I have an electrical engineering question. I have a DialArc HF welder. This
welder has a nominal 30VAC winding which feeds a bridge rectifier, resulting in
DC voltage which is applied via the dial rheostat to a magnetic amplifier bias
winding. The rheostat allows the user to vary the bias, which greatly affects
the output current.
My welder welds fine, but I have to turn the dial up farther than I'd expect to.
The nominal AC voltage is 30V, but mine only measures 28VAC. The DC voltage
taken from the bridge rectifier is unfiltered, which means it contains a large
ripple.
Here is my question: if I add a largish filter capacitor across the DC output of
the bridge rectifier, shouldn't that effectively raise the DC bias voltage by
some amount? I'm thinking maybe if so that would make my dial read more
accurately.
Thanks,
Grant Erwin
The dial indicates current rather than voltage, yes?
I didn't see where you checked the actual weld current against the value
indicated on the dial.
--Winston
Current, yes. I have been welding for many years, and have run a whole lot of
1/8" 7018 rod. My test was to run some 1/8" 7018 and turn the machine up until
it welded correctly. From my experience I know that's about 110-120 amps.
The machine has two amperage controls. One is a big knife switch that has 3
positions. I had the machine in the middle position, something like 60-180 amps.
The other control is the dial, which is only calibrated 0-100%. My intuition
says that if I want a current halfway up the range, that the dial should be
somewhere around 50%. Well, I have to turn it up to about 80%.
I have a dc clamp-on ammeter, but no one to read it while I weld, sorry.
Grant
No camcorder, no sample/hold. For that matter, guys, there is no NEED for a
numerical measurement here. I have all the measurement I need already. The
question is about circuits, not about a current measurement.
Grant
Grant - check the clamp-on - mine has peak current hold.
Maybe you have one.
I've been welding 11018 all week. Big job on armor plate.
I'm doing 130 amps on 1/8 - close to .128" * 1000 as we have
been advised.
Martin
Grant Erwin wrote:
Nope - moved out - in Lufkin Texas now - East Texas.
Meltdown in San Jose and the state just made it hard to come by a new job.
So now I'm my own boss but looking for additional work.
Lufkin cutting edge design LLC.
Martin
Grant Erwin wrote:
My guess - 204V input not 208 or 220 not 240...
I have 204 in the shop. I thought it was 208v.
I believe the largely AC part of the DC voltage - think it as ac biased above
or below ground.
It is likely important to be that way. The dc bias (full dc) biases a
magnetic field - maybe requiring it to be near saturated. Then the AC part
the alternations folded all on one side - pulse that into more or less
saturation.
I think if you put a filter - would have to be a honker - it would mess
up the design.
Looks like you are '10%' low. More or less. Can a tap on the main transformer
primary be changed ? - is it into a 220 and 204 or 208 being applied ?
Likely a delta / wye voltage issue. I'd check the main AC power plug term block.
Otherwise a small 2.5V high voltage/current transformer could be added in series
to boost the output to a higher state.
Martin
Grant Erwin wrote:
Grant, I asked my welding 101 instructor a similar question (about
various machines having somewhat different currents at the same
setting). He said that it is typical, you have to set one machine at
110, another at 100, etc, that's just life. I am not sure if it is
really worthwhile to mess with this, as it seems that your machine
works well but is not perfectly calibrated.
i
There is no such thing as "perfectly calibrated".
Transformer welders are not constant current sources (even though we call
them that). Inverter based welders are closer to constant current sources
by are normally built with software to emulate the effects of a transformer
welder.
They operate on a voltage current _curve_. This allows you to adjust the
power delivered to the weld by varying the length of the arc. Welders that
have dig or arc-force adjustments allow you to change the slope and/or
shape of the curve to give you more, or less, control over the current with
the arc length.
If you try to measure the voltage and current of an actually weld, you will
see it jumps all over the place as the weld arc jumps around and the welder
changes the length of the arc with their movements. It's extremely hard to
come up with a single number with any accuracy. You can't do it with a
meter and get a number that's worth much. You have to average the current
and voltage over some time period so you can't define what the reading
means unless you also formally define how you perform that average. To
meters that are well calibrated for measuring a constant current will
likely show very different numbers if you tried to use them to measure weld
current because they will act differently as the current is jumping around.
I don't know how the industry actually manages to get any real consistency
between machines so that an "amp" setting on one even comes close to an
"amp" setting on another because they really aren't "amp" settings as much
as they are "power curve" settings. I suspect the standard for how these
power curves are labeled with "amp" numbers on the dials has drifted over
time as well.
A more accurate way to label the dial would be with a power numbers. But
even that's hard becuase the power changes with the arc length - so a
welder that tends to keep a very tight arc is going to be welding hotter
than a welder the keeps a longer arc.
Grant - to your question - no I would not suggest you try to modify the
circuit in order to fix the dial position. I don't understand these
magnetic amplifier designs well enough to say if your suggestion would
work, or if it might cause the amplifier to overheat and melt down??? I
don't know if they feed unfiltered rectified DC into it simply becuase it
was easy to do, or if it's actually an important part of the design. I
believe what it will do is cause the transformer to saturate only on the
peeks in sync with the output power. The adjustment rheostat then has
finer control over how much of the cycle is in saturation. If you filter
it, it might break how the circuit works by forcing it to be mostly in
salutation or mostly not saturated depending on knob position - that is
causing a very quick all on, to all off, transition at some point on the
knob. But that's just a guess since I don't fully understand how they
work.
