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