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Reinhard's MMC notes
Terry's few thoughts on Reinhard's great collection of MMC knowledge. Many
snips...
>c.) Leave a space between each cap row. This will allow any created
> heat to be carried away by air convection.
This may also save nearby caps if one blows.
>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.
Poly caps may self heal if one were to short. Interlinking would defeat
this by allowing much greater current to flow into a momentary short which
would reduce the chances for a successful healing. I don't recommend cross
linking now. It also defeats the ability to select various strings to
adjust the caps value.
>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.
I think you can go to any BPS just as long as you do not exceed the RMS
current limit of the MMC. For very high break rates, you will need lots of
strings.
>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).
Of course, I am a BIG advocate of using high value (~10 Meg) resistors
across each cap. They add a very nice safety feature especially when you
have many in dependant strings that can shock you. I also think there is a
very good chance that there can be unequal charges built up that may over
voltage a cap. Everyone will have to decide for themselves on this... I
know one person has already found uneven charging to be rather "shocking" :-)
>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.
I think MMCs can be adapted to running any size or break rate coil. It may
take an unreasonable number of caps in some cases. Other than just too
many caps to deal with, I know of no fundamental power, current, voltage,
or BPS limits that an MMC could not be designed to handle. In fact, an MMC
could be very heavily over designed for critical show coils and such.
>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.)
WIMAs seem to have more air voids in them than other brands. I wonder if
they are more worried about the "ion inception level" (where ionization
starts in small air pockets causing slow damage to the dielectric). I
don't think that would be a concern in our low MTBF (mean time between
failure) use. Everyone in the industry seems to ignore AC ratings and
worry about RMS current.
>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.
I would get very worried if the caps get five degrees C hotter than the
surrounding air. Of course, we won't be using them in 80 or 105 C ambient
conditions.
>
>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.
As long at the RMS current is within the cap's ability, they should perform
just fine in any continuous AC application. That is what everyone else
uses them for.
>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.
I am a big advocate of running the caps close or over the manufactures
ratting and taking advantage of their normally very long rated life. We
don't need the caps to last 50 million years. 50 days is just fine. The
reason for pushing them to the edge is that you can save a substantial
amount of $$$$. If you pay 20 percent what a "properly" rated cap would
cost and the caps live just as long...
If the caps run cool and you don't go over their voltage rating by more
than 25%, they will probably never fail. I would ignore the AC voltage
ratings. The old 3X voltage rating rule was for the handmade caps where
things just were not perfect. We now know that caps do not see large
voltages unless they are resonated without limit but then, they or the
transformer will still blow.
>If I had reposted all this, the mail would have
>been rejected because of the enormous length........ ;o[!].
You can always split it up. I don't know how long a post can be? The
digests are 32 so... As long as the content is reasonable, I don't care
how long posts are.
>
>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.
The series resistance of an MMC is far far below the other series losses in
the primary. If my primary runs at 3.1 ohms and the MMC is 0.1% of that...
>
>
>Coiler greets from germany,
>Reinhard
>
Very good post Reinhard!! I think MMCs will quickly obsolete all other cap
types.
Terry
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