Ransom
Ransom GRM+ Memberand UltimaDork
1/28/20 12:27 p.m.

I've heard these adjustments referred to as related, interconnected, or even interchangeably...

What I want to know is what's happening when I adjust the wire feed speed on a GMAW machine with voltage and wire speed adjustments.

  • Does current/amperage increase just because you're feeding conductor (filler) into the arc faster (perhaps the current can be conducted better/more consistently because it's not waiting for the melted-back conductor to feed until it makes contact again)?
  • Or does the machine make some other internal adjustment besides feeding filler faster in order to work with the faster feed speed? That is, does the machine know that it has to deliver more current in order to keep up with the heat demands of more filler and probably faster travel?

GMAW is nominally a "constant voltage" operation, and I understand that the machine tries to keep near the voltage set by the voltage adjustment (which I gather is also the determiner of arc length). Voltage actually varies a bit at a given voltage setting, falling as current rises. Really curious about whether its "brain" is also taking wire feed setting into account and actively adjusting something (adjusting what?), or just reacting to the set voltage and wire speed...

I'm amazed at how hard it is to find an answer to this question on the web or in the textbook.

Brett_Murphy
Brett_Murphy GRM+ Memberand UltimaDork
1/28/20 1:13 p.m.
Fueled by Caffeine
Fueled by Caffeine MegaDork
1/28/20 1:26 p.m.

For GMAW-CV the welder selects the wire feed speed (WFS) on the wire feeder unit and an appropriate voltage on the welding power supply. The internal circuitry of the power source then supplies an appropriate amount of welding current necessary to maintain a stable arc. The GMAW process variables of current and wire feed speeds are interrelated so one cannot be independently adjusted without affecting the other byjust altering the WFS selector setting on the power supply itself.

 

https://ewi.org/gas-metal-arc-welding-basics-welding-current-welding-voltage/

 

so..  you adjust wire feed and voltage and then amperage is adjusted internally... 

Vigo
Vigo MegaDork
1/28/20 1:32 p.m.

Admittedly i know a lot more about electricity than i know about welding (i've done a bit, not much) but with mig you have to melt the wire faster than you feed it (not necessarily guaranteed) and you have to feed fast enough that it doesn't break the arc by melting faster than it's fed. Since the current and how fast the wire are fed are related in that way, it makes sense that some machines would automate that adjustment to some extent or other. 

I think to get into a lot of detail is going to depend heavily on what specific welder and how much detailed info you can get on how that specific welder works.  Might be tough..

McDesign
McDesign New Reader
1/28/20 1:47 p.m.
Brett_Murphy said:

Subbed.


I watched pretty much every video this guy has.

Jody is the MAN!

 

Forrest

oldopelguy
oldopelguy UberDork
1/28/20 2:00 p.m.

I don't think there's any internal correction being made between the two. Changing one shouldn't have any effect on the other. 

That said, the other factor that the machine has no control over is your speed across the workpiece.  If you have a certain current or voltage setting, depending on your machine, and you linger in one spot on the workpiece you are going to heat up that portion of the workpiece more. If you don't want it to get so hot you can either turn down the heat or dwell less time in the same spot.

That's where wire speed comes in: if you crank up how much filler you are injecting you will have to move faster across the workpiece, which should have the same effect as dialing down the heat.  For thicker metals you get a counter to that, though, because the faster feed rate can sometimes push the arc further from the torch tip and deeper into the weld. 

Your speed of work is most closely related to the wire speed, so I suggest picking a setting as fast as you are comfortable working and then fine tuning your penetration with the heat. If you can't get deep enough, dial back the feed, slow down across the workpiece, and fine tune with heat.

ShinnyGroove
ShinnyGroove Reader
1/28/20 4:11 p.m.

^ what he said.  How fast you're moving the tip, and in what pattern, is the third variable and possibly the most important.

Ransom
Ransom GRM+ Memberand UltimaDork
1/28/20 7:42 p.m.
Fueled by Caffeine said:

For GMAW-CV the welder selects the wire feed speed (WFS) on the wire feeder unit and an appropriate voltage on the welding power supply. The internal circuitry of the power source then supplies an appropriate amount of welding current necessary to maintain a stable arc. The GMAW process variables of current and wire feed speeds are interrelated so one cannot be independently adjusted without affecting the other byjust altering the WFS selector setting on the power supply itself.

 

https://ewi.org/gas-metal-arc-welding-basics-welding-current-welding-voltage/

 

so..  you adjust wire feed and voltage and then amperage is adjusted internally... 

I think this is a lot of what I'm after, but it still seems left ambiguous what it means to supply enough current for a stable arc... Is it reacting to something about the circuit as welding commences, or is it adjusting anything directly based on the feed speed setting?

