Patent GB-567176-A (GB)
Title: Improvements in and relating to electric welding systems
IPC: B23K11/26
Filed: 1942-07-30
Abstract: 567,176. Supply and control systems for resistance welding ; charging condensers. BRITISH THOMSON-HOUSTON CO., Ltd. July 30, 1942, No. 10643. Convention date, July 31, 1941. [Class 38 (iv)] Thermionic control.-A system for charging and periodically discharging through a transformer 8, a condenser 11 to supply impulses of current to welding electrodes 2, 3, has the following features: The condenser is charged through a full wave rectifier comprising controlled valves 29-31 and uncontrolled valves 26-28 connected to the secondary windings of a three-phase transformer 52. The conductivity of the rectifiers 29-31 during a charging period is first held substantially unaltered and then progressively reduced. The maximum charging current is limited by reactors 33-35 and the charging rate can be varied by selection of one or other of two supply circuits 17, 18 of different impedance. The condenser 11, the capacity of which may be increased by additional condensers 12, is normally short-circuited by a relay 95, and upon removal of the shortcircuit by operation of a switch 126 in an initiating circuit, charging is commenced, but not before the operation of a time-delay relay 123 permitting the cathodes of the valves to become fully heated. The voltage of the charged condenser is indicated by a voltmeter 89 associated with a rectifier 90 and is limited (a) by a discharge gap 133 and relay 134 causing interruption of the initiating circuit and (b) by a relay 110 energized by a controlled valve 101 responsive to an adjustable proportion of the condenser voltage. The conductivity of the rectifier is automatically controlled to remain unaltered for a period after commencement of charging and to diminish progressively as charging proceeds and the condenser is also maintained in its charged condition until discharged through the transformer by a valve circuit 66 which varies a unidirectional component of voltage applied, in addition to a pulsating voltage of triangular wave form from transformers 40, 41, to the control grids 32 of the controlled rectifiers. This control is obtained by a resistor 87 which receives the difference between the voltage across a glow discharge tube 84 and a predetermined portion of the condenser voltage (tapping 81), whereby the grid of a control valve 67 is variably biassed to cause corresponding changes in the potential of its anode to which the control grids of the rectifier are connected through an anti-hunting circuit. Alternate discharges of the condenser through the transformer 8 occur in opposite directions through the automatic action of reversing relays 135, 136 which interrupt only low values of condenser current. The discharge circuit is completed by an ignitron 143 which is triggered through valve 145 by the discharge of a condenser 152 through a transformer 150 when voltage is applied to a relay 187. This cannot occur until (a) the condenser 61 is fully charged, whereupon relay 110 energizes relay 117 to apply negative bias to the electrodes 32 to stop the charge and to prepare the discharging circuit for condenser 152 ; (b) a switch 205a, 205b is closed ; (c) the ram operating the movable electrode 3 is in a definite position closing switch 195 ; and (d) the electrode pressure is sufficient to close switches 165 ; alternatively to (c) a manual switch 209 may be operated after a ramstarting relay 157 has operated. Reversal of condenser potential is stopped by automatic ignition of an ignitron 153. The operation of the relay 158 controls the reversing relays 135, 136 and a contactor 207 which removes a short-circuit on the welding electrodes to enable their release to be effected without sparking. The condenser discharge current operates a relay 172 whereby a relay 176 is operated to apply voltage to a tuning circuit-184 and also causes voltage to be applied to a tuning circuit 183, whereby a relay 187 is operated to discharge the condenser 152 and initiate the welding impulse ; switches 190, 191 determine respectively whether relay 187 shall operate with time delay or instantaneously.Tuning circuit 184, after an interval, causes relay 188 to operate, releasing relays 158 and 207 and locking itself in to prevent further action ; if switch 186 is closed, however, voltage is applied to a timing circuit 185 determining the interval before relay 189 initiates a further welding operation, and these operations then repeat while the starting switch 156 is closed. The timing circuits 183-185 are as described in Specification 496,641, [Group XL].