Re: EMMC caution

Hi Finn,

	I was a little surprised that your standard MMC gets so warm.  I think a
string in your standard cap is around 1nF while a string in the Panasonic
caps would be 5.6nF.  The Panasonics should see five times the current!
When the caps cleanly blow apart, that is a sign that they were destroyed
by over voltage.  When a cap puffs up and looks melted and burns, that is
from too much current.  I bet a few overheated and shorted and the others
blew from over voltage.  Your transformer is wound for 20kV, if I remember
right, so the 16kV (rated) strings may have seen around 29kV.  However, I
would expect them to take that which I think they did.  If pure over
voltage was the cause, the caps would have blown instantly when the power
was turned on.  However, in this case it took a short while for the caps to
overheat before all the excitement started.

	I think MMCs are pretty though as far as overvoltage goes but the current
handling and internal heating needs to be carefully considered.  I have
been looking very carefully at the new line of PHE448 caps from EVOX
www.evox-rifa-dot-com .  They have very good data sheets and seem like they
have really tested these parts.  They also have a dv/dt rating of 25000
volts/uS which is very good.  Their thermal dissipation is rather low but
they may have much more realistic numbers too...



At 10:38 PM 7/4/99 +0200, you wrote:
>I ran a couple of EMMC, Identical to Terry Fritz`s, in parallel to my
>standard MMC this weekend, and one of them blew up in just a few
>I had been troubled with internal archovers in my static gap, and fixed
>them again, and set the individual gaplength at 30 mills, 0.75 mm. This
>was "BAD", the safetygaps started flashing at only 7-8 gaps in service,
>so I reset them to 24 mills., 0,6 mm, and that much better: I could now
>use 16 gaps, and the streamers grew to 7`8" , 230 cm.
>After this I plugged the 2 EMMC`s into service.
>These were identical to Terry`s, but one had only 9 caps, the other had
>10 caps.
>After retuning, the first impression was that of the sound, following a
>lower breakrate with the static gap, and a generally shorter
>streamerlength, which was not what I had expected.
>But I didn`t have much time to deal with this, because within less than
>one ninute, the 9 capped string blew to pieces. All the epoxy around the
>cap cracked, and fell off, one cap even caught fire.
>My standard MMC was getting lukewarm, the center strings were 10 - 15
>degrees C warmer than ambient.
>The coil is now at a museum, and a lot of the guests came by to watch
>the show, so the coil took a lot of service that day, and I pushed it to
>the limits, untill I closed the doors to get some time on my own with
>I know this is not definitive of much other than that where 20 caps in
>series work, 9 in series may not, since I don`t have controll of the
>exact voltages that the gaps fire at. There may also be an issue about
>the current, since these two strings had to deal with 25% of the total
>But I would like to post this as a caution against pushing the caps too
>far, particularly when using PT`s, and pigs. 
>Details about my system can be seen at:
>Hope this can be of some help still...
>Cheers, Finn