Re: OLTC update - A problem!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Terry,

On 28 Aug 2002, at 20:19, Tesla list wrote:

> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> 
> Hi All,
> 
> I retuned the coupling and all
> 
> http://hot-streamer-dot-com/temp/OLTC08-28-08.gif
> 
> on the OLTC and the performance was better but still not as I had hoped.  I
> think I know why now.
> 
> Using the output impedance calculation method at:
> 
> http://hot-streamer-dot-com/TeslaCoils/Misc/impedance/impedance.html
> 
> I came up with an output impedance for my coil of 2,044,989 ohms!!  That is
> really high!  My big coil is only 26500 ohms.  So I think the coil's output
> impedance is too high to fry nitrogen the way we would like it too.

I think that consideration should only apply if there is no topload. 
Even then, the coil creates something of a topload all by itself. I 
am betting from what I've seen of the secondary oscillograms that the 
secondary Q is rather low. I'm also betting that based on the amount 
of wire crammed into such a small volume. Have you yet 
measured/calculated Qs? Perhaps I should stop being lazy and do it 
myself.
 
> Output impedance is governed by the Lsec and Csec ratio.  So I think I need
> a smaller secondary L and a top terminal the size of a cow or something...
> I'll have to work on this...  I was worried about this, but I didn't think
> I would make it this far so soon :o))  I suspect this problem plagues most
> low frequency coils.  I was modeling the coil to run into a 220KOhm + 2pF
> streamer load which has a magnitude of 2,191,278 ohms so it "should" have
> worked from that point of view.  Source and load "should" have been closely
> matched.  But it looks like there are a few unknowns still out there ;-)
> With 286 watts in I should be able to get streamers of 28 inches according
> to John's formula.  Much more considering I am not burning power in a spark
> gap.  But either the low frequency (I note that I am at the frequency of
> Greg Leyh's Electrum which seems to come in a little low for streamer
> length given the input power) or the impedance thing is causing a problem.
> Probably time to get out them fiber-optic probes ;-))

I think you are running into conflicts between losses and available 
energy. For example, running a low L coil with a humungous topload is 
going to require a bundle of energy to get to a decent voltage. 
Generally, there are scaling factors at work such as size being 
proportional to required energy and being inversely proportional to 
losses. Sorry for the handwaving. Again I'mm too lazy to go through 
the details.  
 
> 
> In other news...  I designed in IGBT current tester.  Here is the simple
> diagram:
> 
> http://hot-streamer-dot-com/temp/OLTC08-28-06.gif
> 
> Here is the messy one:
> 
> http://hot-streamer-dot-com/temp/OLTC08-28-07.gif
> 
> It is basically a copy of a single section of the coil.  I played the
> values to get an almost perfect matching current waveform to the real one
> but I can push 600 peak amps here on a single IGBT (1200 amps with a slight
> modification!).  I will probably use the coil's drive modules to run it but
> making another drive circuit is pretty easy too.  With this tester I can
> run the IGBTs until they break and determine how hard I can push them.
> apparently nobody knows how hard IGBTs can be pushed in such case.  Be
> interesting to see :-))
> 
> The impedance problem is the first real "obnoxious" problem with the OLTC.
> But it is not a mater if an OLTC works now but rather how to perfect it ;-))

I didn't doubt it would work to some degree. I ran such a coil at 
considerably lower voltages using a MOSFET gap years ago. But that 
was never going to be a match for a system running high primary 
voltages.

Regards,
Malcolm
 
> BTW - For 50 Hz UK operation. the charging inductor needs to be:
> 
> 50 = 1 / (2 x pi x SQRT (47e-6 x L))  Where L = 215.6mH    Big ~6 to 10
> pound split core (E-core may be better)) that does not saturate given the
> current.  I guess it could be designed for a saturable reactor if one were
> really good at that stuff...
> 
> You need to watch that the peak value of the primary voltage does not go
> too high with the longer charging time and the line voltage you have over
> there.  I can't think of anything else that would be affected.  I am not
> sure my IGBTs can take 240VAC yet until I do the destructive testing...
> The IGBTs and stuff will be here Friday so I can do that this weekend.
> 
> 
> I should point out that this OLTC stuff is all brand new bleeding edge
> stuff and I am using a ton and a half of computer models and extravagant
> test equipment to get it to work.  The goal is to make it very simple and
> easy, but right now the guarantees are zero for those that wish to try it
> themselves.  Things have gone extremely well, but if you try it and have
> trouble right now, just don't blame me :o)))
> 
> Cheers,
> 
> 	Terry
> 
> 
> 
> 
>