Method for controlling welding current

Claim 1

In a resistance welder having an A.C. power supply, a switching element for phase-controlling a voltage from said A.C. power supply to supply a welding current to a welding transformer and a phase-shift pulse generator for supplying a pulse to said switching element to control a firing angle so that said voltage is phase-controlled; a method for controlling the welding current to a target value comprising test current supply steps carried out in a first cycle of an A.C. supply and weld current supply steps carried out after said test current supply steps have been carried out, said test current supply steps including: (a) causing said switching element to conduct through a predetermined firing angle during the first cycle of the A.C. supply; (b) detecting a conduction angle of said switching element during the first cycle of the A.C. power supply; (c) calculating a power-factor angle on the basis of the conduction angle detected in said step (b) and a predetermined relationship between the conduction angle and the power-factor angle; and (d) selecting a curve corresponding to the measured power-factor angle from a plurality of predetermined curves representing relations between the welding currents and control voltages to said phase-shift pulse generator for a plurality of power-factor angles of the load; and said weld current supply steps including: (e) measuring a variation of the voltage of said A.C. power supply; and (f) controlling a period of the pulse from said phase-shift pulse generator based on a value determined in accordance with the selected curve and the measured variation of the voltage and supplying the pulse to said switching element.