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Re: [TCML] Findinding the facts of a 3 P unknown transformer 50:1 "black Box" and Charging inductor(s) design.



Hi Bert & all,

as you know I'm also actually constructing a similar DC resonant charging system. At the moment the construction of the 400bps rotary (400bps @ 4000 1/min with very low dwell time) is in progress.
Until it is finished, there are still 2 questions, I'm wondering about.

First question is about the design of a properly rated 6-pulse rectifier. My system is powered by a 3-phase transformer, which delivers 9,44kV output between it's phases. So after 6-pulse rectifying I get 13,35kV DC for feeding the system, so far so good. My first try were 6 diode strings with each only 16 diodes in series, each 1kV / 1A. This 16kV strings died after short time during my initial halogen-dummy experiments, when increasing the voltage more and more. Then I build a new 6-pulse rectifier, constisting of 6 strings, this time each using 39 diodes in series, each 1kV/3A. So I now have strings each 39kV / 3A. Couldn't test it until now, because new rotary is still in progress. But I have the feeling 39kV is still to less, because I saw 6-pulse rectifyers, using 60kV per string and these were only powered by MOTs. So, what are the important design parameters, when building a 6-pulse rectifier for a DC resonant charger? Are there special diodes, one should use? Im actually using 1N5408 1kV/3A Diodes. Do you think, they will survive?

Second question. By studying Greg Leyh's amazing DC resonant charging systems, both the 40kW coil and also Electrum, I recognized, that he uses an aditional air coil inductor and a capacitor, to protect the charging reactor (and the DC supply?) from HF, going backwards. In the case of the 40kW coil he uses a 9,5mH inductor combined with a 45nF cap. Isn't the charging reactor itself blocking all of the HF, because of it's high inductance? Is this realy needed and if so, how critical are the values of L and C of this protection circuit? Because I use a homemade reactor, I want to minimize the risk of killing it, it was a lot of work :-)

Regards,
Stefan


----- Original Message ----- From: "Bert Hickman" <bert@xxxxxxxxxxxxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Saturday, July 12, 2014 8:12 PM
Subject: Re: [TCML] Findinding the facts of a 3 P unknown transformer 50:1 "black Box" and Charging inductor(s) design.


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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