > 2x or more. I though so too. Tucked inside the hole of the toroid like that should be pretty harmless. Now I'll go model the setup with a solid toroid (ie with the hole filled in) to check the max extent of this effect. > I'm suspicious about Marc's coil max field. > Did the both coilers measured secondary base currents > during these tests? No, unfortunately they're all based on estimates of the firing voltage. Hopefully we'll get some peak base current readings so that we can pin down the stored energy much better. Remains to be seen whether that will improve the picture. > This way ,from geometry of system > one can simulate everything else. Indeed, short of a direct topvolts measurement, this is the best way in. > If they didn't ,how did you know how much of primary > orginal energy went into secondary? We don't know for sure. The model's working with estimates of Q for each coil, and these are rough (+/- 50%) estimates. Of course, peak base current readings bypass that issue. There could easily be 10-20% error in topvolts estimate from this cause. > I think someone on pupman reported in past of heaving > big smooth electrode on higher powered system without > any signs of observable breakout.Nearly allways, as > the rule, the coil run in such mode for a few first > seconds that shoot single most powerful arc.The > explanation was 'that coil needs more time to build up > charge on topload'.There could be other reasons to > explain that (Irregularities in bang size comes first > to mind). Yes, or maybe a build up of positive ions, giving a space charge to enhance the surface field on -ve going half cycles? From the point of view of predictability, I hope not. > Having said that,along the lines of the latest tesla > list discussions on atmosphere influence to TC > performance,the differences can't be so large.If you > know what I mean..Someting is "fishy" about all that. Absolutely agree. Too much variation to simply write- off as standard deviation of breakout threshold. Just doesn't seem right at all. I've been assuming that as we refine the measurements, we'll see some convergence. I'd be happy to see a residual spread, as I mentioned, a spread that we could correlate with Fres and k. And underneath that, we would expect to see a smaller residual variation due to atmos pressure and temperature. > Hard to beleive someting like 100% critical field > change can be ever observed for anyone. That about sums up the position. I think we need more accurate data from as many coils as we can measure. We must take lots of coils, tune them for best performance (whatever the operator deems that to be) and then back off their variacs until they're just breaking out, then measure the peak base current at that setting. Obviously the issue here will be to measure Ibase peak with a worthwhile accuracy (say better than 5%). Those possessing a scope will be able to simply put a resistive shunt in the base lead and probe the voltage across it. Each coil would have to be set up so that it can be modeled accurately. Results would go to a page like http://www.abelian.demon.co.uk/tssp/cmod/ which I have scripted so I can just feed in new coil input files and it does the whole thing automatically. Can we do this? Can we extend our database of breakout voltages to get more accurate measurements on more coils? -- Paul Nicholson, --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.