Patent GB-508200-A (GB)
Title: Improvements relating to electric welding apparatus
IPC: B23K11/24
Filed: 1937-12-21
Abstract: 508,200. Control systems for resistance welding. BRITISH THOMSONHOUSTON CO., Ltd. Dec. 21, 1937, Nos. 35390, 35391, 35392, 35393, 35394, and 35395. Convention dates, Dec. 21, 1936, Dec. 30, 1936, Dec. 31, 1936, Jan. 14, 1937, Feb. 3, 1937, and June 18, 1937. Addition to 496,484. [Class 38 (iv)] Switching cor6trol ; inductive control ; transformer arrangements.-In a welding supply arrangement ih which the voltage across the welding transformer is varied cyclically by two modulators, as in the parent Specification, a substantially rectangular curve of R.M.S. voltage is obtained by arranging one modulator to have a modulation of (100+x) per cent of the mains voltage and a frequency three times that of the other modulator which has a modulation of x per cent. A particularly favourable value of x is 15. The rectangular waveform may also be obtained as in Fig. 2 by arranging, in the supply, a third modulator 6 which is mechanically coupled to the other modulators 11 and 12 and has a frequency equal to one of them. Preferably modulator 5 has a percentage modulation of 30 and is arranged to have the same frequency as, but to be in phase opposition to, that one of the other two modulators which has the smaller modulation. With the above arrangements contact switching devices operated in synchronism with the supply are provided to open and close the welding circuit for an adjustable number of modulation periods, such opening and closing taking place at an adjustable point during a zero period independently of the instant at which the welding switch is operated. In the application to spot welding, Fig. 6, a changeover switch 39 is operated manually to effect the weld. When a suitable electrode pressure is reached auxiliary contacts 40 close, thereby energizing electro-magnet 36 to couple switch drum 31 and cam 33 to the modulator shaft. Drum 31 rotates and the starting-point and duration of the welding current are controlled by segment 35. Drum 31 performs only one revolution since cam 33 changes over a switch 38 to de-energize electro-magnet 36. The circuit is thus left in readiness for the next spot-weld which may be effected by changing over switch 39 once more. An alternative switching arrangement is shown in Fig. 7 in which the instant in the cycle at which relay 44 is energized to close switch 42 to start the welding current is independent of the instant at which switch 45 is closed to effect the weld. In this arrangement the termination of the weld is independent of the position of switch 45. After the operation of switch 45 the energization circuit of relay 44 is not complete until a switching device 47, coupled to the modulator shaft, closes. When relay 44 is operated, however, an auxiliary contact provides an alternative energization circuit through a switching device 46. A further auxiliary contact on this relay energizes another relay 50 a contact on which breaks the original energization circuit of relay 44 and leaves relay 44 under the control of switching device 46. When the latter opens the weld ceases. A . mechanically-operated spot welding machine is shown in Fig. 9 in which an electrode 53 is placed on the work or removed from it during the zero period of the modulated voltage. The movement of the electrode is controlled by an eccentric 61 coupled to the modulator shaft.