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Re: Induction heating in toroid / short circuit of secondary
To Dale Hall, "rwall" and all:
I'll repeat here my perhaps-pedantic comment on "ES" vs. "electric": "If
you mean by ES, electro-static field, I don't think there is one:
"static", after all, means "standing still". That is, a bunch of
electrons just sitting there. Not so, in a t.c. But I'm sure you mean
the more general <electric field."
I'm sure we all mean "electric field": technically; in our case, an
alternating electric field--alternating between + and - polarities
(between deficiency of and surplus of electrons, respectively) at the
frequency of the secondary's self-resonance. None of those electrons are
standing still, of course; they are all rushing up & down in the
secondary coil between earth and the top electrode--eventually to break
out (or in!) in a spark. And any "static" electric field, e.g. in a Van
de Graf machine, must, of course, at one time have been not-so-static,
having increased from 0 to its present level over some duration of time.
How quickly-varying a "static" field must be before someone starts to
call it "dynamic" is anyone's guess. So much for quack-Dr. Herrick's
explanation of electric fields.
As to the magnitude of the magnetic field at the top of the coil, upon
reflection I think it must be of much the same magnitude as at any other
place along the coil. After all, current flows serially, so to speak,
from one turn of the secondary to the next. If the secondary coil itself
is a relatively poor radiator of EM energy, as is alleged, then much the
same current must flow in the top turns as in the bottom ones (or at
least, in those quite a bit further down). Thus much the same
self-generated magnetic field must exist.
If there is current flow within the loop of the toroid, then that current
flow must be induced by a magnetic field. Significant current flow
implies significant magnetic field impinging upon the toroid. From
where?...certainly much more from the secondary than from the primary.
I welcome correction, of course.
I'll take this opportunity to relate a bit of insight--perhaps old-hat to
some but perhaps useful, particularly to solid-state t.c.-ers. Up until
recently, I have been at some pains to limit the duration of the
pulse-bursts that excite my MOSFET-driven t.c.--to just a little longer
in duration than the time necessary to create a spark (maybe 350 usec).
That was so that I could keep the average input power down, allowing for
more sparks/second before I pop a ckt breaker.
I found, however, that the sparks were rather pallid (Do I hear, "Of
course!"?) and subsequently I found that when, serendipitously and due to
electric-field interference with my control circuit, the burst length
increased to ~3 ms, I got nice, fat impressive sparks. So I am going to
"sacrifice" spark rate for fatter sparks, and am currently working to
stabilize my control circuit so that when I have a 3 ms burst length it
is >I< who determine that and not stray fields.
Also, it appears that there is some tradeoff to consider between
achieving a high spark stand-off capability with a smooth/large-c.s.d
toroid on the one hand and providing plenty of pulse-burst power
available (a relatively large primary current) on the other hand. The
high spark-standoff capability will tend to allow a relatively long spark
to initially form. But it is the available "punch" of primary power that
is going to "fill up" that initially-pallid spark with additional energy
for an extended period of time, and likely at the same time extend its
length. The latter, because the tip end of an existing spark
constitutes an extremely "sharp point" from which it is relatively easy
to squirt even more electrons if one has the power available to do so.
I don't have a feel for any optimal relationship between large/smooth
toroid and primary power. Help, anyone?
--And thus a final comment: My recently-advertised "novel top electrode"
might not be so much of an advantage--assuming it were to work as
intended. It may be that a power-punch, so to speak, is more important
in extending spark length than the maximizing of the initial voltage on
the toroid.
Ken Herrick
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