Patent GB-729386-A (GB)
Title: Improvements relating to electronic control circuits
IPC: B23K11/26
Filed: 1953-06-18
Abstract: 729,386. Converting. GENERAL ELECTRIC CO. June 18, 1953 [June 21, 1952], No. 16896/53. Class 38 (2). A load is supplied from an A.C. source through a circuit including changeover apparatus which is set after a predetermined time from the start of the supply period and operated the next time a predetermined point is reached in the A.C. supply cycle, whereby the load current is changed in value without interruption. In a welding circuit including ignitions fed from a 3-phase supply, the changeover apparatus includes a thyratron 148 whose anode-cathode circuit is completed at a, Fig. 2, thus setting it, and whose grid is pulsed causing it to fire at that point between b and c at which the C- phase ignition fires, there being 120 degrees between that point and the corresponding firing point in the next A-phase between d and e which interval gives a constant period for the changeover apparatus to operate. A welding transformer 3 has three primary windings energized from a three-phase source L1, L2, L3 through ignitions 7, 9, 11 with current in one direction and through ignitions 8, 10, 12 with current in the other. Each ignition has an exciter circuit comprising a thyratron such as 22 which is normally held off by the voltage bias applied to its grid 30 from battery 76 through diode 58. When this bias is cancelled by voltage developed in secondary winding 64 of transformer 70 from the firing of thyratron 91 in its primary winding circuit, voltage from secondary winding 49 of transformer 55 in a phase-change circuit is applied to grid 30 at a time determined by the values of resistors 127, 128, 129 which are ganged with the corresponding resistances of the other phases, and respectively control the values of welding, post heat and full heat currents. The phase change circuit is energized from half the centre-tapped secondary winding 88 coupled with a primary winding 85 connected between the same supply phases as the associated ignitrons. Associated with each ignitron is a thyratron, e.g. 91, which controls the connection of the same half of winding 88 to the primary winding 100 of transformer. The second and third phase thyratrons 93, 95 are fired from secondary windings 118, 66<SP>1</SP> of the transformers, e.g. 70, 72 of the first and second phases respectively. These operate during supply of current in one direction and thyratrons 92, 94, 96 associated with the other halves of windings such as 88 operate during supply of current in the other direction. The length of each pulse, which consists of the same number of pulses from each phase of the supply, supplied to the load is controlled by the sequence timer shown in the lower part of Fig. 1. This includes two sets of thyratrons 170, 171, 172 and 173, 174, 175, the anodes of which sets are respectively connected to opposite ends of winding 88 and are thus energized by voltages displaced by 180 degrees through inductors so that as described in Specification 659,996 the thyratrons of one set can be fired from those of the other. Operation of starting switch 195 first closes contact 196 to short circuit the bias applied through resistor 194 to the grid of thyratron 169 which fires to energize both the solenoid valve s.v. controlling the application of fluid pressure to the welding machine, and switch 144 whereby antipolar relay 102 is energized through contacts 199 and moved round one step. This operates contacts 101, 104 in series with transformers 70, 71 to connect one and disconnect the other so that the next load pulse is reversed from the preceding one. Closure of contacts 143 of switch 144 connects the anodes of thyratrons 148, 149 to battery 146. Completed operation of switch 195 closes contacts 197 to short-circuit the bias applied through resistor 193 to the grid of valve 170 which fires, and after squeeze time delay determined by network 180 the grid of thyratron 173 is energized and it fires, followed by the firing of thyratron 171. Thyratrons 173, 171 apply turn-on pulses respectively to thyratrons 91, 92 to initiate positive and negative load pulses, which one fires, being determined by the position of antipolar relay 102. Thyratron 171 also fires two valves 174, 175 respectively through welding time delay network 183, and delay network 184 which times out after the combined weld and post heat times. Firing of valve 174 sends a firing, pulse to the grid of thyratron 149, which be comes and remains conducting while relay contacts 143 are closed, to complete the anode cathode circuit of thyratron 148 to battery .146. The change-over from weld to post heat current occurs at the end of a phase cycle, e.g. between the firing of ignitrons 11 and 7. The firing pulse, in firing circuit 20 of ignitron 11 passes through the primary winding 165 of current transformer 163 and induces in secondary winding 163 a pulse which is applied to the grid of thyratron 148 to fire it. This operates relay 140 and resistors such as 128 are introduced in the phase shift circuits of the exciters, to cause the ignitrons to fire in the cycle. When circuit 184 times out, thyratron 175 fires and through transformer 107, applies a turn-off pulse to thyratron 91 to end the load pulse, or if the load pulse is negative, energizes thyratron 172 which, through transformer 114 sends a turn-off pulse to thyratron 92. Each load pulse is initiated by a separate operation of starting switch 102. Specification 292, 110, [Class 38 (ii)], also is referred to.