Re: OLTC - Primer

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

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

Hi Marco,

At 10:12 AM 8/15/2002 +0300, you wrote:
>Terry,
>
>just a simple suggestion for your OLTC prototype. Remember to
>mount/engineer the IGBT string so that the substitution of a damaged
>device is not a pain in the *** but just a fast welder exercise. I'm
>afraid you'll have to replace some of those IGBTs, and having to take
>apart all the string to change one or two devices will make the
>operation more tedious. Use a good glass-fiber PCB, or your pads will be
>scratched away after 1-2 IGBT replacements.
>
>I'm following this project with interest. Keep us informed.
>
>Best Regards
>

I am not too worried about voltage, but right now the current is probably
too high for the IGBTs at full power.  I am leaving it as is and will
change things once I have some real testing done.  The IGBTs are mounted
directly to copper conductors (no PC boards) so they are easy to replace.
I went to K-10 Sil pads to isolate the collectors from the heatsink and to
get ride of that nasty thermal grease.  I suspect I will have to redesign
the IGBT mounting and all later, but what I have now will work for testing
and to collect data for the next revision.  

The thing is on 5 amps fuses, I am thinking the IGBTs will often win if
something does not go right.  Another advantage of having the IGBTs on
independent caps is that a failure will be fairly well isolated and easy to
find.  Having the IGBTs in parallel would have made finding a bad one hard
(of course, they would probably all be bad in that case).  I am trying to
get enough silicon in the IGBTs so that they can absorb the full energy of
the coil (until the fuses blow) if something goes bad.  If I redo it, I
will go to isolated heatsinks so I don't have the sil pads.  They add
thermal resistance which could be critical if the IGBTs become the "load".
I want the coil to be able to survive some pretty nasty fault conditions.
Replacing parts whenever something goes bad will "hopefully" be avoided.  I
want it to be super tough since Tesla coils have a habit of things going
bad, especially when others try to reproduce it :-))

If the IGBTs stay open, there is no problem.  If they stay closed, they can
easily survive the AC fault current till the fuses explode.  They are
pretty hard wired in parallel operation so they should 'all' be either on
or off without any "only three IGBTs on" situations...)  Right now current
is a concern, but that is just a matter of adding more IGBTs.  There is a
situation were the coil could be running without a secondary and all the
power goes...  Somewhere...  I would like the IGBTs to take that power for
a length of time if needed.  They really should not fail easily.  The only
thing I am really not considering right now are direct streamer hits to the
array.  However, if a streamer hits it, the whole project is a success :-))
 The big danger to the IGBTs will be latter if the thing works and
"everybody" starts making OLTCs.  The real test of how tough it is will be
to see if it is so tough the anyone can cobble an OLTC together and have it
work well ;-))  It certainly is simple enough, but the thing has to be
designed so it is very hard to damage the parts no matter what "mistakes
are made" :o)) 

I think I will be able to do lower energy testing this weekend.  That will
determine the losses and give me a good idea of the actual currents and Fo
frequency.  I went dumpster diving this morning and got a whole lot of
cores for the inductors.  More parts are coming tomorrow...

Cheers,

	Terry