Another Migatronic 5000 MX

  1. Rod06 New Member

    Messages:
    4
    Location:
    England North Yorkshire
    Hi,
    I have come on this site to try and help my local garage, who are doing some work for me, when I mentioned that my background is electrical/electronic engineering (now retired) and he asked if I might know what was wrong with this welder. Well, it's a bit of a specialized area, but hope someone might have come across the fault.

    The unit works ok, but he tells me that the gas contactor (relay) suffers from quite bad arcing. I suggested that cleaning the contacts (if this is possible-I've not seen inside the welder) might help. I also see on a circuit for another welder (180 MXE) of the same make (could not find a circuit for the 5000 MX model on line) has a bridge of capacitors, but it looks like these are not to protect the contacts but in fact power factor correction.
    Maybe the contactor/relay simply needs replacing?

    Any help or advice will be appreciated and passed on.

    Many thanks, Rod
     
  2. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,615
    Location:
    Sheffield UK
    I believe you can get semiconductor snubbers which are more efficient than the usual resistor/capacitor types, may be worth a try.
     
  3. mike 109444

    mike 109444 Member

    Messages:
    4,254
    uk Bristol
    Hi Rod. You can think of a mig as a DC power supply with a multi tapped primary (this gives you the power selection) and a heavy current capable secondary (hence the thicker winding. This is followed by the diode pack (rectifier) usually stud or press fit diodes on ally plates. The Dc out from this feeds int a choke and the capacitor bank. Choke is usually a single winding transformer type core.
    The contactor is usually in the feed to the primary and is controlled by pulling the trigger on torch which via the pcb operates the contactor. The gas valve will be a solenoid an so should not be the "sparking" issue and it will be as you suspect the contactor contacts sparking. Head ach will be matching up a replacement contactor rated for the machine. There should be labels and markings on the contactor which should allow for cross referencing a replacement if a direct replacement s not available.
    The link below is from google image search and happens to be from a post this forum. If you look at the second image I think you will find the contactor is hidden under the wiring in bottom left corner next to the aux transformer
    http://www.mig-welding.co.uk/forum/threads/migatronic-5000mx.21831/
    Note there machine MAYBE 3 phase and so would req a 3 phase contactor, check first!
    And welcome BTW :hug:
     
  4. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,615
    Location:
    Sheffield UK
    I think he means the gas relay, rather than the main contactor.
     
  5. mike 109444

    mike 109444 Member

    Messages:
    4,254
    uk Bristol
    Looking at the image there does not look to be a relay on the pcb so guessing it's switch by transistor(s) on pcb or from set of contacts of contactor.
     
  6. mike 109444

    mike 109444 Member

    Messages:
    4,254
    uk Bristol
    Looks like the same pcb is used in the 5000 and the 180 !
    So should be possible to replace C4 and R4 which may help.
     
    • 180 inside.jpg
    • pcb components.jpg
    • 5000.jpg
  7. Rod06 New Member

    Messages:
    4
    Location:
    England North Yorkshire
    Many thanks Rtbcomp, sounds like a good idea but we shouldn't have to modify the existing circuit. Yes, I did mean the mains input contactor/relay to the primary of the main transformer

    Thanks Mike, yes I worked out that the contactor or relay which ever you prefer, simply switches the primary of the main transformer via a signal from the gas valve, which obtains it's supply from the mains input via a smaller transformer, although as I said, I'm using a circuit diagram from a different but I suspect similar model.

    Having spend a little while looking at how it works, I can't really see any reason why this should arc apart from worn contacts unless there are some shorted turns on the main transformer which would cause excessive current to be drawn on the primary.

    Many thanks for your replies and advice. I see that there is a website for Migatronic, so depending how old this welder is, maybe they could supply a replacement contactor/relay?

    Regards, Rod PS I'm more into electronics but like to help where I can. Maybe I'll get a bit of discount on my car repair LOL!
     
  8. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,615
    Location:
    Sheffield UK
    Because you can see it arc I assume you're not welding at the time, so it's only switching the quiescent current. Maybe the contacts are sluggish, is there any dust &c causing a bind between the coil and armature in the contactor?
     
  9. Robotstar5

    Robotstar5 Casanunda Staff Member

    Messages:
    18,071
    Location:
    Birmingham
    Some manufacturers loop across the contacts to spread the load, maybe one set of contacts has failed and the others are arcing under the increased load?.

    ...and welcome to the forum.
     
  10. Rod06 New Member

    Messages:
    4
    Location:
    England North Yorkshire
    Hi rtbcomp, I don't know the answer to that, but I assume he must have seen it when welding since the relay should be open unless the gas is flowing? The control panel etc, as you no doubt know, is powered by a small second transformer derived from the mains before this relay. I don't know much about mig welders but am I correct in thinking the gas flow will only occur when the trigger is pressed to feed the wire? That makes sense since it's the gas valve (according to my circuit) that activates this relay.

    Hi Robotstar5 and many thanks for the welcome (And to Mike, sorry Mike, forgot to say yesterday). Yes connecting unused contacts in parallel sounds like a good idea, and your theory makes sense. I have not seen the cover off this welder yet (the garage owner simply told me about it) so I don't know if it will be possible to get to the contacts to clean them and add some contact (switch) cleaner. If not, I see that Migatronic have a web site and they may be able to supply a new relay? I have no idea how old this welder is, but as long as the current rating and coil supply are the same, then a generic mains relay would probably be ok. The primary current will be much lower than the secondary, but I don't know off hand what it would be. It would be quite easy to work out with more info.

    Once again, thank you for your kind help, but I will have to ask the garage owner for more info and indeed, need to see the problem in action!

    Kind regards, Rod. (PS I spent most of my working life fixing TV's, video's and audio equipment except for my final job at college trying to educate electricians LOL!) Glad I'm retired!
     
  11. bwservices Member

    Messages:
    397
    Location:
    Ireland
    Should be £40 for a contactor so worth a try.it will always have the needed info written on it.
    if you want to spend double,ask migatronic for one.
    contactors wear,theyre a consumable part of an old welder.
    if the contactor has a normally closed auxiliary, it could be to discharge the smoothing caps after each cycle,and are known to spark if a bleed resistor has failed.
     
  12. mike 109444

    mike 109444 Member

    Messages:
    4,254
    uk Bristol
    If you take a look at the data plate on the welder it should show the max current it should draw. This will allow you to sus what (over)rating the contactor will need to be as it's on the primary side. Looks like the contactor coil should be AC which should be cheaper than a DC one!
     
  13. Rod06 New Member

    Messages:
    4
    Location:
    England North Yorkshire
    Hi bwservices, yes they often use such bleed resistors in smpu (switch mode power supplies) which are very common these days. They also use them in microwave ovens since they are potentially lethal (very high voltage but low impedance = high current) and indeed we had an engineer killed with one of these, but not at the service center where I worked. I know what you mean about manufacturer parts. In my previous work, Sony were very expensive for parts, where often a generic part would do but may invalidate the warranty.

    Hello Mike, yes I thought there might be some info in the contactor / relay. I'll have to arrange to see inside this welder.

    I think the real reason I came onto this forum was to try and find out of this is a common fault with such welders? I know in my old trade we had our share of common faults which when known about, saves a lot of time and possibly money in misdiagnosing faults. (who me?.....never LOL!).

    Once again, many thanks for your kind help.

    Old uni joke, Question: How many psychologists does it take to change a light bulb?

    Answer: Does the light bulb really want to change?
     
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