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Re: [TCML] First MMC Design questions

Hi Yesn,

The data sheet for the proposed PHE450 series states:  The PHE450 Series is
a polypropylene dielectric with double metallized polyester film as
The preferred CD942C data sheet states:  Type 942 round, axial film
capacitors utilize a hybrid section design of polypropylene film, metal
foils and metallized polypropylene dielectric to achieve both high peak
current as well as superior rms current ratings.
The critical difference is the electrode plate construction.  A solid foil
electrode (942C) can handle much more current that a metalized film
deposited on the dielectric (PHE450).  While the 942C description also
mentions metalized film electrodes, this is just a "floating electrode"
that is situated midway between the two solid foil electrodes that connect
to the two terminals.  I think the floating electrode is a means to achieve
a higher AC voltage rating.  Bottom line, the 942C is more expensive but is
much better suited to handle the high currents of TC operation.  If the
942C is unavailable, the PHE450 could probably be used, but I would suggest
using more parallel strings to reduce the current per string.  But I can't
offer any more specific guidance for how many more strings.

The three LTR cap values that you cited were all fairly close, I don't
think the cap value would impact safety or performance significantly, but
choosing a configuration with more strings is probably very important if
using a cap with metalized film electrodes.

Regards, Gary Lau


On Wed, Feb 28, 2018 at 3:50 PM, Myfirstteslacoil via Tesla <
tesla@xxxxxxxxxx> wrote:

> Hi all,
> I made a 'post' a while back about my Ricci transformer that wouldn't work.
> After trying long it turned out to be working fine. It is a 9-50 trafo.
> Resonant cap value is 0.017684 uF. (I live in EU)LTR is then: 0.026526 uF.
> My first question is concerning the LTR value. Is it safer to go higher
> than this value? Or does it have no impact on safety (and performance)?
> I did some calculations using a 0.15 uF 2kV cap for the MMC. If I use 2
> strings in parallel with 12 caps in series I get a capacitance of 0.0249
> uF. Using 11 in one string gives a capacitance of 0.0272 uF.
> These options are both very realistic to me but which is the better setup?
> 2 caps more doesn't really disadvantage me in terms of costs. However a 3
> string of 17 is really goes to the boundary. (It would give 0.0264 uF)
> For the MMC I want to use the most popular caps but they seem to be out of
> stock in my country. I decided to look for similar ones. I found one from
> Kemet.
> This is the link: https://nl.mouser.com/ProductDetail/Evox-Rifa-KEMET/
> PHE450SR6150JR06L2?qs=sGAEpiMZZMv1cc3ydrPrF85GIl804cbNdYhnhUR0dRE%3d
> Life time is missing from this page while it is present on the CD 940C
> series caps. https://nl.mouser.com/ProductDetail/Cornell-
> Dubilier-CDE/940C20P15K-F/?qs=11kF6y5Z3OhbthK%252bpYczHA%3D%3D
> It does have a higher VAC of 700. Is this beneficial? And most importantly
> has anyone ever used these? If so can you recommend them? They are
> significantly cheaper.
> In the TCML archives there was already one asking the same. As a reply
> someone answered: "I don't think those capacitor will work for Tesla coils.
> If you look at the curves of applied voltage vs frequency on the web site,
> these capacitors roll off at much lower frequencies than we work with. In
> other words we couldn't apply a high voltage to them at primary/secondary
> frequencies."Links that were provided by him and other repliers do not work
> anymore. (http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/
> PHE450%20Series/$file/F3294_PHE450.pdf).
> However Bert Hickman said the following about the caps: "The series is
> rated for high frequency applications with high current stress. Sounds like
> a potential TC MMC app to me... :^)."
> I did find a page containing the information however I do not know if this
> is the same that was on the site once.
> After some digging I found it in someone's PhD Thesis. In the datasheets
> is the page it's about the PHE450 series. To find it quickly for you it's
> on page 8 of the Datasheets. The Datasheets begin on page 259. ; )
> This is the link of the paper: https://dspace.lib.cranfield.
> ac.uk/bitstream/1826/8574/.../Aldhaher_S_Thesis_2014.pdf
> This person used one of the PHE450 series: https://www.youtube.com/watch?
> v=Uh3sLMV2jeY (The caps don't look similar to the picture on Mouser)
> From what I see on Mouser it should be fine except for that some data is
> not there, for example ESR value. So should I wait for the most popular or
> go with these? It is the first time I am building a MMC so please forgive
> my lack of understanding.
> I hope I have provided enough information.
> My kind regards,
> Yesn
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