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Re: Terry's DRSSTC actually hooked to a coil now >:-))

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Steve,

At 02:29 PM 2/1/2005, you wrote: 
Hi Terry,


> > >by following primary current, you just
> > >"ride along" with the frequency...
> >
> >Yup, this is exactly what happens. But you have to be careful because there
> >are two possible "rides" the coil can go on. If you design the system to
> >give the right voltages and currents on one of these paths, they may be all
> >wrong if it switches to the other one.
>
> I guess I am not understanding what the "two paths" are here. The two coil
> system has primary and secondary resonances which beat against each
> other. One small and the next larger "notches" due to this beating can be
> seen here:

The 2 "rides" are the upper and lower pole of the frequency split that
occures with all TCs.  The idea as i understand it is that you want to
tune the primary to be "suggestive" of what frequency the system
should lock onto.  If you find the lower pole more desireable for some
reason, it helps to tune the primary extra low so that you minimize
the chance of it changing to the upper pole.  I think the problem with
tuning in the middle is that it might flip one way or the other,
perhaps unpredictably, and i think the problem with that is that its
not really optimized for either frequency of operation.  It may also
change poles during the burst, which might lead to some problems (this
just gave me an idea of something i need to test).  So its best to
pick a pole and stick with it i think.

Better let Steve C. come and clean up my partial understanding here ;-)

I think this is a Phase Locked Loop or external oscillator timed system issue... If I have the IGBT timing run off an oscillator and the primary frequency suddenly shifts 180 degrees without the oscillator shifting as well.... It blows up!! If the PLL cannot track the frequency fast enough, that could be real bad indeed!!

But in my case, I run directly off the primary current and if it wants to shift frequencies around quickly the IGBT driver shift right along with it. I can go from 200kHz to 100kHz instantly. Or from 50kHz to 250Khz "instantly". So no problem ;-)) My driver has no predetermined frequencies so the drive frequency is totally up to what every the coil system wants whenever and however it wants it.

Normally I would tune the primary about 5% lower than the secondary to compensate for streamer loading just like in a normal TC. But with an oscillator controlled system, one may want to pick a pole for a "starting" frequency. But I don't think that frequency would be maintained well once sparks start to hit things. However, Steve Conner's PLL circuit is very capable!!


>
> http://drsstc.com/~terrell/pictures/FullSystem-01.gif
>
> I should have let it go longer and you could see lots of beats.  Between
> the two "notches" you can see where the current (yellow) shifts some.  The
> frequency certainly is not a stable value but bouncing around.  But these
> are perfectly normal for a primary current driven system.  The secondary
> current driven system like Dan's is much different in that the beating is
> in the drive signals where I think it "beats up" the IGBTs more since the
> zero current crossing miss in that case.

Well, possibly.  Its hard to say what is better yet until someone
makes primary feedback work properly with no explosions ;-).


Mine has not blown up "yet" :o))

Ive
tried primary feedback on several occasions, but each time found some
small quirk that scared me off.  Most recently i found that using
primary FB and making sparks to ground causes something in my
controller freak out, turning the coil ON for many mS!!

I have current limiting too which protects against the controller going nuts. The fiber-optics also keeps noise away from it. but in general, I think it is best to have "two" or a "backup" protection incase say the controller goes nuts... Of course, more easily said than done ;-)) But I think current trip out is really important!! If I over current, it trips and retries about every 500mS. Might as well be off... I don't think it would work into a dead short though, the di/dt would be too fast. But that is an obvious goof case.

.....

I think you should be able to tune the primary system to stay at just
one frequency, but i can not be certain of this.  I dont have
measuring equipment capable of accurately detecting if the primary
current was jumping from F1 to F2 like you do ;-).

No. I think the two pole system will always dance between the two frequencies. I don't think you can stop that unless you give up zero current crossing as in the case of secondary feedback.

....
> Right now primary and secondary are tuned to 85.1kHz and tuned to within
> 0.1kHz of each other.  It is trivial to mistune, but I just don't see
> "why"?  If the frequency want's to shift, the driver does not care.  I have
> no local oscillators or anything controlling the H-bridge.  It is totally
> timed from the primary current.  It could go from 1Hz to 100kHz and then to
> 200hZ and the driver would not care (1Hz would probably drain the filter
> caps or hit current limits).  Perhaps in the case of an oscillator
> controller like your PLL controller, such shifts are far more of an issue?

I think you just want to generally avoid this because it seems
unstable.  I would also like to hear exactly why this is a "bad"
thing.  But i think its been shown that DRSSTCs produce the best
sparks when operating at one of the pole's and staying there.

"In my case" I just don't think it is an issue. Even if it were band limited chaotic from say 50kHz to 200kHz, my controller would just track right along with it without a care. If I were driving off a PLL, it would indeed blow up since I could not change the PLL frequency fast enough.


>
> If the secondary took a heavy hit, it's oscillations could certainly be
> shifted out of sync with the primary or totally disrupted.

Hehehe... my nightmare.  Those CM300s sure didnt react well to these
extra chaotic situations!

If they don't blow up, they take them perfectly!! That is the advantage of the big silicon!! It can live through such disasters.

Problem with ground strikes and my
particular setup was the secondary jumping to a harmonic (3X Fres
roughly) and getting a mess of "bad" switching transitions fed into my
driver... lots of hard switching and at 3X the normal operating
frequency... just terrible really.

That is bad... The secondary is sort of a "light weight" oscillator and it can be pushed around by streamers and say racing arcs. The primary current is very hard to goof up so that's why I like to use it.



Well, the primary might ring up to some higher than normal currents,
but hopefully your over-current detector catches that.


;-))

.....
>
> I "think" it is riding both poles and the resulting primary current
> regardless of frequency or poles and zeros.  The driver just does not care
> what the frequency is or how much it shifts from cycle to cycle.

Dunno what to think of that yet.

When the theory gets too hard for me to understand, thats when i go
build something ;-).  Still lots of experimentation to be done with
the DRSSTC stuff!


Cool!!  I think I understand the "two modes" issue now *:-)

I don't think it is an issue at all for the type of drive I am using.

Thanks!!

Cheers,

        Terry



Steve W.


> Cheers,
>
>         Terry
>
> >etc
> >
> >
> >
> >Steve C.
>
>