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Message: 2
Date: Sun, 13 Jul 2014 19:15:41 -0500
From: Bert Hickman <bert@xxxxxxxxxxxxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Subject: Re: [TCML] Findinding the facts of a 3 P unknown transformer
50:1 "black Box" and Charging inductor(s) design.
Message-ID: <53C3212D.80400@xxxxxxxxxxxxxxxxxxxxx>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hi Stephan,
Teslalabor wrote:
> 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?
The 6-pulse rectifiers live in a relatively benign environment. The
total PIV rating of each stack must be at least 2xVdc (~26.7 kV), so
using almost 3XVdc (39 kV) should certainly provide adequate voltage
margin for your system. 60 kV strings on a MOT system is definitely
overkill. I haven't analyzed your system, so I don't know what the RMS
current is for your rectifier and dequeing diodes under various
conditions. However, converting from 1A to 3A rectifiers will provide
greater current margin.
>
> 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 :-)
That's a good question for Greg to address. :^)
Remember that the design for the 40 kW coil was done way back in 1990-91
- long before the army of TCML experimenters arrived. Today, Greg might
swap the position of the tank cap and spark gap to prevent the RF that
appears across the tank cap from backing up into the chokes. In his
original design, a lower frequency filter (~8 kHz) was used to help
block TC operating frequency RF from backing up into the charging chokes.
However, even if we swap the gap and tank cap, using a HF filter may
still be a good idea. But use a modern Terry filter (that shunts and
dissipates the transient's energy), since the TC's RF is now shunted by
the gap. But, even with the gap shunting the HV charger output, it still
generates tons of high-amplitude conducted (and radiated) VHF and UHF
"hash" every time it fires or reignites. Unfortunately, the high dv/dt
transients don't uniformly distribute themselves across the choke
winding. Because of inter-turn and inter-layer capacitances and winding
inductance of each turn, the transient voltage stresses become
concentrated across a relatively small number of turns that are
electrically closest to the gap. If not blocked, these have the
potential (NPI) to generate corona and partial discharge damage between
turns and even nearby winding layers in the choke. This can cause
premature failure of the choke's insulation system. Introducing a lossy
low-pass filter harmlessly bypasses (and dissipates) these transients
before they can reach the choke. Consider it as cheap insurance... :^)
Bert