Re: Maxwell Scot D cap dielectric

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Scot and John,

Sorry to hear about your accident, Scot! I'm not sure of the exact
construction used for these particular caps - but you should be able to
tell now that the rolls are "available" for inspection... :^)

Many older style polypropylene (PP) pulse caps were constructed using a
layered system of metal foil, kraft paper, PP, PP, kraft paper, metal foil,
combined with castor oil as the impregnant. The kraft paper provided a
physical path to insure that the oil was drawn by capillary action to fully
penetrate the interior of the capacitor rolls. This construction technique
also provided voltage "grading" - the maximum voltage stress was actually
seen by the PP, while a lower voltage stress was seen by the high
dielectric constant combination of the kraft paper and oil layers. The
oil-soaked kraft paper layer also helped reduce the local E-field stresses
at the edges of the capacitor plates, reducing the incidence of destructive
corona in the areas that normally see maximum stress. 

Newer PP caps now eliminate the kraft paper, and instead use PP with a
roughened surface (sometimes called "hazy film") to help wick oil into the
capacitor roll. Newer caps may also use a thin metalized film in place of
metal foil. The thin evaporated metal film "self-heals" in the event of
dielectric breakdown by simply evaporating the metallization around the
defect, isolating it without significantly affecting the capacitance. The
result is a capacitor with higher energy density and higher reliability.

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<FutureT-at-aol-dot-com>
> 
> In a message dated 4/16/01 8:58:11 PM Eastern Daylight Time, tesla-at-pupman-dot-com
> writes:
> 
> > BTW  these things do get warm when run on this system  they are the 31XXX
> >  series caps ( paper n oil dielectric)
> >
> >  any ideas as to why they are getting warm ( possibly 120+ degrees F )
> >
> >
> >  Scot D
> 
> Scot,
> 
> I didn't know the maxwell 31xxx series was paper and oil dielectric,
> I thought it was polypropylene.  are you sure it is paper?  Maybe
> there's paper with poly.?   The one you have with the hole in it
> from the accident.....  is it still good other than the hole?  Are
> you going to repair it?  If it's damaged inside, can you take it
> apart and salvage the good sections?
> 
> John