Patent GB-1272191-A (GB)
Title: Arrangement for controlling the voltage or current applied to an electrical apparatus
IPC: A63C9/084
Filed: 1970-03-05
Abstract: 1,272,191. Control systems for electrostatic precipitators. SIEMENS A.G. 5 March, 1970 [8 March, 1969], No. 10735/70. Heading H2H. An arrangement for controlling the voltage or current supplied to an electrical apparatus comprises a regulating means for varying the voltage of current, a reference capacitance upon the voltage across which the control of the regulating means depends and a controllable charging means through which the reference capacitance is chargeable, the charging means being coupled to the apparatus so as to be sensitive to the current in the apparatus in such a manner that the rate at which the capacitance charges drops when a predetermined current has been sensed. An electrostatic precipitator F is supplied from an A.C. source U by way of back-to-back thyristors S, a smoothing choke D, transformer T and a full wave rectifier G. The conduction periods for the thyristors S are controlled by signals A, L derived by a control unit St, the input signals to the control unit being a signal B obtained by a current transformer W and representing the precipitator current and a light signal obtained from a lamp l and falling on a photosensitive device in the control unit, the signal from the lamp l being indicative of the absence of a flashover or short circuit in the precipitator F. The voltage, and hence the current, supplied to the precipitator F is normally under the control of the signal A, the signal L being a quenching or inhibiting signal, the arrangement being such that signals A and L cannot exist simultaneously. The signal A is derived by means of a capacitive device in the control unit St and the signal L from a transistor therein. When the precipitator is switched on the charge on the capacitive device increases with a corresponding increase in the operating periods of the thyristors S, the voltage and current applied to the precipitator thus increasing. During this build up the flicker of the lamp L is rendered ineffective so that it does not affect the voltage and current build up. When the voltage and current reach the required values the signal B stops the increase in the charging rate of the capacitive device and introduces a constant charging rate circuit whereupon the voltage and current at the precipitator F is maintained constant. Should the precipitator current tend to increase a discharge circuit is connected to the capacitive device whereupon the device discharges slightly thus returning the current to its normal value. Should a flashover occur in the precipitator the lamp I is short-circuited and becomes extinguished. The signal from the associated photosensitive device then disappears and the quenching signal L is derived and applied to the thyristors to cut off the precipitator supply voltage. The derivation of the signal L includes discharging of the capacitive device and the disappearance of the signal A. After a predetermined interval the signal L disappears and if the flashover has cleared the capacitive device begins to recharge thus restoring the supply to the precipitator.