There was a machine in the welding lab that sounds a lot like the one you
describe. It might even be the same one you are talking about. I'm
guessing it was from the 80's? It behaved much as you describe. Trying to
guess which of the three range settings and what position of the 0-100%
knob to use to get the right current never seemed to make much sense. You
just had to do what you do - adjust it until it welded right and don't try
to guess what "amp" setting it's at. Normally, it had to be turned up
higher than seemed correct by the amp range numbers compared to other
machines. I don't know if it was the age of the machine and maybe
insulation was fading and contacts getting dirty to make it delver less
power, or if it worked the same way when new.
I would suggest you get a sharpie and mark "110" on the dial position you
think feels like "110" to you. :) Use three colors, one for each range
position to mark your reference numbers with.
Thanks, Curt. I appreciate an informed opinion. I think I'll do exactly as you
suggest, especially since it means I don't have to actually do anything. :-)
Grant
What's that Lassie? You say that Curt Welch fell down the old
sci.engr.joining.welding mine and will die if we don't mount a rescue
by 13 Jun 2009 16:16:50 GMT:
Isn't it just the opposite? Longer arc=more resistance=higher power.
(for a given current)
I don't believe so but I really don't fully understand the electrical
characteristics of welding arcs.
If it were a truly constant current power source, then a higher resistance
would in fact create a higher power so you are right if that's what you are
thinking. Power = current ^ 2 * resistance. But maybe less of the power
would go into the weld and more would go into the air in that case? So
even with the power supply putting out more current, you still might get
less in the metal? Or, the heat radiated from the arc would distribute
over a larger areas of the base metal tending to cause it's welding
temperature to drop??? Like I said, I don't fully understand the dynamics
of welding arcs.
If it were a constant voltage power source instead, a higher resistance
would create a lower current. Power = voltage ^ 2 / resistance.
However, transformer welders are neither constant voltage nor constant
current. Their voltage starts at an open circuit voltage of around 80
volts typically and drops as you put a larger load on it. The current
increases as the voltage decreases. The shape of the voltage current curve
controls whether power is increasing or decreasing since it could go either
way when current increases as voltage decreases. Power = voltage *
current. Stick welders are generally designed to allow the welder to
control the power with the arc length. I think some typically very short
length is the max power, with longer lengths causing the power to reduce.
And if you increase the arc force, or dig, control, it makes the welder
increase the power even more at the short end of the arc length.
Here's a web page about this stuff from the Miller site I just found:
http://www.millerwelds.com/education/articles/article108.html
It also points out that welders designed for TIG are closer to constant
current machines and don't vary the current as you change the arc length.
SO if the resistance of the arc goes up with a longer TIG arc, the power
output from the welder will go up, but I suspect the extra power doesn't
effectively make it into your weld with most the heat in the weld due to
the current that's flowing though the metal and not as much the current and
heat in the arc itself?
On Wed, 03 Jun 2009 21:33:17 -0500, "Martin H. Eastburn"
Excellent Idea!! I had the same problem with MY Dialarc250 for a year
or two, then the local power company (Pacific Greed and Extortion)
changed out the transformer and now I have 251 volts AC about 7 months
out of the year and I weld LOW on the dial scale.
Gunner
"Lenin called them "useful idiots," those people living in
liberal democracies who by giving moral and material support
to a totalitarian ideology in effect were braiding the rope that
would hang them. Why people who enjoyed freedom and prosperity worked
passionately to destroy both is a fascinating question, one still with us
today. Now the useful idiots can be found in the chorus of appeasement,
reflexive anti-Americanism, and sentimental idealism trying to inhibit
the necessary responses to another freedom-hating ideology, radical Islam"
Bruce C. Thornton, a professor of Classics at American University of Cal State
Fresno
My shop has regular 220 volt power, generally measures 229-230VAC.
This makes me stop and think, though. In actual reality, my shop wiring has a
welder receptacle run about 12' with 6 gauge wires. But then for convenience
I've been using a fairly long (think it's about 50') extension cord that's only
12 gauge.
I am sure when I'm running 40 or 50 amps through that cord it's dropping some
voltage, and I know the cord should be much heavier, but it never gets hot so I
haven't worried about it. Let's see .. 12 gauge wire is about 2 ohms per 1000
feet, so 50 feet would be 0.1 ohms. Running 50 amps through it would thus drop
the input power 5 volts. In my shop, that would show up as maybe 224 volts at
the welder worst case (this is only running 1/8" 7018, maybe 110 amps DC, not
anywhere near the rated output for the welder). I'll try it with the welder's
plug connected straight into my welder receptacle, see if that helps.
I *could* change the input power connections on the welder from the 230
connection to the 208 connection. That should have the result of making the
apparent input power look like 230/208 * 230 = 254VAC, about like Gunner's.
Jeez, that makes me kind of nervous.
Grant
But then run it though that extension cord and it would be more like
230/208 * 224 = 247 which isn't as bad as 254. :) 224 into a 208 welder is
only (224-208)/208 or 7.7 % over voltage which seems like something a
transformer based welder might handle without much problem.
It would be interesting if you could get a real measurement of the voltage
drop on that extension cord. There may be more combined drop in the cord
and in your shop wiring than you would expect.
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