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Re: More Plastic SWCs
From: "Steve Young" <youngs-at-konnections-dot-com>
William,
So far, your interesting experiments are all at line frequency. I believe
the dielectric losses of your soda bottles will go up astronomically when
your PSWCs are in an actual primary tank circuit at 100 KHz or so. High
losses = melted plastic = very short life = time to try something else.
The wisdom of many coilers is to use polyethylene which has very low losses
even at RF. If you must use salt water, then try using large zip-lock bags
instead of soda bottles.
--Steve Young
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: More Plastic SWCs
> Date: Wednesday, August 26, 1998 6:49 PM
>
> X-Envelope-From: will.e.payne-at-lmco-dot-com Wed Aug 26 09:06:45 1998
>
> Last night, using techniques in my last post, I extended the running time
of
> a soda bottle Plastic SW Capacitor (PSWC) to 30 minutes.
>
> TEST ARTICLE : Outer electrode and vessel was a cheap K-mart stainless
steel
> stock pot, the bottom of which was lined with disk cut from plastic
ceiling
> light grille. The soda bottle was thoroughly stripped and all label
> adhesive removed with methanol. The inner electrolyte started out as
> carbonic acid, magnesium sulfate and sodium chloride. The outer
electrolyte
> was magnesium sulfate and sodium chloride in a solution of water and
> fermented Gatorade. The inner electrode was 10 inch of 1/2 in copper
pipe
> suspended in the bottle neck by a copper tee. The dead ant from last
night
> was retired from duty. Electrolyte levels were equalized within a mm,
and
> no oil layer was added.
>
> THE TEST : The test article was connected in parallel with a 450 VA,
15,000
> Volt NST bridged by a 2 cm static arc gap. During the test, intermittent
> streamers were observed on the upper dielectric, which mostly appeared
> internal. Some condensation was also observed on the internal surface.
> After several minutes the streamers subsided although condensation
remained.
> The plastic bottle dielectric failed at about t+30 min, with a small
rupture
> at the electrolyte surface. All components had about a 10 C temperature
> rise. A narrow band of plastic at the liquid surface had shrunk during
the
> run, making a smooth, symmetrical groove or constriction all the way
aroud
> the bottle at the liquid line. The groove was about one cm in width and
> depth. Crystalline deposits were also observed around the entire
> circumference.
>
> RECOMMENDATIONS : Filter contaminants from electrolyte, use a mineral oil
> film on both electrolyte surfaces to reduce corona. This may aggravate
heat
> accumulation at the top of the liquid level.
>
> I would consider one hour as the minimum acceptable life of a disposable
> dielectric capacitor. I would still like to compare notes with any other
> folks who attempt to use plastic soda bottle SWCs, the more experience i
can
> gather from others, the further/quicker I can progress. Rules of thumb:
> dielectric life goes as 1/e^5 with voltage and 1/2 with each 10 degree
> temperature rise. Any comment ?
>
> Rmember, don't re-invent the wheel on a steep hill.
>
> William E. Payne
> 275 Oak Hills Dr
> Dallas, GA 30132
>
> 770-494-1104 (day)
>
> altair-at-altair-dot-org
> www.altair-dot-org
>
>
> > ----------
> > From: Kip Turner[SMTP:kip-at-mindspring-dot-com]
> > Sent: Tuesday, August 25, 1998 18:15
> > To: will.e.payne-at-lmco-dot-com
> > Subject: more on building capacitors
> >
> > Will,
> >
> > Your extrapolation is interesting but may not work. You have to look
> > at the BIG picture. Capacitors are usually tested at twice working
> > voltage to weed out obvious defects. In going to very high voltages,
you
> > may excite other failure modes, such as arcing other than through the
> > dielectric. Also, using your 6x example, you will get 36 times the
> > normal internal heating, and most dielectrics are poor thermal
conductors.
> >
> > Yes, but I control the construction to eliminate other modes. Your
point
> about heat is well taken.
>
> > I have made PET (mylar film) capacitors for AC operation at 1 KVA per
> > cubic inch. This produces about 10 watts of heat per cubic inch and
the
> > units would increase in temperature at a rate of several degrees C per
> > second. The units were rated for 30 seconds on-time per 10 minutes and
> > required a internal cooling arrangement to even survive that.
> >
> > I consider min acceptable life = 1 hour. Active cooling not desirable,
> mabe passive cooling.
>
> > Since you are operating at a higher frequency and very high fields, I
> > would expect a much more rapid rise to the failure point (150-200 C).
It
> > is possible that, even with a cooling liquid for electrodes, the
interior
> > of the PET is overheating. The plastics are not very good thermal
> > conductors.
> >
> > My observations above point to localized heating at the liquid surface.
> Oil layer may/not help.
>
> > You might want to look it up, and calculate what happens to the
internal
> > temperature, assuming that the heat input is uniform and about 1 % of
the
> > reactive power in the capacitor. You should be able to integrate that
> > and estimate the hot spot maximum temperature. You should keep PET
> > well below 150 C, to avoid damage that weakens it and drastically
> > increases
> > the loss. You might want to check the "bottle" to be sure it does have
> > the lower loss. It may not be as good as the film...which you can buy
> > easily as "drafting mylar". Mylar (PET) is non-linear with voltage.
It's
> > resistance has a significant field intensity related effect, with the
> > resistivity dropping at high voltages. This can help, in some cases,
> > but you are above the fields which give long life.
> >
> > Remember that there are several other causes for failure. Not just the
> > voltage. You may reach gradients which produce field emission,
especially
> > at discontinuities.
> >
> > Hey, I don't care if you pass along my "OPINIONS". Or you can take the
> > credit ( or blame ) if you want to !
> >
> > 73,
> >
> > Kip
> >
>
>