Re: CD 942 failure tonight

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

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

Hi John,

Ok, if the rsg total gap is 0.15" or less, then it's not the problem. 
Phasing may be a possibility. You are correct, the safety gap should be 
firing if the voltage is climbing up (unless it's set too wide). What are 
you using for electrodes in the safety gap which is across the rsg?

The caps rms current rating is excellent "if" circuit resistance is 3 ohms. 
If it's more like 1 ohm, then your looking at 22 amps or so. The tempeture 
rise is then 6.7 per deg C and what you want to keep your eyes on. I like 
to see this value down around 2 or less.

The cap bank voltage rating (18kV) is set near the NST's Vp (17kV), so 
there is basically no derating built in - it's on the edge (which many here 
claim to get away with and obviously do). When I built my Geek MMC, I went 
with 18 caps per string (36kV). For you to do this and keep the same 
capacitance, you would end up with 144 caps (8 strings). If you look at the 
cost, large capacitances and derating can get quite expensive, even for 
MMC's. The only way to reduce the cost for larger capacitances is to have 
individual MMC caps of larger value (maybe someday) or simply take your 
chances with less derating built in.

If you get the same heating with a static gap, then it may be worth 
converting the 4 strings of 9 into 2 strings of 18 which drops the tank 
capacitance to .033uF. This will allow more primary turns (higher surge 
impedance),  a higher cap bank voltage rating (36 kV), and deg C is down to 
1.5 at 1 ohm. It also puts you that much closer to resonance rise problems 
if an srsg is used. The .033uF is about the static gap LTR value and it 
would be prudent to run it first with a static gap to check heating. If you 
then run with an srsg, be aware you may risk the NST as it's twice as low 
as the srsg LTR value.

I'm not sure what the happy medium is, but the wiring and caps heating up 
as you have mentioned tells me the rms current is  high and an indication 
that circuit impedance is low. That coupled with the low voltage rating on 
the cap bank sounds like a good probability of cap failure.


Note, a similarity to consider where the number of caps per string play a 
major role:
1) Cap voltage rating.
2) Individual cap temp rise.

Both need to be considered for MMC's.

Take care,
Bart


Tesla list wrote:

>Original poster: "John Richardson" <jprich-at-up-dot-net>
>Hi Bart,
>
>
>
> > Original poster: Bart Anderson <classi6-at-classictesla-dot-com>
> >
> > Hi John,
> >
> > I went through your specs and I really don't see any problem there. I also
> > don't see an issue with rms current values. It sounds like the caps are
> > overvolting. Definately doublecheck the MMC connections as Terry
>mentioned.
> > Assuming the MMC is ok, then what could cause overvolting? Well, how wide
> > is the total gap of the RSG? It should be set narrow but wide enough so
> > that electrodes don't crash
>
>I would have to say that the total gap spacing is somewhere in the area of
>.15, if not less.  I am just wondering why my safety gaps haven't fired?  I
>would think that they should, as that is there sole purpose, to avoid a
>situation like this.  As far as my MMC wiring, I think everything is fine.
>Lots of spacing between everything.  Caps are all mounted on fiberglass perf
>board, connections good, and each end of the strings is secured to brass
>square stock.  The hot wiring does have me thinking, though.  Must be a lot
>of evergy going thru there to get this warm as fast as it does.  Comments
>for improvements appreciated.
>
>Thanks,
>John
>
> > Take care,
> > Bart
> >
> > Tesla list wrote:
> >
> > >Original poster: "John Richardson" <jprich-at-up-dot-net>
> >
> > >Here are the specs:
> > >
> > >Topload: 4x24  Soon to have an additional 4x36 on top of it.
> > >Secondary:  4.5 OD PVC with 36" of heavy build #24, 1481 turns.
>Plan on
> > >an 8" with an 8 by 36 toroid in the future.
> > >Primary:  .25 Cu tubing, .25 between turns, 16 turns total, seems
>happiest
> > >tapped at 6 and 5/8 turns.
> > >Gap: SRSG
> > >Safety across RSG:  Set at .5"
> > >NST safety gap:  5/8"
> > >Trans:12000/120 NST
> > >Capacitor bank:  4 strings of 9, total of .0667, per MMC chart.
> > >
> > >After rough tuning and ramping up variac, I heard one cap blow.  Black
> > >soot and a hole about a quarter inch deep.  Soldered up a new one, and
> > >ramped variac up to 120 volts.  Sparks about 4 feet, and gap won't fire
> > >until variac is above 60%.  After that it will fire all the way down to
> > >about a quarter turn on the variac.  Checked rotary, and there is no
> > >follow around with the gap arc.  She fires right on electrode alignment,
> > >so I am assuming that phase is pretty close.  Started it up again, and
> > >tried to fine tune.  A half turn either way from the tap point of turn 6
> > >5/8 and performance diminishes.  Put tap back at this point and fed the
> > >trans the full 120 volts.  I stopped things again to check everything
> > >out.  First I checked the variac, an ancient Ohmite, to see if it was
> > >getting warm.  It's only rated for 10 amps, and I made a new brush out of
> > >a piece of brass due to no access to new parts.  Everything looks good
>and
> > >runs cool.  The  lash up wiring to the coil primary is 14 gauge, and it
>is
> > >hot to the touch, and the primary is warm.  Will go to #8 for final set
> > >up.  Looked at cap bank, and the new cap looked like a piece of popcorn
> > >that had just popped, black and all.  The one on the next string over was
> > >bulging like you know what, but no black stuff.  I can see the foil,
>however.
> > >
> > >Why is this happening?  I am not going to re-fire the system until the
> > >last portion of the tank wiring is replaced with #8, PFC caps and meters
> > >installed, phase controller up and running, and new toroid
> > >installed.  I've put lots of work into this thing, including welding up a
> > >nice stand for the controller, gap, trans and cap assembly, and want to
> > >get to the bottom of this cap problem.  Any thoughts?
> > >
> > >Thanks,
> > >John Richardson
> > >
> > >
> > >
> > >
> >
> >
> >
>
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