MMA AC and DC welding current (Theory question?)

[PTvor]

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.

But what about industrial type DC welding machines with rectifiers? Generally I suppose they were few and far between and the usual industrial welder would have been an Oxford (etc) transformer and there would also have been generator sets which produced DC as used in pipelining and other outdoor pursuits. All this must have changed a bit around the mid 60s, when solid state rectifiers started to come in. Before then they must have used motor generator sets or mercury arc rectifiers with obvious problems, expensive, short lived, and environmentally not very nice.

I had an uncle who welded aluminium with what he called argon arc. I saw some of the hot water bottles he'd made as foreigners around 1960 and they were works of art as welding, as hot water bottles they were lacking and burned your feet. That was late 50s/early 60s and he died in 1968. It was obviously TIG and it would be interesting to know what gear he used. I imagine it would have cost as much as a house and would be considered crude by modern standards.

Did you have to design DC only rods?

So with AC with a running rod you have an extremely hot start 100 times a second and the ions in the gas suddenly have to change direction. AC rods have potassium in the composition because it's more easily ionised than sodium aiding the very hot start, the problem is that potassium salts are dearer than their sodium counterparts.

I'm minded of the table on my inverters which reads 80A/23.1V, 100A/24V, 130A/25.1V. I'd guess that was done with a dummy pure resistive load and arcs are going to be different depending on the composition of the rod, but basically you are going to need more voltage to drive more current.

 

[TechnicAl]

But what about industrial type DC welding machines with rectifiers? Generally I suppose they were few and far between and the usual industrial welder would have been an Oxford (etc) transformer and there would also have been generator sets which produced DC as used in pipelining and other outdoor pursuits. All this must have changed a bit around the mid 60s, when solid state rectifiers started to come in. Before then they must have used motor generator sets or mercury arc rectifiers with obvious problems, expensive, short lived, and environmentally not very nice.
Thats right.......most shipyards used what was called AC Pots..........they would run loads from one connection.........There were no meters so the settings were down to the welder.

I had an uncle who welded aluminium with what he called argon arc. I saw some of the hot water bottles he'd made as foreigners around 1960 and they were works of art as welding, as hot water bottles they were lacking and burned your feet. That was late 50s/early 60s and he died in 1968. It was obviously TIG and it would be interesting to know what gear he used. I imagine it would have cost as much as a house and would be considered crude by modern standards.
I think Argon Arc was the start of MIG and Heliarc was TIG

Did you have to design DC only rods?
Well not really. Sometimes the final criteria was that it only need perform on DC but if it was possible to run on AC as well.....all the better....you might find another customer........you would probably find that most 7015s, 8015s etc etc also run on AC

So with AC with a running rod you have an extremely hot start 100 times a second and the ions in the gas suddenly have to change direction. AC rods have potassium in the composition because it's more easily ionised than sodium aiding the very hot start, the problem is that potassium salts are dearer than their sodium counterparts.
To be honest most rods have both. Both Na and K will help ionisation but higher Na usually means more spatter. It all depends on what else is in there. Like most things there are generalisations but many exceptions to the rule. If it was all K the coating would probably suck moisture from the atmosphere like a sponge. Latter Lithium also made an appearance to help with moisture resistance but it seems to have an adverse effect on shelf life......small bits of coating dropping out a bit like woodworm..

I'm minded of the table on my inverters which reads 80A/23.1V, 100A/24V, 130A/25.1V. I'd guess that was done with a dummy pure resistive load and arcs are going to be different depending on the composition of the rod, but basically you are going to need more voltage to drive more current.

 

[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

I'm sure it still must have an effect on how it welds.a higher OCV is almost always nicer to weld with,what the exact relationship between OCV,rod composition and resultant welding voltage I have not been able to find.
There is a very detailed video on arc force on engine drives and inverters I will try to post a link to

 

[wheresmejumper]

 

[wheresmejumper]

It is more engine drive related but still explains inverter based arc force.not for everybody though

 

[HughF]

I'm in China so the YouTube link won't show up, is that travis's video on volt/amp curves on the red face and short hood?

 

[wheresmejumper]

Yeah that's him