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Re: Another MMC Failure
Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
Hi Finn,
See this,
http://hot-streamer.com/temp/CD942C20P15K-MMC-Capacitor-Lifetest.pdf
Of course, we don't know the "function", only the
limits. At 500V (630 V ion inception voltage-
dual series) the plates have no
ionization. They moved down to 500V in the
ratings since the voltage gradient is higher at
the plate edges. But at 500V/cap the life should
be "forever". At 2000V they show a decay step at
75 hours. I never tried any other voltages on the tester.
I can't imagine current had anything do do with
the failure here. What you describe is voltage
failure in many of the caps so it is just not one
bad cap but a real problem going on. The unblown
areas of the dielectric film are probably white
from ion bombardment chewing away at the dielectric.
IMHO - All those life equations that have a "^xx"
term in them like V^19 or I^3.54598 are howg
wash... I am sure they worked once for the guy
making the equation, but I have never seen anyone
"repeat" the results. If they try, they just end
up making a new "different" equation...
I see the data sheet at:
http://cde.com/catalogs/942C.pdf
Has a "plethora" of AC voltage ratings... They
are either 315 or 630 for polypropylene... So I
don;t know if they are fudging 630V caps down or
315V caps up. But you answered that when you
said the caps have an "intermediate"
electrode. So they are dual series types for 630V.
What we need to know is the "AC voltage on
time" It does not matter if the AC is 70kHz or
50/60 Hz. Anytime there is above say 30Hz on the
caps, the clock is ticking... I wish I had run
the above test for a few more data points to
predict the "curve"... Since you don't need
current, you might consider the 940C20W1K types.
I think DC COX is running at 1/3 the DC peak
voltage on advice from experts he knows.
Cheers,
Terry
At 02:41 PM 7/15/2006, you wrote:
All.
I just had a MMC fail on an OLTC coil that I built a year ago.
The coil resonates at 70kHz , primary cap is 16.2µF and the pri-L is 350µH
With a firing voltage of 1150V this translates
into a primary current close to 8000A
The primary cap is made up of 18pcs. 942C12P9K
so each of them sees 450 peak amps.
They are rated at 861Apeak, so it is no wonder
that the cap failiure is consistent with what we
have come used to see from an overvolted cap.
I am sort of surprised they gave in to over voltage......
The caps show multible punctures of the
dielectric that leads to the evaporation
deposited layer of the intermediate electrode, with signs of self healing.
In a couple of the caps, the intermediate
electrode has almost completely lost all of it`s
conductive material, so they started to turn
resistive, and heated up, although not seriously so.
The coil is in adventure park service, and the
duty cycle is low. every 4.5minutes, it puts out
112 bursts of 124 discharges at 500BPS.
This killed the cap in a bit more than a month.
From
http://home.earthlink.net/~jimlux/hv/caplife.htm
I can gather that if I double the voltage rating
of the cap, the expected lifetime will increase
180fold and that suits me fine, because I can
then expect the caps to last 15 years.
What I wonder is this, should I put 942C12P9K
caps in 36 strings with 2 in each string, or
should I rather use 942C20P15K and put 108 in parallel.
A Maxwell cap is another option, of course.
Cheers, Finn Hammer