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Re: [TCML] Help choosing tank caps for dual MOT

On 9/1/11 6:23 AM, jhowson4@xxxxxxxxxxx wrote:

Just to throw my 2 cents in here,

A quick Google search yielded the discovery that option 1 uses paper
as a dielectric, not ideal for tesla use.



Not necessarily... while polypropylene film with extended foil
electrodes is preferred, oil/paper has a long history of successful use.
 The dielectric is oil, which is low loss (the paper is basically a
spacer).  What it really depends on is the electrode design.  paper/oil
DC filter caps, for instance, have fairly high inductance.




option 2 i could find no information on, but be wary because people
on ebay love to put Tesla in the description for things, regardless
of its actual purpose or not.

option 3 well yea its gonna work, i cant really say much about which
size would be better though.


but might i present option 4 A home made poly cap could be made for
your voltages relatively easily and cheaply. I have been doing alot
of digging into the poly caps information as I am having the same
problem sourcing a capacitor. I am assuming that you are seriesing
the 2 mots and doubling them? Running the numbers threw my little
excel calculator I made. and using the advice that your capacitor
should be moderately over rated so it does not cook itself.
rolled poly caps are a lot of work: you're basically trading time and reliability for money. There's a non-zero chance that first one you build will have some crippling flaw or have short life, because it's simple in concept and tricky in execution. Do you have a vacuum pump? A big clean surface to lay the layers of poly out on? Do you have a source for the poly, and for the foil? 





you could make a .1uf 9600V capacitor using 135ft of the 6mil poly
which could easily cost you 50-60 for the plastic. that of course is
excluding the costs of aluminium for etc etc. over all the price
would be almost 60-80 depending on how fancy you want it. The
resulting cap would be rated for ~40kv plenty of head room and be
pretty bullet proof when running off your mot supply. in my opinion.
You could even put it under oil if you were feeling ambitious but at
the lower voltages it should not be necessary.
Running a rolled poly cap without oil impregnation is asking for short life. You will, even at low voltages, have corona at the edges of the foil plates, which will degrade the plastic slowly but surely. Eventually, under the mechanical stresses of the capacitor, the plastic goes away, and you'll get breakdown. I guess, without oil, you at least don't get the burning oil leaking out.. just smoldering melted plastic.
Almost 15 years ago or so, the rolled poly started to be less popular, and there was some interesting failure analysis done on these caps by dedicated list members. There's just a lot of things that can go wrong. 






I personally have used the home brew caps for the past 6 years with
out failture, 2 are open air, 1 is under oil. the air units run off a
obit supply (10kv) and I have had about 10 obits in parallel at one
time or another. I personally love the home brew caps, but other
people seem to have trouble with them.
It's like carbon fiber composites: there's a lot of factors that go into success/failure so it's unpredictable in small manufacturing lots.. Some people roll tighter than others (I'm not sure which would actually be better). Some have different run duty cycles (which changes heating and such). Some have different LC ratios and system Q (which changes di/dt and voltages) 



These days, there's three basic approaches that are fairly low risk:
1) beerbottle salt water (in various forms) - heavy, can be lossy, real cheap, not real durable in general. A variant is the plastic party cup cap. 

2) MMC - labor intensive, expensive for big capacitance

3) commercial caps - very tricky if you're buying surplus

There are other approaches which might be useful, in a particular situation:
1) ceramic HV caps - some work well, some don't, if you have a cheap source, go for it.
2) home built foil caps - rolled poly, some work, some don't, labor intensive, not all that cheap if you have to buy materials
3) all manner of flat plate capacitors - foil/glass is lossy, but, hey, it's worked for people in the past. foil/plastic baggies/plastic sheets,etc If you have a cheap source for the materials (free) and lots of time, it can be worth trying.
Pretty much anything you can imagine has been tried and reported on the list, so a search using the materials you're thinking of will probably turn up someone's practical experience. 
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