[TCML] MMC for 4 mot SRGTC

[bartb]

Hi Kris,

No, you don't have to run LTR. You also don't want to run at or near 
resonance either. Pick something between or think of running STR at 
about 0.1uF or less. For this scenario, ideally use 150 caps for an 
ideal standoff voltage (15 per string, 10 strings). You can of course 
use less caps per string, but I personally would not go below 12 per 
string. That would be 96 caps (12x8). Since resonance is 0.158uF, I 
would suggest staying away from resonance in either direction (LTR or 
STR) by a factor of 1.5 to be safe.

I run STR with my pig. Two reasons why I don't run LTR on it: money and 
too much energy for the gap. As a rule of thumb for myself, I don't 
allow the cap size to get to within a 1.5 factor of resonance. If I did, 
I would risk cap failure.

LTR is for cap safety and for NST safety. MOT's are robust as compared 
to an NST, but still the caps have to be considered. There are benefits 
to running LTR as well, but this is hard to do with large cap sizes 
because of cost. And besides, 0.1uF is near 7 joules at 120 bps! The 
0.4uF size would throw over 14 joules at 120 bps into the bang event 
which would require a serious RSG design to handle the energy. The 
transformer is capable of 28 joules with slower bps. Don't even think 
about it!

So, I guess I'm recommending STR at 0.1uF or less. You also need to look 
at your primary inductance and geometry with the cap size before you 
settle on a value. Knowing your coil is 6" diameter, I guarantee only 4 
turns on the primary with the 0.1uF size. Larger cap sizes would be even 
less turns. You might actually want to go for something less than 0.1uF. 
As a matter of fact, 0.06uF would do well and guess what, requires 60 
caps (5 strings of 12 caps per string). This would be about 5.5 turns on 
the primary, 4 joules at 120 bps, and about 7 to 8 foot streamers 
possible. Not a bad scenario for a 6" diameter coil. BTW, I run 0.06uF 
with a 10kva pig with an 8.5" coil diameter.

Take care,
Bart

Kris Grillo wrote:
> I am putting together a 6" coil with 4 MOT power supply and a 120 bps sync rotary gap.  I would like to run the mots wide open (no ballast). I estimated 8400 V .5A for the transformer output. For the MMC, JavaTC is saying I need .4117uf for LTR! That seems huge to me. I have 60 of the 942's for 5 strings of 12 right now, and I planned on doubling that to get to .1249uf before I get it up and running, but that is still less than resonant. I could probably swing another 84 caps to get to 12 x 12 for a just about resonant cap, but 336 more caps for the .4117uf value is a little out of my price range right now. Is it so important to run larger than resonant cap with a mot driven srg coil? The mots are under oil, if that matters. Any suggestions?
>