Re: OLTC update

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "tesla by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla-at-paradise-dot-net.nz>

Hi Terry/List
The idea of tuning by selecting the Number of IGBT's operating seems great.
You could possibly have "tuning " IGBTs which have much smaller caps which
are selected just to tune the coil.
Best
Ted L in NZ
PS thank you for sharing your work with us all
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, August 12, 2002 7:05 AM
Subject: Re: OLTC update


> Original poster: "Paul Nicholson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
>
> I guess the main difficulty with the OLTC is that a large primary
> cap is required in order to get a decent firing energy.  Thus an
> impractically small primary inductance is called for.
>
> Barry wrote:
>
> > Would it be easier to build a secondary that couples to a
> > higher harmonic of the primary?
>
> Hmm.  Putting the energy into a primary (or any coil) by
> connecting a charged cap across its ends tends to excite the coil
> at its lowest resonant mode, which is what we'd call its Fres.
> Only a little of the firing energy goes into higher modes
> of the primary.
>
> To excite the primary at a higher mode (not a harmonic), one way
> would be to simultaneously discharge several caps, strategically
> placed along the winding, alternated with ground connections.
> The initial firing energy would be shared across all the caps.
> This would seem to be feasible given the use of semiconductor
> switches.
>
> Apart from the difficulty with tuning, so far so good.
>
> The problem then is to couple effectively to the secondary. Now
> with the primary at a higher mode, current in parts of the primary
> will tend to be pulling against current in other parts of the
> primary as far as the inductive coupling to the secondary
> quarter wave is concerned.  And as the mode number gets higher,
> the overall cancellation of the field from the primary becomes
> more complete.
>
> Therefore, it would be necessary, in order to obtain a cooperative
> field from each half-wave current segment of the primary, to
> reverse the winding sense between each firing cap.  But doing so
> would destroy the transmission line properties which were giving
> you the particular mode, and the system degenerates almost into
> a set of parallel primary windings, each with its own personal cap.
>
> So to persue Barry's rather interesting idea, I think it would be
> necessary to make sure that the secondary was closely coupled to
> only one half-wave's worth of the primary, and that the rest of
> the primary contributes relatively less to the coupling.  Acmi
> can be used to model the coupling to a secondary from a primary
> which has caps placed along it's length, so long as these caps are
> large compared to the self-C of the primary itself, which it's
> safe to assume.
>
> Note that this might not be possible with traditional spark gaps
> because of the need for simultaneous switching.
>
> Apart from tuning, I think the other main problem with this
> approach would be how to ensure that large amounts of energy don't
> converge onto a single cap of the primary, which it's perfectly
> free to do so once the thing is released into resonance - given
> unpredictable reflections from the top of the secondary.  We
> wouldn't want any of the caps to suddenly find itself carrying
> a whole lot more energy than it started out with.
>
> But then we do have scope for controlling the timing of the
> switching, so maybe things could be controlled automatically
> somehow.  If you get into that sort of territory, you can
> conjecture building up a traveling pulse waveform along a primary,
> by firing caps consecutively, so that the energy is built up
> into a single broadband pulse.  You'd set things for a steady
> impedance transformation along the way, and the primary would
> couple to or merge into a secondary, which would continue the
> impedance transformation (by some cunning choice of coil profile)
> to give a single huge voltage pulse at the top.  Picture a deep
> atlantic wave approaching a beach, rolling up in the shallows,
> trading width for height.  The deep sea wave sees a gradual
> impedance change as the beach applies some extra boundary
> conditions.  Surfers know the beaches with the best profiles -
> wonder what shape of coil would turn a long duration, low-voltage,
> high-current, broadband pulse applied at one end, into a short,
> high voltage, low current pulse at the other.  Note that this is
> quite unlike a normal TC.  We're now firing up multiple resonant
> modes, and timing them so that they momentarily converge to a
> single giant voltage pulse at one end (preferably the far end)
> of the coil.  But before anyone gets too excited, there are some
> problems with this approach and it's not likely to give you any
> more topvolts per joule than a regular TC, if my sums are correct.
>
> But, I think I like Barry's idea, because it opens up some
> interesting possibilities.  Whether it would bo any better than,
> say, a normal primary with lots of separate caps and switches
> paralleled across it would have to be considered.  All in all,
> I'm following this OLTC stuff with great interest.
> --
> Paul Nicholson
> --
>
>
>
>