The undereducated theory I've had is that given the electrode is the filler and it's being melted back, current is limited by the existence of the filler, whether it's the time between shorts in short circuit transfer or the makeup of the spray in spray transfer... If a given circuit is set to a given voltage, and part of the conductor is constantly eroded by the arc, feeding the conductor faster will reduce resistance and increase current, right? I mean, up to where it's fed as fast or faster than it's melted back...

SkinnyG
SkinnyG UltraDork
1/28/20 7:46 p.m.

I'm not a welder, but the tips I give my students regarding this are:

Follow the chart inside the welder.  It'll get you in the ballpark. I find my welding does better with a tick less wire speed than the chart calls for, on every MIG I've ever used.

Choose the heat based on the thickness of metal.

Choose a wire speed that does not produce undercutting, but does not burn back into the tip.

Welding vertically or overhead you're going to need more wire speed.

But I'm not a ticketed welder; I'm largely self-taught, and watch a lot of Jody.

Ransom
Ransom GRM+ Memberand UltimaDork
1/28/20 7:59 p.m.

In reply to SkinnyG :

I'm actually asking this as a part of getting formal welding training, but the instructor and I are both adrift on this item and he's having as much trouble as I am finding a definitive answer.

There's absolutely an element of the fact that I want (and kinda need for my style of learning) to understand the esoteric parts going on behind the scenes. I mean, it's not literally a requirement without which I can't join metal, but generally each of the parts of a system make more sense to me once I understand the whole thing.

wvumtnbkr
wvumtnbkr GRM+ Memberand UberDork
1/28/20 8:03 p.m.

Heat should be adjusted for metal thickness.  I have found that the hottest setting you can get without blowing through is usually best.

Wire feed rate is adjusted so that your weld bead doesn't mound up from the surface (too fast) or get sucked in (too slow).  

 

Your hand speed can do the final adjustment and "fine tune" the weld.

 

There are some welders that try to compensate and have a brain.  I like to think of them as electronic nannies on a car.  They can be great for a beginner or someone that is unskilled, but generally hold back the skilled people.

 

As far as I know, welders without a "brain" have little or no connection between wire speed and heat setting.   They shouldn't.  They are 2 seperate adjustments for 2 seperate things.

wvumtnbkr
wvumtnbkr GRM+ Memberand UberDork
1/28/20 8:07 p.m.
Ransom said:

In reply to SkinnyG :

I'm actually asking this as a part of getting formal welding training, but the instructor and I are both adrift on this item and he's having as much trouble as I am finding a definitive answer.

There's absolutely an element of the fact that I want (and kinda need for my style of learning) to understand the esoteric parts going on behind the scenes. I mean, it's not literally a requirement without which I can't join metal, but generally each of the parts of a system make more sense to me once I understand the whole thing.

The arc is surprisingly stable over many variables.  You have to pretty far one way or the other on your wire speed to not maintain an arc.  Too fast of wire speed will actually push the torch away and not melt.  

 

Too slow and the filler doesn't keep up and will "self extinguish" (and usually mean a messed up tip).

 

The heat or current or whatever you want to call it will weld and stick metal together on even the lowest setting.  It just won't penetrate thicker pieces and will result in a superficial weld.  Therefore, it doesn't have anything to do with maintaining an arc.

Ransom
Ransom GRM+ Memberand UltimaDork
1/29/20 11:05 a.m.

I was referring to the bit from the article Fueled by Caffeine quoted/linked, specifically:

The internal circuitry of the power source then supplies an appropriate amount of welding current necessary to maintain a stable arc. 

What I'm really trying to get at here is not how to operate my welder; it's to understand the general principle of GMAW with a Constant Voltage power supply. The sentence above seems to sort of suggest an internal compensation for wire speed, but could also just mean that a CV power source of this type will naturally provide an appropriate amount of current...

If we look at it from an Ohm's Law perspective, it does seem like the usable range of wire feed speeds would naturally increase current, as a partially vaporized (or just melted back) wire will have more resistance than one which is complete, and with a "constant voltage" the current would increase as resistance decreases. For some of this we're talking about average current, as short-circuit transfer will be "intact wire making contact" current interspersed with "melted back" almost-no-current.

BUT; it continues to not be entirely clear to me whether that IS the current adjustment, or whether a constant current GMAW machine (in the generic, textbook case) is making internal adjustments either directly affected by twiddling the wire-speed knob, or based on what happens when the arc is struck.

Paul_VR6
Paul_VR6 Dork
1/29/20 11:23 a.m.

The power supply will be constant voltage, but vary by the voltage knob. The current just 'happens' based on your wire feed setting, and as long as the power supply can take it, it will increase as your feed increases. The faster the wire feeds, the harder it is to maintain the arc, so the current increases. 

