Re: DRSSTC thoughts...

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

Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com> 

Mike:

Ever tried hooking one of your 600 kW induction heater circuits to a classic
Tesla coil (minus the spark gap of course)?

It might be an interesting experiment --- just before the secondary melts!

Dr. Resonance

 >
 > Hi Terry, Steve and everyone,
 >     About the cooling issues, especially the die's, and whole devices, I
like
 > water cooling. Distilled water of course and non-conductive hose, ~1 foot
 > per kV in length and then some safety factor. At work we routinely use it
 > cooling the big triodes of induction heaters of 600 kW output class and
also
 > for the "hockey puck" large SCR's controlling the 480 volt 3 phase to the
 > transformer primary. Leakage to ground is not a problem with good water, a
 > non-iron pump and heat exchanger.
 > Even the solid state machines use water cooling, be they the older "fast"
 > SCR type or IGBT type.
 > On the larger tube machines at ~450 kHz and down (280 kHz typical on
 > thermatool pipe welders) we have no problem with water cooling 22,000
volts
 > at 30 amp power supplies.
 > It does help to have a corona ring /cone the first foot from the water
 > jacket (anode) around the feed/drain hoses as they pull away from that
water
 > jacket to the hose trap / coil wound to get enough feet before hitting the
 > feed and drain manifolds, otherwise that end(s) dielectric heat somewhat.
 > But for that kind of voltage DC and RF, this is not so hard to do and one
 > ring protects both hoses.
 > For the power levels spoken of on the coils, a very modest pump and
 > exchanger would be very easy to do and is quiet.
 > Even the CQK-650 tube, tetrode, is rated at 22 kV, 100 amps anode (CW),
1.77
 > output Mw with 900 watts signal grid drive, used on VOA is water cooled
 > without problems. Our old tube re-building division used to rebuild them
and
 > the smaller CQK-450. Point being water and electricity do mix if done
right.
 > Regards,
 > Mike
 >
 > ----- Original Message -----
 > From: "Tesla list" <tesla-at-pupman-dot-com>
 > To: <tesla-at-pupman-dot-com>
 > Sent: Tuesday, September 28, 2004 11:13 PM
 > Subject: Re: DRSSTC thoughts...
 >
 >
 >  > Original poster: "Steve Ward" <steve.ward-at-gmail-dot-com>
 >  >
 >  > Hey Terry,
 >  >
 >  > Some comments:
 >  >
 >  >
 >  > .
 >  >  >
 >  >  > Dan ­ I see you used the model for streamer loading of 220K +
1pF/foot
 > for
 >  >  > a streamer load in your SPICE models.  I was just wondering how well
 > that
 >  >  > worked for you?  There is some concern that model is just "too
simple"
 > for
 >  >  > some things, so I was curious if it worked well in this case?
 >  >
 >  > Steve Conner has been battling this one for a long time.  A real
 >  > streamer model is the #1 hinderance in simulations i feel.  Its just
 >  > hard to get it exact.  And your results can change wildly depending on
 >  > what you used for a streamer model!  Am i gonna see 1000A or 2500A?..
 >  > was my question at one time :-P.
 >  >
 >  >  > I see that super high current IGBTs are really in demand now ;-))
But
 >  >  > monsters like the fabulous Powerex CM600HA-24H my be too heavily
 > designed
 >  >  > for "power" when what we need is "current".  It can "officially" do
 > 1200
 >  >  > amps peak which is nice but I know we don't need the 4200 watts of
 > power
 >  >  > dissipation ;-))
 >  >
 >  > Yeah exactly.  They also didnt strive to minimize stray L inside the
 >  > bricks either.  Greg Leyh wrote an article on this... cant remember
 >  > where to find it though.  As we push these IGBTs harder and harder the
 >  > little things like stray L inside the IGBT will add up quick!  And
 >  > heck, we'd be doing something bad if we dissipated over 50W per IGBT
 >  > (unless it was a REALLY big system).  Even with making 11 footers, my
 >  > IGBTs stay cold to the touch.
 >  >
 >  >    So I wish to remind of the simple IR IRG4PH50UD.  A lowly
 >  >  > 24 amp rated TO247AC IGBT with diode.  But it uses a great big die
in a
 >  >  > small package thus it's 180 Amp peak rating ;-))
 >  >
 >  > Yeah, thats a tough little guy!
 >  >
 >  >    Note that some people
 >  >  > have put a lot more current through them like this one happily
eating
 > 700+
 >  >  > amps :o))
 >  >  >
 >  >
 >  > If only the bigger IGBTs could withstand such abuse.  I only run my
 >  > CM300s at 2X their peak rating.
 >  >
 >  >  >
 >  >  > Above about 740 amps, the signal starts "flat topping" do to the
fact
 > that
 >  >  > the gate structure simply cannot support more current no matter how
 > high
 >  >  > the gate voltage is.  BTW ­ I note a lot of folks no longer worry
much
 >
 >  >  > about the maximum gate voltage specs :o))
 >  >
 >  > Seems that 30V or so is common now, at least thats what i always use.
 >  >
 >  >    But suppose we just
 >  >  > can't find them "cheap" or want new IGBTs without spending $1300...
 > Then
 >  >  > using a few of the little IRG4PH50UD IGBTs may start to be a good
 >  >  > deal.  They cost $16 a "pop".  If you get $1300 worth of them, you
