OLTC Update

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

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

Hi All,

I got the OLTC's primary circuit running off low voltage DC supplies tonight.

	http://hot-streamer-dot-com/temp/OLTC08-18-01.jpg

	http://hot-streamer-dot-com/temp/OLTC08-18-02.jpg


Hard to find the thing under all the leads and test equipment...  Here are
some close ups of the good parts:

	http://hot-streamer-dot-com/temp/OLTC08-18-03.jpg

	http://hot-streamer-dot-com/temp/OLTC08-18-04.jpg

It will look much simpler when cleaned up ;-))  


So no more of those goofy MicroSim charts...  Here is a real waveform from
the primary:

	http://hot-streamer-dot-com/temp/OLTC08-18-05.gif

I was able to do a very important stability test where the IGBT controller
circuits were actually all running but with a bad input waveform.  This
checks to see if the coil would do anything "bad" if the control
electronics go nuts:

	http://hot-streamer-dot-com/temp/OLTC08-18-06.gif

No problem at all :-))


Here is the primary voltage and drive signal at about 50 volts input (all I
can reasonably do right now):

	http://hot-streamer-dot-com/temp/OLTC08-18-07.gif

I will have to figure out the loss and divide it by ten for the 500 volt
case where the coil will actually run.  The losses of IGBTs or more or less
proportional to current, but the power is proportional to current squared.
So as you increase the current (drive voltage), efficiency of the coil
increases.  So right now, the all important primary loss looks fine.

There is one odd thing.  The coil's Fo frequency seems to be 38kHz.  That
is higher than I was thinking by about 6kHz.  I will have to check to be
sure all the IGBTs are coming on, but it may just be due to the
difficulties in measuring (and guessing) sub micro-henry inductances.  When
you dealing with 500nH inductors, you just never know how they are going to
turn out ;-))  Higher frequency is very nice since it reduces the size of
the secondary.  However, current increases too and I am a bit concerned
about that.  In any case, it looks like we have some room to play :-))


The diagram of the IGBTS, caps, and control stuff is here (sorry about the
very rough sketch ;-))

	http://hot-streamer-dot-com/temp/OLTC08-18-09.gif

Don't let this seem too complicated.  Barely 10$ worth of odd parts there
and you could hold them all in your hand.  If you take out the resistors
and divide the basic circuit by ten.  It only has two "important" parts.
The IGBT and the primary cap.


Here is something fun!  Ever try to quench a gap with all the power still
in the primary ;-))

	http://hot-streamer-dot-com/temp/OLTC08-18-08.gif

No doubt about it.  Even though the anti-parallel diodes are still in
circuit, Turning off the IGBTs stops the whole show right quick!  The
trailing "screeeeem!" is the primary wondering where to dump the energy.  I
have no idea where it goes...  Probably would not want to do that at full
power without a secondary :-))


Oh!  It's noisy!!  Forget that "silent" thing!"  The caps squeal like pigs!
 I have not figured out the current yet, but the low frequency and high
current vibrates the caps like little speakers.  It sounds like, and is
about as loud, as a florescent light ballast that has gone bad.  I wonder
if mechanical vibration of the caps is a problem...  Probably no big deal
but I guess solid state coil's that depend on high currents or going to
have noisy caps.  I was only running at 1/10 voltage tonight.  I imagine it
gets really loud at 550 VDC =:O


I could kick myself for one thing...  I have no way of getting a current
monitor in the primary circuit :-((  I have like six Pearson current
monitors here (one is a 50,000 amp!) and no way to insert one in the
circuit.  I guest that is why they make the split clamp one types too (and
charge 5X for them!  They know when they gotcha!)
  
So I decided to stop and think at this point tonight rather than keep
fiddling with it till I blow it up :-))  But all seems to be going fine.

Cheers,

	Terry