Patent GB-642468-A (GB)
Title: Improvements in or relating to electrical control systems
IPC: B23K11/24
Filed: 1946-11-08
Abstract: 642,468. Control systems for consumption apparatus. BUDD CO. Nov. 8, 1946, No. 33228. Convention date, Nov. 23, 1945. [Class 38 (iv)] Apparatus for controlling the supply of electric power to.a load subject to a change of condition which causes a change in voltage across the load, comprises a device for deriving a voltage dependent upon the condition of the load, means for comparing the derived voltage prior to change in the condition of the load with that during the change and providing an output voltage dependent upon the difference between the two derived voltages, and further means responsive to the output voltage corresponding to a predetermined percentage difference between the derived voltages adapted to initiate actuation of the apparatus. In the resistance welding system shown a predetermined percentage decrease in voltage across the electrodes 4, 5, due to fusion of the work 6 is utilized to terminate the weld. A unidirectional reference voltage representative of the alternating voltage across the electrodes is obtained at the points 56, 57 of a " comparator " C by transformer 51, rectifier 52 and condenser 54. This voltage is fed to the points 81, 82 of a potential divider 92, 93 through a variable impedance tube 63 which removes inaccuracies in the reference voltage due to current variations in the work circuit not resulting from physical change of the work. Tube 63 is controlled by a pentode 64, the control grid of which is biassed by the difference in voltage between a fixed source 66 of E.M.F. and a rectified voltage (derived from potentiometer 48) proportional to the current in the work circuit and obtained through current transformer 58. To reduce the lag in the production of the reference voltage due to the charging time of the condenser 54, an auxiliary charging source 195- 197 precharges the condenser to a voltage somewhat below that normally reached during a weld. A fraction of the corrected reference voltage, selected by potentiometer 80, is applied to a condenser 101 through a rectifier 100 which prevents the condenser voltage falling with that of the point 82 when fusion of the work occurs. Valves 84, 85, forming elements of a bridge 84-87, have their grids energized, respectively, from the potential divider 92, 93 and an equal ratio resistive or capacitive divider 98, 97 across condenser 101, the setting of the bridge by potentiometer 89 being such that prior to fusion of the work a negative bias is applied from the points 105, 106 of the bridge to the grid .of a discharge tube 108. When the work fuses the falling voltage across divider 92, 93 varies the conductivity of valve 84 and at a predetermined percentage fall of voltage-as determined by potentiometer 80-the sign of the bias applied to the gas discharge tube reverses. The tube thereupon conducts and energizes a contactor R1 for interrupting the energization of the primary circuit of the welding transformer 1. A modified bridge circuit for operating the discharge tube with rising work voltages is also described. Indicator lamps, shunted by coils operating graphic recorders, may show the stage reached in a welding operation. Thus, an amber light may show during the initial stage of the weld, this light being replaced by a green one when the prescribed percentage fall in work voltage occurs. If, subject to a further period determined by a delay relay R3, 115 energized by the discharge tube, a timing circuit 12 controlling the primary circuit of the welding transformer has not timed out, the green light is replaced by a red one.