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Re: "Enhanced" Multi Mini Capacitors Part 1
Hi Terry, all
First of all, I would like to thank the various members for their on and off
list get-well wishes. Itīs nice to know one has a sort of "family" out there. I
will be processing past mails as fast as possible. This may take a little bit
(just alone the reading will take a while!!), as I have been neglecting a few
things, so I will kind of be on and off the list for a while, but I WILL answer
them for sure. And now back to coiling........
Comments below are "connected" to my past post on the WIMA FKP-1
6kVdc, yet only 700Vac, rated caps. Big thanks to Terry for all those
urls on capacitors. The only thing is, it didnīt really make it easier ;o)),
but thatīs the way it goes. The more you know, the more complex
something "simple" gets. There is another interesting paper on the
Dearborn site in the technical papers archive called
"Paper and Film Energy Discharge Capacitors: An Introduction"
1.) All papers seem to agree the major derating factor from DC to AC seems
to be internal heating problems that occur within the dielectric. However, this
still doesnīt explain why the WIMA FKP-1 caps have an "acceptable" derating
factor in the DC-classes below 2kV, and on the other hand all caps in the 2kv
to 6kV (DC) range have (more or less) ridiculous DC to AC derating factors.
(We (GTL) did send a mail to WIMA, but are still waiting for an answer.)
On the other hand, we donīt use our caps 24hrs/week/year, so maybe we
can get away with it.
2.) dv/Dt
The question is if we coilers can use the equation I= C*dv/dt? Quoting
Panansonic: the given dv/dt values are for less than 10,000 pulses. We
will arrive at 10^4 pulses pretty fast in coiling work. No mention is made
to what derating is necessary in case the circuit sees more than 10^4
pulses. However, we donīt need a 10,0000+ hour lifetime (Please see
other post.), so I think this eq is okay to use.
3.) Panasonic gives a neat frequency to voltage derating equation on
page 3. On page three they say a "ECWF2224" cap has a permissible
current of 2.7Arms at 30khz. If I take a look at the curve on page 4 the
permissible current goes UP (!) as frequency is increased. It peaks
out at roughly 80khz. This, too, doesnīt really help in "derating" a
capacitor properly. The way I understood Panasonicīs data sheet,
this derating factor is applied to the already lowered AC rating.
This one has me a little stumped ;o))
4.) The only real clue that Panasonic gives, is the following statement:
"If a capacitor of DC rating is used in an AC circuit (except....), the
maximum voltage is limited by heat generation or electric discharge"
This sounds VERY simple (and my ERO caps donīt get warm at all
on the outside during 5-7 minute runs). So maybe this is why we get
away with it.
5.) Film or foil:
Panasonc makes it quite clear, what I have already said in previous
posts: caps using metallized plastic end connectors are worthless
for TC usage, because the maximum permissible current is way too
low for TC usage. MMCīs MUST (for longer service) be built with
metal foil caps.
6.) Size:
Quote "The value of dv/dt is mainly determined by the lead spacing
(element width) and element sectional area" This, too, explains what
I have found empirically. The physically bigger capacitors are more
suited for TC usage.
All in all, (as Gary Lau once said), like a bumble bee, this thing
(MMC) shouldnīt fly, but it does!!
Coiler greets from germany,
Reinhard