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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?  >
 > ==========================================================
 >
 >
 >