RE: The OLTC II lives!

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Steve,
            Great work on the OLTC. I am getting inspired to do
something about it when not so busy with other things. You said:

On 30 Jan 2004, at 8:03, Tesla list wrote:

 > Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com>
 >
 > Hi all,
 >
 > In response to your questions:
 >
 > K.C. asked if I had tried it without a breakout point. I have, and it
 > doesn't even break out :( But so far I haven't been able to achieve
 > the full bang energy I designed for, due to the crowbar triggering
 > early. I hope to get it to the point where it will break out without
 > one, or failing that, to use a smooth breakout "bump" rather than a
 > point.
 >
 > Dave Sharpe was asking what blew my IGBT. The short answer is, I don't
 > really know. It was an Infineon BUP314 rated at 1200V 42A. It failed
 > with collector, gate, and emitter all shorted together. If you look at
 > the schematic of the short circuit protection unit
 >
 > http://www.scopeboy-dot-com/tesla/scpigbtprint.gif
 > view along with
 > http://www.scopeboy-dot-com/tesla/systemfull.gif
 >
 > you can see that when the IGBT turns off, its Vce should be clamped at
 > the DC link capacitor voltage, which is never more than 650V. Also, in
 > bench tests, shorting the DC output with an SCR, it has successfully
 > worked every time. It only goes kablooie when plugged into the Tesla
 > coil.
 >
 > I _think_ it is getting killed by not having an undervoltage lockout.
 > When the control circuit power is turned off, the DC link caps stay
 > charged, the gate drive to the protection IGBT dies away slowly, and
 > the main IGBTs switch on. So basically the poor protection IGBT is
 > getting the DC link caps dumped through it while the gate is
 > underdriven. Locking out the gate drive would solve this, but so would
 > getting rid of the IGBT :)
 >
 > Terry mentioned John Freau's sparklength equation. Unfortunately 48"
 > at 2kW is well below par, John's equation predicts 76". But of course
 > his equation assumes 120bps operation, so 2kW would be a bang energy
 > of 16.7 Joules, whereas I was only using 4.5. We already know that
 > power in the form of bang energy is much more effective for spark
 > growth than power in the form of bps. I would like to think that if I
 > ran my OLTC at a lower bps, it would do better in terms of spark
 > length v. power. I have run it at 2bps, and got 28" streamers for a
 > power input of about 10 watts, which is a factor of 5 over the "Freau
 > limit" :)

In theory one can do a lot better than this too.

 > In the light of these results I've been trying to think how Freau's
 > equation could be adapted for coils of different or varying bps. How
 > about L=A*sqrt(E) + B*sqrt(P) where E is bang energy, P is input
 > power, and A, B are empirical constants ;)
 >
 > But maybe there are other factors at work, like the low resonant
 > frequency reducing streamer currents, or the high self-capacitance of
 > the secondary compared to a thinner coil with fewer turns, which would
 > eat my output voltage. I haven't had the chance to explore all this
 > stuff yet.
 >
 > Steve C.

You may find part of the answer in the "gap" losses. Have you
quantified those yet? Your bang energy is similar to that which is
used in one of my coils producing 5'+ at a breakrate varying
erratically between 100 and 200. Primary currents in kA range don't
need a huge gap drop to produce significant losses. Mind you, the
peak primary current in my coil is about 700A across a conventional
single gap. The difference is a <<100V conduction drop vs 7-8kV drop
across the primary coil whereas in the OLTC..... ?

Malcolm