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How to limit current from spot welder?

Discussion in 'Electronic Basics' started by Eric R Snow, Feb 8, 2007.

  1. Eric R Snow

    Eric R Snow Guest

    Greetings All,
    I have a handheld spot welder, the type used for autobody sheet metal
    among other things. I am converting it to a stationary spot welder and
    one thing that would be nice is to be able to limit the current for
    thinner material. The input voltage is 220 volts @ 15 amps. Can I just
    use an appropriately rated power resistor on the input side to reduce
    the current? The welding current is 4000 amps and the suggested weld
    times are specified in pulses at a pulse rate of 60 Hz. For example
    the suggested weld time for .010" mild steel is 4 pulses. This is an
    awfully short time period and I would like to make it a little longer
    so that the timing periods allow more tolerance.
    Thanks,
    Eric
     
  2. Eeyore

    Eeyore Guest

    It depends entirely on what's in the box that 'controls' it. It doesn't sound
    like there's much there. Conceivably your best bet would be to run it on 120V.

    Graham
     
  3. Rich Grise

    Rich Grise Guest

    Tolerance of what? Why do you want to lengthen the time period? If four
    pulses weld .010" mild steel, then why change it? You put your parts in
    the jaws, clamp them down, and weld. Do you want to do .005" mild steel?
    Then try two pulses.

    Or do you just want to slow it down so you can watch the weld taking
    place in slo-mo? ;-)

    Good Luck!
    Rich
     
  4. Eric R Snow

    Eric R Snow Guest

    Greetings Rich,
    I want to slow it down because the timing relies on how fast I can
    turn it on and off. I'm the timer. When doing .005" sheet I melt
    through too often.
    ERS
     
  5. kell

    kell Guest

    Make and model of the welder?
     
  6. Jamie

    Jamie Guest

    get your self a high current mercury relay to run the xformer and
    get 2 adjustable delay on timers.
    the first timer gets started with your clamp switch to insure you have
    full compression. The first timer when it times out and comes on will
    then start the second timer. the Second timer will use a NC (normally
    Close) contact which will operate the main Mercury relay to turn on your
    welder xformer. when this second timer times out, the contacts will
    then open and wait for you to release the clamp to start the next cycle..
    We did this once our self's for a shop, with the use of timers we removed
    the arching that takes place when you're clamping and releasing because
    you never know when the switch gets out of alignment with the electrodes
    and energize under light contact.
    Get your self short delay timers.
     
  7. Ken Moffett

    Ken Moffett Guest

    The problem with electromechanical relays are the delays on
    make and break. In the case of the spot welder, "make" delay
    is no problem. But these delays are on the order of hundreds
    of milliseconds. Since one cycle is ~16 mSec, once the timer
    shuts-off the relay current, it would still power the welder
    primary for 16 or more cycles. What he needs is an
    electronic timer that can have a start-stop response time of
    1 cycle @60Hz, and a solid state relay that can also respond
    in that short of time. The SSR should have a current rating
    twice that of the spot welder.

    I built one of these several years ago, after trying to spot
    weld 0.010" stainless with a large Miller LMSW-52
    (220vac/2.5KVA) spot welder, without a timer. And the timer
    that Miller sold cost more than the spot welder, and stand
    w/foot operated tongs, put together. And it was
    electromechanical, and was never ment to do really short
    cycle times.

    The one I put together is archived in the Metalworking
    Dropbox 2001, under "Miller Time"

    If I were doing a lot of light weight sheetmetal I would get
    a lower powered spot welder.

    Ken
     
  8. Jamie

    Jamie Guest

    yeah, that's all well and fine how ever, you must pay close attention
    to the type of SSR you use to insure zero crossing control and make
    sure there is always a load with a HV suppresser on the electrodes.
    arching loves SSR's
    personally, unless micro spot welding is being done, i don't see why
    the dwell time is so short.
    I mean, it's a manual machine as it is now, how can he even be close
    to that ? I think some one with that kind of speed should be in track :)
     
  9. Phil Allison

    Phil Allison Guest

    "Jamie"


    ** Risky idea - ensures maximum possible " inrush surge " peak currents
    every time.




    ......... Phil
     
  10. Phil Allison

    Phil Allison Guest

    "John Popelish"


    ** Wrong to assume the secondary is always effectively shorted.

    There will be very many occasions when the secondary is open.

    Expect inrush surge peaks equal to the AC voltage peak / primary ohms.

    My well exceed the SSR's repetitive I peak rating.

    Splat.


