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Plasma cutting sequence

Discussion in 'Electronic Design' started by Ignoramus27153, Jun 2, 2006.

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  1. Could anyone say what is the proper plasma cutting sequence? How about
    this:

    when trigger is pulled:

    - open air valve for a certain amount of time (like 0.25 sec)

    - Then start current

    * When arc voltage is above certain value, turn on HF, turn off HF if
    voltage drops below that

    When trigger is released:

    - stop power
    - keep air valve open for a period of time, like 5s

    does it make sense?

    I am programming my microcontroller, which I mostly did for stick and
    tig (proper relays are closed when they should be), but I have no idea
    what to put in for plasma.

    thanks

    i
     
  2. Carla Fong

    Carla Fong Guest


    I've never measured what our machine (Hypertherm) does, but that
    certainly sounds like what the sequence is.

    I think the post cut air purge is 8 seconds on ours... Probably just
    enough time to cool the torch components. Likely not a critical value.
     
  3. Thank you.

    It's OK, the numbers could be fudged with, I just need to get the
    sequence right. I already programmed this particular sequence (with
    delays and voltage thresholds as parameters)
    Sounds good, I will change my value from 5 to 10 then.

    A ignorant question, do plasma cutters need only air? (no second gas
    going through another line)?

    i
     
  4. woodworker88

    woodworker88 Guest

    The purpose of the post cut purge is to cool the torch components
    significantly. Since in a plasma torch the arc is contained within the
    nozzle if you shut off the air, you will have a puddle of copper
    electrode in the tip. I think mine depends on the time the torch has
    been on, but I have heard it go on for as long as 30 seconds. Makes
    sense, the hotter the torch, the longer it should run.

    Plasma cutters need only a compressed gas to plow the hot metal out of
    the cut. Obviously the most economical is clean, DRY compressed air
    but professional plasma cutting often uses dry nitrogen instead (no
    oxidation when cutting Aluminum or Stainless). I've never heard of
    anyone plasma cutting with shielding gasses (argon,etc).
     
  5. Got it. Will increase the limit. Thanks.
    Thanks, that's nice. I do have a compressed air dryer, actually.
     
  6. Pete C.

    Pete C. Guest

    Suggest you check out the plasmacutting group on Yahoo groups. It has a
    couple techs from plasma cutter manufacturers as regulars that could
    give you a lot of info.

    Pete C.
     
  7. Thanks. I am also a member of homemade welders yahoo group, but it is
    pretty quiet. I joined the plasmacutting group and already asked my
    question.

    Thank you!
     
  8. Steve Sousa

    Steve Sousa Guest

    Are you using the same welding machine you talk about here for plasma
    cutting?
    If so, what did it take to make the change?

    Thank you.

    Best Regards

    Steve Sousa
     
  9. Yes, the same machine. Its transformer has six secondaries, two per
    each leg of three phase. If I connect these secondaries in parallel
    (two on each leg would be paralleled), delta connected, I get 66
    VAC. That's for welding.

    If I connect these secondaries in series (that is, two in series on
    each leg), Wye connected, I get 220VAC on each leg. That's what I
    would use for plasma cutting.

    I do this reconnection using contactors, three of them connect for low
    voltage, and two for high voltage.

    This partis already working (contactors are mounted and doing all
    that reconnection).

    i
     
  10. I hope the insulation resistance on those secondaries is high enough for the
    higher voltage series connection.
    220v isn't very high, but the inter-winding insulation might not be designed
    to take that under continuous duty, since the transformer was designed for
    66V operation.

    LLoyd
     
  11. Remember that voltage from one turn of a winding to the next turn is
    pretty small (and does not change when I reconnect the
    transformer). These windings seem to be fairly far apart.

    See a picture here:

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


    i
     
  12. I wasn't refering to the inter-turn insulation; inter-winding. If the
    windings are separated from one-another and any other point that might be
    called "common" by enough insulation, then it'll work. I wasn't nay-saying,
    just raising an issue.

    My first real job was in the test lab of Florida Transitron in DeLeon Spring
    Florida. My job was to "hi-pot" all the prototypes, and to set up burn-in
    rigs for temperature-rise and core-loss determinations.

    LLoyd
     
  13. Wayne Cook

    Wayne Cook Guest

    I know you've had pretty good luck so far on your conversions and I
    commend you on what you've achieved so far. However my experience
    tells me that what you're about to try is going to be fraught with
    disappointments.

    Plasma cutting is much more complex than most people think. But I'll
    try to list the basics real quick (I really don't have time to get
    into details at the moment, we are having to take turns watching my
    wife at the nursing home at the moment).

    To start with you have the air coming on, the high frequency engages
    and initiates the pilot arc (this is something you've not taken into
    account). The pilot arc is a low amperage arc from the electrode to
    the nozzle but it's fed from the main cutting arc (this is where I had
    trouble fitting the new torch to my old PCM-100 plasma cutter). If the
    pilot arc is to hot then it'll fry the consumables, to low and it's
    not long enough to initiate the main arc. The main arc is initiated
    when the pilot arc crosses to the work piece. This is the main cutting
    arc.

