3 phase AC welders - how work?

Discussion in 'Electronics & Electrics' started by RussellK, Nov 18, 2012.

  1. RussellK

    RussellK Member

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    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?
     
  2. desertstalker

    desertstalker Member

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  3. Symon

    Symon (Plugging your Socket)

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    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.
     
  4. simmo2302

    simmo2302 Member

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    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.
     
  5. TERRA Operative

    TERRA Operative Member

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    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.
     
  6. Symon

    Symon (Plugging your Socket)

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

    TERRA Operative Member

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    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.
     
  8. OP
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    RussellK

    RussellK Member

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    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.
     
  9. TERRA Operative

    TERRA Operative Member

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

    RussellK Member

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    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?
     
  11. bobbavet

    bobbavet Member

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    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!

    [​IMG]

    [​IMG]

    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.
     
    Last edited: Nov 18, 2012
  12. OP
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    RussellK

    RussellK Member

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

    DarkYendor Member

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  14. TERRA Operative

    TERRA Operative Member

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    ^^^ 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.
     
  15. RobRoySyd

    RobRoySyd Member

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    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_to_single_phase_transformers.htm
     
  16. KrisT

    KrisT Member

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    Voila!

    haha
     
  17. TERRA Operative

    TERRA Operative Member

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    Most three phase transformer based welders use three phase transformers and a three phase bridge rectifier, I doubt any of the welders in for repair at work have a three phase to single phase transformer inside them.
     
  18. HSV_Enigma

    HSV_Enigma Member

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  19. OP
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    RussellK

    RussellK Member

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    I suggest you visit the local TAFE. Most have these huge old welders (by huge I mean around 4ft high), they are most certainly *not* DC, and I suspect they vary rarely break down.
     
  20. OP
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    RussellK

    RussellK Member

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