Patent GB-582225-A (GB)
Title: Improvements relating to control arrangements for electric welders
IPC: B23K11/24
Filed: 1944-06-10
Abstract: 582,225. Control systems for resistance welding. COSSOR, Ltd., A. C., and BROOKER, M. W. June 10, 1944, No. 11147. [Class 38 (iv)] Recurrent pulses of current generated at a central control station are continuously fed to a number of local spot-welding stations to actuate, at the will of the operator of any local station, control means which allow the release into the 'welding circuit at that station of a single power pulse of duration approximating to that of the time between successive control pulses. The mark-space periods of the control pulses are stabilized by an 'A.C. time wave which controls the pulse generator. Preferably, the power pulses are also supplied by the central station. Fig. 1 shows central station plant operating from A.C. and D.C. mains L, N and A, B, respectively, and providing recurrent power pulses of variable duration to lines P, N and control pulses to lines C, N. Lines P and C extend to all the welding stations as do the mains A, B and L, N, the main N being common with main A. The power pulses are provided by back-to-back ignitrons T2, T5 connected between main L and line P, and controlled by gas-filled relays T1, T3, T4. T1 functions as a saw-tooth generator, firing T2 during fly-back and synchronized by mains L, N by transformers Tr3, Tr4 and phase shift circuit C3, R5. The last controls the period of conductivity of T2 during the positive half-cycle of the A.C. mains voltage. When T2 conducts a condenser C2 is charged through T3 and is discharged a half-cycle later by a synchronizing pulse from Tr3. The discharge fires T5 which then conducts for the remainder of the negative halfcycle of mains voltage. A resistance R1 in the saw-tooth generator circuit controls the train frequency of the power pulses and R5 the power in the pulse. An artificial load resistance R7 across P, N maintains the pulses when none of the welding stations are operating. The control pulses are obtained from the anode of a diode T6, connected across P, N in series with a resistor R6, the anode going negative with respect to N each time T2 conducts. At any welding station, closure of a switch S, Fig. 2, connects a relay Ry1 across lines C, N over a part of normally-closed contacts of a relay Ry2. Ryl picks up on arrival of a control pulse, locks in on the D.C. circuit through R11 and the normally-closed contacts of Ry2 and energizes contactor Cr1 in the primary circuit of welding transformer Tr5, thereby closing the welding circuit and connecting Ry2 to the control line. Action of Ry1 is delayed, if necessary by a special delay circuit, to such an extent that Cr1 closes when there is no power pulse on P, N. The next power pulse effects the weld and the control pulse, arriving at the same time, energizes Ry2 which picks up, locks in on the D.C. circuit over R13 and opens its normally closed contacts to de-energize Cr1. Ry2 and Cr1 must be sufficiently delayed in action to prevent the latter opening its contacts before the power pulse has ceased. For half-wave working relays T3, T4 and ignitron T5 are dispensed with and the power pulses on line C are utilised for control pulses. When power pulses of more than one cycle duration are required, the relays Ry1, Ry2 are preferably A.C. operated both as regards the closing pulse and the locking in supply, in which case T6 and R6 may be omitted, line C is the same as line P and line B is replaced by line L. To avoid difficulties associated with the time constants of the relays, the generated control and power pulses may be phase displaced.