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Re: The Electrum Project

Original poster: Greg Leyh <lod@xxxxxxxxxxx>



Original poster: "Dmitry (father dest)" <dest@xxxxxxxxxxx>

Hello Greg.

if i`m not mistaken, about 92% of the start energy is transfered from
primary to secondary, approximately at 4 periods of Fres, and system
operates in mode 6:7 or 7:8 - is it true? what is the Q (unloaded) of
primary & secondary ?


I would be thrilled if 90% of the energy made it to the secondary, but I doubt that more than 70% actually does, due to the impedance of the rotary gap. The dymanic impedance of Electrum's 8 gaps in series measured about 1 to 1.5 ohms. I never measured the unloaded Q of the primary; I assumed the Q would be completely dominated by the gap impedance. I didn't do a specific msmt of the unloaded Q of the secondary either, but inferring from sig gen msmts right after erecting the tower it looked to be about 100 to 150. Again, this number is probably not very meaningful since loading by the output arc completely dominated the operating Q.
Using solid-state switches in place of the rotary gap however, I would expect the switch losses to be well below 5% of the total throughput.


http://www.lod.org/Projects/electrum/construction/pages/rtarygap.html

here each stationary electrode has its own insulator, but here:

http://www.lod.org/Projects/electrum/NewZealand/pages/nzrotarygap.html

are they all mounted at the same insulating rod? is bottom left
electrode the last in the chain and connected to the primary?
look here:

http://www.lod.org/Projects/electrum/NewZealand/pages/nzxmsnline.html

is the top left electrode the first in the chain &  connected to the
cap? and two last electrodes at the photo are  connected with the bus
(from the side)? are there only 3 buses - one at the left & 2 at the
right? and why i can`t see neither buses nor electrodes here:

http://www.lod.org/Projects/electrum/NewZealand/pages/nzgapinst.html

are they covered by something from the side?

Very keen observations on your part. The original gap design in the first pic used eight separate porcelain station posts to locate the tungsten stator assemblies. However, we noticed that occasionally there was stator-rotor electrode interference that would scrape the electrode faces. One evening, this resulted in a serious hole punched in the side of my truck. Using a synchronized strobe light to illuminate the entire structure, we discovered that the stator supports suffered from a pi-mode vibration as the rotary gap accelerated through its critical resonance around 2200 RPM. We stiffened the stator supports by replacing the porcelain posts with solid 3"x6" beams of G-10, which appear as the 'covers' that you mention in the last picture. The electrodes and Cu busbars are mounted directly to the top surface of the beams. These solid beams have a fundamental resonant frequency way beyond the gap rotational speed, which fixed the interference problem.

The mechanical issues surrounding the rotary gap provide even more reason for moving to silicon switches as the design throughput power is increased. Electrum's rotary gap has about 100kg of rotating mass in the form of 4 rotating disc armatures, and at 3000 RPM stores just over 300kJ of kinetic energy -- enough to lift an 150lb adult 1500ft into the air.

"with 8 gap in series, the effective opening speed of the gap system
is Mach 3.2, and the total mechanical dwell time is comparable to a
single beat envelope at 38khz, with k=0.22"

ok - imagine that after 1 period of Fres electrodes will go out from
"engagement", and after another three periods they will move at approx
3-x their thickness - then summary gap length will be 6" - it should
be enough for arc disconnection, but in reality this distance would be
smaller. it`s at 350bps, and at 100 - less 2" - will be enough?
i.e. this gap is quenching by itself and it`s not neccessary to connect
other in series with it?  did you try to make something similar, but
in smaller scale - ex. at 10kw? coz the disc`s diameter is too large
for me - 71cm %-)

I don't think the gap quenched regularly on the first envelope; I often saw a second envelope, albeit much smaller, like this one: http://www.lod.org/Projects/electrum/techdata/sphere100us.jpg
It's debatable IMO whether quenching any sooner would make a big difference, since the dart leaders appear to die down long before the first envelope ends. Those 2-3 cycles near the peak amplitude seem to be doing most of the work.

-GL