Re: MMC practical apps.
Tesla List wrote:
> Original Poster: Philip <shadow42-at-totcon-dot-com>
> Ive been reading on the recent MMC trend and it seems to me that its a
> great compact alternative. The only problem I see for the MMC is its
> cost to coilers with small budgets like myself. I was reading Terry's
> note on the falling capacitance and dielectric air pockets. Since most
> coils incorporate the rolled poly or poly plate caps I was curious if
> this dielectric bubbling could cause a similar problem for them as well?
> I noticed a C reduction in my rolled cap before it died on my 4" coil
> that I posted specs on a little bit ago. The reduction was quite small,
> so small in fact that I never thought about it till now. I realize the
> oil in the cap should fill these pockets of air as they form (if they
> do) but could periodic re-vaccum pumping could help this? Since im sure
> that not all air can be removed in the first pumping of the cap. My cap
> only got hot and died spilling some smoke filled oil into the catch
> bucket. My dissection of it revealed a spider effect on the poly surface
> that looked like cracking of a car windshield when hit by a rock.
The pattern you is typical of the effects of partial discharge where the
e-field stress is greatest - at the edges of opposing capacitor plates.
It's formed by the decomposition products of successive corona-like
discharges at the oil-polypropylene boundary layer. Over time, these
"Lichtenberg patterns" grow, tree-like, leading to tracking and
carbonization of both the oil and poly, and failure of the dielectric
system. As this occurs, the oil may also decompose, releasing gases
(mostly hydrogen), that can then further promote ionization,
accelerating the process. Basically, you overstressed your cap - this is
the primary reason to use identical caps connected in series to reduce
the e-field stresses seen by the individual caps. Using a single cap
with a thicker dielectric layer simply won't do the trick...
-- Bert --