US2961577A

US2961577A - Electrostatic precipitators

Info

Publication number: US2961577A
Application number: US831514A
Authority: US
Inventors: John B Thomas; Howard T Williams; John W Drenning
Original assignee: Koppers Co Inc
Current assignee: Beazer East Inc
Priority date: 1959-08-04
Filing date: 1959-08-04
Publication date: 1960-11-22
Anticipated expiration: 1977-11-22

Description

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17 Claims. (Cl. 315-111) This invention relates generally to electrostatic precipitators and, more particularly, to an apparatus for controlling the power supply to the ionizing electrodes.

In conventional electrical precipitators, alternating current from a suitable source is stepped up to a high voltage and rectified to provide direct current. This high voltage direct current is applied to the discharge electrode and ows to the collector electrode of the precipitator. As the particles of entrained matter pass the discharge electrode, the particles are charged by means of a gaseous discharge known as corona. These charged particles are collected principally on the surface of a collecting electrode. Periodically, the particles which are collected are removed from the latter electrode by suitable means such as, for example, rapping.

As a result of this gaseous discharge or corona, a current flows through the discharge electrodes. The characteristics of the corona depend to a great extent upon factors such as pressure, temperature, humidity, chemical composition, and particle loading of the gas. A change in one or more of these factors, therefore, will affect the current flow to the precipitator. In many instances, these changes cause a current ow that is in excess of the capacity of the transformers and rectifiers which step up the voltage and change the alternating current to direct current. To avoid injury to the transformer and rectifier,V most power supplies have incorporated therein an over-current tripping device operatively connected with a transformer and rectier. Thus, an increase in current llow beyond the rated value of the power supplies will operate the tripping device and remove the power from the precipitator before damage to the transformer and rectifier can occur. Thereafter, the precipitator remains inoperative until an operator can re-energize the unit by setting the tripping device manually. To prevent such excessive current flow and the resultant inoperativeness of the precipitator, there has been employed a control means for maintaining a constant ow of current in the electrical energizing equipment. Such control means are usually adjusted to deliver maximum possible current to the precipitator and are known generally as current controls.

In some instances there will occur within the precipitator certain conditions of gas compositions, dust loading, and the like which will cause a complete breakdown of the gas between the precipitator electrodes. This breakdown, known as a spark, manifests itself by large surges of current in the power supply and by momentary loss of the voltage on the electrodes. No dust collection takes place during the momentary loss of voltage. In practice, the duration of this loss of voltage is so short that sparking can be tolerated at rates up to about 100 times a minute before the long term collection efficiency of the precipitator is adversely affected.

It is well known that a precipitator operating at and near the sparking voltage so that not more than about 100 sparks per minute occur is operating at maximum cleaning efficiency. Should theV sparking rate exceed l rates Patenti@ F 2,961,577 Patented Nov. 22, 1960 approximately sparks per minute, collection efciency is reduced and in many instances the current surges in the power supply will cause the overcurrent protective device to trip resulting in complete loss of cleaning eftciency.

Accordingly, to achieve maximum elciency, the sparking rate mustbe maintained at an optimum even though the internal conditions within the precipitator may vary. Heretofore, this has been achieved by manually regulating the input voltage to the equipment or, alternately, control devices known as spark rate controls have been incorporated in the power system to automatically provide a predetermined sparking rate.

From the foregoing, it is readily apparent that the basic problems in controlling the power supplies to the precipitator are twofold. Firstly, the current llow to the precipitator must be maintained below some value commensurate with the current rating of the transformers and rectifers to prevent-trip-out of the over-current devices from occurring during load changes; and secondly, the spark rate must be controlled at an optimum rate commensurate with the precipitator conditions as described above.

Heretofore, both of these basic control problems have been met by the use of either a separate current or separate spark control system. This is not entirely satisfactory for primarily two reasons. In most cases, firstly, it is impossible to predict whether the applied voltage will be limited by spark-over or the power supply current rating and, therefore, the type of control required must be determined by trial after the installation is operating. Secondly, on those units which normally operate from a current control, abnormal sparking may occur when large patches of dust are dislodged from the collector plates by the rapping systems. The current control does not readily compensate for such occurrences.

An object of the present invention is to provide a novel means for delivering maximum power to a precipitator incorporating therein a means for controlling the current flow and voltage so as to achieve optimum efficiency of the precipitator. 1

. Another object is to provide a novel means for delivering the maximum possible power to a precipitator during normal and adverse operating conditions.

