Re: CD 942 failure tonight

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

Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> 

Hi John,

Possible causes for blown MMC that I can think of:

1.  Cres for a 12K 120ma tranny is 26.5 nf.  With SRSG you will need ~3.1
times Cres (or 83 nf) to keep voltage rise (inductive kick) from exceeding
rating.  You are using 67 nf which could allow excessive rise.

2.  Irms spec for C is exceeded.  Probably not since you have 4 strings of
.15uf cap.  What is you pps  120 or 240 or more?  Since your lash up wiring
and primary are getting hot, your RMS current must be very high.


3.  Inadequate clearance between CDE cap body and other components (such as
bleed resister).

4.  With too small of a toroid, the number of turns on the primary may be
too low causing peak current to be too high.  6 5/8 turns seems somewhat low

Run JAVATC and it will tell you what peak current and energy transfer time
are.  This will help put somethings into perspective.

Gerry R

 > Original poster: "John Richardson" <jprich-at-up-dot-net>
 >
 > 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
 >
 >