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Hi Jim,
Remember that, like distribution transformers, your large transformer
can easily deliver 2-3X its "nominal" rated power without breaking into
a sweat for typical TC run-times. It takes some time to heat up the
large thermal mass and surrounding oil.
Now, regarding charging chokes...
Although you can wind your own, or use a series of MOT's (with air gaps
added to the cores), it looks like you could reconfigure parts of your
existing charging choke to get the lower inductance you need. Based on a
back of the envelope calculation, you'll need something in the range of
4-6 Henries. A charging choke should not saturate during the charging
cycle, it should not have excessive DC resistance, and it must be
capable of standing off the full DC supply voltage across the
winding(s). Unfortunately, the closed magnetic circuit of your existing
choke will cause it to saturate when used in a DC charging system, and
its high winding resistance will also cause unacceptable losses.
Reviewing the images of your power supply, your existing choke consists
of two identical windings connected in series with fluxes aiding. The
series combination of the two windings is 150 Henries, and the DC
resistance is 563 ohms. Since the windings are identical, we know that
L1 = L2 = Lw (the inductance that a single winding would have on the
same core). The mutual inductance of the pair of windings is M = k*Lw.
Because of the closed magnetic circuit, k for your choke is probably in
the range of 0.95 or more. This allows us to estimate Lw by using the
combined inductance. The case of two identical coupled inductors
simplifies to:
Lseries = (1+k)*2*Lw
Solving for the individual (isolated) winding inductance (Lw) and
resistance:
Suppose we now simply reconnect the individual windings so that they are
in parallel or anti-parallel. For the parallel (aiding flux) case, the
combined inductance equation simplifies to:
Lparallel = (1+k)*Lw/2
= 36.54 Henries - better, but still too high.
And, for opposing fluxes, the equation becomes:
Lanti-parallel = (1-k)*Lw/2
= 0.96 Henries - still too low
However, suppose you disassembled your existing choke. This provides you
with MUCH greater flexibility, allowing you to create a charging reactor
that meets your needs and doesn't saturate. After disassembling, install
each winding on its own independent I core to create two identical
chokes (using some of the surplus core material you mentioned), and then
connect them in parallel. The parallel connection reduces winding
resistance to 1/4 of its current value, or about 140 ohms, and also
doubles the choke's previous current-handling capability - both
important for resonant charging.
You now have some further options:
Case 1: Keep the two chokes magnetically isolated (k=0 in the above
equations)
Assuming similar core size and closed flux paths for each choke, each
new choke would have about 38.5 Henries, and the two in parallel would
have 19.3 Henries. This is getting closer, but is still too high.
However, if you insert small gaps into the magnetic paths of each choke,
you should be able to reduce the inductance down to the to the range of
8 - 12 Henries for each so that the parallel combination falls in the
desired 4 - 6 Henry range. Core saturation is also now avoided.
Case 2: create two magnetically-coupled windings (0 < k <= 1)
You will gain much greater flexibility by magnetically coupling the
cores through adjustable air gap(s) that link the two cores. You can see
by the equations above that reducing the gap(s) (i.e., increasing k with
flux aiding) INCREASES the combined inductance while reducing the gap
with fluxes in opposition REDUCES the combined inductance. As in the
above case, introducing the air gaps dramatically reduces the effective
inductance of each winding. It will take a few inductance measurement
and gap adjustment cycles, but you should be able to dial in the
combined inductance to the 4-6 Henry range that you need.
Resonant charging calculations:
Plugging in some numbers into a resonant charging system calculator:
Based on a 0.05 uF tank cap, a desired max break rate of >600 BPS,
maximum power output output of 10 - 12 kW, and a 14 kV HVDC supply, a
charging choke in the range of 4-6 Henries is suggested. Assuming 140
ohms of choke resistance, the above inductance range supports a maximum
break rate of 650 - 730 BPS, with peak choke and dequeing diode current
in the range of of 1.2 - 1.5 amps. Bang size will be about 17 joules at
a maximum tank voltage of about 26 kV. Charging inductor power
dissipation ranges from about 11 watts at 200 BPS to about 100 watts at
600 BPS. YMMV based upon the actual 3-phase output voltage from your
transformer, but these numbers should be in the ballpark.
Hope this helps and best wishes,
Bert
--
Bert Hickman
Stoneridge Engineering
http://www.capturedlightning.com
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Jim Mora wrote:
Hello List,
I have been fascinated by 3 Phase, 6 pulse resonance changing since I first
read Richie Burnett's pages 11 years ago. Bert Hickman has been very helpful
in calculating the parameters of my repurposed charge transformer.
Important things to know are the cross-sectional area of the core of coarse
(known).
The turns ratio (approx determined).
Another thread suggests measuring the inductance of a primary winding
(shorted in) and its DC resistance (Z) - good idea!
I have learned a lot from this exercise. My core cross section area is bit
disappointing. But on the other hand, I don't have enough room for very long
streamers but hot ones would be awesome. Bert had an excellent suggestion to
rewire the secondary from wye to Delta which he points out has a 58% drop in
voltage and a bonus substantial output charging current increase.
The likely output will be 10KVA based on the core. I have both a .125"
electrode x 12, and a .5" really beefy x 8 rotaries. Richie suggests paying
attention to the dwell time to avoid streaming power arcs. I guess that will
be tested "empirically" (I have lots of cut .125" electrodes for distructive
testing) and 3/8" for dual series fixed. I haven't made the .5" towers yet,
I'm thinking 9/16"
*** I now want to turn attention to the charging coil ***
My parameters are not too different as Richie's test case. I have a new .1
uf (50KV)General Atomics that can be series to .05 (100KV). I think having
(2) inductors is a good thing for voltage stand off and flexibility and a
variable gap too.
*** I am going to study the many inductor design sites but any school of
hard knocks would be welcomed. I have quite a bit of core material around
both EI and C cores for low freq operation.
*** I fully intend to over build the diode strings but will start a new
thread on the Dequeing diode robustness. I have (500) UF5804 which are
milspec 3 amp 1000v pk inverse diodes, 500 RMS. I intend to over build
beyond that. 3 amps forward(1.5) "should" be plenty for three phase 6 pulse
current at 10KVA.
Thanks for all you do! I am having fun getting back in Tesla coiling!
Jim Mora
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