RE: MMC advice

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Chris,

On 1 Oct 2003, at 16:50, Tesla list wrote:

 > 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).

On the face of it, the MMC looks OK if a bit marginal on the voltage
rating side. The clue would seem to be the cap heating which together
with the 2" output suggests the coil wasn't in tune. I have been
running 15 strings of 7/string 47nF at about the same peak voltage
(8.5kV) into an 18uH primary running 130kHz without a problem and
just a tiny bit of heating (maybe from heavy conducting wires running
heat into it from the gap mounting blocks).

Malcolm