Re: [TCML] DRSSTC tuning at high power

[Michael Twieg <mdt24@xxxxxxxx>]

Yeah I've always been on the watch for detuning as power level changes.  The
weird thing is I observe that as power and streamer length increases, our
measured operating frequency also increases.  Completely counterintuitive;
it should decrease as streamers grow.

Is it really true that a higher K makes the system more immune to detuning?
I can't think of why this would be the case, but I think I've observed it
myself (originally we tried tuning at k=0.1 and had a real trouble at low
power, but at k=0.12 we were able to nail it).  How would I know if the
tradeoff between tuning and voltage gain no longer worth it?

-Mike

On Tue, May 24, 2011 at 4:36 PM, Eric Goodchild <ericgoodchild@xxxxxxxxx>wrote:

> Oh yeah you definitely need to take advantage of the voltage rating of your
> caps and IGBTs. I run a 650VDC to 800VDC bus on my large DRSSTC. The large
> sparks don't start to kick in until I get to at least 600VDC or higher.
> This is partly because my system is detuned so that it won't start creating
> larger sparks until a suitable amount of power is applied.
>
> Eric
>
> On Tue, May 24, 2011 at 8:25 AM, Michael Twieg <mdt24@xxxxxxxx> wrote:
>
> > I always thought k=0.12 was a bit on the high side already.  It should
> > nominally take about 8 cycles and 270us before the notch.  And that's
> what
> > I
> > see at lower power levels.  Interestingly, I see at higher power levels
> > that
> > the primary current notches and the secondary current peaks like 40us
> > sooner.  This seems really weird to me since that time should depend only
> > on
> > k, which shouldn't change with power level.
> >
> > We can raise the primary up a few more inches to get a higher k, and yeah
> > we
> > can raise the voltage quite a bit.  So far we're normally using a bus
> > voltage around 450V, but the PFC as it is can go up to 600V.  We tried
> > running it at that level before but it didn't seem to get us much longer
> > sparks.  Once we get up to tone frequencies above a few hundred Hz the
> > voltage starts to droop anyways because the PFC starts power limiting (we
> > were limiting ourselves to 50A off 240V, which is based on the limitation
> > of
> > our available AC line), so we lowered it down to 450.  If we wanted we
> > could
> > probably take the voltage as high as 800V (our caps are are rated to 450V
> > x2
> > in series, and we're using 1200V igbts on a very low inductance laminated
> > bus).  I think I see what you mean and trying a higher K with a higher
> bus
> > voltage might give overall better efficiency.
> >
> > -Mike
> >
> > On Tue, May 24, 2011 at 1:43 AM, Steve Ward <steve.ward@xxxxxxxxx>
> wrote:
> >
> > > I would try more tuning, ive seen that response before and ive seen it
> > get
> > > tuned out a lot better than what i see here.  Though at the same time
> > that
> > > is a lot of cycles and its hard to keep it in tune on that time scale.
> >  Im
> > > really surprised you need that many cycles of drive even.  From the
> looks
> > > of
> > > it, i'd probably shut off after 5 cycles.  What bus voltage do you run
> > at?
> > >  Maybe you just need more voltage drive and less cycles.
> > >
> > > Steve
> > >
> > > On Fri, May 20, 2011 at 10:08 AM, Michael Twieg <mdt24@xxxxxxxx>
> wrote:
> > >
> > > >  found some primary current waveforms I had taken recently.  They're
> > all
> > > > for
> > > > the same tuning configuration, but at different repetition rates.
> > > > At 208Hz:  http://imgur.com/TpjEY
> > > > at 330Hz:  http://imgur.com/R9ieE
> > > > at 523Hz:  http://imgur.com/pMq2j
> > > > Scale is 200A/V or 400A/div
> > > > You can see how the notch is barely there to begin with at 208Hz, and
> > at
> > > > the
> > > > PRF gets higher it disappears and the peak Ip grows greatly.  The
> first
> > > > 270us of the waveform is being driven by the H bridge, the rest is
> just
> > > > freewheeling.  I can't tell if the power in the secondary is being
> > > > recovered
> > > > or not...
> > > >
> > > > Do these waveforms look reasonable?
> > > >
> > > > -Mike
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