    Things to consider are to much amperage or voltage, to sudden a shut
    off, to sudden a ramp up, and many other little details play hell with
    the consumables on a plasma cutter. This is what makes the more modern
    machines so much better than the old ones. They have better control of
    the whole process which is what makes the cuts cleaner, the
    consumables last longer, and the overall process much nicer.

    Good luck.
     
  14. Thanks Wayne. Yes, I have to consider the low amperage pilot arc
    indeed. I will try to rewrite my procedure to account for it. In any
    case, I think, it makes sense to get welding right (stick welding is
    already working, but for TIG I would use the controller). So I have
    quite a while before I can try plasma. As for modern plasma cutters,
    mine is controlled by a microcontroller, so I can program it to do
    anything I want (knowing what to do is a problem). I wish that I could
    have a local friend with a plasma cutter to see how they work.

    I will post more on this.

    i
     
  15. Jim Thompson

    Jim Thompson Guest

    [snip]
    [snip]

    My sympathies. Just did that with a son... it's really tough.

    ...Jim Thompson
     
  16. Wayne Cook

    Wayne Cook Guest

    Thanks.
     
  17. I'd consider a ramp up and hold ... then ramp down (when trigger released)...

    The ramps prevent surge currents that might bite the electronics and might punch
    holes through the metal...

    I'd go to some of the sites - Hypertherm is a good site - see tech docs for how things work.

    Martin

    Martin H. Eastburn
    @ home at Lions' Lair with our computer lionslair at consolidated dot net
    NRA LOH & Endowment Member
    NRA Second Amendment Task Force Charter Founder
    IHMSA and NRA Metallic Silhouette maker & member
    http://lufkinced.com/
     
  18. Kevin Y

    Kevin Y Guest

    There's an guy on ebay every once awhile selling plans to convent your
    welder to a plasma cutter. There like $3 plus shipping maybe there
    something useful in his plans I don't know. I thought I'd throw that out
    there.

    His torch is made from PVC pipe which bring a smile to face every time I see
    his item for sale.
     
  19. Oh, and I must say that I am sorry about that.

    i
     
  20. kidkv

    kidkv Guest

    PLASMA GAS SELECTION CHART
    Gas Recommended For Advantages Disadvantages
    Air

    * Carbon Steel
    * Stainless Steel



    * Clean Fast Cut on C.S.
    * Affordable
    * Convenient



    * Short Electrode Life
    * Nitriding on Cut Surface
    * Oxidation on S.S./ AL

    Nitrogen

    * Stainless Steel
    * Aluminum
    * Carbon Steel



    * Excellent Cut on S.S./AL
    * Excellent Electrode Life
    * Affordable



    * Nitriding on Cut Surface

    Argon/Hydrogen

    * Stainless Steel
    * Aluminum



    * Excellent Cut Quality and Speed on Thicker Material (> 1/2 in.)
    * Less Smoke/Fumes



    * Expensive
    * Not for C.S.

    Oxygen

    * Carbon Steel



    * Clean Cut
    * No Surface Nitriding
    * Fast on C.S.



    * Short Electrode Life
    * Oxidation on S.S./AL
    There are various types of torch cooling systems. Low-amp torches (150
    amps or less) can be cooled by channeling a secondary gas through the
    torch. Higher-powered torches use water and require a cooling system
    with a reservoir, pump and heat exchanger. It's very important to use
    de-ionized water for cooling, since the coolant (water) may contact
    both negative and positive potentials inside the torch.
    Theory: Sequence of Operation

    When an operator gives a start cut signal, the system energizes and a
    prepurge of gas lasting a few seconds will flow through the torch. This
    ensures that proper gas flow is available before an arc is struck. The
    initial arc, which ionizes a portion of the plasma gas, is generated by
    a high-voltage spark between the nozzle and the electrode. Current
    flows through the ionized gas (plasma) to the nozzle.

    The gas flow pushes the arc out of the orifice where it reattaches to
    the outside of the nozzle, forming a J-shaped pilot arc. In some
    systems, the pilot arc is controlled by a timing circuit. Other systems
    have an arc transfer sensor to detect the current change (when the
    cutting arc takes over) and switch off the pilot circuit. Some systems
    also have an automatic restart pilot, which is useful when cutting
    grating or expanded metal. With automatic restart, the torch can cycle
    back and forth between "pilot" and "cut" modes as long as the start
    signal is present.

    The pilot arc forms a "pathway" to the metal. If the torch is close
    enough to the workpiece, then the arc will transfer from the electrode,
    through the nozzle, to the metal. Once this transferred arc is
    established, it will continue as long as there is metal to transfer to.
    The constricted plasma jet concentrates the energy of the arc on a
    small area of the workpiece, heating it to melting temperature and
    blasting the molten material out of the cut.

    When a stop signal is given (or the start-cut signal is shut off), the
    cutting arc stops and the gas continues to flow for a few seconds to
    cool the torch and torch parts. It also shields the electrode,
    preventing outside air from reacting with the electrode element as it
    cools.
    hay i hope this helps out some:))
     
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