RE: MMC advice

["Tesla list" <tesla-at-pupman-dot-com>]

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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