From: Paul
Date: Sat, 07 Oct 2000 14:01:24 +0100
Subject: Re: [TSSP] Progress report 28th Sep 2000
Terrell W. Fritz wrote: > I would like to submit my paper at: > > http://users.better.org/tfritz/site/papers/modact/modact.html > > for perusal... Good stuff Terry, a few comments... On the equivalent circuit: You've not taken account of the proximity effect on Rs, so the 69 ohms should be more like 150 ohms. On primary waveforms: I guess the large current spike at the start of the primary current waveform is just circuit parasitic capacitance getting charged up very quickly. If so that pays a compliment to the bandwidth of your current pickup. I'm puzzled by the apparent non-coincidence of the current and voltage notches. DC offset to the primary ring voltage - the gap is rectifying? On secondary waveforms: The notches do coincide on this one. What's with the DC offset appearing in the secondary top voltage trace? Probe artifact? I see no way for the secondary to sustain DC for so long. On the V/I to the secondary arc: Looks like the I is very slightly leading the V, so there's some voltage dependent energy storage going on - some justification for a capacitive equivalent circuit. Terry, do you have data files (csv format?) containing these arc V/I curves as arrays of numbers? If so we can work out the effective R and C of the arc fairly accurately. If you repeat the run with different variac settings you'll soon get some evidence for arc Z as a function of length. Perhaps you've already done this? Terry, I'm well impressed with your high current pickups - I'm assuming the current spikes are genuine and not spurious pickup. If so I'd like to consider using them for permanent instrumentation in a CW driver. Can you point me to some details? Terry, I think I could do some number crunching on your trace data. They should reveal the effective average Z of the secondary arc, and, after a bit of shuffling, the dynamic V/I curve of the primary arc. Just confirm that upper and lower traces are always simultaneous sweeps and are not on alternate trigger. Some arithmetic: Primary tank volts at trigger, 14kV, across 17nF gives a bang energy 1.67 Joules. Secondary peak top voltage 260kV across (16.7+10.3) pF gives 0.91 Joules. Therefore overall efficiency of energy transfer primary tank to secondary tank is 54%. Seems reasonable. Terry wrote: > ...and state that gap losses often account for ~50% of the loss > in Tesla coils. If you can probe for the V/I across the primary gap, and bottom any DC offset issues in the V probe, we can integrate the product of the two traces to yield the total loss in the gap, and then compare that with the total loss as calculated above. You would predict 50% of (1.67 - 0.91) = 0.38 Joules. Also, calculating the correlation function of the two traces would lead to the gap V/I phase angle. (PS, How quickly does the 500V probe recover from a severe over- voltage? Quick enough for the above?) I suspect Malcolm may already have such a procedure in mind for his experiments. > These models seem very accurate but in this case are > "lumped parameter". I don't think thats a problem, providing we are aware of it and take due account when necessary (eg I avoided using the base current to work out (above) the stored secondary energy, we must use the top V instead). > I apologize for not testing my secondary outside yet to find the > free area Q but rain, snow, hail, and it is... yes, indeed... now > sleeting outside right now :-P have prevent this experiment... No worries, its been raining here too, most days. Means I can't get my coil out to play either. I've plenty to do indoors - I've a big HF coupling transformer to wind if I get really bored! > My "200000 ohms plus 1pF/foot" value for a steamer impedance is > very rough but I think it is the best estimate out there at the > moment. Well we've got to start somewhere! Doesn't seem at all unreasonable for a first approximation. Cheers, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.