have
 > 20
 >  >  > IGBTs per leg at 3600 amps peak "rated" and 14000 amps "unrated"!!
So
 > it
 >  >  > may be useful to consider a "multi-mini IGBT" (MMIGBT ;-)) array in
 > some
 >  >  > cases.
 >  >
 >  > Yeah, that certianly is impressive.  And it seems that you could take
 >  > more control of problems with stray L.  Then again, how to minimize
 >  > inductance on a layout consisting of so many individual devices is
 >  > quite a feat!
 >  >
 >  >   However, Dan's data shows the primary current can almost
 >  >  > double during a ground strike!!  That is a critical bit of info!!
Do
 > the
 >  >  > models show that behavior too?
 >  >
 >  > Ive simulated this in some of my models.  My big coil has active
 >  > current limiting to keep things "safe" under these conditions.
 >  > Shorter, heavier arcs make things much worse as they present lower
 >  > impedances.
 >  >
 >  >  >
 >  >  > Of course, if off the self IGBTs just are not right, check this
DigiKey
 >  >  > part out ;-))
 >  >  >
 >  >  > IRG4CC50UB-ND
 >  >  >
 >  >  > 600V 55A for a buck ;-))  I bet you can find a wire bonder on E-bay
for
 > a
 >  >  > song (Yipps!! guess not
 >  >  >
 >  >
 >
(http://cgi.ebay-dot-com/ws/eBayISAPI.dll?ViewItem&category=45045&item=384297461
 > 1)...
 >  >  > Just use the silver epoxy for the pad bonding too...)...  Silver
filled
 >  >  > epoxy for mounting, and your there baby!! :o)))
 >  >
 >  > Oooh, now THAT can get interesting.  Talking to Jimmy Hynes, the idea
 >  > came up to make our own bricks with some dies, but instead of
 >  > designing for lots of power, design it to withstand very high
 >  > temperature transients.  That is, put heatsinking on BOTH sides of the
 >  > die to remove heat even faster.  Or even just some sort of small
 >  > aluminum block on top of the dies to wick away heat faster... then IT
 >  > can be dealt with via forced air cooling.  I wonder if they come in
 >  > 1200V versions :-).
 >  >
 >  >  >
 >  >  > Also note that the primary cap strings could be distributed between
the
 >  >  > IGBTs to force current sharing like the OLTC does if that would
help:
 >  >
 >  > Not sure if there would be much reason for that one, though if we were
 >  > paralleling a bunch of small IGBTs it may somehow work.
 >  >
 >  >  > Apparently, primary to secondary arcing is a  problem.  This is
often
 > due
 >  >  > to incorrect primary to secondary tuning.
 >  >
 >  > Or your secondary throwing sparks 3X its length!  Thats when my coil
 >  > starts to show some signs of being stressed out ;-)
 >  >
 >  >    Since DRSSTCs are self tuning,
 >  >  > there may need to be a little "tweaking" the that circuit to better
 >  >  > match/lock the primary and secondary frequencies.
 >  >
 >  > Well, the tank circuit cant tune itself.  I think that is what was
 >  > meant.  Tweaking the primary L is what is needed... or just turn down
 >  > the variac a bit ;)
 >  >
 >  >  >
 >  >  > The SPICE models can be used to force all kinds of fun fault
conditions
 >  >  > too.  Primary to secondary arcs and such are just t timed switch
 >  >  > away.  These studies tend to reassure one that such events usually
are
 > not
 >  >  > too bad and can also find a few real doozies!
 >  >
 >  > Ooh, i will have to try that out in my next sim.
 >  >
 >  >  >
 >  >  > One may "possibly" be able to use 60Hz resonant primary charging for
 > the
 >  >  > DRSSTC like the OLTC used.  That would eliminate a lot of the DC
 > charging
 >  >  > and giant cap stuff.  But I do note that "I" seem to be the only one
 >  >  > foolish to go that way ;-))  But I have not had any problems with
it...
 >  >
 >  > I dont think *my* line can supply 1200A for even very short periods of
 >  > time.  I think we need these big DC caps there to be a low Z power
 >  > supply.  Maybe im wrong...
 >  >
 >  >
 >  >  >
 >  >  > It would be neat if there were a fairly off-the-shelf system that
could
 > be
 >  >  > hooked up to any already made coil to get rid of the HV stuff and
 > simply
 >  >  > convert it to DRSSTC operation right then and there.
 >  >
 >  > The main problems are 1) tank impedance for DRSSTCs are usually less
 >  > than that of a SGTC.  2) operating frequencies can tend to be too high
 >  > for efficient use of a solid state driver using slugish IGBTs (though
 >  > they are indeed getting much faster!).
 >  >
 >  > I too must think that the days of the
 >  >  > spark gap coil are numbered.  Not because there "are" good SSTCs out
 > there,
 >  >  > but because the will soon simply be the "best" TCs...  Even "big"
 > systems
 >  >  > are coming to speed fast!!  The record for a point to point arc is
59
 >  > feet...
 >  >
 >  > 59' for what???  a solid state system?  I was pretty sure that SGTCs
 >  > were beyond those spark lengths for some of the larger systems, but
 >  > maybe not?
 >  >
 >  > Steve
 >  >
 >  >  >
 >  >  > Maybe this all will be a bit of help, forget any parts that are
 > obviously
 >  >  > stupid :o))
 >  >  >
 >  >  > Cheers,
 >  >  >
 >  >  >        Terry
 >  >  >
 >  >  >
 >  >
 >  >
 >
 >
 >