    ......... Phil
     
  11. Yep. A shorted secondary, current limited transformer (I
    think that is a pretty good description of a spot welder) is
    mostly inductor. If you could add logic to fire the SSR, on
    the first half cycle, near peak voltage, the transformer
    would get less hot and so would the SSR. But the shorted
    secondary also reduces the tendency of the core to saturate
    (except for the small magnetic shunt), so it may not be so
    important.
     
  12. Jamie

    Jamie Guest

    The problem i originally was stating is that Welders arc at various times.
    We have copper tape welders at work that one of our electricians
    tried to use a 3 phase SSR to drive the xformer. Not only was the idea
    bad to start with, the xformer was a single phase type and his thinking
    was that combining all three legs of this SSR would triple the duty of
    the SSR was idiotic. I gave up trying to explain to this individual the
    theory of diodes, SCR load sharing, arching, HV problems etc...

    Needless to say, after shorting a few of these expensive SSR units,
    the problem was past to me. To solve this problem i simply used a large
    mercury relay. Problem solved.

    We do have induction heaters at work that blow an SCR in the bridge
    once in a while from arching. i must say, you should hear and feel the
    HUM that comes out of that before it takes out the fuses!
     
  13. Ken Moffett

    Ken Moffett Guest

    Phil,

    In mine, I use the switch that is built into the lever
    mechanism for the jaws. In the original design it was just a
    power switch in the primary. Jaws close=power on...jaws
    open=power off. But the switch operates after the jaws make
    contact and before contact is lost. In my design it just
    triggers my timer. No timing or primary current unless the
    jaws are closed...and I don't open the jaws until the timer
    has cycled off. I suppose this where you could get powered
    open secondary...if you let up on the jaws on a long time
    cycle.

    Ken
     
  14. Phil Allison

    Phil Allison Guest

    "Ken Moffett"

    ** If the jaws close onto an insulator ??

    Even lightly rusted, paint coated or grime/oil contaminated surfaces are
    insulators, particularly at low voltages.




    ......... Phil
     
  15. Si Ballenger

    Si Ballenger Guest

    You might try making a gizmo to shunt some of the output to the
    weld contacts into another load. Maybe a carbon rod or some thing
    that can handle the short burst of current and reduce the amount
    being put into the weld itself.
     
  16. You seem to imply you believe you can reduce the current and allow for
    longer weld times. That is doubtful. You would need a tapped weld
    transformer or at least a choke to do this.

    A resistor in the primary is difficult. Possibly an electrical heater
    element could be used but you may find the welding performance falls off
    quickly.
     
  17. Jamie

    Jamie Guest

    Phase controlled SCR's can adjust the voltage to the primary side.
     
  18. How well will that work with such a device?
     
  19. amdx

    amdx Guest

    The following response assumes there are no control circuits that require
    the full 220v input voltage.

    Try to find a large Variac, a voltage variable transformer, usually
    configured
    as an auto-transformer. See http://www.elect-spec.com/variac_tutorial.htm
    for more information. This would give you fine control of your spot welding
    current.

    Or,
    you could wire in a series opposing (bucking) transformer on the primary
    side. For example, If you had a transformer with a 40v 15a secondary, this
    could be
    wired in series (with opposite phase) with the primary of your spot welder.
    This
    would lower the spot welder input voltage to 180 volts and the output
    proportionately
    lower. But this only gives you one fixed voltage, and I don't know how do
    you would
    pick the right bucking transformer without testing.

    Or,
    Since large Variacs can be expensive, find a 220v to 110v stepdown
    transformer with
    a 15a secondary rating. Wire this series opposing. Then connect a smaller
    Variac to the
    primary of the stepdown transformer. I think the Variac could be much
    smaller.
    (This off the top of my head)
    If you had the Variac at full voltage, your spot welder would be supplied
    110 volts,
    the spot welder primary current would be 1/2 or about 7.5 amps. That means
    the
    current on the primary of the step-down transformer would be 3.75 amps,
    which is
    the same current supplied by the Variac. So a much smaller Variac.
    I have not crunched any other examples, so I'm not sure this is the worst
    case scenerio,
    I may have it backwards and this could be the best case.
    Group, HELP verify a couple more scenerios)
    Mike
    I got a brand new 240volt 9amp Variac at a hamfest for $15. The seller
    commented,
    "oh, I should have looked in the box before I gave you the price."
     
  20. John Fields

    John Fields Guest

    ---
    Hardly.

    Since the resistance of the materials is causing heat to be
    generated when charge flows through them, the rate of change of
    temperature is going to depend on the quantity of charge flowing and
    the amount of time that charge is allowed to flow.

    In other words, it will take the metal longer to melt with 2000
    coulombs per second flowing through it than it would with 4000
    coulombs per second flowing through it.

    If changing the current doesn't change the time, what do you suppose
    does?
     
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