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Re: Double humpin'

Bert, Richard,
                A few observations from the scope...

> > I get the impression that some of our folks think that you get a double
> > hump or spliting of frequencies at tight coupling.  Double humping only
> > occurs in spark systems when we exceed the point known as "critical
> > coupling".  critical coupling has only a little to do with
> > actual inductive coupling (about 50%)
> > 
> > If we throw a fixed gap of a fixed dwell/quench in a system, critcal
> > coupling occurs at some fixed coupling coefficient K=X.  If we have a
> > variable dwell/quench gap, and a fixed tight coupling, by varying the
> > dwell we can make the system go from  below critical coupling to well
> > beyond.  In short, critical coupling is a sliding point based on actual
> > inductive coupling and dwell/quench time of the gap's realizable
> > quenching ability.  In theory we can have a single frequency output (no
> > splitting) at k=.65.  This was the struggle in the early days of spark
> > transmitters in the 100KW-.5MW class.
> > 
> > Richard Hull,TCBOR
> 
> 
> Richard,
> 
> Excellent point Richard! Seems like every time I think I'm beginning to
> understand how these things work, mother nature, or you, slap me upside
> the head with a dose of reality!
> 
> A lot of coilers, myself included, have _lousy_ gaps which quench poorly
> and non-repeatably. With longer-than-desired quench-times, most of us
> seldom observe the phenomenon you describe! I needed to do a series of
> PSPICE simulations to confirm this - I have NOT been able to confirm it
> experimentally, since I don't have my high-speed rotary constructed as
> yet (a Winter project...). 
> 
> The PSPICE simulation shows that, if the gap dwell-time is reduced to
> the ideal time or somewhat less (i.e., the end of the first beat IF the
> gap were to continue firing), the double frequency humps do, indeed,
> disappear! Even if the primary is set to a somewhat lower frequency, the
> secondary/toriod will ring up at its single natural frequency ONLY. 
> 
> This seems to imply that frequency splitting and secondary coil
> flashover _could_ be reduced by consistently quenching quickly enough.
> Previously I believed that secondary flashover was caoused by the
> secondary being driven to the upper "hump" frequency, which in turn
> caused the 1/4 Wave point to be lower on the coil. NOW, I'm no longer
> sure that this is actually the case! Or maybe splitting only occurs
> during subsequent beats (at lower energy??), but this doesn't sound
> right either!! Hmmmm!! What seemed to be so simple before, turns out now
> to be another layer to peel on the TC onion!

Attached sparks produce practically the same result (ideal dwell 
timewise) as optimally quenching which is why it seems largely 
unnecessary for what we are doing. I guess if the topload ROC and 
capacitance is _just_ big enough, trapping the energy in the 
secondary using optimum quenching would produce a better result.
That seems to be borne out by Richard"s experience with the huge 
toploads.
     While the amplitude of the oscillations is changing in the 
coupled system before quenching, a split spectra must IMO be produced
(during ringup). Could you check the spectral output on SPICE Bert?
I still think the top frequency is exciting the resonator in ways
we don't want during ringup.

    Meanwhile, here are a couple of thoughts to consider on critical 
coupling. In a CW system, it is easy to arrange this by simply 
adjusting k=kc where kc is Q dependent and the primary and secondary
impedances are well defined and fixed. At critical coupling, primary 
losses=secondary losses.
    How about a disruptive system where the Q's are high before 
breakout and k>>kc? The scope shows the ringdown/ringup is relatively 
lossless compared with the losses when a discharge breaks out. Energy 
transfer is a good deal more efficient than critical coupling. During 
this period and under these conditions the system is definitely 
overcoupled. 
    Suppose we now quench at the end of ringup. We cannot say the 
system is critically coupled at this point because to all intents and 
purposes, coupling between the two circuits no longer exists. 
Moreover, secondary losses will be far higher than primary losses in 
toto. 
    Suppose a secondary discharge starts while the system is still 
ringing up (scoping suggests this will be the case for most coils, 
particularly those without a very large topload). Under this 
condition, any secondary energy not participating in the discharge 
will, as usual, be reflected back down from the top, but it will now 
be with a phase shift as the top end impedance has changed (zero 
phase shift for the unloaded case). Now the still-coupled primary 
"sees" an impedance different than that which it saw when the 
secondary was unloaded. Under this condition, the primary can no 
longer unload its energy at the same rate as with the unloaded 
secondary case and dissipates more as a result. Here we are 
approaching critical coupling IMO.
    I've written this mainly to clarify my thinking. Any other 
thoughts on this most welcome.

More melting pot stuff,
Malcolm