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Re: Started winding today....
At 10:49 AM 3/22/99 , you wrote:
Bert, All,
Yes, that is indeed the reality. It took several rolled caps too knock some
sense in my head though. Well, that and discussing the issue with Condenser
Products. One can vacuum degas all they want, but those sharp edges will
get you any time you push the envelope. The only insurance is as Burt says,
keeping the voltage as low as possible by series combinations.
Regards,
David Trimmell
>Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com>
>
>David,
>
>The real secret is in splitting up the voltage stress among
>equally-sized capacitors connected in series. These caps fail when oil
>breakdown is induced at the very edges of the capacitor plates - this is
>where the e-fields are the strongest. These stresses are sufficient to
>induce ionization breakdown of the mineral oil itself, even though your
>poly thickness may be quite substantial. Under the applied stress, the
>oil will self-generate gaseous breakdown products (typically hydrogen
>and methane gas) which promote further ionization, and the destructive
>UV and ionization byproducts quickly attack the LDPE dielectric.
>
>Using equally sized series-connected caps divides the voltage stress and
>reduces the localized e-fields below the point where the oil breaks
>down. Commercial capacitor manufacturers understand this principle quite
>well, and they will typically construct their AC and pulse caps so that
>each individual capacitor section sees no more than 3-5kVRMS. And each
>individual capacitor must use at least 2 layers of dielectric to prevent
>a single defect from breaking down the dielectric layer. If you design
>your caps so that each section "sees" no more than 5 kVRMS and use
>multiple layers of thinner material in each capacitor module, you should
>be able to run with 15 kVRMS on 80-90 mils of total LDPE with no
>problem.
>
>Safe coilin' to you!
>
>-- Bert --
>