GB429586A - Improvements in and relating to safety control devices for metal-vapour or gas-filled electrical discharge vessels - Google Patents

Abstract

429,586. Rectifying alternating current. SIEMENS-SCHUCKERTWERKEAKT.- GES., Siemensstadt, Berlin. Aug. 30, 1933, No. 24068. Void [Abridged as open to inspection under Sect. 91 of the Acts, March 1, 1934]. [Class 38 (ii)] Relates to an arrangement for obtaining back ignition and overload protection of a rectifier by the application of a negative bias to the grids. A condenser 14 is charged through a rectifier 10 from a cur. rent transformer 9 in the supply to the main rectifier 5. When the load current exceeds a predetermined amount the bias across the condenser 14 exceeds that of a battery 12 and a negative bias is applied to the grids 7. In a modification the battery 12 is replaced by a glow discharge tube. When the voltage across it exceeds a predetermined amount, conduction takes place and the grid becomes negative. In a further modification the battery 12 is omitted and the rectifier 10 is composed of glow discharge tubes which break down when the voltage applied to them from the current transformer 9 exceeds a predetermined amount. In a modification shown in Fig. 5 a battery 28 is connected in circuit and its voltage is sufficient to maintain the glow discharge tubes 24-27 conducting once they had been rendered conducting by an overload. With this arrangement a negative bias is maintained on the grids until the switch 29 is opened. In a modification, Fig. 6, a relay 30 energized from the current transformer 9 controls the connection of a battery 28 across a resistance in the grid circuit. This relay operates slowly and is provided with a maintaining circuit. On the occurrence of the first overload a negative bias is applied to the grids momentarily to remove the overload, this bias however is removed before the relay closes. When the next overload occurs the battery 28 is sufficient to maintain the glow discharge tubes 24-27 conductive thus maintaining a negative bias permanently on the grids. A number of relays may be provided so that several overloads may occur before the negative bias is permanently applied. In a further modification, Fig. 7, the bias on the grid 7 is controlled by means of grid controlled discharge devices 34, 35 supplied from the A.C. supply, the grids of which are controlled by current transformers 19, 20 in the supply to the main rectifier 5. In the event of overload the valves 34, 35 become conductive to apply a negative bias to the grid 7. In a further modification a positive bias proportional to the supply voltage opposes a negative bias. During normal operation the positive bias is greater so that the grids of the main rectifier are positive. A reduction of the supply voltage below a predetermined value causes the bias applied to the grids to become negative.