The contactor was definitely on the secondary side. With all 3 circuits
paralleled, it had about 240A rated capacity. I think it had problems
with extinguishing the arc because I had to dress the contacts 3 or 4
times over the 15 years that I had the machine. I never had the AC feed
capable of running it at full capacity, and most of my welding is
smaller stuff anyway. It probably would have been a lot worse if I was
running it close to capacity all the time.
I'm rather surprised: Seems like there's be less inductance to fight on
the primary side. The secondary side is _designed_ to maintain an arc.
Leakage between primary and secondary would give the primary a little
I have been looking at the remote Control Board PC3 schematic. I think
it is just a simple PWM generator. Q1 and the nearby R's and C5 look
like a sawtooth generator. A1A is operating as a comparator with the
sawtooth in the + input and the wiper voltage from the adjustment pot
driving the - input. The remote contactor input (J) gets grounded when
the pedal is not in the off position, pulling the anode of D4 close to
ground. This reverse biases D4 and allows the comparator output pulses
to go to the - input on A1B. The + input voltage is set by the
optocoupler that is driven from the panel/remote switch. In the remote
position, the + input will sit about 2.7V and the PWM will pass through
A1B and drive the IGBT module gate to pulse the field. It doesn't look
like there is any feed back on the current flow or voltages.
I suspect that the caution about putting the fine current control at
100% is a leftover from before they added CR3. With CR3 there, it makes
no sense from what I understand (which may be wrong still).
It looks like the power while welding and current to excite the welding
generator field come from the coils on the left side of center and the
welding current and aux power come from the coils to the right of
center. I think that the voltage regulator board is just operational
when running as a standby generator and not as a welder. When CR4 opens
up, it looks like it turns off most of the regulator board. I haven't
figured out the SCR/UJT and NPN stuff on the right side of the print yet.
I am getting close to all I can skull out of the prints without a
machine in front of me to poke at and test on.
That seems reasonable. I wondered the same thing but couldn't see through
most of the details. A DC motor speed control module could probably be
adapted to do the same job. Possibly it could be made to do both the
regulator's job and the welder control's job by selecting either current
set pot or output voltage error amplifier as signal source.
That's for another day...
I took a look at S1B today and it makes even less sense 8-) The panel
labels agree with the schematic markings, which make no sense. Even
better, the switch is open! The contact slider for S1B appears
to be missing. Everything else is there, S1A is fine.
That's my impression also. The welder is open loop, mostly just a fixed
voltage on the current setting choke. Any feedback would be too slow.
That has me stumped as well. I have been told it's possible to build a
PWM circuit using SCRs, either by gate-turn-off or current diversion,
but either approach seems to require more parts than are shown.
For the moment I'm tempted to leave well enough alone. As Iggy pointed
out, the last think I want to do is mess up a working welder. In the
meantime you've given me some good things to think about.
I'm probably better off practicing than tinkering. Google found a few
examples of folks hooking up remote rheostats, so when the carbon pile
arrives it makes a low risk experiment. At this stage just keeping the
tungsten out of the puddle takes most of my attention; another control
to think about could easily make matters worse.
You're welcome. Welding machines interest me a lot. A slightly different
tack on this might be to try building your own current control/HF system
to run off the weld outputs of your Legend, something like:
That way, your machine would stay unmodified and retain what value it
I have wanted to build my own design inverter TIG machine for a long
time, but I wanted to have one that would always work and not be a
perpetual project in the way of just going out and welding. As a result,
I bought a Miller machine. Maybe after I retire...
That's an astoundingly ambitious project! Given what could be bought
in 2011, it might have made economic sense. Given what can be bought
today it's not so obviously worthwhile. The challenge might be satisfying
but I'm not sure there's any functional gain to be had.
The machine I have is no collector's item. There are a few photos here:
http://www.zefox.net/~bob/welder/ (The reddish stuff in the photos is
paint overspray; what got inside didn't stick and cleaned up with a
soft brush but the outside has pink highlights.) The un-grooved slip
rings are what persuaded me to buy it, suggesting relatively low run
time. The exterior is very beaten up. The HF-251D-1 is less bad but
also somewhat beaten up.
Mostly I like to play with old machinery. If it can be made easier to
use on the cheap I'm not afraid to make changes. What the AEAD-200LE's
taught me so far is that the faults are in the bum on the seat....
There are times when it's possible to build better than one can buy.
It was true of hi-fi speakers in the '70s for a short time, maybe ten
years. Probably true of welders when IGBTs first appeared, but now?
I don't have any illusions about being able to build a better machine
than I could buy. Mostly, it would be a very cool technical challenge.
