Patent GB-418920-A (GB)
Title: Improvements in or relating to electric arc discharge devices
IPC: H01J13/48
Filed: 1933-06-15
Abstract: 418,920. Converting ; protective arrangements. WESTINGHOUSE ELECTRIC & MANUFACTURING CO., East Pittsburg, Pennsylvania, U.S.A.-(Assignees of Slepian, J. ; 1115, Lancaster Street, Pittsburg, and Ludwig, L. R. ; 738, Princetown Boulevard, Wilkinsburg, both in Pennsylvania, U.S.A.) June 15, 1933, No. 17102. Convention date, July 30, 1932. [Classes 38 (ii) and 38 (iv).] An arc-discharge device is provided with a starting-electrode permanently in contact with one of the main electrodes and made of poorly conducting material such as carborundum, Nernst filament material, ferro-silicon or galena, as for example a composition of about 70 per cent carborundum, 30 per cent clay and some lamp black, the resistance should lie between 10<-2> and 10<-1> ohms per centimetre cube or per centimetre of length ; the invention is described with reference to mercury-vapour rectifiers and to lightning-arresters. It is stated that an arc forms between the cathode and the portion of the auxiliary electrode near it, thus forming a starting cathode-spot. Fig. 1 shows a rectifier in which the auxiliary electrode 17 is carried by a tubular metallic holder 15, 16 serving as a lead and enclosing the main cathode lead 13, from which it is insulated by a sleeve 18. The mercury or like cathode 2 is contained in a quartz vessel 3 in a metal tank 1, provided with a steel guard ring 4. The cover 6 of the vessel 1 forms the anode and is cooled by a waterjacket 8, the temperature of which is regulated to somewhat above 100‹ C. by a spring- controlled escape valve 9. To prevent formation of an arc between the auxiliary electrode 17 and its holder 16 with consequent backfiring, a rectifying valve 23 is placed in the electrode lead. Fig. 2 shows an arrangement in which the cathode 49 is cooled by a water jacket 51, 53 and tubes 52 connecting different compartments of the jacket. The tubes 52 are plated or covered with molybdenum, tungsten or other non-sputtering material which is welted by mercury to fix the cathode spot and prevent burning of the pipes ; the temperature of the mercury apart from the cathode spot, should not be above 80‹ C. and preferably about 50‹. Cooling fins 56 may be arranged between the tubes 52. The anode 63 is formed as a hollow projection on the cover plate 57 and is cooled by air circulation. The plate 57 and the cathode-container 48 are separated by a corrugated ring 58 of insulating material protected by a guard-ring 61. The joints between the ring 58 and the container 48 and plate 57 are cooled by water circulating in channels 82. The auxiliary electrode 67 is carried on a rod 69 enclosed in a shield 78<1>. The end of the electrode 67 attached to the rod is thicker than that dipping into the mercury, and it is stated that this arrangement may allow the rectifying valve 23 shown in Fig. 1 to be dispensed with. In the arrangement shown in Fig. 5, the cathode chamber is formed of two metal portions 140, 141 separated by a porcelain ring 142, which is protected from the heat of the arc by overlapping rings 144, 145 and a quartz ring 147. The body of the rectifier is formed as a water jacketed cylinder 130 and a water chamber is formed in the bottom of the member 141. The anode 134 is supported by an insulating bushing and is screened from the cathode by a water-cooled baffle 160. The auxiliary electrode 151 is carried on a steel rod 153 passing through the wall of the chamber where it is protected by a quartz tube 154 and steel tube 155 and connected to the lead through the member 157. These three constructions are described in connection with arrangements for converting three-phase into direct current or conversely. In each case a battery of six rectifiers is employed of which one is shown in detail and the remainder diagrammatically. In the arrangement shown in Fig. 1, the mid-points of the triple two-phase secondary winding 31 of a transformer are connected through an interphase transformer 33 to the positive side of the load line ; the end points of the windings are connected to the cathodes 2 of the rectifiers, the anodes of which are connected through a bus 24 to the negative sides of the load line. The auxiliary electrodes are fed from the bus 24 by a rheostat 26, bus 27 and rectifiers 23. Since the greatest danger of back-fire is immediately on the extinction of the arc and diminishes rapidly the establishment of the negative voltage on the main cathode is delayed by connecting the electrodes through a capacity 44 and resistance 45. A small saturable choke 46 may also be employed. Fig. 2 shows an arrangement for converting direct current to three-phase. The primary 85 of a transformer similar to the secondary 31 of the transformer Fig. 1, is connected to the main electrodes of the rectifiers in a similar way. This transformer is connected to the direct-current supply line 83 by an interphase transformer 86 in Scott connection. The auxiliary electrodes are supplied through a series of rotary commutators 91 driven from the line 83 by a variable speed motor 91<a>. Fig. 5 shows a connection similar to that of Fig. 1, but in order to regulate the period at which the auxiliary anodes 151 are excited they are connected to the supply through valves 166, the grids of which are adjustably biassed by a battery 170 and charged from the next leading phase of the transformer 165 through small transformers 173. By adjusting the voltage of the battery 170, the period of the cycle at which the grids allow the valves to pass current, and consequently the voltage of the power transmitted to the line 164 can be regulated. Auxiliary electrodes of the type described may also be used with circuits of known type for transforming threephase current to direct in which the rectifiers have a common cathode. Arrangements for transforming single-phase current to direct and conversely are also described, including an arrangement with a glass container with two mercury electrodes, each of which is provided with an auxiliary electrode and can be used as the cathode. Fig. 3 shows the application of the invention to lightning arresters. A perforated casing of fibre or the like is closed at each end by a metal plug 98 to each of which is attached a carborundum rod or the like 101 secured to a piece of brass 102. The pieces 102 are connected through a high resistance 105, and on an overload occurring one of the pieces 102 becomes a cathode and starts an arc. When the overload ceases the gases from the burning casing blow the arc out.