MMA AC and DC welding current (Theory question?)

[premmington]

I have been thinking about this.

Do you need less amps for DC MMA welding than AC MMA welding?

With AC current switching on and off at 50 htz, would you get less energy than a DC current at the same amp settings?

 

[rtbcomp]

AC is amps is quoted as RMS, which has the same heating effect as the same DC current. Whether this affects an arc I'm not sure.

The big difference when using DC is that the heat is split unevenly, most of the heat going to the positive side.

 

[TechnicAl]

Most rods will run on less amps DC than they do on AC.........the arc is more stable
But it depends on the rod

 

[500e]

Good Thought PR
Al are you saying that the same rod with less DC Amps will penetrate as well as same rod with higher AC Amps/
If so are there DC rods you would recommend or (standard) rods that would give appreciably worse results on DC.
Like the more stable bit.

 

[TechnicAl]

Yes but its not massive. Some rods perform badly on one or the other. Some that are good on AC can "fingernail" on DC. To be honest those Im sure of are Lincoln and as we know from other posts they have changed.

 

[PTvor]

I seem to recall reading somewhere, it might have been a Lincoln document, that AC/DC rods need about 10% less current on DC than on AC. It's been my experience that they run better with lowish amps on an inverter compared to a buzz box and up to about 10% less seems right.

However, note that the current indicators on welders may not be that accurate, so while the current from the AC set should be measured at RMS and should deliver the same energy as the same value in amps DC, if the scales are inaccurate anyway, comparisons are dubious.

Then with DC, you can run electrode positive or negative, at least with electrodes which allow it, such as 6013. Another thing to factor in is that transformer type welders have a drooping characteristic, that is the current will change with arc length, but an inverter, which is the commonest DC welding source, will usually attempt to maintain a constant current at all arc lengths.

Generally, I think it's true to say that you need slightly less amps for MMA on DC than AC.

 

[premmington]

I seem to recall reading somewhere, it might have been a Lincoln document, that AC/DC rods need about 10% less current on DC than on AC. It's been my experience that they run better with lowish amps on an inverter compared to a buzz box and up to about 10% less seems right.

However, note that the current indicators on welders may not be that accurate, so while the current from the AC set should be measured at RMS and should deliver the same energy as the same value in amps DC, if the scales are inaccurate anyway, comparisons are dubious.

Then with DC, you can run electrode positive or negative, at least with electrodes which allow it, such as 6013. Another thing to factor in is that transformer type welders have a drooping characteristic, that is the current will change with arc length, but an inverter, which is the commonest DC welding source, will usually attempt to maintain a constant current at all arc lengths.

Generally, I think it's true to say that you need slightly less amps for MMA on DC than AC.

PTvor Quoted "Generally, I think it's true to say that you need slightly less amps for MMA on DC than AC"

This is what I have found!

But is it because in AC, the arc is trying to extinguish itself 100 times a second, making it harder to strike and hold an arc length. Or is it because there is less heat energy created by the arc?

I am an electrician and understand about RMS (root mean square).

My thoughts are, if the arc is trying to extinguishing itself at a higher rate, it must run cooler?

But I can't get my head really around my thoughts about the above statement though.

Is AC cooler? Or is it easier to hold a tighter arc length in DC? (so thus the 10% drop in guessed AMPS)

You can change penetration by changing electrode polarity in DC. In AC it does not make that much difference.

I do allot of MMA AC welding due to arc blow in DC when welding semi-magnetized materials.

 

[TechnicAl]

PTvor Quoted "Generally, I think it's true to say that you need slightly less amps for MMA on DC than AC"

This is what I have found!

But is it because in AC, the arc is trying to extinguish itself 100 times a second, making it harder to strike and hold an arc length. Or is it because there is less heat energy created by the arc?

I am an electrician and understand about RMS (root mean square).

My thoughts are, if the arc is trying to extinguishing itself at a higher rate, it must run cooler?

But I can't get my head really around my thoughts about the above statement though.

Is AC cooler? Or is it easier to hold a tighter arc length in DC? (so thus the 10% drop in guessed AMPS)

You can change penetration by changing electrode polarity in DC. In AC it does not make that much difference.

I do allot of MMA AC welding due to arc blow in DC when welding semi-magnetized materials.

This is my view. It is fairly easy to make 6013s that only run on DC.........its the AC bit thats the challenge and especially on low OCV sets.

We had to have a stabilised supply unit built so we could test rods at 42V OCV for the German market.

Apparently the supply changes all over Germany so some regions have quite low average Voltages (I cant remember the numbers). When this was first mentioned I didnt believe it so I shot off over to Germany. I got the reps from different areas to arrange appointments and went in armed with my rods ready to kick ass. I came home with my tail between my legs and a big problem. The rods just wouldnt run in some areas, some places they were great and in some so so.

 

[TechnicAl]

Increasing the OCV increases the energy so striking is easier. I dont think it has any effect on bead shape but if it increases the arc voltage it should spread a bit wider. The flux tends to prevent this though.

The OCV doesnt affect DC

Constant current means that the current stays the same whatever arc length you pull (unless it snuffs or sticks) and the arc voltage changes to compensate. This is the same whatever the machine (Inverter or Transformer) or the current type. If you measure it accurately there are small current variations until it catches up (cant think of the correct term).

 

[wheresmejumper]

Ocv and arc force are different variables,arc force wont effect the rods ability to strike up.hot start will.ocv will but not to the same extent in inverters

 

[PTvor]

But is it because in AC, the arc is trying to extinguish itself 100 times a second, making it harder to strike and hold an arc length. Or is it because there is less heat energy created by the arc?

