This photograph is of a wire feed welder. This unit consists of a step-down transformer with a bunch of taps on the primary side. It is fed from 115 Volt 60 hz supply. The switch on the left allows the selection of the tap on the primary side.

The output side is an unusual arrangement. The device on the right appears to be two SCRs, connected to the AC output of the transformer. The trigger wires lead back to the circuit board on the left side. One output of the SCR arrangement leads to the ground clamp, but the other leads to an additional winding on the core of the transformer. This then passes through an under-rated relay and out to the wire feed apparatus.

What purpose does this additional winding serve? Unlike some transformers used in welders, this one has no shunts in the core to cause leakage inductance. My guess is that the SCR arrangment is used to vary the output of the duty cycle to provide a Constant-Voltage power source for wire feed welding. What I can't understand is why this additional winding is needed on the core of the transformer. Some welders use an additional inductor on the output, but that only applies to DC welders. Also, it is usually isolated from the main transformer core.

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    \$\begingroup\$ You should put a schematic of what you see. \$\endgroup\$ – MikeTeX Apr 21 '17 at 13:57
  • \$\begingroup\$ A second secondary could be used to provide a different set of voltages to drive the controlling system. It can also be used as a feedback system to measure current in the secondary. \$\endgroup\$ – Trevor_G Apr 21 '17 at 16:56
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    \$\begingroup\$ I think it's used for controlled saturation of the transformer in order to allow selective control of the output welding current. I think there's an old patent on the idea dating into the mid 1960's. If this is enough for you, I won't bother with an answer here. If not, I suppose I could go look it up and provide more thinking. But that's my suspicion, right now. \$\endgroup\$ – jonk Apr 21 '17 at 19:47
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    \$\begingroup\$ It's not 'a transformer' because there is not ONE magnetic circuit with two or more windings. There are TWO magnetic circuits, and probably the intent is to saturate (drive nonlinear) the core. \$\endgroup\$ – Whit3rd Apr 22 '17 at 9:17
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    \$\begingroup\$ Look up the US patent, "Controlled saturation welding transformer," US 3147455 A. \$\endgroup\$ – jonk Apr 24 '17 at 16:17

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