Re: Primary current feedback on the DRSSTC-3

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Hi Steve,

Some comments:


On Mon, 14 Feb 2005 07:24:30 -0700, Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>
>
> >im getting about 30" sparks
>
> Wow, that is hardcore stuff considering the small size of your DRSSTC-3. I'm
> really not surprised that it detunes by 22% :-o
>
>
> >i noticed the primary
> >current had a much nicer linear ring up, instead of the choppy looking
> >current ring up when tuned in the middle
>
> The transient simulation I have predicts this. It also says that the lower
> tuning you're using produces less output voltage for a given primary
> current.

Yep, my simulations say the voltage will be less too, but spark length
is all i care for ;-).

 That explains why you have to turn the DC bus voltage higher to
> achieve breakout, and also explains the "snap" effect. When the streamers
> form, they pull the secondary down to match the primary which has the effect
> of increasing the output voltage, hence reinforcing the streamer growth. So
> your streamers explode out all of a sudden.

Yeah, the best tunings so far need about 75% buss voltage to really
get solid streamers forming.  Running about 500A then.

>
> They don't keep growing infinitely because pulling the secondary down lowers
> the primary current too (and also the real part of the streamer load damps
> the resonance)

Yes, exactly what appears to happen in reality.  Interestingly, i
noticed that my "notch" was dissappearing at really high powers.

>
> You need to be careful going down this road. You'll probably find that the
> tuning for best eventual streamer output produces insanely high primary
> currents prior to breakout.

Oh yeah :).  It sort of makes me think back to Terry's first sim of a
DRSSTC, where he accidentally set an initial condition on his primary
inductor of a few hundred amps effectively.  The spark only occures
near the end of the burst, after the primary has some 500A stored in
it.  Unfortunately, the sparks appear somewhat thinner in this way,
though longer.

 I expect the pre-breakout primary current will
> be what limits how far you can go with the detuning thing. Eventually you'll
> reach a point where the IGBTs explode before any sparks appear at all.

Already did that on accident (no exploding IGBTs though).  In the
midst of tuning, i accidentally clipped onto a turn lower than i
wanted (1 full turn, honest mistake).  Before i knew it, i was reading
off 800A in the primary with almost no action on the toroid =\.
Obviously tuned a bit too low!  I think the ZCS basically makes the
IGBTs much harder to destroy though i still have about 300nS delay in
my driver, so its not exact ZCS, but its close ;-).

I think i will further explore more "inbetween" tunings and also see
how it works on the upper pole tunings, though i suspect it might work
poorly there due to detuning.  Just have to try it and see.  By the
way, Jimmy said he tuned his DRSSTC to the lower pole and eventually
ran out of primary turns as he was pushing up near 100" sparks or so.
He also didnt get much for streamers until high buss voltages were
involved ;-).  I think he mentioned running about 1300A in his primary
at that point (150A IGBTs).

Steve Ward

>
> Steve C.
>
>