DialArc magnetic amplifier?

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
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Grant Erwin wrote:
(...)

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
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Winston wrote:

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
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Grant Erwin wrote:
(...)

Does your ammeter have a 'Max' function that records the maximum amount of current it sees?
This one would probably do the trick for $14.00: http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber683
--Winston
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Winston wrote:

That clamp meter is AC only. DC clamp meters (Hall effect) cost a fair amount more.
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Grant Erwin wrote:

Do you have a camcorder, or even a digital camera with self timer? Aim at the meter and weld.
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Pete C. wrote:

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
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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:

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Are you still in California? Is it hot down there? If so, it must be miserable working with plate you have to preheat.
Grant
Martin H. Eastburn wrote:

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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:

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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:

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Good guess, Martin, but wrong. 230VAC from my wall, machine set up for .. 230VAC.
Grant
Martin H. Eastburn wrote:

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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
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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.
--
Curt Welch http://CurtWelch.Com /
snipped-for-privacy@kcwc.com http://NewsReader.Com /
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Curt Welch wrote:

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
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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)
--

Dan H.

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snipped-for-privacy@privacy.net (dan) wrote:

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?
--
Curt Welch http://CurtWelch.Com /
snipped-for-privacy@kcwc.com http://NewsReader.Com /
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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

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Gunner Asch wrote:

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
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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.
--
Curt Welch http://CurtWelch.Com /
snipped-for-privacy@kcwc.com http://NewsReader.Com /
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