I am an electrician and understand about RMS (root mean square).

My thoughts are, if the arc is trying to extinguishing itself at a higher rate, it must run cooler?

Don't forget there are also peaks where the power is higher and the arc isn't really extinguished. All the gases are very hot and ionised. I think the current disappearing and reversing a hundred times a second has to do with the arc being less stable.

Is AC cooler? Or is it easier to hold a tighter arc length in DC? (so thus the 10% drop in guessed AMPS)

Inverters have arc force and anti-stick. When the arc is detected to be very short, they increase the power. Presumably, DC needing less amps is also noticeable on the old industrial DC rectifier and transformer sets.

The same power should be going in on the same amps DC or AC, so there should be as much heat. There's a difference between welding rod positive and rod negative.

You can change penetration by changing electrode polarity in DC. In AC it does not make that much difference.

I do allot of MMA AC welding due to arc blow in DC when welding semi-magnetized materials.

Surely some mistake. You can't change polarity in AC.

I'm wondering if there's a difference between current requirements rod positive as opposed to rod negative. I usually use rod positive, apart from on thin stuff, so I've noticed rods needing less amps with DC but I've only really looked at rod positive. I haven't tried using the same electrode rod +ve and -ve and comparing the current needed.

 

[premmington]

To conclude.

My DC inverter has a higher OCV voltage than my AC transformer (Fact from rating plate) so thus making it easier to strike an arc.

I have it in my mind, that arc force settings change welding voltage not current. Correct or not? Higher welding voltages make the weld pool flatter, yes or no?

Also, I know that some rods perform better in DC than AC at the same amps. And I tend to have to raise the amp settings in AC. So the assumption AC is cooler!

Oh... Don't give me a welder with too many controls! I spend more time playing with them than welding. Teehee...

Thanks to you all for your input

 

[wheresmejumper]

Increasing arc force reduces voltage,you normally run with just enough for the rod not to stick when you dig it in.
lowering the arc force will let the pool wet out more

 

[PTvor]

Here's Miller's explanation of arc force (they also call it arc control).

https://www.millerwelds.com/resourc...nt-current-cc-dc-welder-for-training-purposes

The voltage drops as the arc gets shorter and the inverter compensates by increasing the current which it can only do by increasing the voltage. However, this only happens at short arc lengths and the extent to which it happens is either preset, or set via the arc force control.

There may be more to hot start than having a high OCV, for instance, the inverter may detect that the rod has touched the work and throw everything at it for a split second, to establish the arc, then return to the settings on the dial. As I understand it, OCV only really applies to AC welding.

 

[Sarto]

Hi,
I have some 6010 electrodes, and they won't lit at all in the old tranfsormer unit (AC).
On the inverter, they run poorly with low arc force setting, while they run reasonably
(for the clumsy me, at least) if I set arc force kind of 70%.

So are they DC only? It's better use DCEN or DCEP?
What kind of characteristic should have a machine to run them well? (OCV, etc)

If I am too off topic, tell me!

SARTo

 

[wheresmejumper]

Here's Miller's explanation of arc force (they also call it arc control).

https://www.millerwelds.com/resourc...nt-current-cc-dc-welder-for-training-purposes

The voltage drops as the arc gets shorter and the inverter compensates by increasing the current which it can only do by increasing the voltage. However, this only happens at short arc lengths and the extent to which it happens is either preset, or set via the arc force control.

There may be more to hot start than having a high OCV, for instance, the inverter may detect that the rod has touched the work and throw everything at it for a split second, to establish the arc, then return to the settings on the dial. As I understand it, OCV only really applies to AC welding.

As far as i know,hot start is only ever amp related.it will only ever adjust how much extra amps is given and for how long.
Why do you think ocv only applies to ac welding?

 

[TechnicAl]

Hi,
I have some 6010 electrodes, and they won't lit at all in the old tranfsormer unit (AC).
On the inverter, they run poorly with low arc force setting, while they run reasonably
(for the clumsy me, at least) if I set arc force kind of 70%.

So are they DC only? It's better use DCEN or DCEP?
What kind of characteristic should have a machine to run them well? (OCV, etc)

If I am too off topic, tell me!

SARTo

6010 DC
6011 AC/DC

 

[Bill Edwards]

Hi,
I have some 6010 electrodes, and they won't lit at all in the old tranfsormer unit (AC).
On the inverter, they run poorly with low arc force setting, while they run reasonably
(for the clumsy me, at least) if I set arc force kind of 70%.

So are they DC only? It's better use DCEN or DCEP?
What kind of characteristic should have a machine to run them well? (OCV, etc)

If I am too off topic, tell me!

SARTo

As TechnicalAl says 6010 are DC only. Also, a lot of inverters will not run them, ones that do state that they are cellulose compatible (6010 is a cellulose rod).

 

[PTvor]

As far as i know,hot start is only ever amp related.it will only ever adjust how much extra amps is given and for how long.
Why do you think ocv only applies to ac welding?

The point of hot start is to increase the current, but you can't do that without voltage to drive the current.

Something Technical said, but if you look, they never specify OCV for a rod for DC, only AC.

 

[TechnicAl]

In the olden days some sets used to have 2 tappings 80 and 100 volts OCV. 7018s ran better on 100V, in fact some wouldnt run on the 80 or lower tappings.
As far as I know the OCV keeps the arc flowing during the switching from + to - on AC (sine wave). With DC you need to do that so the OCV isnt as relevant.
Later HSE wanted low OCVs so instead of raming down they were changed to ramp up but they werent as good.