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RE: MMC advice
Original poster: Thomas <tom-at-pwrcom-dot-com.au>
I'd junk those caps, and make a fresh (conservatively rated) MMC.
Dr R's advice about 2 to 2.5 times de-rating the DC voltage is a good one.
I worked out that mine are de-rated by about 1.7x (the RS 22nF 1500V DC
caps, same as you) and run without a problem.
18x 1500V DC = 27kV DC
11kV*sqrt(2) = 15.6kVp
27/15.6 = 1.7
Plus I only use my main spark gap as the safety gap across the caps.
15" is a good start for that size transformer.
> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Thursday, 2 October 2003 08:50
> To: tesla-at-pupman-dot-com
> Subject: RE: MMC advice
>
>
> Original poster: "Mudford, Chris" <chris.mudford-at-agresearch.co.nz>
>
> Hi Malcolm,
> Total value of the MMC is 14.7 nF made up of 4 strings of 6
> individual arcotronics 0.022 uF 1500 V (RS 114-468). By
> calculation the
> primary inductance is ~58 uH where I'm tapping at for my new
> secondary.
>
> When I'm here, after replacing the blown cap, I set my main gap to 2.2
> mm (single static gap, fan forced) and initially set my
> safety gaps such
> as it was just firing at 105% mains input. I had also reconnected the
> mains earth to the transformer as the ground electrode of my
> safety gap
> is connected to here also. When I started up and ramped up
> the variac I
> was getting about 2" from a break out point at 30% on variac, but, the
> safeties were going constantly so I opened them up and tried again. I
> tried to get a "finer" tune but it didn't seem to matter + or - half a
> turn. So I set up an object and pulled up the power, but the MMC was
> getting warm, and suddenly crack, pop smoke flames etc.
>
> I get the feeling my MMC is just too close to the limit and previous
> damage has meant that caps are weakened and with the new
> secondary they
> are just failing. But, just before it failed I was getting 15" to the
> steel end of a fluorecent tube (not grounded).
>
> Cheers, Chris (NZ).
>
>
> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Thursday, 2 October 2003 2:15 a.m.
> To: tesla-at-pupman-dot-com
> Subject: Re: MMC advice
>
>
> Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz>
>
> Hi Chris,
> What is the total value of the MMC and how many strings in
> parallel make it up? Also, how many caps per string? What is
> you primary
> inductance? For the record, I use the 47nF Arcotronics caps and have
> never blown one.
>
> Malcolm
>
> On 30 Sep 2003, at 16:47, Tesla list wrote:
>
> > Original poster: "Mudford, Chris"
> <chris.mudford-at-agresearch.co.nz> >
> > Hi everyone > > Still playing with my little coil (NST 6/30) and
> wound a new secondary > (90 mm PVC, aspect ratio of 6, 1600 turns).
> Connected everything up and > retuned to what looked to be
> pretty good
> at turn 13.5 (this is what I > wanted, as my previous
> secondary lead to
> a tap point of 3.5). However, > my MMC let out smoke again. To
> refresh, my MMC utilises arcotronics > 0.022 uF 1500V (RS 114-468)
> which are meant to be 1800V/us. Although I > haven't heard of anyone
> using these exact ones, somebody on the list was > using .047 uF.
> Across each capacitor is a 33 mOhm 0.5W resistor. It > has been
> running in a 6x4 array and periodically sets fire to the odd >
> capacitor. Admittedly I have been running at resonant 50Hz
> and I did >
> have a safety gap across the capacitor (now removed). I do
> run a filter
> > (as per greg hunters page) and a safety gap to ground (set
> to fire at
> > 105% mains voltage). > > On pulling the blown capacitors
> apart they
> comprise of two seriesed > rolls and there is no aparent
> damage within
> the rolls but a lot of > damage between them. Would this suggest too
> much current? > > When running a safety gap where each HV leg is to
> "jump" to an electrode > which is connected to the NST case, where is
> the NST case connected to? > Should it be mains earth, a separate
> ground, RF ground? On writing this > I realise I have
> disconnected my
> mains earth from the NST case having > had a few electrical
> mishaps on
> home appliances. In which case my > safety gap is probably
> ineffectual. > > Look forward to peoples ideas. > > Cheers, Chris
> (NZ). > > PS I have 72 WIMA FKP1 1600 V (650 V AC) 68 nF
> caps which I
> could > utilise for a new MMC. Would people agree to go for 1.5 x
> resonance and > to still use peak ac supply voltage divided by DC cap
> voltage for the > number of caps per string? >
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