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I have an old, transformer-based welding machine, which is rated to draw about 70A at 240V, at full output. It uses an electro-mechanical relay (contactor) to energize the main power transformer, and depending on the point in the AC cycle that the contacts close, it may have a high inrush current, well in excess of the rating of the circuit breaker that feeds it. When it was built, in 1960, fuses were the norm, and one would simply use a time-delay fuse, to tolerate the spike. Since it's in my garage at home, and not in a commercial environment, I don't have the ability to use a breaker with a D trip curve, without replacing my entire breaker panel, at the cost of hundreds of dollars US. I'm looking for a more elegant solution.

Ideally, I'd like to find an off-the-shelf solid-state relay, which is designed specifically to power inductive (single-phase) loads, by switching on and off only at peak voltage, where the current is at a minimum. Does such a device exist, or would it be realistic to design an analog circuit that would accomplish this?

A resistive (NTC) inrush current limiter isn't really suitable for this application, because sufficiently high current devices would cost as much as replacing the circuit breaker panel, or more, and I need the ability to rapidly turn off and on the main power transformer primary as many as 20-30 times per minute, and NTC resistors at that power level tend to have cool-down times in the range of minutes.

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  • \$\begingroup\$ 70A*240V = 16.8kW... what in the world is being welded, three-inch thick plate? \$\endgroup\$
    – rdtsc
    Mar 31, 2021 at 20:08
  • \$\begingroup\$ I think you mean that it should switch at zero crossing of AC voltage not peak? Something like Crydom CWA24125 with appropriate heatsink and accessories (zero turn on) could work. Why not just purchase new welding machine with likely safer design than fifty-year-old+ equipment? \$\endgroup\$
    – Ernesto
    Mar 31, 2021 at 21:01
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    \$\begingroup\$ @Ernesto, Nope, turning transformers on at the peak of the AC voltage waveform gives the lowest inrush current. \$\endgroup\$
    – Aaron
    Mar 31, 2021 at 21:02
  • \$\begingroup\$ @Elkvis Okay, I am not familiar with that type of SSR. There should be a away to add a 90 degree phase delay (at mains frequency) before control input so that zero corresponds to peak of mains voltage \$\endgroup\$
    – Ernesto
    Apr 1, 2021 at 1:47
  • \$\begingroup\$ @rdtsc This machine is capable of TIG welding 0.5"/12.7mm aluminum in a single pass \$\endgroup\$
    – Elkvis
    Apr 1, 2021 at 7:14

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There are many Soft Starter available in very reasonable price. You can search the Term "Single Phase soft starter". or "Three Phase Soft Starter" according to your welder specifications.

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  • \$\begingroup\$ Unfortunately, those invariably use an NTC resistor or a timed relay with a current-limiting resistor, with a fairly long delay. Neither of these will work for my application, due to the requirements I laid out in my post. \$\endgroup\$
    – Elkvis
    Apr 28, 2021 at 17:37

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