GB674043A - Improvements in protective system for three-phase/single-phase machines - Google Patents

Abstract

674,043. Converting. SCIAKY, D. July 2, 1948, No. 17902/48. Classes 38(ii) and 38 (iv). A resistance welding load is supplied with current impulses in alternating directions from a polyphase source through two groups of valves, one from each group being connected in each phase, there being timing sequence and control means for firing the valves, with an associated protective circuit which prevents the firing of one group of valves before the current has died away in the other group, which produced the last current impulse. Welding load 10 is supplied through a transformer 18 with single - phase current or unidirectional pulses from a three-phase source 23 through ignitrons connected in anti-parallel pairs 32, 34; 37, 39; 44, 46; one pair to each phase. Ignitrons 32, 37, 44 are controlled by thyratrons 51 and ignitrons 34, 39, 46 by thyratrons 52. The thyratron grids are energized from timing sequence and control grid mechanism 58 in such order that a succession of alternately-reversing current impulses passes through the primary winding of transformer 18. A voltage developed across parallel connected resistance 67 and capacitor 68 through transformer 61 and rectifiers 65, 65 from the secondary (as shown) or primary windings of transformer 18 when any current impulse is flowing, is fed to the mechanism 58, which operates to prevent the next firing of the ignitrons until the current impulse has died away. In a modification the voltage applied to mechanism 58 is obtained from a battery by closure of a relay energized from transformer 18, Fig. 5 (not shown). Alternatively this voltage may be obtained through an amplifier and rectifier from the ends of a resistance connected in series with transformer 18, Fig. 6 (not shown). In a further modification, there is only one ignitron per phase, but the current impulses they pass are switched alternately to one end and the other of the primary winding of transformer 18 by two switching ignitrons, the return path being from a centre tap on this winding. The main ignitrons are operated by a relay circuit such as is shown in Fig. 3 of Specification 674,042, but coil D is now connected in series with the contacts of the relay in the return path across the auxiliary supply to the relay circuit, so that the next impulse cannot be generated until this relay opens, when the current impulse through transformer 18 has reached zero, Fig. 7 (not shown). This circuit may be slightly modified by using four switching ignitrons to reverse the current after each impulse through the whole of the primary winding of transformer 18 which now has no centre tap, Fig. 8 (not shown).