Re: Cap life testing - Re: MMC cap bank - ion inception testing

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

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tesla-at-lists.symmetric-dot-net>

On Sun, 6 Jul 2003, Tesla list wrote:

 > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
 >
 > Hi Ken,
 >
 > At 08:35 PM 7/6/2003 +0000, you wrote:
 > >>>...........
 > >>I was thinking of something like:
 > >>http://hot-streamer-dot-com/temp/CapLifeTest.gif
 > >>I think that main fuse should be 7.5 amps not 0.75...
 > >>lots of fuses...
 > >
 > >This setup does not tell you when a cap has reached end of life by losing
 > >too much capacitance. While losing say 11% capacitance may not sound too
 > >bad, you've lost conductive metallization in the cap, and it's ESR has
 > >gone up. Also, we know that voltage is highest across the smalles value
 > >caps when used in series. You end up with a downward spiral effect when
 > >one cap starts to wear out.
 >
 > For the ionization problem, the capacitance does not decrease. We are not
 > punching holes in the dielectric and blowing away metal which causes
 > capacitance decrease in caps that are over voltaged a lot.  Here, we are
 > just chewing on the edges of the dielectric.  They start off with say 5500
 > volts of stand off.  When the damage gets so bad that they fail at 2000
 > volts, "I" would say the cap is pretty "well done".  I was thinking that
 > the first failure would cause a current pulse in the fuse that would blow
 > it indicating the failure.  A neon bulb with like a 10Meg resistor across

This depends on how the cap fails. It could short, it could try to heal.
Dielectric failure from corona or overvoltage is still dielectric failure,
and you're going to get a spark.

 > the fuse could make a nice easy lamp indication.  Also, the fuse should
 > "save" the cap so it can be taken apart to check out the damage first hand.
 >
 > Normally, we don't overvoltage caps to the 5500 volts needed to start
 > punching holes in the dielectric.  Our MMCs will probably never see a
 > voltage punch though.  But if we do have a problem like a short in an array
 > or a streamer hit.  Our MMCs will laugh it off ;-))  Josh's filter will all
 > the popped caps was pretty cool!  Far cooler was the fact it still worked!!
 >
 > The dissipation factor may go up some over time, but most of the current is
 > in the solid plate area where the ionization is far less.  We might not see
 > much dissipation increase from dielectric damage just at the foil edges.

Hybrid caps have two types of edges, foil over plastic and metallization
over plastic. One the metallized parts can heal, but they are on the other
side of the foil, so it's about the same.

 > A zero voltage crossing solid state relay may fix the line transient
 > problem right away.  It would prevent full voltage from going across
 > discharged caps during turn-on or a momentary loss of AC which might blow
 > the fragile fuse prematurely.  There is the HV transformer there too, but I
 > think it would still work.
 >
 >
 > I was thinking about how long of life we want...  Assuming a museum coil is
 > run for 1 minute out of every 10 minutes, 12 hours a day, 365 days a
 > year...  That is 438 hours/year for a very heavy use.  For the home hobby
 > coiler, we are probably looking at maybe 5 hours/year.

This may be, but if you are really pushing the caps hard with corona and
the average life is 10 hours, you don't really have a big margin there.

 > We probably would like our hobby coil to last 50 years at which time "we"
 > won't care anymore ;-))  So that is 250 hours total run time.
 >
 > For the museum, assuming the coil is real popular... The capacitors
 > probably have to last till a big overhaul at which time the gap motor,
 > relays, switches, etc. need to be replaced.  I am thinking every 5 years
 > our museum coil will need a pretty big OEM overhaul which could include a
 > new primary cap bank...  Maybe DC or another pro coil maker could tell us
 > if that is reasonable to assume.  So we could say 2500 hours.
 >
 > So we have a "wish" for 250 to 2500 hours of life.  Not too bad and pretty
 > easy to test.  Basically 10 to 100 days.  Of course, we could make an MMC
 > to run at far lower cap voltage too vastly extending life.  But then it
 > starts to get so expensive you might as well buy commercial caps unless you
 > need an odd value, ultra reliability, or like to hit the caps with
 > streamers ;-)).  If you made a cap with only 500 peak volts per cap, the
 > life would be...  just about forever...  It would be interesting to compare
 > the price of a commercial cap and an ultra over-designed MMC which could
 > easily outlive it.*
 >
 > BTW - Don't let Mark and I steal all the fun here.  If you Ken (or anyone
 > else) wants to do this experiment too, feel free!!  When it really gets fun
 > is when we all get different numbers :o))
 >
 > If CD does not already know these numbers, just watch them use "our"
 > numbers themselves too ;o)))  Maybe we can get some free caps out of it
 > ;-))  Then watch them send us other caps to be tested too!!  Vendors love
 > this kind of free testing ;-))  (as long as the results are good...)
 >
 > Cheers,
 >
 >          Terry
 >
 > *A Maxwell 37667 30uF 35kV could be done with 70 caps per string in 14 rows
 > to give only 500 volts peak per cap.  A $3000 cap!!  But at 200Arms and
 > 6000 amps peak rated and a "self healing" voltage of say 385kV...  You just
 > are not going to wear that cap down ;-))  It would last forever especially
 > considering a number of caps coil fail without really affecting the whole
 > array.  You could hit the array with a shot gun blast and the remaining
 > caps would easily still run it all...  Hit that Maxwell with 35.001kV
 > Pooff!!  It's done....  Of course, the BIG MMC array would be far far

Umm no, a 35kV GA cap will not self destruct at 35.001kV. Please refer to
their life deration charts for the math. The same science applies to tiny
foil caps. Life falls fast with overvoltage conditions, and extends a huge
amount with derated voltage use. As you can see from the CD AC rating of
942C caps, corona probably sets in way before the DC rated voltage.

KEN

 > larger than the 37667...
 >
 > But, the MMC could also be a 15 x 3 array...... ;-))  $135, breakdown at
 > 82.5kV (self healing)...
 >
 > BTW - Maxwell, now general atomics, has a pretty cool web site at:
 >
 > http://www.hvpower-dot-com/capacitors.htm
 >
 >
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