TC as pulse transformer

From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Saturday, June 06, 1998 3:00 PM
To:  Tesla List
Subject:  Re: TC as pulse transformer

Tesla List wrote:
Jim and all,

My comments are interspersed below. Very interesting thoughts, Jim!

> ----------
> From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
> Sent:  Friday, June 05, 1998 2:34 PM
> To:  Tesla List
> Subject:  Re: TC as pulse transformer
> > > At some point, particularly as the Ctop gets big, can't the TC be
> > > considered as a air core pulse transformer.
> > >
> >
> > A closer model seems to be a dual-resonant air-core transformer because
> > of the way energy actually transfers from the primary to the secondary
> > over a number of half-cycles of Fo.
> Only if k is low. Why not make k=1?

The main inhibitor appears to be the great difficulty in electrically
insulating the primary from the secondary while also achieving a high
turns ratio. A k of about 0.8 seems to represent the limit which
balances coupling and insulation limitations for a high turns-ratio
air-core transformer. However, by using tapered coils, E-field shaping,
complete oil immersion, and similar tricks, coupling coefficients of 0.6
are routinely achieved. Now a dual-tuned resonant transformer with k=0.6
will transfer virtually ALL the primary's energy to the secondary within
two half-cycles of Fo. This operating mode has the distinct advantage of
permitting primary commutation at a zero current and zero voltage point
- in other words it's possible to do with physically realizable
switching devices on a repetitive basis.
<SNIP>> >
 Even if we stopped the process at 1/2 cycle (at the
> > first point of zero primary current), only a relatively small portion of
> > initially available primary energy will have been transferred to the
> > secondary.
> But, you don't need to have a low k. That is just an artifact of how
> TC's have been built in the past, probably due to quenching
> requirements, etc.

I agree. However quenching becomes a formidable problem at higher values
of k. It's likely that large back-to-back hydrogen thyratrons could
handle the standoff voltage, large tank circuit currents, and still be
able to recover in time to block energy return from the secondary. It's
unlikely that any gaps operated within STP air will recover
sufficiently. At least with this mode we don't have to also contend with
a blocking a huge voltage spike while attempting to commutate maximum
primary current...

> But the di/dt is exactly what you want, that is what produces the high
> voltage on the secondary. You want that energy to couple into the
> secondary.

Whether you "bang" the secondary with a di/dt impulse, or do a smoother
dual-resonant transfer of energy over one cycle of Fo, the maximum
voltage on the secondary is still constrained by the "bang size" in the
primary and the total output capacitance of the secondary. Whether the
secondary is shock-excited or resonantly excited, the peak voltage will
be about the same. Oscillatory behavior will occur in the secondary
unless we can somehow "load it" to such a point that it becomes
critically damped or under damped. A power arc to ground would
accomplish this. Simple air streamers will not, since non-connecting air
streamers seem to result only in underdamped behavior irrespective of
the output power level. Operating the coil in a more easily ionized
media (argon, neon, or helium?) might do it, however...

> If only 10-25% of the lines engage the secondary, then the k is low (.10
> to .25??). Why not build a coil with a k of 1, which is an impulse
> transformer.
> >Practically speaking, [barring a single-shot explosive-type opening switch] it's not
> > possible to open the primary circuit at a current maximum with any
> > switching devices typically available to coilers, including
> > semiconductor or gas tube devices.
> Well, of course, that IS the problem....

A physically realizable compromise might be k=0.6...


> However, this would seem to be more in the fashion of generating
> > repetitively arrested streamers, each one following part of a weakenned
> > trail blazed by its predessor, with far ends "blindly" searching for
> > ground.
> It is unlikely that a streamer can repeat the path of a previous one.
> The ion recombination time in air is very fast (on the order of
> microseconds). Also, the spark leaders in a typical tesla coil are small
> enough in diameter that they cool almost instantaneously after the
> current is removed.  More likely, the apparent behavior is due to the
> sparks tending to follow Efield irregularities, and in a statistical
> sense, they tend to always go in the same place.

This may be a possibility as well. Whatever the reason, at least the
"roots" of the streamers seem to follow similar paths bang after bang.
The dielectric recovery time of air is almost certainly longer than a
few microseconds, otherwise main-gap quenching would be considerably
simpler than it seems to be.
> In the case of lightning, you do see current flowing through the channel
> in between strokes,and the strokes making up a flash are essentially in
> the same place. However, lightning dissipates something in the area of
> 100 kJ/meter, which makes a fairly large column of heated air, which
> tends to cool more slowly than the very thin sparks of a tesla coil.  It
> is the classic surface area/volume thing.

Interestingly, in the case of lightning, this current seems to be
attributable, in part, to the discharging of the distributed charge
associated with the path itself, and the various tributaries/forks.

> In some exploding wire experiments, I (and others) have created multiple
> sparks in the same ionized channel, but my energies were in the 10's of
> kJ/meter range, and my "strokes" (to use lightning terminology) were
> separated by a millisecond or so.

I've seen similar behavior as you've described above for exploding wires
during power arcs from a Tesla Coil terminal to ground. In a medium or
larger coil, the peak currents appear to reach hundreds, or even
thousands, of amps, and are separated by the typical 2-3 millisecond BPS
rate. These successive arcs follow virtually the same path for large
numbers of bangs. This also would seem to imply that air does not
recover its dielectric strength even after after 2-3 milliseconds...

-- Bert --