It's the opposite in gtaw from what I remember, as that's constant current welding.

 

Also, read this https://www.lincolnelectric.com/en-gb/support/process-and-theory/Pages/constant-current-vs-constant-coltage-output.aspx

Ransom
Ransom GRM+ Memberand UltimaDork
1/29/20 1:30 p.m.

In reply to Paul_VR6 :

Good link! Thanks! I think this might be the kernel I'm after:

With CV output, voltage and WFS are your preset, controlling variables and current is simply measured while welding.

Which seems to state at least a little more clearly that current is a byproduct of voltage and wire feed speed, not something that the machine is specifically trying to control or adjust.

Regarding your summary, it seems to agree with this, then right at the end seems to switch cause and effect! My impression was (is?) that the faster you feed the wire the more current would flow through the resulting circuit based on the set voltage from the power supply. That is, current is increasing as faster feed reduces resistance. The way you've phrased it there, it sounds like the machine is proactively making an internal adjustment due to incrasing difficulty in maintaining the arc...

Put yet another way, higher wire feed speed doesn't require more current to be sent, it causes more current through the system. Of course if more current didn't then flow we wouldn't keep up the arc, so the additional current is indeed necessary; it's a matter of whether it's a result of feeding wire faster, or some logic in the machine sensing something about the arc and compensating. I think it's the former, not the latter at this point.

Vigo
Vigo MegaDork
1/29/20 2:36 p.m.

I haven't seen inside a lot of very expensive welders. However, I have seen inside a lot of very cheap welders and they have no way to control current to any fine degree (as in instantaneously in response to conditions, that happens but it is passive, not controlled by the machine). There will always be a variation of voltage (below a max which actually can be clamped) and current because of the millisecond-level variation in resistance across the circuit, but I don't think that's what you're concerned with.  If you've ever looked at the 'spark' waveform of a spark plug circuit and if you understand why that does what it does, that's sort of relevant here too. I see a lot of stuff online about the relationship between voltage and current and how changing wire speed changes current, but it's a passive result of the changing resistance across the changing arc distance, not something the machine is doing in response to your changes. I think the short answer is that cheap welders have no capability to raise voltage in order to control current in proportion to wire speed. Expensive welders COULD do that, but i dont know for a fact that any do. I does seem like it would be a feature that would be resented by a lot of 'professional' welders. 

 

Ransom
Ransom GRM+ Memberand UltimaDork
1/29/20 3:10 p.m.

In reply to Vigo :

That's exactly what I'm driving at, and I think you just clunked my brain into gear.

I'm not really interested in some esoteric specialty or high-end machine, or any specific machine; I'm trying to understand the basics of a constant voltage power supply, and how the whole system works.

The part that finally clunked is that if it *were* making some kind of adjustment, it would necessarily affect voltage. I'd been handwaving some sort of rheostat or maybe PWM or something that would reduce the current while leaving the voltage the same, and I don't think it makes any sense. The rheostat would, by adding another resistor in the system, cause a voltage drop, which would just be an adjustment of voltage, which is (fairly) constant in use.

So, I'm pretty well convinced that current really is just a side-effect of wire speed. I think.

Thanks, everybody.

NOHOME
NOHOME MegaDork
1/29/20 3:17 p.m.

My welder has an auto tune feature in that when someone starts welding in my shop, another person tends to walk up and fine tune the wire speed to get the right sound for however they are welding. We all know that sound when the welding machine  is is in the zone.

If I am by myself, I tend to just muddle through with my best guess and get the job done.

 

 

Streetwiseguy
Streetwiseguy MegaDork
1/29/20 3:18 p.m.
Ransom said:

In reply to Vigo :

So, I'm pretty well convinced that current really is just a side-effect of wire speed. I think.

 

I probably shouldn't chime in here, because late to the party, but wire speed has a very small effect on amperage, based on the distance the arc has to jump.  You can pull the stinger away, or move it closer, which will change the power required to jump the gap.  Wire speed is there to provide the correct amount of wire at the correct time, as far as I know.  Too fast, you can feel it push back as it hits the bottom of the puddle.  Too slow, and it creates a big ball of fire that melts the wire into the tip.  Either way, it doesn't make as good a weld as having it set correctly.

SVreX
SVreX MegaDork
1/29/20 4:38 p.m.

I think you are trying to measure the wrong end. 

The output of the MACHINE  is constant voltage.

You seem to be asking about the voltage being transferred to the work piece (which theoretically could vary based on wire, speed, feed rate, etc).  The machine doesn’t measure this, and it doesn’t adjust for constant voltage at the work piece.

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