Re: OLTC Multi Primary

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

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

Hi Winston,

At 12:49 AM 8/14/2002 -0700, you wrote:
>Hi All,
>
>	I don't know whether or not the drawing/idea was mis-understood or not,
>but I'll explain the idea again.  I discussed this with Terry a few
>weeks ago, and he thought that it was too complicated.  I still don't
>think so, but then again, I'm not the one trying to build it and get it
>to work without smoke entering the picture ;-)).  I also don't have the
>fancy computer modeling stuff that Terry has (I can't even imagine how
>much that aids in designing circuits).
>
>	Anyway, the idea was not only for each transistor to have its own
>capacitor, but for each one to have its own complete tank circuit,
>identical to the others.  The main point to this was keeping the
>resonant frequency in the "normal" TC range by keeping the Fres of each
>tank high, while keeping the total energy in all of the caps (tanks)
>high enough to do some good at low (120 BPS) break rates.  That way, the
>secondary coil wouldn't need to have 0.5 Henries of inductance =:-0, and
>would therefor be less of a pain to build.

I think if they were really close together, they would just act as a single
big block of copper.  The inside turns may be shielded by the outer turns
in the bundle.  I also do not really want the turns isolated from each
other.  With either the caps or primary inductors in parallel, I have a way
of forcing everything to stay in phase.  If they were separate, a misfire
where two were to go 180 degrees out of phase could be "odd".  It may work
fine, but there were just to many unknowns for me to go that way right now.
 A future thing.

>
>	Also, using this method, power-per-break can be increased without
>further lowering the operating frequency, if the coil is running
>directly from the line and not DC.  The current consensus (I think) is
>that the prototype OLTC will run on DC.

It is synced to the AC line.  Sort of DC pulses in a resonant charging
mode.  Pretty much locked in at 120 BPS.

>
>	The idea was to wind all the "separate" primary coils into a bundle,
>like Litz wire, so that no one tank would see too much more kickback
>than the others, and so the secondary coil would "see" only one
>primary.  Running at such a low frequency means that, using a
>"conventional" TC tank setup, you must lose either primary inductance,
>or capacitance.  Each one has its inherant problems.  The multiple tank
>circuit method allows you to in effect, lose neither one.  I know that
>an IGBT is a little different from a sparkgap ;-)), but their function
>in the circuit is the same.
>
>	Current sharing is less of a problem, since there are no transistors in
>parallel.  The firing timing issue is the same as with the current
>setup.  And, a problem with one circuit should not affect the others, so
>a catastophic "dominoe effect" failure would be less likely.  The wire
>used in each tank would be much smaller than that used in a single
>larger one, so making and manipulating the setup would be easier.  There
>were some other perks to this (I think), but I've forgotten them.
>
>	I did some math, and worked out a coil that runs at 70kHz, with a TOTAL
>primary capacitance of 30uF.  It has 10 tanks, each with a 3uF cap, and
>a 1.75 turn, 4.5" radius, 1.72 uH primary.  The total energy stored at a
>peak voltage of 320 is 1.56 joules.  The primary would in reality be 1
>turn.  The extra 0.75 is to account for off-axis inductance (tank
>wiring), and other stray effects.  If anything, this should be lower. 
>The secondary is 27.5" (wound), 2175 turns, and 6" in diameter.  The
>toroid is a modest 6" by 20".
>
>	Lastly, I should say that I'm sure that there are problems with this
>idea that I can't see, since I'm not know for my intelligence ;-)).  

It may work perfectly fine.  I really don't know.  In this case, I just
wanted to spend time on other things so I went with something I knew would
work.  Once it works, then everyone can figure out ways to improve it.  

>So,
>I invite the gurus of TC'ing to pick this clean.  I know I'll learn
>something from that.  I also don't mean to question Terry's judgement. 
>I wouldn't have mentioned this idea on the list, but since someone
>brought it up, I though I'd explain it.  Terry is right in keeping
>things simple for the first try.  This can be explored later (if at
>all).  I left some stuff out, but I'm getting tired (it's 12:45 am, and
>I've got work tomorrow).  

No problem at all :-))  If it works, there will be all kinds of new and
fancy improvements added.  But right now, the key is getting it to work at
all...  Besides, I already made the primary, so it's too late :o))

Cheers,

	Terry

>
>	Sorry for such a long post.  I didn't intend for it to be that way
>;-)).  Thanks for reading,
>Winston K.
>
>