Re: OLTC - Re: Primary resonators with very low L/C ratio

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

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>

Hi,

Has anyone ever thought of using a marx type setup? It seems like that 
would pretty much fix the primary inductance problem.

How much would adding another layer of wire on the secondary help? You 
might have to get the amount of turns pretty close, because the coupling 
could be pretty tight, and you would want the current to share evenly. 
There's also proximity loss, but it's probably worth looking into.

Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "Terry Fritz"

Hi Paul,

I think there are three "limits"...

At 06:57 AM 7/9/2003 +0100, you wrote:
 >How do we construct a primary tuned circuit which has a very low L/C
 >ratio? We come up against this question when we try to take an
 >OLTC and either raise the frequency or raise the power level.
 >
 >As a target for discussion, how would a bang size of 50 Joules,
 >firing at 300V, and resonating at 100kHz be dealt with?

E = 1/2 x C x V^2 ==> 50 = 1/2 x C x 300^2 C == 1111uF

Assuming a BPS rate of say 100BPS gives an input power of 5kW.

Fo = 1 / ( 2 x pi x SQRT( L x C )) ==> 100000 = 1 / ( 2 x pi x SQRT( L x
1111E-6 ))

Lpri == 2.28nH !!

For the secondary...

Assuming the biggest topload that is reasonable is 50pF, we get an Lsec of:
100000 = 1 / ( 2 x pi x SQRT( L x 50E-12) L == 50.66uH

 From the above, the first limit we run into is how "small" of a primary
inductance can we use in a real coil. 2.28nH is far below what a real
primary would be. If Steve had used one turn in his smaller OLTC, he would
have gotten ~560nH. My larger 3 parallel turn primary hit
~450nH. Antonio's program could figure it out far better (almost exactly!)
but I would think ~~300nH is the practical limit for how small a primary
inductor could be made.


100kHz is a pretty good frequency to shoot for so we now get a primary
capacitance of 8.44uF. If we want to run 50 Joules, we just have to up the
firing voltage to 3442 volts. However, things get messy over about 1000
firing volts in that caps and IGBTs start to fall out of a reasonable range
of what is available. With 1200V IGBTS, we may have to limit the firing
voltage to 1000 volts or 4.22 Joules/bang. Hopefully, streamer le! ngth "is"
proportional to the square root of input power in this case too. So all we
have to do to get to 5kW is up the BPS to 1185 Bangs Per Second. Since
OLTCs like high coupling and fast quenching (fast primary firing cycle
duration), we can really push the BPS rate way up to get high power as long
as we can charge the primary cap fast enough. So the second limit is how
high of a firing voltage we can tolerate. Newer IGBTs may got to
3000+volts, but right now on the "cheap used" market, 1200V IGBTs seem to
all we can find. So an upper limit of 1000 firing volts is our second limit.


If we are unable to get a giant top load capacitance, we may have to go to
a larger secondary inductance like I did. However, the larger inductor
will reach a point were the losses get so high they steal the coil's power
like mine did. A better (not cardboard) form would help that, but there is
still a limit to how large you can make the seconda! ry inductance before the
effective series resistance eats the power. Higher Fo frequencies really
help this! I was trying to get away with 120BPS (AC resonant line
charging) so to get power I needed larger primary caps. Which lead to
lower Fo, which lead to very large Lsec, which lead to High series losses
in the secondary... For higher power coils, resonate charging (even at 360
BPS three phase) may not be an option. To get very high BPS rates we seem
to need a type of DC charger like Steve used. My OLTC was a little too big
for resonant charging and it's 120BPS limit. But resonant AC charging does
vasty reduce weight and complexity. Probably great for a smaller or even
"table top" OLTC...

BTW - Dave Sharp has suggested that adding ferrites to the center of the
secondary could improve primary to secondary coupling, reduce the secondary
loss, and maybe aid in tuning. Unexplored territory there...

So you have a low l! imit of about 300nF for the primary inductance, a firing
voltage high limit of about 1000 volts (but you can have high BPS), and an
upper limit to how high of an inductance the secondary can be before the
series losses get you. Our L/C ratio for 100kHz is about 300nH / 8.44uF =
0.0355 (H/F).

Cheers,

Terry

 >--
 >Paul Nicholson
 >--



Jimmy