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RE: [TCML] Is tuning different for SRSG than for Static Gap?
There's no need to put MMC's in oil. That was the big advantage of MMC's over rolled poly caps - no more leaky messy oil and bulky containers! Just follow good layout practices and there should be no arcing problems. Maybe better to bite the bullet now and drain it than to wait for a leak.
As far as looking for data about MMC voltage ratings, I think the most definitive post on the topic was an experiment performed by Terry Fritz. He ran three CD942C20P15K MMC capacitors at 1414 VAC (60Hz) which is 2000Vpeak.
http://www.pupman.com/listarchives/2005/Sep/msg00917.html
The capacitance did not degrade until about 75 hours, which is a lot longer than most of us will ever run our coils for. As always, YMMV.
Regards, Gary Lau
MA, USA
> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Brandon Garretson
> Sent: Sunday, March 14, 2010 8:01 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Is tuning different for SRSG than for Static Gap?
>
> I must admit when I started out designing the cap, I had
> indestructibility in mind.
> But I had very little understanding of much else.
> TeslaMap told me 12nf, but I will take real-world experience over a
> modeling program any day.
> I have been planning on taring down the cap and making modular strings
> so I could experiment with different transformers.
> The original open frame MMC I designed had some arcing issues so I
> tanked the whole thing in oil using a small acrylic pet enclosure and
> sealed the top with lexan. It actually looks really cool and fits
> perfectly in the frame of my coil.
> I will search the archives for the peak voltage vs cap size debate you
> mentioned.
> I am NOT looking forward to draining, cleaning and de-soldering 75 caps!
> Live and learn...
>
>
> On Sun, Mar 14, 2010 at 12:00 AM, Lau, Gary <Gary.Lau@xxxxxx> wrote:
> > Hi Brandon,
> >
> > Glad to be of help.
> >
> > You mentioned that with a 12/30 NST and SRSG, you are using a 0.012uF MMC.
> That sounds somewhat on the small side; I would suggest something in the
> neighborhood of .017uF.
> >
> > Your MMC appears to be way over-designed. With strings 25-long, you have a
> 40KV rating, while your peak voltage is only ~17kV. There is debate over how high
> the MMC rating should be relative to the peak voltage, but unless you plan to
> operate your coil for MANY continuous hours, you could do with a lower rating and
> use fewer caps. If you reduce the number of caps per string from 25 to 18, you'll
> arrive at a .0167uF/36kV cap - spot-on for value and more conservatively rated
> than most of us use.
> >
> > Of course changing the cap value will need to be compensated with the primary,
> but you really want to get the cap value right from the start.
> >
> > Regards, Gary Lau
> > MA, USA
> >
> >> -----Original Message-----
> >> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> >> Behalf Of Brandon Garretson
> >> Sent: Saturday, March 13, 2010 8:45 PM
> >> To: Tesla Coil Mailing List
> >> Subject: Re: [TCML] Is tuning different for SRSG than for Static Gap?
> >>
> >> Thank you Gary!
> >> You gave me just the info I needed to move forward.
> >> I will just short out the safety gap and use my TCT to get it close, I
> >> have no problem with that.
> >> I was mostly worried about having to find the sweet-spot within two or
> >> three turns, but I think I
> >> will likely find it within one turn or less if the tuning method
> >> doesnt change with the SRSG.
> >> My coil seems to be pretty sensitive to the tapping point...
> >> As far as the increased cap goes, I actually overbuilt the cap,
> >> acording to "Teslamap", when I started this project long ago in hopes
> >> that I would someday add an SRSG. I just wasnt sure why it neede to
> >> bigger....
> >> My MMC is comprised of 75 .1 MFD 2Kvdc Cornell Dubiliers, 25 in series
> >> by 3 sets parallel for a total of 12nf.
> >> I guess the total capacitance was a little high for the static setup
> >> but should be on the money for the SRSG.
> >> I would assume that voltage shouldnt be an issue as the x-former
> >> currently in use is a 12/30.
> >> Thank you also for pointing out that increasing the topload could
> >> quickly compensate for the size of the primary.
> >> I will keep that in mind if I ever hit a tuning wall.
> >>
> >> As an added note, if anybody is keeping record;
> >> In regards to the Freau phase controller, I am using a 1/13 hp 1.3A
> >> 115V 1800rpm Xerox P/N 127K07993 synchronous motor
> >> made by Reliance Electric and a 2.25A model 10C Powerstat.
> >> It seems to work best with a 67MFD PFC throughout the full range.
> >> Although both the motor and Powerstat do groan a little throughout the
> >> middle part of the sweep.
> >> I freeze-framed it with a strobe light (propeller type gap by the way)
> >> and was able to achieve close to 45d of phase shift.
> >> More or less capacitance doesnt seem to change the shift much, only
> >> seems to effect voltage.
> >> With 67MFD I get a 5V rise at aout "25", and at "98" on the Powerstat
> >> it runs well at about 90V, at "100" it starts to stall out.
> >> I also hooked up the transformer (only) to the SRSG with the strobe
> >> flashing on it and was able to watch how the spark reacts before and
> >> after the stationary electrodes while shifting the phase.
> >> Thanks to my background knowledge of automobile ignition systems it
> >> came as common sense to me that I would want the timing to be slightly
> >> retarded as the arc occurs prior to the electrodes centering.
> >>
> >> Thank you again for the info. I have been reading the Pupman archives
> >> for about two years now and Im very happy with the answers to my first
> >> posted question!
> >>
> >>
> >>
> >> On Fri, Mar 12, 2010 at 7:54 PM, Lau, Gary <Gary.Lau@xxxxxx> wrote:
> >> > Hi Brandon,
> >> >
> >> > As far as tuning the primary & secondary resonant frequencies, no, the type
> of
> >> gap wouldn't matter. Despite having all sorts of test equipment available, once
> I'm
> >> in the ballpark with the design, I always use trial & error to select the best
> primary
> >> tap point.
> >> >
> >> > You should realize that when moving from a static gap to an SRSG, the
> primary
> >> cap value needs to be higher with the SRSG, or else you'll overvolt the cap &
> NST.
> >> So with the same NST and secondary system, you'd need a larger primary cap
> >> and comparably smaller primary. Of use the occasion to upsize your topload to
> >> compensate.
> >> >
> >> > I'm glad to hear that you're using a Freau phase controller. While technically
> not
> >> tuning, I find that the RSG phase is extremely critical, and I use the phase
> control
> >> much like a Variac - set it to an early phase, then slowly advance it until it "goes
> out
> >> of whack", then back off a bit.
> >> >
> >> > Regards, Gary Lau
> >> > MA, USA
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