Patent GB-1402149-A (GB)
Title: Circuits for controlling the power supplied to an electrostatic precipitator
IPC: G05F1/66
Filed: 1972-07-06
Abstract: 1402149 Control systems for electrostatic precipitators ENVIROTECH CORP 6 July 1972 [12 July 1971] 31695/72 Headings H2H and H2K A system for controlling the A.C. power supplied to a transformer 13 and a rectifier 18 which energize an electrostatic precipitator 20 comprises a circuit for generating a reference pulse at each zero crossing of the voltage supplied to the transformer, a circuit 22, 80 for generating signals representative of the precipitator current, a gate 77 which provides output signals when the reference pulses coincide with the precipitator current signals as a result of sparking in the precipitator and means responsive to the gate output signals for reducing the power supplied to the precipitator. When a start switch 32 is closed a main relay MR is energized via a source 10 and a transformer 24 to close contacts MR1-MR3 thus connecting the source to the transformer 13. A control circuit 29, shown in detail in Fig. 3, is also supplied with power via leads 28, 33. A short circuit alarm receives signals from a transformer 155 which monitors the voltage across the primary winding of the transformer 13 and from a resistor 22 through which the precipitator current flows. In the event of a short circuit, a relay (YR), Fig. 2 (not shown), in the circuit 29 is de-energized to close contacts YR1 and operate an alarm indicator 37. A first control relay ICR is energized via the closed contacts TDR1 of a time delay relay TDR to open contacts 1CR1 and close contacts 1CR2, 1CR3. The relay MR is therefore de-energized to disconnect the transformer 13 from the source 10 and the energizing of a relay 2CR causes closure of contacts 2CR2 to indicate the short circuit condition to the user. A transformer 60 is connected across the primary winding of the transformer 13 and during normal operation of the precipitator, the output pulses from the zero crossing detector circuit do not coincide with the precipitator current pulses. However if sparking occurs in the precipitator and is sufficient to reduce to zero the primary voltage across the transformer 13, coincidence occurs and the gate 77 provides an output pulse to a circuit 83 connected to a magnetic amplifier winding 84. The winding 84 controls a pair of reverse parallel connected thyristors in a circuit 14 to fully phase retard the firing of the thyristors and reduce the precipitator current. The occurrence of a low energy spark is detected by a high sensitivity spark detector circuit connected to an amplifier 88. The amplifier also receives inputs from a double integrating network and a signal mixer 122 which receives a manual input from a potentiometer 50 or an automatic input from a capacitor 115 charged from the output of a multivibrator 110. The amplifier 88 is connected to a winding 99 which controls the phase angle at which the silicon controlled rectifiers in the circuit 14 are fired. The double integrating network enables the precipitator current to be rapidly increased after detection of a spark has caused the control circuit to decrease the precipitator current to a low level whilst the discharge of the capacitor 115 enables a further slow increase of the current. The thyristors are protected by a thyrector (204), Fig. 5 (not shown), and the circuit 83 may include a transistor switch. In a modification the thyristors are replaced by a saturable reactor.