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Registered Member #2566
Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147
Hi people,
I noticed Mr. Conner is an active member on this message board ,so I'd like to ask him few Qs about his (retired?) big OLTC: Steve,the primary characteristic impedance of the coil seems to be very low : Zc<0.1 ohm. Is that the most important thing why the coil uderperformed?At 4 kw+ power levels efficient tesla coil systems hurl sparks about 8-9 feet long. You wrote the coil couldn't do better than 6.5 footer.Second,did some primary components (caps,cables,IGBTs) get quickly warm during 4kW operation?And have you tried running the coil 100 BPS with 10 J bang size,and single shot tests with it?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No, it underperformed just because the bang energy wasn't big enough.
To run 4kW, it had to do 400 10 joule bangs per second. What you call an "efficient" coil would be running 100 40 joule bangs per second. At the time I built the OLTC2, it wasn't generally known that high BPS and low bang energy led to inefficient spark growth, in fact the OLTC2 helped to prove it and convince people of the fact. If I had known it was going to suck, I wouldn't have built it.
The primary impedance was not too low. In fact, to increase the bang energy it would have had to get lower still!
Yes, everything got warm except the secondary. The primary coil got hot enough to melt the plastic supports. However, most of the energy actually went into the discharges, I estimate about 80%. It's just that low bang energy, high BPS tends to heat the sparks instead of lengthening them.
At 100BPS and 10 joule bangs, it did 39" if I remember right. I got Jimmy Hynes and Steve Ward to try this same test with the DRSSTCs they were building at the time, and they got about the same. The extra performance comes purely from the fact that you can get more bang energy from the same silicon in a DRSSTC.
And yes, it was dismantled several years ago, however a similar coil with the same drive electronics lives on as an exhibit at a science park in Denmark.
Registered Member #2566
Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147
Thanks for sharing the info,even though I don't think it is just becouse of "the small bang size for the coil size".Two examples are known very efficient SGTCs.R.Hull's Nemessis TC ,with 11-12 kW input generated up to 15 feet point to point sparks.Break rate was between 350 and 400 BPS.The thing is that Nemessis had huge toroid installed (I think 63"x13") and total secondary capacitance almost 100 pF.Another example is 12 ft tall Greg Leyh's SGTC with 100" dia (!) toroid on and total capacitance aprox 150 pF.With 70-75 J bang size and 350 BPS (rms power aprox 25 kW) it hurls 25 feet long arcs.70 Joules looks just as small for the coil of this size as 10 Joules is for your big OLTC,and yet the spark efficiency is excellent.I think you had efficiency problem becouse of very low Zc and losses,not just due to small bang size.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, I do think that was the reason.
On SGTCs running any break rate other than twice the line frequency, not all electrode presentations result in a bang, and not all of the bangs are the same size, because the tank circuit is charging at a 60Hz rate. The builder could quote 400BPS because he has 400 presentations per second, but some of them could be bigger than he thinks from measuring the input power, some could be smaller than he thinks, and some could be missing altogether. The oversize bangs will help the efficiency.
Because the OLTC had a DC resonant charging system, all the bangs were the same size (and that size could be measured more accurately than ever before in the history of tesla coiling)
I maintain that the coil didn't have an efficiency problem, so much as a bang energy problem. SGTCs have parts that get hot too...
Registered Member #2566
Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147
Don't know of Nemesis ,but Leyh's 25 kW SGTC has a stable DC charger which gives pretty same cap voltage and bang sizes as the coil runs .Bigger bang size for the same input energy is usually better,but not necessarily much.
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Qs 4 Steve (Mc)Conner
