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Re: [TCML] DC resonance charging reactor
Bert, I really appreciate your taking time to guide me to right answers. My deep
thank!
Rechie stated in his site the lake of his experience in DC based TC, though I
also highly appreciate his effort. I was trying to replicate his basic circuit @
BPS=1200. My practice confirms your explanations. Cdc, sub-sequentially Ctank at
such high break rate need much more capable source than NST. At present, aside
of NST or MOT, am trying to make use of induction coil (Ruhmkorff coil) as a
possible source of pulsed DC to supply a 30 kW battery driven TC.
Thank you again for the references you gave.
Ibrahim
________________________________
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Tue, September 28, 2010 7:32:52 AM
Subject: Re: [TCML] DC resonance charging reactor
Ibrahim Khaleel wrote:
> Hi all,
>
> Thanks to fleps& Roger for the info. Still need to clarify, if anybody would
>confirm the equations stated in
> http://www.richieburnett.co.uk/dcreschg.html#resonant ?
Most of the DC resonant circuit equations shown on Richie's site are exact.
However, the equation for the peak tank cap voltage (Vpk = 2*Vdc) is indeed only
an approximation.
A brief history:
A huge research effort was conducted during the 2nd World War to develop
efficient methods of high voltage capacitor charging to support pulsed radar
systems. This led to the study and development of AC and DC resonant charging
circuits and their associated design equations. The best treatment of resonant
charging circuits (as well as excellent discussions on Pulse Forming Networks
(PFN's), charging choke design and testing, fixed and rotary spark gap design
considerations, and thyratrons) can be found in the book "Pulse Generators" by
Glasoe and Lebacqz. Even though it was originally written in 1948, this title is
still referenced by virtually all modern texts on pulsed power and radar systems
design. The book can be purchased via Amazon or other used book sources, or it
can be downloaded from MIT:
http://cer.ucsd.edu/~james/notes/MIT%20OpenCourseWare/MIT%20Radiation%20Lab/PREF5.PDF
http://cer.ucsd.edu/~james/notes/MIT%20OpenCourseWare/MIT%20Radiation%20Lab/V5.PDF
> Take the design example Richie is giving on his page:
> Lp=5.1H and Cp=80nF the RMS current Irms=886 mA
> I could not find answers for 2 questions:
> 1. The required current would not be allowed by his NST (10 kV-100
>mA).
> Does that means NST is not usable for DC resonance charging reactor?
> (shunts limiting the current)
Richie's design parameters were for an 8 kW system, so a single 1 kW NST will
not be able to provide sufficient power. However, NST's and NST farms CAN be
used within DC resonant charging systems as long as one is willing to accept
some performance penalties. The DC storage capacitor can briefly supply much
higher peak current than the NST during tank capacitor charging. The problem is
that internal current limiting within the NST can prevent the DC storage cap
from getting fully recharged on each half cycle of the supply mains. This causes
the DC rail voltage to sag, reducing the TC bang size during continued
operation. And, voltage sag becomes progressively worse with increasing break
rates.
The magnitude and rate of voltage sag can be reduced by increasing the number of
NST's in the bank and, to a lesser degree, by increasing the value of the DC
storage capacitor. However, better performance is obtained by using a low
impedance single-phase plate, MOT, or distribution transformer. And, best
performance (particularly at high break rates) is obtained by using a
three-phase HV transformer bank and a 6-pulse or 12-pulse HV rectifier, or by
driving a suitable HV transformer from a higher frequency single-phase source.
>
> 1. How "We know that the peak capacitor voltage will be twice the DC
> supply voltage."? What would be the basic formula for this statement?
It isn't for real world systems, but it can be fairly close. On the referenced
page on Richie's site, the fourth equation down (Vpk = 2*Vdc) actually assumes a
"perfect" DC resonant charging system. Specifically, it assumes that the voltage
on the storage cap does not change during the charging cycle (that Cdc is MUCH
larger than Ctank or we have a very "stiff" DC source), that the charging
inductor resistance is negligible, and that the charging inductor does not
saturate during the entire charging cycle. More exact design formulas (that take
some of these real world complexities into consideration) can be found in
chapter 9 of Glasoe's "Pulse generators" book. When these effects are taken into
account, the actual peak tank voltage can be significantly less than 2*Vdc.
Real world results:
Depending on the quality of the components, selected values of Cdc, Ctank, L and
R (of charging choke), and "stiffness" of the HV source, TC hobbyists will
actually see initial tank cap peak voltages that are in the range of 1.7 -
1.9*Vdc instead of the ideal 2*Vdc. However, these may be further reduced if the
DC supply voltage sags under heavier loading.
>
> RGDS.
>
Bert
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