Patent GB-664302-A (GB)
Title: Improvements in or relating to electric valve converting apparatus especially for resistance welding
IPC: B23K11/24
Filed: 1949-07-19
Abstract: 664,302. Converting; supply systems for resistance welding. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. July 19, 1949 [Sept. 30, 1948], No. 18975/49. Classes 38 (ii) and 38 (iv). A system for supplying current from a single phase source to a load, particularly for resistance welding, comprises two pairs of back-to-back discharge valves connected in parallel between the source and load and controlled such that the first valve of each pair are rendered conductive successively at each half-cycle for a predetermined number of half-cycles to supply a unidirectional current of one polarity to the load followed by the successive conduction of the second valve of each pair in a like manner to provide the load with a unidirectional current of opposite polarity. As shown, the two pairs of ignitrons 11, 13 and 15, 17 respectively supply current to the primary winding of the welding transformer 9, the first pair supplying current through the upper portion 7 and the second pair through the lower portion 19. Each ignitron is fired by a thyratron 177, 251, 245, 195, respectively, held off by a bias 179 until a pulse is received from control thyratrons 21, 25, 24, 23, respectively, which are triggered from a frequency control circuit comprising thyratrons 35, 37 and 39, each having included in its anode circuit condensers 127, 227, 211 connected respectively in the cathode grid circuits and thereby control the conduction of associated thyratrons 39, 35, 37, respectively. Thyratron 39 is normally conductive and condenser 211 is charged to hold off thyratron 37 until contact 143 is closed, whereupon thyratron 35 becomes conductive and sends a pulse at each positive half-wave via transformer 149 to thyratron 21. Condenser 127 then becomes charged to cut off thyratron 39, and condenser 211 is discharged over resistance 203 to allow thyratron 37 to become conductive and so provide a pulse at each cycle to thyratron 24 and simultaneously charge 227 to cut off thyratron 35, and allow thyratron 39 to reconduct and so repeat the cycle until relay contact 145 is opened. The number of successive unidirectional pulses provided over transformers 149 and 221 is determined by the discharge times of condensers 211 and 127, respectively. The pulses applied to the grid circuit of valve 21 causes the valve t'o conduct to cause (a) the firing of thyratron 177 over transformer 171, to set off ignitron 11 and (b) also apply a pulse over transformer 171 to valve 23 which in turn sets off via thyratron 195, ignitron 17. Each pulse applied to valve 24 similarly sets off ignitrons 15 and 13. The sequence timing circuit including relay contacts 139 and 145 comprises thyratrons 43, 45, 47, 49 which respectively control the " Squeeze," " Weld," " Hold " and " Off " periods by controlling relays 77, 83, 89, 93, 119. Each thyratron is controlled by a condenser 59 in the gridcathode circuit and which is connected to a line 63 over a relay contact 75, 81, 87, 91, respectively, on a preceding relay, the time interval being determined by the discharge time of condenser 59 through resistance 61 when the relay contact is opened and allow the thyratron to conduct and energize the relay coil in its anode circuit. A connection is made between point 123 in the grid circuit of weld thyratron 45 via the grid circuit of weld thyratron 39 to ensure that the weld is not terminated until thyratron 45 conducts. The sequence of events is initiated by depressing push-button 128, which effects energization of relay 77. To obtain repeat of the sequence of operations the switch 257 is placed in its lower position.