GB443518A - Improvements in or relating to electrical welding or like processes - Google Patents

Abstract

443,518. Supply systems for resistance welding. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, and GUTTRIDGE, E. J., c/o General Electric Co., Ltd., Wembley, Middlesex. Jan. 8, 1935, No. 649. [Class 38 (iv)] Thermionic and like control.-In an electrical welding or like process in which each weld is effected by current flowing during a regulatable number of A.C. cycles and in which a controlling gas-filled grid-controlled discharge tube is arranged in the primary of the welding transformer, the grid voltage is dependent upon (1) an alternating potential substantially opposed to the anode voltage, (2) a positive igniting pulse applied during a positive part of the anode supply, and (3) a potential which becomes steadily more negative. The discharge tube is rendered conductive by (2) during each positive half cycle and is extinguished at the end. When (3) attains a sufficient negative value, (2) fails to ignite and the welding flow then ceases. A second oppositely acting discharge tube enables the negative half cycle to be utilized for welding. When the main and welding switches are operated independently, a negative potential (4) sufficient to prevent ignition, is applied to the grid by the closure of the main switch and removed when the welding switch is closed. Closure of the main switch W1 connects the primary of the welding transformer T1 to the power supply, but operation of the oppositely acting valves V1, V2, which complete the circuit, is prevented by the application to the grid of the valve V1 of the negative potential (4) by means of the condenser C1, resistance .R2, the vacuum double diode V3, and the secondary winding S1 1 of transformer T2. This secondary winding causes the plate of condenser C1 connected to resistance R2 to become negative and it is connected to the grid of valve V1 through the secondary of transformer T3 and the secondary S2 of transformer T2, but these last two secondaries are then ineffective. When the welding switch W2 is closed to engage contact 2, condenser C2 is placed in parallel with condenser C1. As it is of much greater capacity, the potential difference across C1 is reduced and the negative potential effect (4) on the grid of valve V1 ceases. The secondary S2 then provides the alternating potential (1) which prevents ignition until the secondary of transformer T3 applies the positive igniting pulse (2). Transformer T3 is so designed that its iron core is saturated before the applied voltage reaches its maximum, whereby a positive pulse is applied sufficient to start the discharge in the early part of each positive half cycle of the anode potential, the discharge being extinguished at the end of each half cycle. This is repeated for each positive half cycle until the growing negative potential (3) finally neutralizes the pulse. Charging of the large condenser C2 and small condenser C1 from the secondary S4 starts when welding switch W2 is closed, and when transformer T4 is first supplied with current as the result of the ignition of valve VI. This is effected through the second diode element of valve V3 and resistance R7 which is smaller than R2. The condensers thus charge up at a rate depending upon the adjustable resistance R7 and extending over several cycles until the negative potential (3) attains such a value that it stops re-ignition, thus completing the weld. The second valve V2 is so connected that it is ignited during the negative half cycles following the positive half cycles when valve V1 is operative.