Re: LTR Pig Project - Long Post!

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 2/16/01 10:20:28 AM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

> Hi Shad, John Freau, ...

>  ...your LTR pig project is just "after my heart": I like it! I
>  follow John Freau's efficency design rules with strong interest,
>  accepting (and even admiring :-) ) the good mix of theoretical
>  argument with experimental proof. Part of it was leading me in
>  the design of my bigger coil B&W. John's approach proves to look
>  very successful for (mainly-) NST powered coils of moderate
>  power, up to a few kVA. 

Kurt, Shad, all,

Well, thanks Kurt for those nice comments.  If I say anything below 
that is known to be false, please let me know.  I don't want to be
clinging to incorrect beliefs   :)

A couple of good examples of low bps large coils are;  Kevin
Eldredge's Biggg Coil, and Greg Leyh's Electrum.  But yes, it
does appear that neither of them use reso, or LTR caps.  In
Greg's case, it doesn't matter, because he can get a good power
factor anyway because of the DC drive.

Kevin's coil runs at 120 bps with large caps, and gives 24 foot
sparks.  Greg's coil can run down to around the same bps, and
gives about 50 foot sparks.  Regarding LTR operation;  I don't
consider LTR to be a main aspect of my designs.  Rather it's 
an incidental point.  LTR is good for NST's because it keeps 
the voltage lower.  Some degree of LTR is good for low bps
pig coils because it keeps the power factor good, at 120 bps.
But I only use a small degree of LTR for PT or pig coils, maybe
25% LTR or so.   I use
the degree of LTR that gives the best power factor, and smoothest
operation.  The ballast setting is what determines if it's LTR or not 
anyway in a pig coil.  I don't really see the term LTR as having
real meaning with pig coils.  One just adjusts the ballast for
best results with a given cap size.  But if one does use the 
reso-calcs for a pig, the actual input power should be used, not
the pig's rated power, unless they happen to be the same.  I would 
say that one does not want to run a large degree of LTR in a pig
because you want a high voltage for better primary efficiency, and
you want to keep the power factor good.

Consider Kevin's coil.  His pig combo rating is 14.4kV at 20kVA. His 
cap is 0.138uF or something similar.  He draws 100 amps from a 
240 volt line.  He runs at 120 bps.   I just did the calcs, and a
matched sized cap for Kevin's coil would be 0.31uF or so, and
a typical LTR sized cap would be about 0.6uF or so.  This is 
huge cap, and I don't recommend anything that large.  His cap
seems to be less than 1/2 the resonant size.  (If you calc the
reso-size based on the pig's specs.)  You may want to check
my calcs here in case I made an error.  But if his reso-size
would be 0.31uF, I certainly would not suggest using anything
larger than 0.375uF or so.  Since Kevin's cap may be about
1/2 the reso-size, it's possible that his power factor might not
be that good, but I have no real idea if it is or not.  It is also 
possible that Kevin's coil may work much better with a 0.375uF 
cap, if it greatly improves the power factor.  It's too bad we don't
know what the power factor is on Kevin's coil. Then again, it is
possible that a reso or LTR cap is NG for a big coil.  It may hurt
the quenching too much or something, who knows.  Maybe 
someday someone will build a big coil with LTR.

The work I've done at low power, suggests that a low bps large
coil should be much more efficient than a high bps large coil.
So far I haven't really seen this in any large coil.  But then again
no large coil that i know of has been built according to my
suggestions.  It is possible that large low bps coils are not
showing the efficiency I'd expect *because* they are using 
STR (therefore too small) caps.  To me, the power factor is an
important question.  If the power factor is bad with an STR cap
at 120 bps, then this is not good.  Richie's simulation software
should be able to answer this question.  Actually I think he
already answered it, and said the power factor would be bad,
but my memory may have failed.

>  There seems to be a discrepancy of
>  practice vs. prediction in the higher power range: if, i.e., I
>  look at Bart Anderson's list of coils 97-99, from the first 20
>  TC's with the biggest sparks and power, there cannot be seen ONE
>  with a LTR, even not with a resonant cap, relative to the power
>  (and ballasting) used, but only smaller than resonants (STR's)!

Maybe no one has yet possessed the boldness to try such a large 
cap on a large coil?

