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Re: [TCML] gaps
Hi Bart,
And, with gaps and NST's, YMMV... :^)
bartb wrote:
Hi Bert,
Excellent assessment as I always expect to see in any of your postings!
Your first two paragraphs sort of stuck out to me and didn't quite match
my experience (the note about the most efficient static gap appearing to
be a single gap). For that particular issue, I disagree. If within a
single or very quick time frame, then I would agree. But there are
various types of coilers (and by various I mean how they run their
coils). For me, I like to run "long runs" and for others, 20 seconds is
a long run. For "those" extremely short runs, a single gap would be
efficient, but for those of us that run the 5 minute or longer runs, a
single gap will never do well except maybe at low power.
From a practical standpoint, I agree with you. I've also had very good
results using multiple static gaps. As you suggest, the challenge with a
single gap is cooling. Simply using high velocity air flow is not
sufficient, particularly at higher power levels. A successful high power
single gap (be it sucker type or air blast) requires using comparatively
massive electrodes (i.e., low thermal resistance and high thermal
capacity) combined with sufficient overall cooling. The electrodes need
to be comparatively massive, with little distance separating the active
region and the main body. The key is to prevent forming incandescent
regions on the electrodes since, once this occurs, the gap then loses
its ability to efficiently quench, and coil output plummets.
Static gaps often use thin or pointed electrodes. Since either have
higher thermal resistance, overheating may occur within seconds. The
electrode tips simply can't get rid of heat quickly enough, even with
heroic levels of air flow. To achieve low thermal resistance, electrodes
must be comparatively large and made of copper or copper tungsten alloy
so that heat is rapidly extracted from the active region. Cooling air
then removes heat from the larger electrode assembly as well as removing
hot gases from the gap itself.
The gap has to find a thermal equilibrium. That's when heat
dissipation/time becomes important for an efficient static gap.
I agree. It ultimately comes down to power level, thermal resistance,
thermal capacitance, and sufficient air cooling. You still reach
equilibrium, but at a lower electrode operating temperature, with better
quenching.
A single
gap would only do well in this situation at low power.
It ultimately depends on the robustness of the gap design. When properly
designed, a high power single static gap can work very well, even at
high power and high rep rates.
BTW, there have been
Higher powered
NST coils need quick thermal stability. For myself, I've found largish
copper tubing to be good for a static gap.
I've seen the same thing. And, I suspect that most coilers would see
better performance when using a typical multiple gap versus a typical
single static gap. But I still contend that a properly designed single
gap can ultimately outperform a multiple gap by a measurable amount.
It's quick to heat but quick
to dissipate also. With the right air flow, rather high current can be
used efficiently. I have tried the same air flow on 1/2" tubing and the
gap was lousy! Then tried the same on solid stock. Great at first, but
then suffered immense losses due to heat build up in the material (about
15 seconds into the run, efficiency dropped like a rock). Surface area
makes a big difference. But even 1.25" tubing surface area without the
right air flow is no better than small 1/2" tubing. The air flow and
tube size are very important for an efficient static gap that will rival
the best SRSG.
I agree, and bigger is usually better... :^)
As far as "chaotic firing" of static gap systems, I "sort of" disagree.
It is a bit "higher" bps than one might calc, but I have not found it
all that high. There is a pattern to the break.
Interesting. I've found that at some variac settings, the gap "sort of"
synchronized, showing similar sequences of firing patterns over 2-3
mains half cycles. Then, at other settings, there seemed to be no
discernible pattern, while input mains current fluctuated wildly.
However, I made these measurements on a system that used a mains
resonant tank cap, and that may also have worsened the degree of
instability. It wouldn't surprise me to hear that LTR systems exhibit
more uniform behavior.
Because of that, it's
not so "chaotic" in my measurements (so far). I still need to measure
current and bps concentrically to get a handle on it. My most basic
thoughts are current is dramatically increased (ferro resonance?), the
gap breakdown voltage decreased for a moment (?), or the gap had a
transient occur resulting in a hv situation that forced breakdown.
Always, this occurrence occurs directly "after" an expected break down.
This is the not so chaotic behavior. There is a reason for it. It's just
a matter of determining if it is a sudden current or a voltage
controlled issue.
I mostly agree. Seemingly chaotic behavior oftentimes has underlying
structure. We may be seeing something closer to "deterministic chaos". :^)
Take care,
Bart
Best wishes,
Bert
Bert Hickman wrote:
Hi Mike,
As with life, many Tesla Coiling questions have no simple black or
white answers... :^)
A well-designed static spark gap can provide excellent performance.
The most efficient static gap appears to be a single gap with high
velocity air flow (a "sucker gap" or air blast type). Because of
individual voltage drops within the gaps, multiple static gaps can
have higher overall voltage drops (thus being lossier), but they can
have superior quenching capability at less than heroic air velocity.
Under similar quenching conditions, the single gap may slightly
outperform a multiple gap that has a similar breakdown voltage. Static
gaps are also recommended for first-time coilers for reasons discussed
below.
Static gaps can't wring out maximum performance in NST systems. Static
gaps tend to fire chaotically when used with an inductively
current-limited transformer (such as an NST or ballast-limited pig).
This causes multiple bangs during each AC mains half cycle that are of
varying (and suboptimal) size. In order to get maximum spark length
versus input power, best results are obtained using a synchronous
rotary spark gap (SRSG), or a synchronously triggered static gap,
combined with a larger size tank capacitance (see below). A properly
adjusted SRSG consistently forces the gap to fire so that each bang is
of the same, optimal, size - once on every incoming half cycle of the
power mains.
In earlier days of coiling, many folks destroyed their NST's when they
converted from static to rotary gaps. These were also the days when
the accepted practice was to design coils where the tank capacitor was
"tuned" to resonate (at mains frequency) with the NST's leakage
inductance, a practice known as "mains resonant charging". Indeed,
many TC design tools defined this as the optimal tank capacitor size
for a given NST voltage and current. For a 15 kV NST, this was about
0.01 uF for every 30 mA of output current for a 60 Hz supply.
Unfortunately, if the main gap was set too wide, the rotary gap was
improperly adjusted, or the RSG was merely running too slowly, the
voltage cross the NST could rapidly grow to ridiculously high
voltages. This usually resulted in overvoltage failures of NST's, or
for pig-driven systems, tank capacitors.
Over the years, it was learned that, by using a larger sized capacitor
(called Larger Than Resonant or LTR), mains-resonant overvolting could
be avoided. Adding a properly adjusted safety gap placed directly
across the NST output terminals protects the NST from overvolting
under other/abnormal circumstances. Terry Fritz's "Terry Filter" adds
an additional layer of protection, especially against high speed
transients that can cause inter-turn corona damage within the
outermost turns of the NST windings. It even protects against an
improper safety gap setting.
The bottom line line:
A system using a synchronous rotary gap, an LTR tank cap, a properly
set safety gap, and a Terry Filter is every bit as reliable as a
system using a static gap. The SRSG system also provides higher
performance than a similar static gap system.
Bert
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