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Re: Geek .15uF caps Was: "plate" capacitors
Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
Hi Al,
At 09:21 AM 4/22/2001 -0400, you wrote:
>Hi David, Terry; All, So in other words, you are saying the MMC's will
>NOT handle long duration current usage in a Tesla unless you seriously
>under-rate them for current?
The rating the caps are designed and specified to are for 100% duty cycle
with a life of say 10 years 24 hours a day 7 days a week. We don't run
Tesla coils that long :-)) Ion inception damage will start to get bad at
our high overload levels in maybe 100 to 1000 hours of use.
>They start to fail, first just one or two of
>the caps, then several more; then a whole bank breaks down after five
>minutes Therefore, you must double, triple, or quadruple strings of the
>MMC's just to borderline handle a long sustained run of a few minutes or
>more?
Even a well designed minimal EMMC will run for 50 hours continuously. We
are simply reducing the life from say 10 years to 10 days of operation. If
one needs a coil to run for 10 years full on continuously then you really
will have to beef up the array. But not all that much really. The gap
motor, NSTs, variac brushes... will probably fail before a big under
stressed MMC would.
>Gee, sounds awful expensive to have a MMC bank that will give more
>than a few minutes action.
Some MMCs like Ted's hunted house coil have been run very long and hard
without a problem. Any well designed MMC or EMMC should easily last
"forever" for the average coiler.
>And then without swelling and self healing(how
>long can these things self heal without permanant damage?)
I have pulled poly caps out of electronic equipment that has malfunctioned.
They test fine on high pot, the values are fine but slightly low if your
look carefully, and they still run fine. However, when they are taken
apart, the guts look like Swiss cheese. Tens of thousands of holes blown
and healed through the dielectric... As long as you don't hit them so fast
that heat builds up an the pulse is limited, they really can easily take
tens of thousands of hits just like they are designed too. That took about
10 years of R&D buy the best cap manufacturers to perfect...
>, lets be
>realistic; WORD IS THAT THE MMC's can handle hundreds, if not thousands
>of over voltage punches. Well, do you folks realize that you are getting
>hundreds of multiple punctures in those MMC caps every time you fire your
>coil? Sure now, they self heal, all well and good; but how many times
>can the skinny little cap string do this? And for how long of a period?
I have pulled MMC caps out of running Tesla coils and compared them to
never used caps from the same batch. I saw no evidence of punch through
but only slight ion damage after hard use. I have popped them with high
voltage 10 times and gone in and counted ten holes in the dielectric... A
1600 volt cap can take about 3500 volts before it snaps. Thus I would have
to get about 35000 volts across my EMMC. However, I just don't ever get
over 22000 volts and there are no spikes are other odd voltage transients.
And yes, I have really looked ;-)) Saftey gaps and MOVs would prevent over
voltage any ways. Only direct streamer hits would be a concern and MMCs
seem to be fairly good at taking that too.
> Well, I am waiting for answers from the one minute run or less MMC
>crowd! And please do not tell me that nobody needs to run their coil
>for more than a minute or two for maximum scare of the multitudes. We
>are not trying to scare the masses. We simply want to develop a better
>system of power for all the people. Do you get it now???? Al.
There are ways to make MMCs so they blow right up in seconds ;-)) The most
popular way is to use the "wrong" type of cap. All metal film caps (no
sold metal but only deposited metal film) will blow the end connections
darn fast. Garry Freemyer sent me the MMC he had terrible troubles with
and this is where a lot of the knowledge there came from. Since then, I
think he has had much better luck with MMCs :-)) The second way is to use
too few string such that the caps simply run too hot. After a few minutes
they start to fail as the poly inside them melts. We have spent a lot of
time rating caps for RMS current and finding ways to determine the currents
needed for various coils so we can design them right the first time. The
Panasonic caps will take 3.5 amps RMS. Programs like MMCcalc and Kurt's
new programs will determine the needed number of strings for various
NST/gap configurations. Those programs are pretty accurate and they do
work and the caps made form them don't blow up...
