OLTC update

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi All,

Today I explored ways to raise the OLTCs resonant frequency and increase
the power.  I found that I could go to a smaller 28.2uF primary cap (6 x
4.7uF) which would raise the resonant frequency to around 35kHz.  So, maybe
I will only need "one" mile of secondary wire :-)  Here is the schematic:

	http://hot-streamer-dot-com/temp/OLTC8-8-01.gif

Here is something interesting about power!  Since the quenching is
electronically controlled and we are using resonant charging, we can turn
the IGBTs off any time we like.  Such as, when the caps are at a far
negative voltage:

	http://hot-streamer-dot-com/temp/OLTC8-8-05.gif

What is stunning here, is that this gives us an effective firing voltage of
1018 volts while still staying reasonably in the safe region for 900 volt
IGBTs!  1080 volts, 28,2uF, 120BPS...  That is a coil primary system power
of 1973 watts (16.4 joules per bang)!!  Assuming a 90% coil efficiency,
that "suggests" a streamer length of over seven feet!!  However, there is a
problem with the primary current:

	http://hot-streamer-dot-com/temp/OLTC8-8-04.gif

Looks like about 4500 amps peak =:O  One would have to parallel up about 25
IGBTs.  Not impossible or that expensive...

I will not go this high in power for this coil (yet :o)) since I want some
pretty good safety margins during 1st explosion...  I mean "light" :-))
But really, the only change would be another pound of IGBTs and heat sinks
for another $75.  It is bazaar what you can do when things are all
electronically controlled.

I have a little more confidence in the models at 25kHz.  They predict a
400kV peak output voltage with about a 500uS ringdown:

	http://hot-streamer-dot-com/temp/OLTC8-8-03.gif

No problems there ;-))  Although, I imagine getting hit with a coil at that
frequency and power would not be fun...

The system charging current is very tame with no surprises:

	http://hot-streamer-dot-com/temp/OLTC8-8-02.gif

This is interesting.  Here is the AC line voltage and current into the system:

	http://hot-streamer-dot-com/temp/OLTC8-8-06.gif

No need for line filters or PFC caps :-))  I am think this coil will be
very electrically quiet compared to the spark gap types.  There really are
not any terrible RFI generators in it except the streamers.  Remember that
the IGBT "main gap" gives off no real sound, heat, or light (at least, not
more than once :o))  There is a slight "clicking" sound as 2000+ amps pulse
through it and maybe 20 to 30 watts of heat, but that's it...

System complexity is still very low.  I am a bit worried that one may need
a scope to check things in such a coil during construction, but I'll try to
make it "just work" without any fancy stuff.  Protection against faults is
fairly good but only "real" testing can verify that...  The theory may be
"new" but it is much simpler than a conventional coil.  There just isn't
much hardware to it.

System weight is also very low.  There are two transformers.  The current
sense CT, but it is less than an inch square ;-)  There will have to be an
AC resonant inductor that will probably be about 4 pounds.  I could, and
may, use a small variac for testing, but the inductors value will never
change normally, so it could be a simple fixed or tapped inductor.  I have
a variac core (4 pounds) and some smaller ones that would probably work.  I
was trying to sneak by without using any iron stuff ;-))  However, the
"iron" weighs only 3% of what a 15/120 NST, SRSG motor and big variac does
(160 pounds!)  Of course, the super seven foot arc version would weigh
"nothing" compared to a pig, 25 amp variac, controller, and an arc welder
:o)))  I will guess the whole thing will be about 25 pounds with most of
the weight in the secondary and top terminal.  The secondary will probably
be like a 12 x 48 inch Sonotube.  The top terminal will be a big as
possible to help push that frequency down. 

Cost also seems low.  The secondary and top terminal are big and
conventional.  Probably blow $50 there.  The controller is almost nothing
to $100 if you make a fancy one like I did. The electronics is maybe $40
with nice die cast boxes.  $30 just in power cords and the dryer plug.
"stuff" is probably another $50.  Caps and IGBTs are anybody's guess.  I'll
just say they are about $5 each if you look around.  Probably $100 total.
So that adds up to... $370  Probably could do it for $250.  Of course,
"extra" IGBTs can add up, but at least things are pretty well isolated so
"failures" are contained in just that part.  Insuring reliability of the
IGBTs needs to be the big priority once it is working.  Not really much
else that could fail...

Three areas of concern are:

1.  Primary circuit resistance and loss.  28.2uF at 25kHz as a reactance of
0.226 ohm.  Not much room for a lot of extra resistance there!!  I am
trying for a primary circuit resistance of about 0.01 ohm.  That is really
low, but the primary circuit does resemble a big copper short ;-))  This
actually goes right along with a primary inductance of only ~750nH!

2.  Control, faults, and the unexpected...  The "computers" say it will
work ;-))  But there are a lot of things that can go wrong between theory
and reality.  I think the big basics are solid, but there certainly may be
some "technical" problems that will need work.

3.  The IGBTs.  Currents of thousands of amps and many hundreds of volts...
 Current sharing...  If there is anyway for it to fail, it certainly will.
I "think" I have things covered, but I can use pretty light fuses to save
the parts for analysis if they don't.  I think if one IGBT "goes away", the
rest will be safe.  No too bad really but at these powers and this level of
"experimentation", blown IGBTs are expected till the bugs are worked out.

Of course, this all only exists in hard drives and dreams at the moment,
but parts are coming in ;-))

Many thanks for everyone's ideas and thoughts on this, I have used them all!!

Cheers,

	Terry