Patent GB-619040-A (GB)
Title: Improvements in and relating to electric control circuits for supplying current to areactive load
IPC: B23K11/24
Filed: 1944-11-01
Abstract: 619,040. Supply systems for resistance welding and the like. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 1, 1944, No. 21367. Convention date, Nov. 1, 1943. [Class 38 (iv)] A control circuit, e.g. for a resistance welding transformer 13 supplied through reversely connected valves 10, 11 from mains 12, comprises means for controlling the instant of initiation of conduction of the valves and means for indicating when the instant of initiation of conduction is advanced to the point corresponding to the full heat setting. The cathode 15 of valves 10, 11 are each connected through separate secondary windings on three transformers 19, 21, 23 to their respective control grids 16. Thus the conductivities of valves 10, 11 are controlled in accordance with the. resultant of three voltages, the first from transformer 19, in phase with the anode voltages of the valves and under the control of a timer 25, the second from transformer 21 in phase opposition to the anode voltages, and the third, a peaked voltage from saturating transformer 23, the phase position of which controls the instant of initiation of conduction of the valves. Phase-shifting means.-The peaked voltage component is produced by, and its phase position controlled by, a phase-shifting circuit 31 comprising two valves 32, 33 rendered conductive during positive and negative halfcycles respectively of the mains voltage supplied from transformer 38, so as to shockexcite the primary 34 of transformer 23 connected across valves 32, 33, the instant of initiation of conduction thereof being controlled by a combination of an alternating and a direct current component impressed on their control grids 40. The A.C. component is derived from a transformer 46 in turn connected to a secondary winding of transformer 38, and the D.C. component is derived from a tapped resistor 50 energized by a full-wave rectifier 53 connected through a transformer 52 to mains 12. Hence adjustment of the magnitude of the D.C. component by means of a tap 51 on resistor 50 controls the instant of initiation of conduction of valves 32, 33 and in turn the phase position of the peaked voltage produced by transformer 23. Indicating means.-The approach of the phase-shifting adjustment to the full heat setting is visually indicated by a glow-discharge device 56 energized over positive half-cycles of the mains-voltage by a transformer 58 and rectifier 63. In order to prevent breakdown of the discharge device until the voltage applied to the grids of valves 10, 11, has advanced to the desired phase position, a valve 64 is connected in shunt with device 56 and rectifier 63, having a control grid 65 energized with a negative bias by means of a capacitor 66 and by the voltage of a capacitor 67 charged in accordance with the voltage across valves 10, 11. This latter voltage is obtained by utilising a component in phase with the supply and derived from a winding 71 on transformer 58, and a component derived from the load circuit from a winding 72 on a transformer 73, since the difference between these two voltages is the voltage across valves 10, 11. Windings 71, 72 are chosen such that their voltages are equal and of opposite polarity when the valves 10, 11 are rendered conductive at the phase position corresponding to the full heat setting, under which conditions the net voltage across capacitor 67 will be zero. By proper selection of windings 71, 72, valve 64 may be rendered conductive to remove the voltage from device 56 before it breaks down, until the conduction of valves 10, 11 approaches the full heat setting. In a modification of the indicating circuit (Fig. 2, not shown), a discharge device and a condenser are connected in parallel directly across valves 10, 11, the discharge device being adjusted by means of a series resistor to break down at a point retarded with respect to the full heat setting so that it becomes extinguished at the latter point.