Patent GB-1527792-A (GB)
Title: Arc welding control circuit
IPC: B23K9/20
Filed: 1977-01-14
Abstract: 1527792 Control circuits for arc welding apparatus; converting CROMPTON PARKINSON Ltd 14 Jan 1977 [23 Jan 1976] 02722/76 Headings H2H and H2F A circuit for supplying power from a three phase source to arc welding apparatus 4, 5 comprises a three phase bridge including controlled rectifiers SCR1B-SCR3B and a control circuit for setting the conduction angle of the controlled rectifiers, the control circuit including an adjustable first pulse generator for supplying firing pulses to the rectifier of one phase at an adjustable time in the cycles of that phase, and second and third pulse generators controlled by the first generator for supplying firing pulses to the controlled rectifiers of the second and third phases respectively at phase angles of 120 and 240 degrees after the pulses from the first generator. When the three phase supply is connected, a cooling fan is turned on and closure of a switch SW1 supplies power via a transformer T2 to the control circuit. A trip reset push button PB2 is pressed to set a hand held tool ready to weld a stud 4 to a workpiece 5. The tool is then moved into position to contact the stud with the workpiece and a push button PB1 is pressed to initiate a weld cycle. Relays RLA, RLC and RLB are energized thereby triggering thyristors SCR4B-SCR6B to supply a pilot arc via a resistor RIB and starting timing circuits including unijunction transistors TR1, TR2. The energization of the relay RLB closes contacts RLB/1 to energize a coil LS which lifts the stud away from the workpiece and draws the pilot arc. A timing circuit including a transistor TR4 is started when the circuit including the transistor TR2 has completed its timing cycle, the point of conduction of the transistor TR4 in each cycle of the supply being determined by the setting of a variable resistor network R52-R74 selected by a switch SW4B. The conduction of the transistor TR4 supplies a trigger pulse to the main rectifier SCR2B. Timing circuits including transistors TR5, TR6 are also started and supply trigger pulses to the main rectifiers SCR3B, SCR1B at 120 and 240 degrees with respect to the trigger pulse for the rectifier SCR2B. The main controlled rectifiers SCR1B-SCR3B therefore all have the same conduction angle in their respective phases. The timing circuit including the transistor TR1 sets the overall weld time, which time is selected by a switch SW4A ganged to the switch SW4B so that the weld time and the arc current are set by one operation. At the end of the weld period, the transistor TR1 switches on a thyristor SCR7 thereby deenergizing the relay RLB and de-energizing the coil LS so that the stud engages the workpiece under spring pressure. The arc is shorted out and the current flowing through the welding circuit is limited by the main transformer T1. Relay RLC is then de-energized to switch off the main arc and the pilot arc, and removal of the welding tool from the stud de-energizes the relay RLA. In the event of a short circuit in one of the main or pilot arc rectifiers, a contactor RLD/3 is de-energized thereby opening contacts RLD/1-3. These contacts are also opened if malfunction of the transistor TR1 fails to terminate the weld.