Building ambitious projects has been one of my favorite ways to learn
new stuff and increase or keep current with stuff that I can use in my
day job or maybe my next day job.
The Dynasty box that I got will run 250Hz on AC and you get
significantly better penetration on aluminum at 250Hz. It also sounds
like a hive of really angry bees. I don't know how far upward that
extends in frequency though.
There's a remark in one of the welding forums that Lincoln built a
400Hz welder at some point. The writer praised it considerably, though
he offered no details.
I was thinking in terms of considerably higher frequency, high enough
so that electrode capacitance, rather than electron emission, contributes
to the discharge. I'd think that would be in the hundreds of kHz if not
higher. Admittedly, I do not know if it's worth the trouble. But, inverters
weren't worth the trouble twenty years ago, now the trouble is much less
and the virtues seem considerable.
Once the frequency goes over 20kHz at least you won't hear the bees....
Shielding would likely be a significant problem. However,
if the advantages justify it the work could be done in a
Faraday cage. They can be made of screen or metal covered
wood and aren't really very exotic, apart from being low
production items. Think of a carefully costructed screened
porch. Or, a sandblasting cabinet.
There are some FCC allocations for industrial heating applications.
One at 13.56 MHz is used quite widely, though that frequency may
be impractically high. Induction heating is lower frequency, but
I'm not sure how or if it's regulated. Microwave ovens have a slot
at 2.4 GHz, but I'm pretty sure that's too high for convenience,
if not for effectiveness.
What would be the advantage of capacitive coupling? I suspect that the
plasma in a normal TIG arc is so conductive, that the difference would
be lost. But then I don't understand why the higher frequency AC
penetrates better. I'm not criticizing here, I am just trying to piece
It just occurred to me that the voltage drop between the electrode and
the work times the current flow is dissipating a LOT of power, that may
be the primary source of the welding heat. This is pure speculation on
my part though, I am just an engineer, not a plasma physicist.
The power dissipated is distributed between the electrode, the workpiece
and the arc itself. I think the major effect of raising the AC frequency
would be to change the distribution of power dissipation among those
components. If you could build a 13.6 MHz welder I'm pretty sure it would
behave very differently than a conventional one. Better or worse, and
for what purpose, I'll admit to being unsure.
In a DC arc electrons have to be emitted by the cathode and collected
at the anode. Most of the current is carried by the electrons and most
of the power is delivered to the surface they land on. To make the cathode
emit electrons either ions must strike it with enough energy to dislodge
sufficient electrons or the cathode must be hot enough to emit thermionically.
This remains true in an AC arc up to the frequency where electrons don't have time
to transit the arc before the voltage reverses. Then, they're trapped, sloshing
back and forth in the arc, collisionally ionizing the gas and ensuring an adequate
supply of charge carriers to keep the discharge going. The result is that the
electrodes don't _have_ to supply the electrons. This allows the cathode to
run colder than required for thermionic emission and avoids the erosion caused
by ions hitting the cathode. More power goes into the arc, less into the electrode
and workpiece. That opens an opportunity to manipulate where the heat goes.
At the same time, ion cleaning of the workpiece would decrease, perhaps to
the detriment of the welding process.
The frequency at which electrons don't have time to transit the arc isn't known
to me and is probably rather hard to estimate accurately. Ordinary fluorescent
lamps have an efficiency peak in the tens of kHz, electrodeless lamps have been
commercially available in that frequeny range. Microwave frequencies have been
tried but far as I know they were a technical success and a commercial failure.
I'm not sure plasma physics is much help in a puzzle like this; it's a very
messy problem. Direct experimentation is apt to be easier and more persuasive.
Quite possibly experiments have already done, probably for somebody's Defense
Department during the 1950's. They'd have been stuck using vacuum tubes, which
would have made the project much harder than it would be today.
Apologies for the length, thanks if you read this far!
The speed of electrons in a solid wire was one of my college physics
"In the case of a 12 gauge copper wire carrying 10 amperes of current
(typical of home wiring), the individual electrons only move about
0.02 cm per sec or 1.2 inches per minute"
You'd have to know the free electron density in the plasma channel to
figure it for an arc.
That's an interesting paper, I wish I could understand the
math better. It seems to treat only the DC case, however.
I wonder if there's an AC treatment around somewhere. It'll
surely be much messier! One of the guiding assumptions is
that the arc is a resistive fluid obeying Ohm's law. Arcs
generally don't obey Ohm's law.
As the authors note, the paper is a "useful intermediate step".
Thanks for writing!
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