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MIG Welding, CC Inductor, and bypass thereof

Discussion in 'Electronic Design' started by Ignoramus20083, May 14, 2007.

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  1. This is in regards to an old Hobart CyberTIG welder that I modified by
    putting in a new microcontroller that I programmed in BASIC.

    http://igor.chudov.com/projects/Welding/11-New-Rectifier/

    Basically because I have a separate SCR controller, and separate
    module for controlling it, I can program it to do anything I want, to
    supply any voltage and any current, within my power limits, of course.

    So it is basicallly an arbitrary DC power supply, with a few extra
    relays that control high frequency arc starting and gas valve.

    This thing works just fine in constant current mode, letting me stick
    weld as well as TIG weld.

    Note that it has a big inductor that helps with constant current
    welding.

    Now that I am done with CC mode welding, I started exploring MIG
    welding, which requires constant voltage. Found a LN-25 wirefeed unit
    on Craigslist.

    The issue at hand is that the output of my welder goes through a large
    inductor that is helpful in CC mode, but counterproductive for wire
    feed welding. This inductor weighs perhaps 50 lbs. (?)

    So, for nice wire feed welding, I would need to bypass it. I see two
    ways of doing it:

    1) Make a simple bypass circuit using a big contactor and big cable
    (to be turned on when MIG welding is being done).

    2) Instead of bypassing the inductor, install a capacitor that would
    "cancel out" the effect of the inductor.

    Can that be done at all, is this an insane idea or not? My hope is
    that it would be done with a smaller "work envelope", utilizing
    smaller wires and a smallish capacitor, instead of a bulky contactor
    and big wire, etc.

    Any thoughts on bypassing the inductor?

    thanks

    i
     
  2. Guest

    In order for the capacitor to cancel the current going through the
    inductor, the current through the capacitor would have to be as large
    as the current going through the inductor. So a lot of capacitance and
    large wire.

    Dan
     
  3. The capacitor cancels the inductor impedance, only at one
    frequency. I don't think this approach is feasible for this
    application. I would be thinking in terms of splitting the
    inductor winding and connecting the halves in series for
    stick or tig welding and in parallel (1/4 the inductance)
    for mig welding.

    But you might just be able to short the inductor with a low
    voltage contactor, similar to what is used for cranking
    motors. As long as you don't open it during welding, you
    don't have to worry much about its voltage rating, and the
    current rating will be pretty conservative.
    http://www.mouser.com/catalog/630/1640.pdf
     
  4. Not true actually, the inductor is there to smooth current
    fluctuations due to change in arc length, etc, and keep the arc going,
    so the capacitor would only see action when welding parameters change
    -- hopefully only a fraction of the time. I may very easily be
    mistaken, though.

    i
     
  5. Yep, you are 100% right, a cap won't work. I realized it too. :-(
    Yes, I have a few suitable contactors, one three pole motor control
    type contactor with 65 amp rating on each pole and 110v coil. I would
    parallel all poles. I also have 24v coil, 400A rated DC contactors.

    I collect contactors and motor controls.

    Thanks. I will just do that bypass -- shoould not be a problem, it is
    simple work.

    i
     
  6. Nick Mueller

    Nick Mueller Guest

    Did you *ever* look at the parts list of a MIG/MAG welder before making that
    clueless claim?
    I bet not!

    Oh yes, and voltage is measured in gallons, amperage in miles...


    Nick
     
  7. Don Young

    Don Young Guest

    Why not just install a separate output terminal not including the inductor
    for the MIG?

    Don Young
     
  8. Don, that's another great idea, the simplest so far. The minor
    disadvantage is that it would not go along well with the polarity
    commutator that I have, but it is surely no big deal.

    i
     
  9. Mark Rand

    Mark Rand Guest


    Note that MIG/MAG welders still have inductance in the output, they just have
    less than stick welders. Decent ones also have variable inductance.

    Can you rebuild the inductor to make part of the core movable?

    Mark Rand
    RTFM
     
  10. John Husvar

    John Husvar Guest

    Just peeking at the parts lists on lincolnelectric.com indicates the
    difference between CC and CV welders seems to be capacitance in the
    output circuit at a casual look.

    I'd imagine the inductance would be smaller than that in a CC welder
    too, but adding in switchable capacitance should smooth output voltage
    variations to some degree.

    The question, of course, would be: how much capacitance and how to wire
    it so you can switch between modes easily.
     
  11. Yes. A point was well made that the capacitance value would depend on
    "frequency", but since this is not something where there is a fixed
    frequency, we do not know the needed capacitance.

    Practically speaking, I should simply bypass the inductor by adding
    one more output that bypasses the inductor. It is a $10 solution and
    is totally bulletproof. All I need it bolt a welding cable connector
    to a piece of thick insulation board and mount it into a hole on the
    welder, and connect it properly to the output of the SCR rectifier.

    i
     

  12. Not easily, no, it is all well bolted down to the case of the welding
    machine. I thought that I did not need the inductor at all?

    Esp. considering that I am rectifying three phase (smoother voltage)?

    i
     
  13. Glen Walpert

    Glen Walpert Guest

    The frequency issue was WRT "cancelling" the effect of the inductor,
    not really pertinent to your DC application. The function of
    stabilizing the output voltage against fast arc impedance fluctuations
    suggested by John is not the same and I recommend you give it serious
    consideration.
    The suggestion of looking at how other machines do the CC/CV switch
    was also a good one IMO.

