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Hobart TR-250-HF Inrush Current

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  • Hobart TR-250-HF Inrush Current

    Hi all -

    I’ve recently picked up an old TR-250-HF (without power factor correction caps) that had been sitting unused for a while, as part of a surplus auction.

    I started to comb through the machine, and found/replaced several things that need replacing; some of the control transistors, one of the big mica capacitors, some of the 0.8ohm biasing resistors feeding the control transistors.

    However, any time I turn it on and activate the contractor (either with the foot pedal, or flipping the switch to local), my 100A breaker feeding the machine will trip immediately.

    I’ve put a nice fluke clamp on it, and the inrush current on this (strapped for 230V) is reading somewhere around 195A (!).

    For fun, I pulled out the 4 rectifier diodes and tested them (all of them tested good, open when they should be, and ~0.42V breakdown in the other direction) and decided to test the machine with them left out - now I can use the machine in AC mode (a little, still trips the breaker sometimes while clicking over the contractor), and the inrush current is only ~120A.

    1) Is there something obvious that I’m missing? I don’t have a wiring diagram/schematic or manual for this machine.
    2) at a glance I’m having a hard time figuring out why it works fine(ish) with the diodes out, yet the diodes test fine - I think the only thing after the diodes would be the DC stabilizing winding.

    help! Tearing my hair out with this one.

  • #2
    What is the expected duration of the inrush current? What is the rating of the breaker? Some breakers are designed for inrush tolerance. It might even be the correct breaker, but just an old, tired one. This is outside my electrical experience, so I'm just spit-balling some ideas out there.


    • #3
      A good callout - not sure on the expected duration, but I’ve tried a couple of different brands/flavors (and sizes, don’t tell the safety czars), and this just seems to be a massive inrush current regardless.


      • #4
        The more I think about it; what I’ve done by disconnecting the diodes is taken the “stabilizer” (massive inductor) out of the circuit; so I should be expecting the startup current to be a little lower.

        I’ve seen forum posts where people are running this machine off of 30 or 50A RV/range outlets; seems weird to me that I can’t get this to behave on a 100A service.

        I’ve ordered some large caps to install as power factor correction - maybe this will help soak up some of the inrush from the mains transformer -> weld voltage transformer.


        • #5
          Posting my thoughts here (maybe they’ll help someone else down the road).

          Looking through the limited diagrams (the one plastered on the inside of the machine), it looks like the models which came with power factor correction installed also had a tertiary winding on the welding transformer (and this is what the caps were installed on). This is pretty smart for a few reasons 1) it provides the necessary capacitance to counteract some of the inductance in the system and 2) it allows a permanently pre-charged core for the welding transformer; this would effectively eliminate any massive inrush currents that are due to the transformers getting spooled up/magnetizing (smart!), as whenever the machine was on, this smaller winding would be be keeping the welding transformer magnetized.

          Sadly, this means that machines not having them equipped at the factory are SOL on a quick add-on.

          Maybe there’s something cute that can be done with keeping the secondary side slightly pre-charged...

          Ramblings aside, I’m still feeling like something’s not right with this machine.


          • #6
            Well, my saga comes to a close -

            After all was said and done, I added a few things, and this has a startup current of ~7A now (!) rather than the 200+, and runs an arc like absolute butter.

            1) ~35mfd, 440v cap across-the-line of the inbound (pre-contractor side). Sort of sized this experimentally, but figured it would help even out the inductive load of the huge inductor coil in there. Running at full snot, it might want some more cap in there, but I’m usually under 150A for most things.

            2) 6x MS35 5R025 NTCs, to limit the inrush current (placed after the contractor, so that they can a, chill out that massive inrush and b, cool down between welding periods).

            This machine runs like a top now, and I can run it (in the lowest settings) on a 30A breaker with no trouble - though, it’s running from a 60 right now for the sake of flexibility. I’ll come back through and attach photos of the modifications...