Re: Another MMC cap candidate (UK)


The MMC photos you saw are of my 6.5" coil. The saltwater caps are only
part of the post-neon filter circuit, and have no role in the main tank
circuit. You can just see my wirewound resistors smouldering away in
the photo! It runs well with my MMC, I have now got a record spark of
72" for about 2.2 kW input. I managed to raise a few eyebrows!

Alex Crow

On Sat, 05 Jun 1999 15:08:25 Tesla List wrote:
> Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> 
> Hi Alan, all,
> A little quicker (and longer) response this time ;o)).
> >Original Poster: Alan Sharp <AlanSharp-at-compuserve-dot-com>
> >I've been following the correspondence on multi-mini- caps.
> >And I was very impressed by the performance of Alex Crow's
> >coil at the UK teslathon.
> Very nice pictures on Mikeīs site. I hope we will get around
> to organizing a Teslathon, here in Germany, soon. Big
> compliment to all those shown. I esp. liked Vivianīs plexiglas
> SRSG. Great work Vivian!! You seem to be able to find
> LOTS of plexiglas for cheap ;o)).
> I noticed you guys were running the MMC in parallel with
> some saltwater caps (if I made it out correctly) on the 9.5"
> coil. Did you try running the coil soley on MMCs (i.e: w/o
> the SW caps attached)? Even though the capacitance would
> have been lower, the performance should increase, because
> SW caps are very lossy. This will noticeably reduce the
> possible spark length.
> >What I have summarized is that the caps will not be killed by
> >overvoltage
> >If the 1.414 * AC is these than the DC voltage rating of the cap.
> >So for my 8 KV AC transformer I need 8 of 1500V DC caps in series,
> >I'll probably use 8 by 8 in series / parallel.
> >They will also not be killed by over current is dV/dT is sufficent.
> Here is the way I build my MMCīs:
> I.) General comments:
> ----------------------------
> a.) Go for poIypropylene only.
> b.) Get physically big caps. The bigger surface:
> b1.) will allow a better heat disappation (prevents premature cap
>        death).
> b2.) usually is a sign for metal FOIL endplates (very important!).
> b3.) From b2, these caps have MUCH higher dv/dt ratings.
> b4.) Using metallized plastic ENDPLATES WILL result in cap failure
>        (I tried and lost some Philips caps.).
> b5.) If you find any cap having a dv/dt less than 1000V/ĩsec, you can
>        bet on them not having metal FOIL endplates. DONīT use these.
> c.) Leave a space between each cap row. This will allow any created
>      heat to be carried away by air convection.
> d.) KIS (keep it simple). While I did propagate using interlinking of
>      the cap strings a while ago, I no longer believe this is worth the
>      trouble, because:
> d1.) it complicates the whole thing. Removing an interlinked string (for
>        tuning or repairs) envolves lots more work.
> d2.) If an interlinked cap in a string blows(opens), you get unequal
>        voltage distribution, that might lead to more problems.
> d3.) Design your setup, so that it is easy to pull a complete string. I
>        mount mine upside down and use a friction fit to hold them in
>        place. Pulling a string is a matter of seconds (only two
>        soldering junctions).
> d4.) Following d1-d3 makes the MMC very universal. This means you
>        can use it for more than one coil.
> d5.) Cap killers are high BPS. This being said, stay away from BPS
>        rates >>480+. Go for a bigger cap instead. Following John
>        Freauīs posts have shown me that high BPS with a small cap
>        will NOT lead to longer sparks than a coil with low BPS and a
>        "correctly" sized cap.
> e.) I find equalizing resistors unecessary for equalizing the voltages
>      across the caps. They do, however, safely discharge the whole
>      MMC within a few seconds. The actual resistance is uncritical.
>      This will vary with the number of caps used in a string, as you
>      donīt wanīt to get too low on total resistance (wastes power
>      and heats up the capīs surroundings).
> Specific comments for DW (spark gap driven coils) only:
> ----------------------------------------------------------------------
> To make things easy (for various explantions), letīs use
> this example cap:
> WIMA 100nF
> 1650Vdc/650Vac
> dv/dt: 3500V/ĩsec
> Array: 10x10 for 10nF per row and 100nF per MMC
> Vdc(rated)= 16.5kV
> Vac (rated)= 8500V
> Imax per string: 350A
> Imax per MMC: 3500A
> 1.) dv/dt rating:
>  -------------------
>  a.) In order to stay universal, try not to exceed dv/dt too
>        much. While (as Terry has shown) you CAN exceed
>        dv/dt, you loose versatility doing so. What I mean is,
>        if you build your cap on the limit (there are some $$
>        reasons making this legimate), for e.g. 120 BPS, then
>        you wonīt be able to run it at 240 bps, etc.
> b.) Calculate the maximum primary current.
> c.) Calculate the allowed current per cap (from C* dv/dt)
> d.) As the current flows through each cap (in a string), the
>      maximum allowed current is the same for one or 1000
>      caps in a string.
> e.) From b, now calculate the number of strings needed.
