Patent GB-878672-A (GB)
Title: Supply circuits for electro-precipitators
Inventor: WATSON GORDON
IPC: B03C3/68
Filed: 1956-12-15
Abstract: 878,672. Supply systems for electrostatic precipitators; automatic voltage control systems. SOLUS ELECTRONIC TUBES Ltd. March 17, 1958 [Dec. 15, 1956], No. 38316/56. Class 38 (4). An electro-precipitator has in series between its high-tension generator and its field electrodes the anode cathode path of a grid controlled thermionic valve capable when cut off of withstanding the voltage of the high-tension generator and means controlled by the current through the precipitator for changing the grid voltage from a value adequate to pass the dust charging current of the precipitator to a negative value sufficient to cut off the valve. The auto-transformer 3 (Fig. 1) feeds the primary of the high voltage transformer 6 via the contacts 7a, 7b and the coils of a thermal switch 8. A volt-meter 11 is connected across the primary of the transformer 6, protected by a non-linear resistor 12, and having a current transformer 9 to compensate for load variations. The secondary voltage of the transformer 6 is applied to the cathode of the valve 25, which applies a half-wave rectified voltage to the precipitator electrodes 4, 5. The valve 25 has a limited emission, its cathode may be of tungsten. The other end of the transformer 6 secondary is connected to earth via the resistors 13-16 and ammeter 17. A gas discharge tube 18 protects the ammeter and a capacitor 23 connected across the resistors is protected by a non-linear resistor 24. Under fault conditions the current through relay winding 19A operates the relay, closing contacts 19a, 19b, 19c. The contacts 19a close the circuit of another winding 19B on the relay. The components connected to the grid of the valve 25 bias the valve slightly positive when the anode is positive with respect to the cathode, but when the contacts 19b are closed the voltage across capacitor 29 biases the grid negatively causing the valve to be cut off. The starting circuit includes a relay coil 7, starting contacts 53, holding contacts 7c, breaking contacts 54, thermal overload contacts 8a, thermal switch contacts 52a, and a manual switch 34 mechanically coupled to a switch 33 arranged to earth the precipitator electrodes. The relay 19 is slow to release, giving time for ionization to die away but if it is repeatedly operated the contacts 19c operate the thermal switch 52 which switches off the apparatus. In a further embodiment (Fig. 2, not shown) the voltage drop across a resistor, inserted in series with the valve, is used to ignite a gas triode which develops a voltage pulse which is amplified and fed back as negative bias to the grid of the valve. In another embodiment (Fig. 3, not shown) two valves are used in a full-wave rectifying arrangement, excessive current through the valves operates a relay which applies a negative bias to the grids of the valves. An automatic voltage control circuit is formed by the rectifier 37 (Fig. 1) which feeds a voltagesensitive relay 36 which has contacts 36a, 36b, one of which closes when the voltage is too low and the other closes when the voltage is too high. The closure of one of these contacts operates either relay 41 or 42, causing a motor 35 to move the tap on the auto-transformer 3 so as to correct the voltage. Specification 689,798 is referred to.