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More Plastic SWCs

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
>