    And don't forget the adjustable droop :).

    G
     
  14. Glen, I am a little lost, John talked about using only a part of the
    inductance?
    Good idea. I looked at the Miller Deltaweld 451 manual:

    http://www.millerwelds.com/om/o232s_mil.pdf

    Please see page 23.

    Its schematic is really very simple.

    It has a three phase transformer, three phase SCR rectifier and a
    control board -- all like I have in my current welder. If you recall,
    I threw away the six phase rectifier and put in a regular SCR
    rectifier with a modern SCR controller.

    Now look at the top right corner of the picture. The first thing you
    see is a fuse-protected capacitor bank C6. (part number 29,
    6*16000uF). Then to the right, you see a transformer Z1, with its
    primary being the positive welding lead and secondary leading into
    "CR1". That "Z1" is called a "stabilizer" and is part number 22.

    CR1 is a 24 VAC, definite purpose contactor (!)

    I am not sure WTH this contactor is doing here.
    G, I will definitely work on the droop and constant power welding in
    general. I need to make some changes that I am afraid to make. That
    is, make the voltage and current adjustment pots be inputs to the
    BASIC controller, as opposed to being inputs to the SCR firing
    controller (as it is now). I am afraid that a bug in BASIC may make
    the welder fry something or someone by outputing a lot more than it
    was asked for. Now, at least, I know that output will be no more than
    what I dialed.

    i
     
  15. Putting an L-C circuit with an arc, you might end up building a power
    oscillator. Look up "Poulsen arc transmitter" in the usual places.
    An arc can exhibit a negative AC resistance: this was used in the very
    early days of radio, to build CW transmitters before tubes were
    available (at that power level, anyway).
     
  16. Joseph Gwinn

    Joseph Gwinn Guest

    When the contactor activates, the second winding on the inductor is
    shorted, which will cause the inductance to become very low. Shorting a
    winding rather than the main winding allows one to use a relatively
    small contactor.

    Joe Gwinn
     
  17. Martin H. Eastburn
    @ home at Lions' Lair with our computer lionslair at consolidated dot net
    TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot"s Medal.
    NRA Second Amendment Task Force Charter Founder
    IHMSA and NRA Metallic Silhouette maker & member.
    http://lufkinced.com/

    Before tubes was hardly radio. That was massive RF noise bursts hoping
    that someone got the interference. Tubes refined to bands and channels.
    Transistors to sub channels while tube and transistors had sidebands.

    When high power is needed, tubes are still used. Solid state is catching
    up but RF power is still complex for solid state for big stuff.

    Martin
     
  18. Jasen

    Jasen Guest

    does it really need to be constant?
    that sounds simplest.
    this will only work at a single frequency.
    The bypass current has to flow somewhere, thin wires won't cut it.

    Putting the capacitor to ground may be more productive, you'd get the
    basic buck regulator topology.

    how much voltage regulation is needed for wirefed welding anyway?

    Bye.
    Jasen
     
  19. Glen Walpert

    Glen Walpert Guest

    Pardon the slow response, I have been in an internet free zone
    (Adirondak mountains).

    Consider what is going on in the arc in GMAW welding: There are
    variations in arc impedance which occur on a fast time scale due to
    the motion of metal droplets within the arc, and there are variations
    on a slower time scale due to changes in average arc length due to the
    difference in wire feed rate and burnback rate.

    As the wire gets closer to the puddle you want the current to increase
    and cause the wire to burn back faster; as it burns back you want the
    current to drop so that the arc does not get too long. Thus the wire
    feed rate determines the average current and the power supply voltage
    determines the average arc length in the CV mode used for GMAW -
    nominally.

    On a faster time scale however a true constant voltage characteristic
    would cause problems due to excessive current fluctuation. Consider
    the 3 "modes" of GMAW: short circuiting mode at low power for thin
    materials where the liquid metal droplets form repetitive short
    circuits, droplet transfer modes at medium powers where fairly large
    droplets periodically break off and transfer without shorting, and
    spray mode at high powers where there is a continuous spray of fine
    droplets in the arc. (Not that there is a sudden transition between
    modes as power is increased, it is actually a gradual and continuous
    decrease in drop size as power is increased.) The variations in arc
    impedance due to droplet position in the arc are obviously worst in
    short circuiting mode and reduce in severity as power is increased.
    But in all cases you do not want to allow droplet position to have a
    big effect on arc current, which should change more slowly with
    average arc length - at a speed which cam be handled by your SCR
    controller.

    The variations due to droplet movement are way to fast for your SCR
    controller to respond to, and you don't want the arc current to
    fluctuate that fast anyhow, so on the fast time scale you want the arc
    characteristics to approximate constant current. Hence an inductor.
    It is there to vary the inductance of Z1, as Joe Gwinn already
    mentioned. When the contactor is closed the current induced in the
    second winding tends to cancel the flux produced by the main winding,
    hence reducing the inductance. Presumably CR1 is closed only in CV
    mode.
     
  20. Grant Erwin

    Grant Erwin Guest

    If you add a capacitor, you are making an LC filter which may strongly affect
    your waveform. When you want a nice rising edge of current, you might get a
    sinusoidal pulse followed by ringing. I think the capacitor idea might be doable
    but it would require a ton of electrical engineering.

    Grant
     
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