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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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