Patent GB-688874-A (GB)
Title: Improvements in three-phase welding system
IPC: H05B7/02
Filed: 1950-12-15
Abstract: 688,874. Converting. WELDING RESEARCH, Inc. Dec. 15, 1950 [Feb. 9, 1950], No. 30662/50. Class 38 (ii). A transformer supplying a welding load is energized from a circuit comprising two phaseshift circuits, one of which controls the magnitude of the welding current and the other the magnitude of the post-heating current. The welding transformer comprises three pairs of primary windings such as 14, 15, connected in # to three phase lines L, L2, L3, each pair being in series with two back-to-back ignitrons 16, 17 which form two groups of jointly controlled ignitrons. Each ignitron is fired by an associated thyratron 26, 27 which have screen grids 28, 35 connected to secondary windings 29, 36 of the same transformer, e.g. 30, and control grids 31, 37 connected respectively through secondary windings 32 and 38 of transformers such as 33, 40 to common grid points B, C. To fire ignitrons 16, 17, points B, C which are normally very negative are made respectively slightly positive compared with common cathode point A which is a star point formed at the junction of primary windings of transformers 33, 40, 48 connected to lines L, L2, L3. Each has two secondary windings, one in the control grid circuit of a thyratron in each group, e.g. 32, 53, whereby the control grids are given the same volts as the cathode. To prevent voltages induced in the welding transformer windings by the unidirectional current flowing, from making the thyratron grids negative to their cathodes and stopping the ignitions from firing, the voltage across welding transformer winding 23 induces through transformer 75 voltages in windings 77, 78 connected in the leads 80, 81 controlling the voltages of B, C. The magnitude of the welding impulse is controlled by varying the firing angle of the ignitrons. This is done by applying to the screen grids 28, 35 through windings 29, 36 of transformers 30 voltages of variable phase produced in their primary windings 82 from the welding current phase-shifting network formed by the # connected pairs of resistances 88, 90 of which 90 is variable, or from the post-heating current phase-shifting network formed by # connected pairs of resistances 103, 104 of which 104 is variable. One or other of these networks is effective, according as switch 100 or 101 is closed. To start an " on " period, switch 142 is opened, either manually or automatically and condenser 130 which has been charged from source 135 discharges through variable resistance 134, whereupon grid 128 of valve 127 through lead 140 becomes more positive until a pulse from peaking transformer 41 fires the valve, and a circuit is established through valves 125, 127, 131 across rectified supply from transformer 121. This makes either B or C (through lead 80 or 81) as selected by valves 168, 170, positive with respect to A and one group or other of the ignitrons conducts. The adjustment of resistor 148 controls the " on " time. While valve 127 conducts, current flows through resistor 143, and condenser 147 charges at a rate controlled by resistor 148, and then discharges through valve 146. With switches 152, 153, 154 and 159, 160, 161, 162 in No. 1 position opening of switch 142 produces one or more welding impulses without post-heating. Valve 146 discharges through windings 155 of transformer 157 to pulse the grid 126, cut off valve 125 and end the welding impulse, and through winding 156 of transformer 158 to reverse the state of valves 168, 170-one being conducting and the other non-conducting-and thereby change the potentials on leads 80, 81 so that the conducting group of ignitrons is switched off and the other switched on. Thus the welding impulses alternate in direction. Each pulse is applied to the grids of the valves 168, 170 and fires the non-conducting one when the other is extinguished by commulating condensers 177. The potentials applied to points B and C are the sums of the voltages between slider 120 and the +ve end of resistor 119, the voltage between slider 145 and the +ve end of resistor 143 and the voltage across resistors 175 or 176. To introduce a post-heating period for aluminium alloys and the like, the various switches are put into the No. 2 position and the discharging pulse of valve 146 is now applied through transformer 157 to grid 185 of valve 184 which fires. Current flows through resistor 188, fires valve 193 through conductor 192 and energizes coils 200, 201 of relays 100, 101 (Fig. 2) which open and close respectively so that the ignitrons operate under control of the postheating phase-shift circuit. The length of the post-heating period is set by varying resistor 191 and the charging rate of condenser 190 which supplies anode potential for valve 202. When this fires, an impulse through windings 207 and 156 respectively induces a pulse in winding 209 to cut off valve 125 and terminate the welding procedure, and reverses valves 168, 170 to reset for arc reversed welding pulse. When switch 142 is open, the " cool " tune between welding current pulses is controlled by the time it takes to discharge condenser 130 through resistor 134 which controls the voltage applied to grid 128 of valve 127. While switch 142 is closed, valve 127 is held non-conductive and no welding impulses can flow. Specifications 654,583 and 681,813 are referred to.