The relationship between volts and amps on a MIG - could you connect two machines together?

[Bill Edwards]

As some may know I'm currently modifying a nice, new 4 roller wire feed unit to run off a cheap 150A welder.

Now, I've just had a thought:
I have two 150a SIP units that have the usual wire feed issues are are essentially scrap - they've been sat for years as a result, but I'm sure the transformer side of things will still be fine.

Could I connect them both together in parellel - just connect the earth leads together and the welding output - and have a welder with 300 amps?
If they were MMA welders on DC I couldn't see any issues with two units giving the same voltage, you'd have double the amps, or the ability to run both machines at a lower setting and have an increased duty cycle.
But MIG seems to vary the voltage rather than the amps and I don't understand how that all works, so what would happen with double amps but the same voltage?

 

[rtbcomp]

1) If for arguments sake you connected 20v and 30v together the 30v unit would supply all the current, until such time the load dragged it down to 20v, then both units would supply current, subject to 3) below.

2) If you connected 2 x 30v units together they would both supply equal current provided their internal resistances were the same.

3) If their internal resistances were different then they would share current inversely pro-rata to their resistance.

If the units are identical then you should be OK as long as you kept the voltage selector switches set to the same on each unit.

 

[Bill Edwards]

Thanks, I have no trouble understanding that.
What I struggle with is the importance of the voltage in relation to the amperage. I can understand with stick welding it's mainly about the amperage. You want more heat, you increase the amps, but voltage doesn't come into it. OK, it's still important as it makes for easier and smoother welding, but it isn't something you deliberately alter when you change a setting.

But with MIG it seems that you change the volts rather than the amps - though I'm not sure if this changes too? Certainly a machine with higher amps is needed for more heat - i.e big stuff - but does this require a corresponding increase in voltage too?
Does voltage have a direct effect on the method of transfer in use?

 

[rtbcomp]

The amps will change as a consequence of changing the voltage. Increasing wire speed and size also increases welding current, so by altering these parameters you can get diffferent currents for a given voltage.

 

[Bill Edwards]

Thanks. So will doubling the amperage but not the voltage (i.e two 150A units in parallel work OK at maximum with sufficient wire speed and size, without being low on voltage? Or does the voltage also need to increase?

 

[rtbcomp]

An amperage figure of 100 amps is saying there are 100 amps available, it does not mean you're going to draw 100 amps, so if you're welding at upto 100 amps with machine that is rated at 100 amps max. doubling the supply from 100 to 200 won't make any difference. (It may make a small difference in reality at max output because you'll be reducing the internal resistance of the setup, but I doubt whether the difference would be significant).

 

[Bill Edwards]

So are you saying that by connecting two 150a machines together I won't be able to use thicker wire and effectively have a 300a machine instead? I'll still be stuck with a low powered (150a) setup?

 

[rtbcomp]

Just the opposite, you'll be making 300A available.

 

[TechnicAl]

Bill...........a MIG set is CONSTANT VOLTAGE so the current varies......you set the wire feed speed and the voltage and the machine sets the current needed to melt the wire. The ESO (distance between the copper contact tip and the workpiece) also affects the current, closer and it increases, longer and it decreases (resistance heats the wire)

a Stick set is CONSTANT CURRENT so the voltage varies. The voltage is by and large determined by the electrode and the way it melts. The OCV also affects the voltage.

 

[Bill Edwards]

Ah, thanks - I'll get on with checking they still work then, while dreaming of a 300A MIG! In fact, if I got a third one - I guess I need to stop somewhere

So I've got it the wrong way round? Why is it that stick controls always tell you that you are altering the amps, not the volts?
Years of welding hasn't taught me much about how they work

 

[rtbcomp]

When we say stick is constant current we mean the current stays constant for a particular setting, eg set it to 90 amps and the current stays at 90 amps whilst welding, even though you may use different diameter rods or hold the electrode a varying distance from the workpiece.

With MIG you set it to 30 volts (or whatever the switch settings allow you to) and the voltage stays constant, but the current varies as described above.

On most MIG sets the switches tend be labelled arbitarily as 1,2,High,Low but the actual current in amps is indicated on stick sets.

 

[Bill Edwards]

Ah, thanks for that, much more clear!

 

[the snooper]

don't forget you will need a rectifier capable of handling 300amps

 

[rtbcomp]

He will need to retain the original rectifiers and connect the DC outputs together, as suggested in an earlier post.

Otherwise you'll get a backfeed through the transformers, and if they were connected out of phase a very loud bang when it was switched on.

 

[AndyStobbs]

Might start pushing the limits of the supply too!

 

[the snooper]

given the fact its is 2 separate transformers each with their own rectifier, wired correctly you could power each transformer via separate sockets

 

[Bill Edwards]

I was thinking that if I take the connections from after the rectifier (i.e the normal torch and earth connections) then there shouldn't be any way that they could backfeed, I'll just be connecting two DC supplies together with no way back. It'll only be me using it so I'm able to make sure both machines are run at the same power settings, and when I want low power I can just shut one off altogether. I'm thinking that not only will I have a lot more power available (300a ) but it must greatly improve the duty cycle when running at say 150 amps, split between both at half power.

I think I'm about to get a very good MIG setup at last, for very little money.
It's a pity that the nice MB15 welding torch I've just bought is only rated to 150 amps though

 

[the snooper]

you could also set up relays to one power switch so both are operated from one switch

 

[Bill Edwards]

I think both units are fan cooled so should be on all the time, but I already have a relay ready to connect up to the wires that will tell it to supply power (and normally feed wire, send gas etc). Sadly, my bright idea and your confirmation of such has left me with a relay with one set of contacts that now needs to switch two welders, not one. It looks like I'll have to add another to get everything working as it should when I pull the trigger.

I'm looking forward to this welder. It should only pull around 26 amps flat out (in theory, as both welders run off a 13a fuse), plus a little for the wire feed - should be very minimal.

 

[the snooper]

wire both fans to which ever transformer you will use a s the primary unit when you don't need high amps