> f.)  Find the necessary (wanted) total capacitance
> g.) From b-f back-calculate the needed single capacitance
> h.) This sounds a lot more complicated than it is. You will need
>      to fiddle with the numbers, but there are various possible
>      combos.
> i.) In DW coils, the peak current is not being fed continiously.
>     This is one of the reasons why we can run them "on the edge"
> j.) Richard Hull once said that MMCīs are only good for coils in the
>     <2kVA range. I disagree on this. The larger the input power, the
>     larger the coil will be (following simple reasoning). The larger the
>     coil, the lower the FRes is. As T = 1/F, the lower Fres is, the
>     easier it is on the caps, because the cap has "more time", so
>     the dv/dt rating becomes less critical. As we are using more than
>     one string, we will divide the current up between the strings. In my
>     present case, I am running ~600A primary current, but I am using
>     13 strings, so each string only sees 46A. I have run these caps with
>     up to 65A per string and have had no failures.
> k.) Looking at T (from j), the steepest rise occurs during the 1st
>      quarter, so this period is of true interest.
> (My) conclusions:
> dv/dt should be kept within limits, in order to stay universal.
> You donīt need to be super religious about it, tho. A 1.1x
> overrating surely wonīt lead to premature cap death. Looking
> at our example cap, we see it will be very hard to actually
> exceed the rated dv/dt of this cap. (Okay, Greg Leyh might
> be able to with his ALF coil).
> 2.) Vdc rating:
> -------------------
> a.) Try to stay within the voltage rating. Due to the high quality,
>      (Terry, once again) you can exceed this limit, too,
> b.) but doing so, will once again be a trade-off in universatility.
> c.) Most manufacturers test their caps at 2-3 x Vdc. They must
>      survive this for 2sec (IEEE standard).
> d.) Following a and d will give you a good measure of safety
>      against strikes, kickbacks, etc.
> (My) conclusion:
> Keeping Vdc within limits gives you great amounts of safety,
> in case anything goes wrong. However, there is absolutely no
> need to go for a rating equal to the one we are using for rolled
> poly caps. The rolled poly caps have the BIG disadvantage of
> being self rolled and using non uniform, non virgin material. The
> MMC does not have these disadvantages. Letīs take another
> look at our example cap. It is rated for 16.5kVdc. Terry has
> shown, these will fail at 5kV PER CAP, so our example cap
> will fail at 50+kV. This (with the proper use of safety gap) rating
> should be high enough for at least a 12kV, if not even for a
> 15kV xformer. It is exactly this DC failure rating (5kV for a
> 1650V rated cap), which lets me say it is useless to go for
> really high voltage DC caps. Some of the GTL members
> do not agree, for whatever reasons, and they went ahead
> and ordered the 6kVdc units. (which is why I sort of lost
> out, as I too ordered caps) If you give Richie and my 4x
> Fmains SRSG idea a try, the "failure time" is only 5ms.
> This means, if you can keep the voltage within limits for the
> 5ms, the SRSG needs to fire the next time, there will be no
> danger for the MMC. Vdc is should be rated so that you
> *should* never exceed it in normal usage (1.41*Vac) and
> 2-4x this DC rating should match the possiblilty of a strike
> to the primary (so that the MMC will survive). Greatly raising
> the DC voltage rating (i.e: using caps that have very high
> DC voltage ratings; WIMA 6kV) is a waste of money, because
> the DC rating really only tells you the voltage peak (single
> pulse case) the caps will survive. There is no use in designing
> your caps, so that they will survive 150kV, if your xformer
> lets go at 20kV. (I hope you see what I mean to say).
> 3.Vac rating:
> -------------------
> a.) Vac rating (as far as I understand this slightly
>      mysterious rating) has to do with:
> b.) internal heating.
> c.) partial discharges.
> (Side comment: DC to AC derating factors on WIMA
> caps seems to be very reasonable up to and including
> the 2kV units. The 2kV units are rated for 700Vac. What
> I ABSOLUTELY do not understand is why the 4kV and
> 6kV units are also only rated at 700Vac. Even WIMA
> could not answer this question directly (they used vague
> words). For a 6kV cap, the 700Vac is a derating factor
> of 1:8.5, where as the 1650/650V is a much more
> reasonable factor of 1:2.5. This is another reason why I
> donīt have much faith in the 6kV units.)
> b1.) Internal heating: As we are using one of the lowest
>        loss materials (PP) and our run times are limited to
>        <30 minutes, this is of no real concern. Unless we
>        really push them hard, they should not get too hot.
>        PP should not be run above 105°. However, this is
>        internal temperature. As we do not know how fast
>        the heat convects to the outside, I would say the
>        outside should stay below 40°C. 40° is slightly
>        uncomfortable to the touch.
> b2.) Keeping the rows seperated by 1/2-1" will allow plenty
>        of ambiant air to circulate and cool the MMC. I donīt
>        think a fan is necessary. If a fan (must be) used, I would
>        go to the trouble to filter the air. You donīt want dirt, etc
>        to build up on the caps. This will reduce heat transfer and
>        might lead to flash overs.