A further object is to provide a novel electrical precipitator wherein the current flow of the electrical energizing equipment is 1 continuously sensed to determine the load conditions and corrections are made in accordance with the load condition sensed to adjust the voltage when the need arises to maintain thecurrent flow constant.

In many applications of an electrostatic precipitator,

the characteristics of the precipitator structure such as the clearance between the electrodes, the conditions of the gas as, for example, chemical composition and the like, and the dust concentration, may be such that sparking (also known as Hash-over and spark-over) occurs. Also, frequently even in the absence of prevalent spark-over conditions, spark-over may occur when large-patches of dust are dislodged from the precipitator plates. .This spark-over is in the nature of a short circuit and if it is not rapidly corrected, may cause damage to the precipitator or the power equipment. In addition, when sparkover occurs too often, the average voltage on the precipitator is substantially-reduced thereby reducing cleaning efficiency. This spark-over may be controlled by presetting the voltage which is applied tothe discharge electrode to a low value. However, when the applied voltage is preset to a value at which no sparking occurs, the eciency of the precipitator .is again materially reduced.

Accordinglyit hasbeen found that in precipitator 'applications where the conditions are such that sparking occurs, maximum precipitator efiiciency may' be achieved by controlling the applied voltage such that intermittent sparking occurs. To this end, most efiicient performance is obtained when the voltage between the electrodes of the precipitator is raised to a value at which sparks Y'start to jump between the electrodes.

Still another object is to provide a novel power control unit for a precipitator wherein there is employed a common current sensing means which effective to energize selectively in accordance with the load conditions sensed, a system for controlling the voltage so as to maintain the current substantially constant or to extinguish sparkover, whichever operation is warranted by the load condition sensed.

A still further object of the invention is to provide a novel power unit for a precipitator wherein the current flow is maintained substantially constant or wherein upon spark-over, the voltage is regulated at a predetermined rate so as to deliver the maximum possible power under the prevailing load conditions.

Yet another object is to provide a spark rate control incorporating a novel means for setting the amount of voltage correction and restoring rate.

The present invention contemplates a novel precipitator for removing foreign particles from a gas wherein provision is made for sensing the liow of electrical current to the electrical energizing equipment and comparing the sensed signal with a current standard; should the comparison relative to said current standard reveal an error to be present during normal current flow, provision is made for the flow of current to be adjusted so as to remove the error and maintain the current flow at the standard; should the comparison relative to said current standard reveal that a spark-over resulting in a surge of current exists, the current adjusting provision is inoperative and provision is made to lower the applied voltage and extinguish the spark-over.

The above and further objects aud novel features of the invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are not intended to be a definition of the limits of the invention, but are for the purpose of illustration only.

In the drawings:

Fig. 1 is a schematic block diagram of one embodiment for carrying out the present invention.

Fig. 2 shows diagrammatically a circuit embodying the invention illustrated in Fig. l.

Fig. 3 is a graph showing the characteristic saturation curve of the self-saturating magnetic amplifier employed in the electrical energy control device.

Fig. 4 is a schematic block diagram of a second ernbodiment for carrying out the invention.

Fig. 5 shows diagrammatically a circuit embodying the invention illustrated in Fig. 4.

Referring to Figs. 1 and 2, the electrical precipitator is represented schematically as comprising generally a source for supplying alternating current, a transformer 12 for raising the voltage level of the alternating current, a rectifier 13 for changing the alternating current to direct current, a discharge electrode 15 for charging the particles, a collecting electrode 17 for collecting the particles, a current control circuit generally designated as C for maintaining a constant current flow to the discharge electrode, and a spark-rate control system generally designated as V for regulating the voltage in accordance with a predetermined spark rate.

Source 10 for supplying alternating current may be a conventional alternator. The alternating current is supplied to a current ow responsive device 19, to transformer 12 and after transformation and rectification, to electrode 15.

Transformer 12 and rectifier 13 may be of conventional types. For example, mechanical, selenium, silicon, or

vacuum tube types of rectiers may be used. Such transformation and rectification units are normally provided with an over-current device (not shown) for their protection from overload currents.

Discharge electrode 15 is represented as a wire extending axially into tubular collecting electrode 17. As the gas, bearing suspended particles of matter, passes through electrode 17, the particles are charged by electrode 15 and deposited principally on the inner surface of electrode 17. The material collected on electrode 17 is removed by rapping or irrigation at selected intervals.

