How does a welding rig control current?

[MarkS]

I've been interested in learning how to weld. While doing research, I found out that most welding rigs have very wide (<20 to hundreds of amps) , adjustable current regulation. They also operate at varying frequencies from below mains frequency to hundreds of Hertz, but this is accomplished via IGBTs. How do they regulate current to such a wide and adjustable band? Most units appear to be digitally controlled, even if it appears to be using a potentiometer. What sorcery is this?

[coppercone2]

The measurement of the electric current can be classified depending upon the underlying fundamental physical principles such as,

    Faraday's Law of Induction
    Magnetic field sensors
    Faraday effect
    Transformer or current clamp meter, (suitable for AC current only).
    Fluxgate sensor, (suitable for AC or DC current).
    Hall effect sensor(suitable for AC, DC, or pulsating current), a type of current sensor which is based on the Hall Effect phenomenon discovered by Edwin Hall in 1879.
    Shunt resistor, whose voltage is directly proportional to the current through it.
    Fiber optic current sensor, using an interferometer to measure the phase change in the light produced by a magnetic field.
    Rogowski coil, electrical device for measuring alternating current (AC) or high speed current pulses.
    Giant Magnetoresistance(GMR): Magnetic field sensor suitable for AC & DC Current with higher accuracy than Hall Effect. Placed parallel to the magnetic field.






Imagine if a BIG LM317 in current mode, got rid of the resistor, and used a magic box across the bus bar connected to the 'sense' pin.

[MarkS]

I know how it's measured. How it it created?

[coppercone2]

big magnetic device

A server power supply gets you close to a thin sheet metal welder

[rstofer]

One control scheme uses a special transformer called a saturable reactor where a small DC current controls the much higher AC current



I ran into these back in the early '60s on precision TIG welders.  The foot pedal controlled the DC current and the reactor controlled the much higher arc current.

[SoQ]

I don't know what they use in commercial welding rigs, but I'm interested in this question since I am in the research phase of building a small hobby-scale spot-welder.

IGBTs appear to be used because they have lower switching losses than MOSFETs.  I'm not sure how true this is; there are so many devices.

I'm guessing they pulse the electrode to generate the plasma arc with an effective average current matching the panel setting.  If I was making a stick welder, that's what I might try to do.  (I haven't done any research on this.  I'm curious to learn whether a chonky inductor on the output wouldn't help starting the arc.  Edit: An IGBT doesn't have the body diode, which I'm going to suggest is the main reason to use them in this kind of scenario.) 

In my application I want lots of amps at a low voltage so there are two basic strategies.  A timed short-circuit through the workpiece  or a beefy DC-DC converter to do essentially the same.

I'm told that two or three car batteries can drive stick welding but I have not tried it.  I'm a total noob at electronics so this may be completely wrong.

[tautech]

I've been interested in learning how to weld. While doing research, I found out that most welding rigs have very wide (<20 to hundreds of amps) , adjustable current regulation. They also operate at varying frequencies from below mains frequency to hundreds of Hertz, but this is accomplished via IGBTs. How do they regulate current to such a wide and adjustable band? Most units appear to be digitally controlled, even if it appears to be using a potentiometer. What sorcery is this?

There is no universal answer.

Arc welders come in various forms, AC, AC HF, and DC, voltage or current controlled.
Arc in various forms; stick, TIG or MIG are the most common.

One need select the type of welding needed and then select the type of welder best suited to the task.

Historically welders used sliding chokes or tapped outlets to vary current and/or voltage.
They were large heavy beasts now much smaller and lighter due to the use of silicon based devices to control output.

Think SMPS.....now much more reliable than early designs.

[coppercone2]

if the inverter is designed sanely, sure