Re: Paper/Oil Capacitors

["Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Tesla list wrote:
> 
> Original poster: "Duke, Ronn (CCI-San Diego CCC)" <Ron.Duke-at-cox-dot-com>
> 
> Snip--
> The Kraft paper used in these caps is made from high-grade wood pulp that's
> been specially processed to have few conducting particles or pinholes, low
> porosity, and uniform density. Because it readily absorbs atmospheric water,
> it should be carefully dried before use (capacitor makers typically used
> vacuum drying ovens). One pound of capacitor paper can easily contain a
> couple of teaspoons of water at equilibrium prior to drying!
> Typical paper thickness is 0.3 - 0.5 mils (0.00030 - 0.0005 inches).
> Multiple layers are always used to prevent single-point defects from causing
> failures and to increase operating voltage. For utility PFC use, voltage
> stress was limited to about 300 volts/mil on the paper dielectric, and no
> more than 1000 volts/mil in a controlled DC environment (Brotherton).
> Longland indicates that the dielectric constant of mineral oil-impregnated
> capacitor paper ranges from about 3.0 (low-density) to 4.3 (high-density).
> ----
> I thought it would be a good idea to make a rolled poly cap with
> paper-poly-paper layers. I figure this would help keep the heat and corona
> away from the poly that would prompt premature failure, and also improve the
> voltage rating and dielectric properties.
> 
> Comments?
> 
> Ronn
> Dukester-at-home-dot-com

Ronn,

This is an excellent approach that has been used very successfully in
older commercial pulse capacitors. The techniques helps "wick" oil deep
inside the capacitor roll, and helps share the total voltage stress in a
manner which is consistent with the voltage breakdown strength of the
oiled paper and plastic film. Most of the voltage stress appears across
the plastic film dielectric because of it's lower dielectric constant.
The presence of the kraft paper also reduces local voltage gradients
near the edges of the capacitor plates, helping to inhibit damaging
corona. When most folks "rolled their own" oil-film tank caps instead of
building MMC's, this was a sophisticated way of making very robust
capacitors. While commercial capacitors tend to use higher dielectric
constant (and more expen$sive) capacitor oils such as Castor Oil or
Dioctyl Pthalate, the approach will work almost as well using simple
mineral oil as the dielectric fluid due to the higher dielectric
constant of the oiled kraft paper.  

Good luck - let us know how it goes!

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