>  The question of
>  
>            SCALEUP and SCALEDOWN of TC's,
>  
>  for biggest spark at smallest input power, looks to me not yet
>  having a definite answer, neither theoretical nor just "crude
>  engineering design". (Don't mean, you cannot just design a
>  successful high power coil a rational way...). One of my main
>  questions is, if you extrapolate, say, a successful LTR-desingn
>  for 1kVA to 10..20kVA, with corresonding huge bang-size, and
>  shift of the whole frequency domain: how will the secondary
>  behave? Is linear extrapolation feasible? Won't different
>  mechanisms and constraints come to bear, like racing sparks or
>  power arcs between prim and sec?

My guess is that the basically linear (considering spark length),
relationship will hold from small to large coils.   I don't think
racing sparks will be a problem.  But a low bps coil, (I call them
low bps coils, rather than LTR coils)....  will need a larger taller
secondary because of the larger bang size.  (Compared to a high
bps coil.)  In any case, I don't suggest using more than 25% larger
than resonant cap on a pig coil.  A low bps pig coil will need a giant
massive toroid for two reasons;  one, due to the large bang size, two,
because of the greater difficulty of coalescing the spark at a low bps.
Consider  Kevin's toroid which is 2 or 3 feet thick, and 8 feet in diameter.  
I think Kevin's coil would benefit from an even larger toroid maybe.

One disadvantage of low 120 bps operation, is the need for the large
secondary, which tends to limit the Ctop voltage.  But I think the
better spark growth characteristics at 120 bps more than make up
for that.

> - I recognize, i.e., Robert W.
>  Stephen, in his BC-1 design procedure, obtained a reso-cap of
>  0.128uF, but reduced it 'wisely' to only half, 0.064uF, for a
>  10kVA design proposal. Even, if set to 0.12uF in the MTC-1 coil,
>  which looks fairly similar to the BTC-1 design, it was yet just
>  below the (dangerous) resonant value. I also see Richard Hull's
>  very successful designs of higher powered TC's and maggies
>  following an alternative way to LTR's, comprising high BPS and
>  small (shurely stressed!)caps.

Well, a low bps coil will indeed use a more expensive cap, but if
it outperforms, it may be worth the cost.  As far as stressing the
cap, it is simply necessary to use a cap of suitable ratings.  Some
would say that the low bps stresses the cap less.  It depends on
what factors you look at.  A book by Morecroft (I think), explains
that a cap should be about 25% larger than resonant, or maybe
the reso-freq was supposed to be 25% lower than resonant, I'd 
have to check my notes.

>  On the other hand, scaling down to tiny coils, there might be a
>  problem with high inductance secondaries of > 1500 turns, as
>  mentioned by John F. himself, lately, 29th jan 2001, "2400 turn
>  sec report":
>  
>  "I am not surprised by the results, because I've noticed in past
>  work that large secondaries do not work well with small toroids
>  for low powered work.  Low power and small toroids need to have
>  small secondaries."

I think that what hurt the 2400 turn coil was the large secondary
which had too much self C in it, compared with the small toroid
I was forced to use because of the low input power.  It is basically
an incorrect design when built that way.  But it is possible that 2400
turns is too many anyway.  But I don't see any reason why 1600
turns would not be good for a pig coil.  It is true it will demand the
use of more primary turns, but the primary wire should simply
be made thinner to compensate.  The losses overall will still be
much lower.  I think it is very very difficult for folks to accept the
concept of using thin wire for both the primary and secondary of
a large pig coil.  I don't mean real thin wire of course, but just 
thinner than usual, i.e. the thickness of wire that gives 1600 turns
should be used.  I think it's important to use the following combo:
Low bps, large cap, many secondary turns, many primary turns,
high tank voltage, and monster-sized toroid.  These factors should
all work together to give the best efficiency and longest sparks for 
a given power input.  I don't know of any large coil that was ever 
built that follows all these guidelines.

I think it's even more important to use many turns in the secondary
of a low bps/large cap coil, because the large cap tends to lower
the primary surge impedance, and increase the gap losses.  By
using many turns in the primary and secondary, we compensate
for that, and bring the surge impedance back up again.

>  
>  In chemical-engineering, for example, I see scaling dominated by
>  dimensionless numbers (i.e. Reynolds for fluid flow). Do we need
>  dimensionless numbers for TC's? - My first guess is: no, - but
>  might be, we need to consider the change of limiting mechanisms
>  (-constraints-, like HV-breakdown in High power or mechanical
>  distances in low(-and high) power) for the different regimes of
>  power.
>  

I think the breakdown characteristics will be basically linear with
spark length, and follow a square law for input power.

Well I think that pretty much sums up my views.  Comments are
welcomed.

Cheers,
John Freau