Re: CD 942 failure tonight

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi John,

If a CD942 cap is driven far over voltage (like 6000volts each) the 
dielectrics will constantly arc inside the cap and cause bulging in the 
center.  This sounds very very much like a wiring problem in the 
MMC.  Carefully go over the connections to insure the rows and strings are 
properly wired as are the drain resistors.  Also note the tips at:

http://www.thegeekgroup-dot-org/mmc/constrguide.html

A 12kV NST should put out a peek voltage of 17kV.  That should divide 
through the 9 caps to get 1889 volts peak across each cap.  Please save the 
blown caps incase we have to examine them.  But for now, triple check the 
MMC's wiring and be sure that wiring is not arcing to some nearby grounded 
thing too.  If you can take front/back pictures, I would be happy to look 
it over too.  Or, see exactly how the caps are blown.

Cheers,

         Terry



At 05:23 PM 2/28/2004, you wrote:
>Hello,
>
>Copnsidering the recent MMC debate,  I figured that I would share this 
>evening's run.  This is a first fire, and tuning was done at partial 
>variac setting.  Volt and amp meters are not in yet, so I can't give any 
>real numbers.  Also, SRSG was set using a flourescent bulb, as I am still 
>waiting on my motor run caps for John's phase controller.
>
>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