Abstract: A multi-phase SCR-rectifier controlled arc welding power source having improved operating and internal protection characteristics. The transformer has an improved core and winding design having a built-in internal reactance and a core arrangement which is scrapless and very compact. The output has an adjustable, drooping, volt-ampere curve such that the welding current may be readily preset or adjusted to any desired value.The power source includes an unusually high inductance for a three phase source so as to give a very stable arc with a control circuit having provisions to properly control the conduction angle of the SCR's so as to overcome the disadvantages of such a large choke on starting or short circuiting so as to give optimum welding characteristics. Further the ratio of short circuit current to welding current is controlled and decreases with an increase in the welding current. Protection against damage of the power source is provided in the event of extended short circuit or overload.