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Re: Check Your MMC Bleeder Resistors?
Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
Hi All,
I didn't get much response to the post about checking MMC bleeder resistors for
resistance changes, but no one seems to have noticed any problems. - Terry
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Date: Sun, 08 Jul 2001 11:47:48 -0600
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From: "Ed by way of Terry Fritz <terry-at-hot-streamer-dot-com>" <edtrind-at-hotmail-dot-com>
To: terrellf-at-qwest-dot-net
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Subject: Bleeder Resistor check
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Hi Terry,
I checked my bleeder resistors on my MMC this morning.
I am using the same ones as you from Digikey.
My MMC consists of 5 strings of 14 Panasonic caps.
I measured 2 resistors from each string (in circuit). All measurements were
within the range of 9.38-9.49 M-ohms.
I also measured the total resistance of the MMC .... it calcs at 28 M-ohms, and
measures 27 M .... same as it did the day I made it. Measured with DMM.
My MMC has seen 115 minutes of use with a 12/60 NST (and I believe managing to
suck more out of the NST with my new Vac. gap.. ..Input current to NST is
13.5 amps (panel meter), 140 Volts).
The caps do get noticeably warm.... measured external cap case temperature at
107 degrees (non precision measurement) after 5 minute runs, so I imagine the
insides are warmer.
Hope this helps, and hope things are well with you,
Ed T.
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Date: Sun, 08 Jul 2001 18:26:03 -0600
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From: "Sundog by way of Terry Fritz <twftesla-at-qwest-dot-net> by way of Terry Fritz
<terry-at-hot-streamer-dot-com>" <sundog-at-timeship-dot-net>
To: terrellf-at-qwest-dot-net
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Subject: Re: Check Your MMC Bleeder Resistors?
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Hi All!
I got a ton 'o MMC here. I checked all the resistors I could reach (i
didn't really count on having to measure 'em), and aside from the dead ones,
they're all within specs. They measure a little low, as normal, but the low
reading is consistant across 'em all (i measured over 100 resistors). As
for the level of abuse they've gotten, everything from a 4/30 all the way up
to a 14.4kv pole pig at 8kva, when RMS current got the best of a few
strings. Oopsie!
So my un-professional opinion, the resistors (I'm using the Digikey ones
Terry sent with the panasonic caps) are good to go till they physically
smoke-test. Mine only died as the strings died and at that point, the
fireworks showed through :)
Some considerations,
- make your solder joints as smooth as possible. I had some arcing because
of the little points I neglected to smooth on my solder joints.
- twist the leads. Yes, it *IS* a PIA for a big MMC, but I've had some
crazy current across my MMC, and it was enough to de-solder the primary tap
on long runs. (wholy cripes!) I would not want my MMC to start falling
apart.
- put a resistor on every cap. I tried to cheat and put one every other
cap. I got jolted for my trouble.
- do keep some space between the caps, about 1/4" or so. I crammed some of
mine together to save space, but it makes visual inspection hard when you
see and hear a failing string, but can't find the caps that are split open.
and the panasonics can have most of their shells split open and the core
melted and *still* test good. Trust me on that one.
I think that's got it all :)
Shad
----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, July 07, 2001 10:42 PM
Subject: Check Your MMC Bleeder Resistors?
> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
> Hi Ed,
>
> At 11:36 AM 7/7/2001 -0700, you wrote:
> >
> >Terry
> >
> > I still worry about deterioration of those resistors due to operating
> >at much beyond their rated voltage. Has anyone ever measured any after
> >a few hours of operation? In my experience they usually fail open, an
> >unsafe thing if you're expecting them to discharge the bank.
> >
> >Ed
> >
>
> I just checked the resistance values on my coil's MMCs. The big coil has
> not gotten much use in recent years but my small coil with it's EMMC
> certainly has. The resistance values are all within 2% of their markings
> and they are 5% to begin with. So, I cannot detect any deterioration.
>
> However, maybe it is a good idea to ask ALL users of MMCs to check their
> resistors just to be sure and see if there are any problems out there.
>
> One can simply ohm across each resistor with a meter that reads resistance
> high enough for the resistance being measured. The values you read will
> look lower than spec since they are in circuit with the rest of the
> resistors in the array. I think the formula is:
>
> Rm = 1 / (1 / (Rr x STRING / (ROWS - 1) + Rr * (STRING - 1)) + 1 / Rr)
>
> where:
>
> Rm = Measured resistance of a single resistor
> Rr = Rated resistance of a single resistor
> ROWS = Number of rows in the MMC
> STRING = Number of strings in the MMC
>
> Of course, the resistance tolerance rating gets into that too...
>
> That failing, the resistance will look "around" 5 to 10% low... If any
> resistors measure "above" their rating given the tolerance, they are
> definitely bad.
>
> I would ask all users of MMC with bleeder caps to please check them to see
> if the resistors are holding up ok. If your's are good or bad, please let
> me know either way at the e-mail address below so we can all get some idea
> of the number of people that did this and their results. I will report
the
> numbers back to the list in a few days. Also include a brief description
> of your MMC and how much use it has had. Hopefully, we can get a real
good
> idea of how they are doing and if we need to be concerned or not.
>
> terry-at-hot-streamer-dot-com
>
> Cheers,
>
> Terry
>
>
>
>