Re: MMC cap bank

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi,

I guess "I" should say something...

Geee....  I don't know ;-))

I sort of grazed through the giant data sheet but did not see any "useful" 
information.  I tested some of these style caps a while back but I'll be 
darned if I can find that post now.  The "google" search thing is sort of 
weak on finding things that have not had a lot of "hits"...

In general, the voltage of MMCs is just the DC voltage for our uses.  You 
can completely disregard the AC voltage rating.  The voltage rating is 
easy...  What worries me here is that you are running them with a 10kVA pig 
system that might push one heck of a lot of current through the 
MMCs!  There is an RMS current your system will have and there is an RMS 
current limit to a given MMC.  If fear the MMC may not be able to handle 
the current in this case.

Two things to watch for:

Heating - If they start to get hot, STOP!!  Poly melts at like 85C.  If the 
center of these caps (deep inside) melts.  They will die (very dramatically 
with a pig hooked to them!!).  Do real short time runs and feel the caps 
(after they are very safely discharged of course).  If they seem to be 
getting hot, you will have to let them cool.  One of those aquarium fish 
tank liquid crystal stick on thermometers might be perfect for this...

http://www.mccg-dot-com/Merchant2/merchant.mv?Screen=PROD&Store_Code=HPC&Product_Code=XA1221&Category_Code=fish1-10

They cost like a buck or two and every pet fish store on Earth as 
them...  Just slap a little one on any cap.

Second, keep the BPS rate LOW.  Capacitor current is directionally 
proportional to BPS.  3000 BPS has 10X the current of 300 BPS and 100X the 
cap heating!!!  High BPS rates need SUPER high RMS current caps.  I don't 
think we have a lot of RMS current here to play with so keep the BPS 
low.  Again, cap heating will tell the story...

Cheers,

         Terry


At 07:15 PM 7/2/2003 -0500, you wrote:
>Hello all,  I'm constructing my first mmc bank. Cap's are .056 uf. each, 
>2500 vdc. 900 vac. each. 10 per string .0056 total per string, a total of 
>10 strings max. .056 possible, probably tap out a smaller value at first 
>although. Cap's are digi-key # BC2048-ND, AC and pulse double metallized 
>polypropylene film. Bleeders are 10 megohm 1/2 watt. Using 14.4 kv. 10 
>kva. pig, MOT. ballast on primary, asynchronous gap, speed is ajustable so 
>it may be in synch. at some point. Will it last?  Comments? 
>Thanks    Arcing in Wisconsin.......