References are at:
http://hot-streamer-dot-com/TeslaCoils/MMCInfo/mmcinfo.htm
http://hot-streamer-dot-com/TeslaCoils/MyPapers/modact/modact.html
http://hot-streamer-dot-com/TeslaCoils/MMCInfo/MMCPower4.html
http://hot-streamer-dot-com/TeslaCoils/MMCInfo/rel/LT_POW_CAP.html
http://hot-streamer-dot-com/TeslaCoils/MMCInfo/other/
Cheers,
Terry
BTW - Hot-streamer-dot-com should be all better now. :-))
>On Fri, 20 Apr 2001 12:28:39 -0600 "Tesla list" <tesla-at-pupman-dot-com>
>writes:
>> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
>>
>> Hi David,
>>
>> Just for the record I'll give the long answer to you short
>> question :-)
>>
>> Metal foil/film caps can self heal after a short over voltage spike
>> which
>> is a tremendous benefit to us. They can absorb thousands and
>> probably
>> hundreds of thousands of sort over voltage punches. It appears the
>> only
>> way to kill them with voltage is to get the voltage so high that
>> they
>> constantly arc internally and burn up. That is at about 3X their DC
>> voltage rating. Hard to do in our case unless you really try. The
>> transformer or something else will blow up first.
>>
>> Current is our main worry and it comes in two flavors dV/dT and RMS.
>>
>> The first concern is that the cap must handle short ~1% duty cycle
>> bursts
>> at hundreds of amps at say 120BPS. Even though there is not much
>> heating,
>> those giant spikes will easily blow poor connections inside the cap.
>> That
>> is why we want caps that have a high dV/dT rating. The problem area
>> is
>> were the leads attach to the end plates (easy to fix) and where the
>> plates
>> attached to the end plates (not so easy). Cheaper film caps use
>> evaporated
>> film for the electrode plates which have a very poor attach that
>> will fail
>> under very high current. The few millionths inch thick metal film
>> on
>> plastic is just hard to connect to. The foil/film caps have solid
>> metal
>> foil for the end electrodes that is easy to connect to and will not
>> easily
>> burn up. That is why we always want "metal/film" high dV/dT types.
>>
>> The RMS current is the "real" limit. All caps have some loss which
>> is in
>> effect like an internal resistor. Polypropylene has the lowest loss
>> for
>> common capacitor materials at about 5X better than anything else at
>> TC
>> frequencies. Thus, they run 5X cooler. In Tesla coils, the RMS
>> currents
>> can be very high and we push it to the limit since caps are
>> expensive and
>> we want to run as much current as we safely can through the minimum
>> number
>> of cap strings. The power lost to heating the cap is simply
>> proportional
>> to the RMS current squared as in a simple resistor.
>>
>> Polypropylene is a soft low temperature plastic that does not like
>> heat at
>> all. 85C is the limit although they play a few games to get that as
>> high
>> as 105C. One should not run them with the outside temperature
>> getting much
>> over 10C above the air temperature. Even thought the cap is say 40C
>> on the
>> outside, the poly film is a very good insulator so the heat
>> generated in
>> the center gets traps and the middle may be cooking at 110C where it
>> melts
>> and fails. So the thermal design of the things could stand a ton of
>> improvement to even out the temperature. The metal foil plates help
>> draw
>> heat out a little but it could be far far better. "i" would make
>> them
>> hollow so the cap is more of a tube. Then the heat would not build
>> up in
>> the center and I think you could get far higher currents through
>> them
>> without overheating. Most of the capacitance is in the outer layers
>> anyway
>> so making them hollow would not reduce the capacitance/volume that
>> much. I
>> would not be surprised if you could get 4X the current through one.
>> Maybe
>> Chris could mention this to his pals at CD ;-)) You need fancy
>> stuff to
>> make poly film/foil caps so it is not a home project...
>>
>> Unlike tcp/ip DNS server network stuff, I do know a bunch about poly
>> caps
>> :-)) The hollow cap idea seems to me like a really promising thing
>> to try.
>> Maybe it could be patented buy some big cap manufacturer and make
>> them
>> rich. Of course, we could use them too. If it were a big deal
>> idea, I
>> would not care about the patents and all that crap. I just want the
>> caps
>> for my coil :-))
>>
>> Cheers,
>>
>> Terry
>>
>>
>> At 08:59 AM 4/20/2001 -0500, you wrote:
>> >> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
>> >> Personally, I would go by RMS current vs. center temperature
>> rise. A
>> >> hollow capacitor, sort of like a tube, should have much higher
>> dissipation
>> >> since the highly thermally insulated center is removed
>> >
>> >Is the power handling primarily thermally limited?
>> >
>> >DK
>> >
>>
>>
>
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