Patent GB-567209-A (GB)
Title: Improvements in electronic systems for controlling current supply to welding and other loads
IPC: B23K11/24
Filed: 1943-05-11
Abstract: 567,209. Control systems for resistance welding and other loads ; automatic regulation. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. May 11, 1943, No. 7490. Convention date, May 21, 1942. [Class 38 (iv)] Thermionic control.-Valve means controlling the supply of current to a welding or other load circuit from a source of periodically pulsating potential, have control means supplied with periodic impulses having sloping wave fronts and with a continuous potential which varies in accordance with the effective value of the same potential so that the net potential applied to the control means rises above the initial value in each period and the instant of ignition is determined according to said effective value and the power delivered is maintained at a preset value. Variations in effective value of source voltage are amplified in the control described. Ignitrons 17, 19 controlling the alternating current from a source 15 to a welding circuit are controlled by firing valves 21, 23, the grid circuits of which comprise a negative biassing resistance 67 and a resistance 65 receiving unidirectional impulses adapted to render the valves 21, 23 conducting when desired ; a transformer 59 gives voltages to balance the potential drops in a resistance 49 in the grid circuits. The impulses are controlled by a valve 77, the grid circuit of which includes a resistance 101 receiving unidirectional impulses as indicated at 221, Fig. 2, a resistance 103 giving a constant negative bias, a resistance 91 giving a negative bias variable with the supply voltage and a resistance 107 which gives a positive bias when the ignitrons are to be ignited. The potential across resistance 91 is determined by a potential divider supplied at constant voltage from source 15 through a voltage regulator 178 and a resistance 175 in series with an amplifying valve 173, the drop in resistance 175 being balanced against a tapped portion of the potential divider. The negative potential of the grid of valve 173 varies with the supply voltage being derived from a potential divider 199 supplied through a three-phase rectifier and phase-displaced transformers 207, 209 connected to the source 15 so that the negative bias from resistance 91 increases with a rise in source voltage and vice versa to the extent required to shift the phase of ignition of valve 77 to compensate any change in amplitude of the source voltage. To start a weld a push-button switch 135 applies D.C. potential from a circuit 121 to the anode circuit of a " start " valve 131 which includes the resistance 107. The valve 131 does not ignite until an impulse from a phasedisplacing circuit 157 applied across a resistance 151 has overcome the negative bias from resistance 103. This impulse is arranged to occur at an instant corresponding with the power factor angle of the load circuit, whereupon a positive bias arises in resistance 107 which causes the valve 77 to ignite. During subsequent half-waves, ignition of valve 77 takes place according to the point at which the sloping wave front of the impulses 227, Fig. 2, crosses the critical ignition voltage curve 219 which is determined by the supply circuit voltage 217. The switch 135 also energizes a " stop " valve 159 which does not fire until a condenser 143 charged from the volts on resistance 107 has raised its grid to the necessary potential ; this is determined by resistance 141 which accordingly times the duration of the welding current. Release of switch 135 closes a discharging circuit for condenser 143 and de-energizes the start and stop valves. The grid circuit of the valve 77 includes a normally balanced circuit 93 comprising a resistance 95 which may be unbalanced to the degree necessary to accommodate inequality in the characteristics of the ignitrons 17, 19 by adjusting the tapping point on the resistance, so causing difference in phase of the firing point of these valves.