Patent GB-657810-A (GB)
Title: Improvements in or relating to control systems for supplying current from a three phase source to a single phase load
IPC: B23K11/24
Filed: 1949-07-13
Abstract: 657,810. Supply systems for resistance welding; converting. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. July 13, 1949, No. 18496. Convention date, Sept. 30, 1948. [Classes 38 (ii) and 38 (iv)] In a system for supplying from a 3-phase source L1, L2, L3, a single phase load L, particularly a resistance welding load, at least one group of three are discharge devices 1TU, 3TU, 5TU, e.g. ignitrons, connected to supply unidirectional current each have an electronic control device 1CT, 3CT, 5CT associated therewith and connected such that upon the first control device 1CT receiving a controlling pulse from a timing device over lines 66 the first control device renders the first ignitron conductive and at the same time controls the second control device 3CT to render the second ignitron 3TU conductive and the second control device controls the third control device 5CT to render the third ignitron 5TU conductive. In the arrangement described a second group of ignitrons 2TU, 4TU, 6TU are connected in reverse parallel to provide reverse half waves to the load transformer I and each ignition is triggered by an associated thyratron FT, each under the control of its associated control thyratron CT arranged so that upon application of a pulse across lines 66 to transformer IF the bias on the first thyratron 1CT provided from rectifier RX is neutralized and the thyratron fires providing a pulse to transformer IT which is transmitted from the secondary 52 over lines 21 to the triggering thyratron 1FT and from secondary 51 to the grid circuit of control thyratron 3CT to cause the latter to fire and so on until the third ignitron 5FT is fired. The anodes of the control thyratrons are supplied from the 3- phase lines over a phase shift network comprising transformer T2 and the delta connected resistance potentiometers 36 to obtain adjustment of the welding current. The reverse half waves are dealt with in a similar manner by providing a pulse across lines 80 to cause ignitrons 2TU, 4TU, 6TU to fire. The pulses are derived from thyratrons 115 and 143 and upon depressing push switch SW thyratron 130 conducts to charge condensers 136, 139 until contact 113 opens at commencement of weld time whereupon thyratron 115 conducts for two half cycles and sends two pulses along lines 80 over double diode 125. In the meantime condenser 120 charges up and stops the charging of condensers 136, 139 by blocking thyratron 130. One and a half cycles later when condenser 136 has discharged, thyratron 141 conducts and charges condenser 148 which cuts off thyratron 115 after the two half cycles and so condenser 120 commences to discharge. One cycle after thyratron 141 commences to conduct thyratron 143 conducts, when condenser 139 has discharged at the slower rate and thyratrons 141 and 143 conduct together for two cycles the latter sending two pulses over lines 66, one cycle after those over lines 80 and until condenser 120 is discharged. Thyratron 130 then reconducts as condenser 120 is now discharged allowing condensers 136 and 139 to recharge and cut off thyratrons 141 and 143. Since condenser 148 charges whilst thyratron 141 conducts and discharges through resistance 147 thyratron 115 reconducts when condenser 148 is discharged and restore the circuit to initial condition which will repeat until the anode circuit of thyratron 115 is opened at relay contacts 163. The time cycle of welding is effected by thyratrons ST, WT, HT, OT, respectively controlling the squeeze, weld, hold and off periods and respectively actuating relays STD, WTD, HTD and OTD. Upon pressing button SW relay CR is energized opening contacts 173, thereby disconnecting resistor 181 from the A.C. line 111 and allowing the grid condenser 183 to discharge over resistors 184 and 185 and allow the squeeze time thyratron ST to fire and operate relay STD and open contact 113 to initiate the pulsing circuit and disconnect resistance 192 from line 111, and allow thyratron WT to tune out, the welding period, fire to energize relay WTD and open contact 163 and initiate the hold period when contact 197 is opened which is tuned out. over thyratron HT when the cycle ceases. To obtain repeating cycles of current the switch R is placed on contact 201a introducing the off time thyratron OT in circuit which when fired a short time after contact 204 is opened actuates relay OTD to open the normally closed contacts 203 included in the anode circuit of thyratron H. T.