Patent GB-730064-A (GB)
Title: Improvements in or relating to apparatus for controlling the supply of current from a polyphase source of alternating current to a load such as electric welding apparatus
IPC: H05B7/02
Filed: 1953-01-07
Abstract: 730,064. Converting. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Jan. 7, 1953 [Feb. 21, 1952], No. 470/53. Class 38 (2). [Also in Groups XXII and XXXVII] The single primary winding P of a welding transformer W is fed from a polyphase source L1, L2, L3 through a bridge arrangement of electric  discharge devices such as ignitrons I1-I6, there being reversing switches K1-K4 between the transformer and the discharge devices so that the polarity of the current supplied to the load can be reversed periodically at a frequency substantially lower than the frequency of the three-phase supply. A firing thyratron such as FT1, FT4 is associated with each ignitron and is energized when voltage is developed across a resistor, e.g. 107, 95, 101, 113 connected between, its cathode and grid. These voltages are derived from secondary windings, e.g. 1SO1, 5SO1, 2SO1, 4SO1 of transformers whose primary windings, e.g. 1PO1, 2PO1 are connected in one of three heat control networks, e.g. N1. Each such primary winding is in series with one of a pair of thyratrons, e.g. 1NT1, 2NT1 and a common thyratron, e.g. 3NT1, supplied from a secondary winding, e.g. 1SS1 of a transformer such as TS1. The firing of thyratrons 1NT1, 2NT1 is controlled by a phase shifting network which comprises a capacitor 151 and resistor 153 connected to opposite ends of the secondary winding 2SS1 of transformer TS1. In series with resistor 153 is a weld variable resistance network PHW1 or a post heat variable resistor network PHP1, one or the other being shorted out by switches 155, 157, the strength of the weld and post heat currents being controlled by rheostats 159, 161 respectively. The leading heat control network N1 shown in Fig. 1c is set in operation upon energization of a resistor capacitor network IN1 from weld thyratron WT of the sequence time (Fig. 1A) through conductor 263. Two following heat control networks, Figs. 1D, 1E (not shown), similar to N1, are energized following a rise of volts across resistor RO1 and the three networks cause firing of the main ignitrons in the proper sequence. The sequence of operations of the whole circuit is controlled by the sequence timer, Fig. 1A, which comprises main thyratrons ST, WT, HT, OT connected so that when rendered conductive they initiate respectively, the squeeze, weld, hold and off intervals, and at the same time start the timing operations of the squeeze, weld, hold and off timing networks SN, HN, ON respectively. The supply for the sequence timer is derived from buses AL1, AL2 connected to phases L2, L3. To produce the sequential operation of these thyratrons and timing networks, several intermediate operations are carried out by auxiliary thyratrons AT2-AT6 and auxiliary networks AN2-AN5. The timing and auxiliary networks each comprise in general a capacitor shunted by a resistor. The capacitor is charged or not according as one associated thyratron is conducting or not, and prevents or allows conduction of another associated thyratron according to whether it is charged or not. A further part of the timing system, Fig. 1B, includes post heat thyratron PT, post heat timing circuit PN, and auxiliary thyratron AT7, AT8 and auxiliary network AN6. The reversing switches K1-K4 are operated by relays SK1, SK2 energized through a step-by-step switch with a moving contact 31 alternately driven by ratchet device RL to engage contacts 32, 34 alternately. Operation of the welding circuit is initiated by first closing switch 416 which operates relay RWN to connect the firing thyratrons to the main ignitrons and then by closing switch FS which starts the tuning sequence by causing squeeze thyratron ST to conduct. Start relay SR is actuated, and locked in at contacts 412, and closure of contacts 21 operates relay AR which results in operation of ratchet device RL and reversal of changeover switches K1-K4 so that the main ignitrons are reversely connected from their positions in the previous welding cycle. This reversal takes place during the squeeze time while pressure is building up in cylinder C following closure of contacts 19. The sequence timer then operates with its set timing, but closure of the pressure switch PS is essential for the welding thyratron WT to initiate operation of the heat control networks as described. The switch RWP is operated by the post heat thyratron PT at an instant before the weld time controlled by timing network WN, expires, and the control of the firing of the main ignitrons is changed from rheostats 159 to 161. Each cycle may comprise any number of times of firing of the ignitrons under the control of PHW1 followed by any number of times of firing of the ignitrons under the control of PHP1, and may be followed after a hold period in which the components of the sequence timer reset and during which the metal cools, by a reversed cycle, after the reversing switches K1-K4 have operated. With repeat-non- repeat switch RNR suitably set these alternating cycles may continue. In a modification, Fig. 4 (not shown), the ratchet relay RL is operated while the weld is being fused, and the operation of the reversing switch is effected just after the flow of welding current has ceased, so that the time added to the welding cycle by this process is reduced. The reversing switch (see Group XXXVII), may be operated by D.C. derived through rectifiers. In a further modification, the ratchet relay RL is replaced by a pair of co-operating relays, Fig. 5 (not shown).