> b3.) Following b1 and b2, a small HF coil, powered by a tube or
>        FET, will need other considerations. As I have not built one
>        of these, I canīt comment on them, but I would think an MMC
>        for such a coil will need different construction thoughts to keep
>        the temperature down low.
> b4.) Higher temperatures will soften the dielectricum and degrade
>        itīs electrical properties. This will cause a change in
>        capacitance and punch-through resistance.
> (My) conclusions for internal heating:
> This is of no real concern in the AC (de)rating for CW coils. We
> should never experience ANY sort of real heating within the caps.
> Caps that get hot, are telling us something is wrong and you had
> better find out what the problem is.
> c1.) Partial discharges. This is a problem, because it is the slow
>      death of a cap. This doesnīt become obvious on the first
>      run.
> Side comment on WIMAīs 6kV vs 2kV: Partial discharges also
> can not explain why the DC->AC derating factor is so rotten
> for the 6kV units.
> c2.) Partial discharges occur within the cap in voids, air bubbles
>        and entrapped dirt particles (see my mail on partial
>        discharges in the archives). Fortunately for us, the
>        manufacturer does everything, he can, to prevent this from
>        happening. Commercial cap material never sees human
>        fingers. The high tech envolved (thin layers, virgin material,
>        high pressure rollers, vacuum degassing chambers, etc,
>        etc) lets the cap survive the hell we put it through.
> c3.) One thing to remember is that the manufacturer (de)rates
>        his caps for 24/7/365 usage. Our coils do not see this
>        kind of operating enviroment. Further more, caps are rated
>        for 10,000-100,000 hours of operation. The MTBF (mean
>        time between failures) is rated in Gh!!. To make things more
>        clear, lets use an example. For our needs, 2000h are more
>        than enough. Furthermore, letīs say we run our coil for 30
>        minutes per day, every day. For 2000h, this means we can
>        run our coil for 2000h/0,5h = 4000 days. 4000/365 = 10.95
>        years!! This is more than enough, I think ;o).
> c4.) In order to keep partial discharges within (what I claim to be)
>        safe values, I would suggest staying below the factor 2.5. I.e.:
>        your 1500Vdc caps (450V ac) should be run at or below 1125
>        Vac. The lifetime does not go down linearly. I havenīt made a
>        graph, but from gut feel, I think with a 2.5x Vac overrating, you
>        will be able to reach a 1000 hr. life span with ease, running the
>        coil as described above. Hardly anyone runs their coil EVERY
>        day of the year. If we further say, we only run every other day,
>        we will still get a lifetime of ~11 years.
> (My) conclusion partial discharges vs. AC rating:
> This value should not be totally overrated in order to get a reasonable
> lifetime out of the MMC. A factor of ~2-2.5 seems "about right". We
> shouldnīt forget this is a statisical value and not ALL caps will go at
> once. As the MMC is built in a modular form, replacement of single
> (even if it is a few) caps is not too difficult or expensive. If you are
> (generalizing here) short on cash or just want to experiment around
> and take a shortend lifetime into account, you can exceed this rating up
> to maybe a factor of 3-5 (i.e: your caps 1300-2250Vac each).
> End conlusions to MMC construction:
> ----------------------------------------------
> Considering the cost of a rolled poly AND the fact that it
> becomes a doorstop, once it has failed, I think the MMC
> has good chances to become THE standard in coiling. I
> can see no problems with current capabilities (section 1),
> peak (DC rating) voltage capability (section 2), as long
> as it is sensibly matched, and no real problems in lifetime
> vs AC rated vs. AC useable voltage (section 3).
> >Polypropylene Axial Caps
> >dv/dt = 1800V/uS
> >PF at 10KHz < 7*10,000
> >0.047uF Capacitance tolerance   ą20%
> >Rated voltage   1500V d.c. working (450V ac.)
> >5-45 -at- 46p, 50-95 -at- 41p, 100-495 -at- 35p  +VAT
> Alan:
> While the long mail above might not directly answer all your
> questions, it pretty much sums up the knowledge and
> experience, I have gathered with MMCs. Other info, which I
> have posted over the past weeks, (on actual construction,
> spark length vs. input power, calculations, etc) can be found
> in the archives. If I had reposted all this, the mail would have
> been rejected because of the enormous length........ ;o[!].
> Your dv/dt sounds good, the 47nF is a sensible size (I think
> 50-100nF per single cap is a good range to shoot for) and
> the cost is not a real prohibitation either. Going for an order
> with several coilers, will reduce the price to 35p (test a few
> units before you buy the bulk packs ;o)). Judging from dv/dt
> and your description, these seem to be caps with metal
> FOIL endplates. Axial caps will take a bit more space to
> mount (in comparison to radial caps) and might be difficult to
> friction fit them into an enclosure (for ease of servicing), but
> this wouldnīt stop me from using them. Of course, if you can
> get radial ones (retangular form) for 2-5p more, I would
> rather use these.
> >I'll probably buy 10 to start with and try them in parallel with my
> >existing Cap.
> Donīt be surprised if your spark length increases (for similar
> capacitance), when you start feeding your coils on MMCs
> alone. This is exactly what happend to me.
> Coiler greets from germany,
> Reinhard