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Overheated Transformer or Failed Power Factor Capacitor, Diagnose & Repair

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  • Overheated Transformer or Failed Power Factor Capacitor, Diagnose & Repair

    Will a welder continue to work if the Power Factor Capacitor fails? What exactly happens when a transformer overheats?

    I just picked up a 1954 61F. It looks like someone tried to repair it. To open the welder, the top handle needs to be removed. It is held with a cross-pin that rusts in place on all of these units. On mine, the sheet-metal cover around the shaft the handle attaches to is seriously bent on both sides, from someone trying to pound out the rusted handle pin. Then they used a grinder to cut off one side of the cross-pin and handle base, started on the other side, and gave up. Someone really wanted to open this welder, probably because it stopped working.

    There are only 3 things inside a 61F: On/Off switch; PFC; and transformer. The switch is accessible through the vent opening in the back of the shell, and no evidence that someone tried to work on it. That leaves the PFC and transformer.

    Looking at the electrical schematic it seems the PFC is additional to the main wiring, not carrying all the electricity flowing through the welder. Or maybe that is just the way it looks because the PFC wiring diagram is on the same diagram as the non-PFC unit. It was built in 1954, so the capacitor is 62 years old, and they do not last forever. Hopefully that is what failed, and bypassing it will get the welder running again.

    The welder weighs 263 lbs, and I’d guess the transformer unit is 175 lbs of steel and copper. Not easy to overheat while stick welding at 180 amps max output. (Miller Thunderbolt or Lincoln AC-225 at 225 amps, same 20% duty cycle, are only 110 lbs for the entire unit). There is no fan or thermal overload switch. It is cooled by convection, which explains the low 20% duty cycle for a welder this size (and weight).

    What exactly happens when a transformer overheats? What damage is done, and what will happen when I attempt to power it up again? Is there any way to tell if it is damaged before doing all of the steps below?

    Examination & Restart:
    Once I finish drilling out the handle and get the cover off, I will vacuum out all the dirt.
    Disconnect the PFC and look for wiring problems: arcing, corrosion, failing insulation, etc.
    Clean all electrical contacts.
    Clean the transformer jack screw and guides.
    Set the transformer to its lowest setting and plug it in.
    If nothing bad happens, crank the transformer to its highest setting.

    At that point is there anything I really need to do, or just plug in the leads and see if it welds?
    I do have a multi-meter, but with such a simple welder, there is not much to diagnose.

    This is the first time I have opened up a welder, so I am a bit nervous. But the 61F supposedly has one of the smoothest arcs of the small AC units, and it is an interesting piece of Miller history.

    Thank you for your help.

    Image 1: 61F transformer. 180 amps max, 20% duty cycle. (same as mine)
    Image 2: Lincoln AC-225 transformer. 225 amps max, 20% duty cycle.
    Image 3: 61F transformer, close-up.
    Image 4: 61F wiring diagram.
    Last edited by Stefen7; 08-21-2016, 10:12 AM.

  • #2
    The wiring diagram thumbnail did not appear above.

    Image 1: 61F wiring diagram.


    • #3
      1. If the PF cap fails open (no connection inside ) the welder will work fine, will just a bit more current when welding and less when idling. If it shorts, it might pop the breaker (best case) or might toast the PF winding on the transformer due to over current, which in turn may or may not hurt the other windings. Practically, if it shorts and the breaker doesn't trip, it might just explode-which is messy but way better than toasting a winding. For normal home use there is no reason to have a PF cap on a welder in my opinion, shared by many.

      2. The wire from which transformers are wound (magnet wire) is coated with a varnish based insulator material which keeps adjacent turns of wire from shorting to the neighboring turns. If the transformer is overheated, the varnish melts and the turns short together, which reduces the inductive reactance, current increases, more heat is generated, etc-the temp runs away until all is a charred mess. Transformers can also fail from a poorly wound and installed coil vibrating against the iron core, destroying the insulation and generating a short circuit, which causes overheating as above. Generally there is varnished paper or fiber material between the winding and the core to prevent this.

      3. Inspection: just look for signs of overheating at connections and on the windings themselves. Smell is also a great assessment sense-if it clearly smells burned it probably is.

      You could check resistance between each coil and the core-should be infinite (OL on a digital meter). If there is a measurable resistance between a winding and the core, insulation is compromised somewhere. If there is a short, Depending on the meter you use, there could be an inductive "kick", a high voltage surge, when you remove the meter leads-hold only the insulated part of the probes. The math here is e=-L* di/dt. Where e is the inductive kick voltage, L is the inductance of the coil, and di/dt is the rate of change of current with respect to time. Since the current supplied by the ohmmeter will change from whatever value it was when measuring to zero when you pull the probe off, the rate of change is very high. There are other mitigating factors so this is probably not a big risk, just something to be aware of.

      I think your well--thought-out testing plan is good. Anxious to hear how it works out.


      • #4
        Thank you Aeronca41 for your help.
        I am still working on removing the cover. Then need to inspect it, and buy a different style of plug, so I will not be able to test today.

        With the effort someone put into trying to remove the cover, something must have failed. I was hoping it was the PFC, which I intended to bypass anyway, (following info from the Dialarc thread). I can peer through the cooling vent on the rear. The PFC canister is intact, and the transformer does not show obvious signs of failure.

        My main concern is to ensure powering up a potentially damaged welder does not injure me or cause damage to the electrical system in my house. I will check the parts I can with the meter before powering up, but I am wary of checking the insides of a powered machine. I would prefer checking for current flow by attempting to strike an arc, with an electrode 10' from the unit, rather then probes on a multi-meter with the transformer in front of my face and hands around 62 year old wiring. (As I noted in the other thread, I had one close call with electricity, I have no intention of testing my luck again).

        Thank You.


        • #5
          All sounds good. If the transformer primary is shorted internally, depending on how badly, I suppose there could be some flying hot metal on power up. Not likely but possible. I would stand back the first time. Or slip the cover back on for first start. If the secondary (welding) coil has a problem, it should not be so bad as long as you have the coils widely separated. I would definitely disconnect the cap before applying power.


          • #6
            Will do. Thank you. Stefen.