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Racing sparks without terminal sparks

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Scott Fusare

Tue Jan 12 2010, 02:15PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

Yes, I had forgotten that the next mode was not exactly the third harmonic so my square wave argument doesn't quite fit.

As for the PSD, yes the only energy getting through to the secondary is that which fits in the pass band. But the energy available at a particular frequency is fixed, the system cannot "pull in" energy from the frequencies that do not pass through, that will be conserved in the fundamental.

Personally I like the bandpass filter model and prefer Terman's less exact but more tractable approach to Paul's outstanding analysis. At any rate, given the Qs involved the pass band is pretty darned narrow. I still find it a bit of a stretch that enough energy gets through on the sharp edge to cause the racing sparks effect.

All that said, I have no idea what does cause it and am simply waving my hands here. Time for some well instrumented experiments me thinks...

dex

Tue Jan 12 2010, 07:42PM

Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147

Steve McConner wrote ...

The point I'm trying to make is that the voltage profile on the secondary is the sum of the instantaneous voltages due to all of the resonant modes. If any of the overtones are excited, the voltage profile will be uneven, with ripples in it. My hypothesis is that these ripples cause the local racing sparks. If the voltage gradient due to the fundamental was already almost high enough to cause flashover of the whole secondary, then even small ripples could start local breakdowns.

I tend to agree with u .Albeit with a slight of change of this view .Any of a very high overtone alone of 1/4 wave SGTC,even if excited significantly,is hard to have enough energy (when superimposed to basic voltage profile),to cause dangerous local voltage ripple.Besides,let us not forget overtones above 10xFres are damped quickly and distorded considerably.Therefore,I'd think that more then one of very high overtones got to be excited significantly ,and if they are given chance to constructively interfere they can cause trouble near the top of the secondary (where voltage is already high).This way we have mode of "traveling beats" superimposed to sum of all other normal secondary tones.That would be a rough picture of mine (which is possibly wrong too).Moreover, this would be very hard to detect experimentally.Currently,I'd be satisfied just with experimental observations concerning my questions 1. and 2. .

@scott fusare:
What Terman's approach you are talking about?Provide links or references please.

Scott Fusare

Tue Jan 12 2010, 07:58PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

Terman, "Radio Engineers Handbook", 1943 McGraw-Hill
p154 and surrounding text.

A lumped approach to be sure but more than adequate as a basic treatise on dual resonant networks.

EDIT & Mea Culpa: After rethinking this and reviewing both Antonio and Paul's excellent work I have to retract my recommendation of Terman for TCs. He deals only with the steady state case which is inappropriate - my bad!

While we are on the topic of links and references I am still awaiting further enlightenment from you (as far as references are concerned) back on the RF spark topic.

dex

Wed Jan 13 2010, 02:24PM

Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147

scott fusare wrote ...

Terman, "Radio Engineers Handbook", 1943 McGraw-Hill
p154 and surrounding text.

A lumped approach to be sure but more than adequate as a basic treatise on dual resonant networks.

Ah that ...Well,just for the record ,I find Drude's approach,even though it is much older,more important and better in that matter:

P.Drude :"Uber induktive Erregung zweler elektrischer Schwingungskreise mit Anwendung auf Perioden und Dampfungsmessung,Teslatransformatoren und drahtlose Telegraphie",Ann.Phys.,13,pp 512-561,1904.

dex

Wed Jan 13 2010, 03:00PM

Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
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Posts: 147

Do bipolar ,1/2 wave excited, SGTCs experience racing sparks problems too?

Scott Fusare

Wed Jan 13 2010, 03:38PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

dex wrote ...

Ah that ...Well,just for the record ,I find Drude's approach,even though it is much older,more important and better in that matter:

Am aware of but never have read Drude's classic paper (other than what appears in Craggs and Meek) . Is it available in English or did you read it in the original German?

rulmismo

Tue Aug 14 2012, 10:00PM

Registered Member #4187 Joined: Fri Nov 04 2011, 08:08PM
Location: Spain
Posts: 43

dex wrote ...

First remark (about modelling SGTC as bandpass filter) has a very little if anything to do with voltage gradients on distributed secondary coil structure.OTOH,the link you provided really deals with effects and simulation of distributed secondary circuit.But,where do you see in it a direct simulation indications that increasing coupling from k=0.1 to k=0.22 significantly affects local voltage gradients?I do think that racing sparks has something to do with constructive interference of very high overtones (higher than 10x fundamental res. frequency) but connection of this thing with increasing coupling (k=0.1...0.2 is the range of interst for coiling) is WEAK.For example it is weaker than the effect of k increase on the increase of the efficiency of energy-transfer from primary to secondary.As k is increased from 0.15 to 0.22 top volts increase by less than 10% in a typical SGTC.That returns us to two already asked questions I opened this thread for:

1.
Can racing sparks appear along secondary coil if topload has large enough ROC to prevent streamer formation from TC terminal?
2.
With coupling as low as k=0.1 one can get racing sparks too if the bang energy is too high for secondary structure.Isn't that right?

Without much philosophy ,I'd like to know of clear experimental confirmation.

I just switched on my first SG coil today.

I have coupling about 0.12 (javatc) and get racing sparks at the middle of the secondary (not shown in the video), about 10cm long sometimes. I do not see any big spark from the top load.

If I use a pointy target near the toroid the problem of racing sparks dissapears, and clear streamers form from the torus to the target (see video).

I post as it seems related to question b 1) above. I will try to reduce coupling anyway as recommended in this other post about the same topic:

I aslo note that my gap between the primary and secondary is VERY narrow (I use insulated HV wire for the primary) so this also could be related as referenced in that post

Comments welcomed
Regards!

dex

Sat Sept 22 2012, 02:25PM

Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147

Thank you for your input rulmismo.
It is clear now that experimental evidence says YES to both questions.

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