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:
Lw = 150/(2*(1+k)
Lw ~ 38.46 Henries
Rw = 563/2 = 281.5 Ohms
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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