Wire speed controlling amps?

[malcolm]

Inspired by the last post in rtbcomp's switch positions thread:

I didn't realise that rate of feed affected the welding current

This one has been bugging me for ages. Some welding Overlord once described the power knob on the welder as the thing that controls the voltage (which it does) and then went on to describe the wire speed as the thing that controls the amps (which it doesn't in my view).

My thought is if you up the volts and the resistance stays the same then the amps will go up so more metal can be melted. (Dr Ohm, his law, and all that).

The only way the amps could go up for the same voltage is if the wire got less hot, or the arc gap got smaller. Although that would probably happen to an extent if the wire speed was turned up. I'll move on quickly in the hope that doesn't defeat my argument.

My take on wire speed is it's something that should be set to suit the power setting. Low wire speed for low power setting and high speed for a higher setting. But there does seem to be room for maneuver: for welding end on to an edge I've found I can reduce the current by slowing down the wire feed and holding the tip very close to the workpiece (shortening the arc? - but that makes no sense). Overheats the tip but the welds are neat.

Any ideas on this?

 

[BillJ]

Is the current constant in dip-transfer mode? If not, then maybe the wire speed can affect the average current by altering the percentage of time that the voltage sees a (near-) short-circuit?

 

[malcolm]

Sounds like a possible, though "dip transfer" is another thing that's been bothering me. The Portamig wire feed is so nice and smooth that I can see where the end of the wire is, and it's not going near the weld puddle even on minimum. Seems to be a tiny little ball on the end of the wire a short distance from the weld puddle with an arc coming off it.

The welds work out OK though so hopefully I've not been doing it too wrong.

 

[Cappy]

I'm not fully up with all of the science behind it, but I think you'll find that as MIG has constant voltage so the amps will alter dependent on the length of the arc. As wire speed can be thought of as setting the arc length so it 'appears' to be controlling the amps.

This, as opposed to stick welding where the amperage is constant and the voltage increases with arc length. Often catches people out when they start thinking that if it's all getting a bit too hot the thing to do is to pull away, when in actual fact reducing the arc length is the better action to reduce the heat.

I think that's right, but it could all be garbage

 

[malcolm]

Thanks Bill & Cappy. Seems I haven't understood dip transfer. A little reading of my book makes things clearer:

Turns out dip transfer happens so fast that you can't see it. 100 times a second a little bit of metal streams off the end of the wire and makes contact with the weld pool before leaving the end of the wire. This provides a short circuit while the end of the wire looks like it is a short distance from the weld pool.

The voltage in MIGs stays fairly constant, so there is more current when you get the short circuit than when it's arcing. (current = voltage/resistance) so current goes up when resistance goes down.

The book didn't go so far as explaining whether increasing the wire speed shortened the arc or the increased metal transfer increasing the time spent in short circuit mode. I guess probably both things happen, and both would increase the current for a particular voltage setting. The extremes would be a really long arc and no metal transfer (burn back) or really short arc length and loads of transfer (stubbing).

The current would also go up and down with the power (voltage) control on the machine, although open circuit voltage only doubles between low and high settings, and the max output current can be 6 times the min output current, so the wire speed must be responsible for tripling the current.

I suppose the effect on wire speed on current must be largely academic as there is a fairly narrow range of wire speed for each voltage setting between burn back and stubbing out. Some scope for fine tuning there though.

 

[Cappy]

I suppose the effect on wire speed on current must be largely academic as there is a fairly narrow range of wire speed for each voltage setting between burn back and stubbing out. Some scope for fine tuning there though.

Think you've got it in one

As with so many things in this life there are the basics that can be learnt from books, courses, chatting and writing to other people, and splendid forums like this of course. Eventually though it all comes down to practice and experience as I am sure you are well aware

My general 'rule of thumb' works out at approx (and only approx it is ) 30/35 amps per mm of thickness on mild steel as a setting to start off at anyway. Then it is, as you say, fine tuning, which I am still learning.

Try this

http://www.weldingengineer.com/1mig.htm

About halfway down the page there's a link to the three sounds of MIG welding, at least it gives an idea of what to listen for.

 

[rtbcomp]

Very interesting - I should imagine the science going on in the weld pool is very involved.

The faster the wire moves the quicker it pulls electricity out of the transformer, hence the higher current - only joking - I think!

 

[TechnicAl]

Very interesting - I should imagine the science going on in the weld pool is very involved.

The faster the wire moves the quicker it pulls electricity out of the transformer, hence the higher current - only joking - I think!

Thats right

The stick out also influences the current. If you pull a long stick out the current drops and a shorter stick out will increase the current, its called the IR2 effect.

Simplified, you have constant Volts, constant WFS and the Current varies. This gives you a constant deposition rate with variable penetration

 

[peter9763@tiscali.co.uk]

Hi lads, new on here, got some info for you regarding the above. In my third year doing MAGS, MMA and TIG overhead. Hope it helps.
The power source in MIG/ MAGS controls the arc length. To be able to do this it has to keep the operating voltage constant. Say you move the nozzle a short distance away from the work for whatever reason increasing the arc length and so the voltage rises in sympathy. The voltage being constant as set by the welder at the start forces the arc length to return to its pre-set value even if the welder has not restored the gun to the original position. This happens within one- hundreth of a second and is said to be self adjusting.
The machine will attempt to keep the voltage stable but cheaper machines will be influenced by fluctuations in mains voltage which can cause lack of side and inter run fusion. More expensive machines sense changes in mains voltage and adjust in fractions of a second.Setting welding current is achieved with wire speed control.
Arc voltage for dip transfer 20 volts max. Too high a voltage gives a long arc with irregular metal transfer, poor deposit quality and excessive spatter. Weld tends to be flat and bead wanders. Too low a voltage gives stubbing into the weld pool with a high weld bead which does not flood in at the edges.
Too large a weld = wire speed too high or speed of travel too slow.
Hope this was of use.

