3 phase AC welders - how work?

[RussellK]

Back when I did a welding course at the local TAFE, the machines were massive old 3-phase machines, which we were told provided a smoother arc than the single phase AC machines we'd use at home, due to the combined phases not having the same zero-crossing extinguishing the arc 100 times a second.

So I assume these machines have a 3-phase to single phase transformer in them - does anyone know of a diagram for one of these, and what the output waveform looks like?

[desertstalker]

Welders are DC aren't they?

If so they likely use this http://en.wikipedia.org/wiki/Rectifi...ase_rectifiers

The resultant DC is smoother with more phases, as the waveforms will overlap. I doubt they bother to do much smoothing on welder supplies (the caps would need to be pretty massive).

[Symon]

Originally Posted by desertstalker

Welders are DC aren't they?

Not all of them.

The older 3-phase welders were just a single phase welder with a 415V primary (only used two of the three phases). The more expensive ones had a rectifier that gave you DC put still was only single phase on the primary.

Later on the DC units had a full 3-phase rectifier to give an unregulated DC output, but the ones these days have a full active front end which regulates the DC output voltage.

[simmo2302]

Originally Posted by desertstalker

Welders are DC aren't they?

welding mild steel, stainless steel and titanium are DC negative i think. electrode is the negative pole, the earth clamp is positive pole.

aluminium welding is high frequency AC.

[TERRA Operative]

Smoothing is used on many welders, using big capacitors almost the size of a coffee cup (I have a few sitting here).

The output is smoother on a three phase machine due to the DC from the rectifier being smoother as mentioned above.
However, with inverter technology, it's all pretty much the same. The thee phase units just kick out more current.


For MIG, mild steel uses a positive electrode, as does stainless and aluminium.
However, when migging gasless/cored mild steel wire, you go for negative electrode.
For TIG, steels use negative electrode and aluminium uses AC.

[Symon]

I always thought that for consumable electrode it was positive to the electrode, and for non consumable electrode it was negative to the electrode?

[TERRA Operative]

Not for cored or gasless wires in MIG machines. They need reverse polarity (negative electrode).

Other than that, what you say is pretty much true.

[RussellK]

To clarify, these were AC output units used for stick welding. You can still buy cheap-arsed versions of such things, often referred to as buzz boxes.

If it is the case that these so-called 3-phase units were in fact just using 2 phases, the claimed difference in zero-crossing compared to a single-phase unit is a load of crap.

With regard to polarity, in stick welding the general principle is that running the electrode negative (workpiece positive) puts more heat in the work - this is often called straight polarity. Conversely with the work negative and the electrode positive, more heat goes into the electrode, and can help with welding thinner sections - it's also called reverse polarity.

All that said, you'll find plenty of welders who swear by one polarity or the other.

[TERRA Operative]

Originally Posted by RussellK

If it is the case that these so-called 3-phase units were in fact just using 2 phases, the claimed difference in zero-crossing compared to a single-phase unit is a load of crap.

That depends if they are actually three phase, or 415v single phase.

[RussellK]

Originally Posted by TERRA Operative

That depends if they are actually three phase, or 415v single phase.

Well, that was my question - if they are indeed using all 3 phases to produce a single phase AC output, how is the transformer configured, and what does the output wave look like?

[bobbavet]

A single phase is the same frequency wave as the input power 50Hz. I should imagine a 3phase being three evenly superimposed waves of 50 Hz. Effectivelly creating a 150Hz power , hence a more stable arc and easier arc starting.

As far as I know a step down iron core transformer the output wave characteristics remain the same, It is the Voltage that is lowered and Amperage adjustable.

Wahla!





Some of the early "switch type" 415 AC welders also had a High frequency unit built in to bump the power to the 1000's of Hz so they could be used for Aluminium Tig welding.

[RussellK]

But wouldn't there be a summing effect, assuming the step-down transformer had a single winding for the output?

[DarkYendor]

Rectified single-phase (lots of zero crossing):

Click to view full size!
Rectified 3 Phase (never gets near zero):

Click to view full size!

[TERRA Operative]

^^^ This.

Of course, with the smoothing capacitors (if fitted) the waveform won't get near zero, but it is a lot smoother with 3 phase still.

[RobRoySyd]

Originally Posted by TERRA Operative

^^^ This.

Of course, with the smoothing capacitors (if fitted) the waveform won't get near zero, but it is a lot smoother with 3 phase still.

Yes, but with high current DC supplies, a 3 phase bridge rectifier gives around 3% ripple and at 300Hz. If you want to filter out that last bit of ripple you need way less capacitance than for a single phase supply.

Yeras ago now I built a 110V 50A variable supply. Just a 3 phase Variac feeding a 3 phase tranny and a fan cooled bridge rectifier. From memory I also put 4 pretty big caps on the output, mainly because we had them lying around as spares for our big UPSs.

Anyways none of this is answering the OP's question which comes down to "is it possible to make a transformer that has a single phase secondary and a 3 phase primary that spreads the load over the 3 phases". I think the asnwer is Yes but it's a beast of thing.

This might be it: http://en.wikipedia.org/wiki/High-leg_delta

Pictures of them from one supplier: http://www.polyphaz.com/Three_phase_...ansformers.htm