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Re: Variacs, HV Jell, and LTRS Caps / oil for MMCs / Etwas trost

Hi Stefan,

At 06:25 PM 5/21/99 +0200, you wrote:
>Hi GSTCs, Terry,
>
>TF> Also, I am working on a new MMC cap design that will require 
>    that they be submersed in oil.  
>Be careful as the mineral oil will be perhaps somewhat aggressive 
>to the housing of the caps! I destroyed my homebuild LDPE/oil-cap
>before applying the first volt because the PS I used as 
>compression plate and cover of the box got brittle due to the 
>mineral oil and cracked all over.

A very good point!  On reason I wanted to use, Vaseline as so many suggest,
is that it will not soak in through the leads or other weak areas of the
epoxy covering like oil would.  I want corona control, heat removal,
electrical insulation, and something to hold the assembly mechanically.
Solid substances will not leak either which is a BIG plus.  Since this
thing should never heat up more than 5 degrees C, most things will remain
solid.  I may add something to it so the trunk of my car will not melt it
either...  I am still thinking about this....

>
>TF> I am running them above their ratings (in the "enhanced"
>    mode :-)) taking advantage of the fact that "I know things" :-))  
>    Since oil is such a mess, I was wondering if mixing mineral 
>    oil and say paraffin would produce a jell that could be heated 
>    to flow but would solidify like grease when cooler?  The jell 
>    would not be a dielectric but more of an insulator and mounting 
>    medium to put the caps in nice PVC cases.  The advantage would 
>    be it would not run, spill, leak, etc.  If this works, I can 
>    make a 5.6nF cap for about $25!!  If it doesn't, I don't want 
>    to have to clean up a bunch of oil :-))  This will test if the 
>    manufactures ratings really do mean anything in Tesla coil 
>    applications.  I am running just over the edge in voltage and 
>    current but theoretically they should hold up very well.  
>I think a fan should do the job. Just make a top and a bottom plate
>and let the air blow through the rows.

Fans help, but the temperature in the center of the cap is the important
part.  The cap is a pretty good insulator so external temperature my not
mean much if the center is roasting.  Of course, that is a matter of
watching how much RMS current it is dealing with more than anything.  I
also want to avoid having AC driven fans and other parts if they aren't
needed.  One problem with my old experimental coil is that it has zillions
of little pieces.  Good for experiments, but not portable...

>
>TF> My present MMC runs soooooo cool I think it may be a waste 
>    of money to over design these things too much.  More 
>    exploration of their limits is called for.  Since I have all 
>    my stuff modeled, I can predict RMS currents and voltages very 
>    well so as to push the rating of the parts "gently"...
>Still a problem is to tell how long a cap will last if it blows
>not in the first hour or so. If it will be stressed over the limit
>the 'time to death' will become shorter at least exponentially
>with the 'overstress' because of the weakening of the dielectric.

I am a reliability engineer at a company that is very expert at stressing
poly caps... Like I say, "I know things" :-))  What I am trying to do is to
take a $200 cap that will last a million hours and make it a $25 cap that
will last 10000 hours.  If a cap lasts 400 continuous days of running or
40000 days, the person who runs a TC an hour a month will never know the
difference.  However, they will notice the $175 price difference real quick!!!

All is going very well so far....

Cheers,

	Terry

>
>Stefan
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