 

[malcolm]

Thanks for that Peter and welcome to the forum.

One thing I've always wondered is why MIG welders only go down to around 25 amps minimum current, where you can get TIG welders that will weld tin foil. I've always assumed it is down to the need for a certain amount of power needed to melt the wire evenly.

But having thought about it a little, the wire could be thinner - I've had no problem with 0.6mm steel wire - 0.8mm aluminium wire is a lot more prone to buckling. The voltage is down to where the transformer tapping is, and the amps we've decided are down to wire speed and thickness.

Pricier welders seem to be easier on thin which would suggest it's a control thing. Or maybe there is no demand for MIG welding on really thin metal.

 

[peter9763@tiscali.co.uk]

Response to the wire size is buckling below 0.6 Pushing smaller wire 3/4 metres through a tube obviously does'nt work.

 

[TechnicAl]

As I posted earlier the current also varies with the stick out, that is the distance between the workpiece and the contact tip. Closer increases the current.
This effect makes welding with wires smaller than 0.6mm very difficult. Basically the wire burns back into the contact tip too easily.
I have seen welding with 0.3mm wire but it was experimental and a fancy bit of kit.

 

[peter9763@tiscali.co.uk]

Sorry Al. I thought wire speed controls amps and the variety of stick out is for various applications i.e. stick out of 3-9mm for beyond the end of nozzle in dip transfer for greater visibility and on spray 3-9mm within nozzle for better gas shielding.

 

[TechnicAl]

Wire speed does control amps but the stick out also has a small effect. Its called the I2R effect (I squared R) and basically the resistance heats the wire (like in an electric fire) and the longer the distance the hotter it gets. If its hot it needs less current to melt it which in turn makes it possible to have a constant wire feed speed.
Forget about the nozzle its the contact tip to workpiece distance that matters. The current is picked up in the contact tip and the resistance depends on the distance from there to the arc.
Hope I have explained it well enough to understand.

 

[peter9763@tiscali.co.uk]

Thanks Al. Never seen that equation. Would it be amperage squared times the resistance? Say you were running 80 amps at 20 volts resistance would be 0.25 ohms would you square the amps and times by 0.25?

 

[rtbcomp]

Wire speed does control amps but the stick out also has a small effect. Its called the I2R effect (I squared R) and basically the resistance heats the wire (like in an electric fire) and the longer the distance the hotter it gets. If its hot it needs less current to melt it which in turn makes it possible to have a constant wire feed speed.
Forget about the nozzle its the contact tip to workpiece distance that matters. The current is picked up in the contact tip and the resistance depends on the distance from there to the arc.
Hope I have explained it well enough to understand.

ER the longer the wire, the higher the resistance but we have a constant voltage therefore less current and less heat so lower temperature.

V = IR, I = V/R so higher R less I

Substituting P = IV in the above we get P = (V^2)/R again more R less P, constant volts.

P = power (heat)
I = current
V = voltage
R = resistance (proprtional to length of stick out)

So I would reckon that longer stickout you lose power at the weld.

The wire is melted by the arc, which is hotter than the resistive heating, when you're stick welding and the stick sticks it just gets red hot but when there is an arc it melts readily.

 

[TechnicAl]

rtb.................im glad you did that

So practically, what happens is you get constant deposition rate (constant WFS) but variable penetration because as you say a longer stick out reduces the current at the arc

Peter......thats your answer

 

[peter9763@tiscali.co.uk]

Thanks for that lads. I guess thats why the more expensive machines have inductance settings you can alter to prevent servere rise and fall of current where the cheaper ones are set. More spatter due the explosive fuseing. Another reason not to buy cheap machines.

 

[poor farmer]

In Dip Transfer Mode you are controlling the globular transfer rate and the pinch effect.
The wire short circuits to the work piece,
Without an inductance transformer there would be a massive explosion of energy, leading to lots of vaporised metal in the arc some of which ends up in the weld pool
With a “choke” the explosion is controlled (stalled) so that a globule forms on the end of the wire
As the globule forms (fed by the wire behind it) the pinch effect starts to come into play.
The arc is providing the energy for the globule to grow, the globule is being fed by the incoming wire; as the globule grows it absorbs more wire, the wire immediately behind the globule starts to reduce in diameter (the pinch effect) until a point is reach when the diameter is to small to carry the current. It’s at this point when the metal (globule) is transferred to the weld pool.
The optimum frequency for this process is 200-225cps.
By varying the distance the torch is from the work piece you are varying (slightly) the parameters of the above cycle, more stick-out equal’s greater resistance equals a slower globule growth, short stick-out equals less resistance equals faster globule growth.

In the Spray Transfer Mode (normally a longer arc) you increase the welding voltage to a point where a sort of equilibrium is established between the torch and the work piece. You have more volts/energy available which enables a longer arc, once the arc is established instead of a globule developing the incoming wire is immediately vaporised and transferred via the arc to the weld pool.