As discussed before, the characteristics of the corona may vary with changes in the gas concentration and composition. These changes may greatly affect the power input to the precipitator. In accordance with the present invention, the power input is controlled to obtain maximum efficiency by the current control circuit C which is operative to maintain the current ow at a preset value in the absence of sparking and spark-rate control circuit V which is operative to regulate the voltage at sparking to periodically extinguish the same and maintain a predetermined sparking rate.

The changes of current fiow caused by normal variation in corona discharge are reflected in gradual variations in the current flow from the preset value. These gradual variations in current are in contrast to the transient surges of current occurring at spark-over which result in substantially instantaneous increase in current iiow usually of greater magnitude than the current ow variation during corona discharge.

The changes in normal current flow and at sparkfover are sensed by the current sensing device 19 which includes a conventional current responsive transformer whose primary winding 21 is connected in the line leading to the discharge electrode 15 and in whose secondary winding 23 is developed a voltage corresponding in amplitude to the amount of current flow through the primary winding 21. By way of potentiometer 25, the amplitude of the voltage is adjusted to a desired level and applied through a coupling transformer 27 to a full wave rectifier 25.

Connected to the rectifier 29 is the current control circuit C and the voltage control circuit V.

In accordance with the embodiment illustrated in Figs. 1 and 2, the current control circuit C employs the current sensing device 19, a reference signal device 44 for developing a signal corresponding to the preset current ow to the discharge electrode 15, a wave forming device 34 for developing a signal corresponding to the actual current flow to the discharge electrode, a comparator 41 for comparing the reference current signal and the actual current ow signal, an amplifying device 54 for developing amplified signal corresponding to the amount by which the current flow exceeds the reference signal at the cornparator, and an electrical energy control device 20 responsive to the amplified signal for maintaining the current at said preset value.

The positive output of the rectifier 29 is in the form of a pulsating voltage having a basic frequency of twice the supply frequency and a peak amplitude substantially proportional to the peak alternating current fiowing to the transformer 12. As is well known, the pulsating direct current voltages originating at the rectier 29 have an alternating current component. For the purpose of forming the alternating current component into a substantially sinusoidal wave pattern, the pulsating current is passed through the wave forming device 34, which is an incomplete filter comprising an inductor 35, resistor 37, and capacitor 39. The resulting positive sinusoidal alternating current wave formed by the incomplete filter has an amplitude corresponding approximately to the root means square value of the load current at the discharge electrode. Accordingly, any change in the load current is reflected in the amplitude of the alternating voltage signal passing through the filter 34.

The alternating voltage signal from the wave forming device 34 is applied by way of a coupling capacitor'40 to the comparator 41 which comprises the amplifier tube 43, the grid resistor 41a resistor 45 and capacitor 47. The capacitor 40 serves to block out any direct current voltage signal and permits only the sinusoidal alternating current voltage signal to be applied to the comparator 41.

Connected to the comparator 41 is the reference signal device for applying a reference voltage signal proportional to the desired current flow to the discharge electrode 15. The reference signal device comprises a po tentiometer 49 which is connected to a suitable source of negative direct current. The potentiometer 49 is adjusted to provide a constant level negative bias on the grid resistor 41a opposite to the peak amplitude of the pulsating alternating current voltage signal corresponding to the preset current ow to the precipitator by an amount equal to the cutoff voltage of the tube 43. With this arrangement, no voltage signal is transmitted through the pentode amplifier tube 43 until the total algebraic sum of the positive alternating current and the negative bias signal exceeds the cutoff voltage of the tube. In this connection, it is to be noted that no voltage signal or, as will be more fully explained below, no control effect is transmitted through the pentode tube until the amplitude of the positive alternating current voltage signal exceeds that corresponding to the preset current ow.

In carrying out the invention, the current flow is preset at substantially the maximum current rating of the transformer 12 and rectifier 13. Accordingly, when the sum of constant level bias signal and the positive alternating current voltage at the comparator 41 is less than cutoff voltage of the pentode tube, no voltage signal is emitted therefrom. However, should the sum of these voltages exceed the cutoff value of the pentode tube 43, a pulse signal is emitted from the plate 45. This pulse voltage signal corresponds to the error between the desired preset current flow and the actual current flow to the transformer 12.

The voltage pulse error signal emitted from the plate 45 appears at the resistor 51 as an amplified signal, by Way of the amplifying device 54 comprising the resistor I53 and the capacitor 55 each of which elements with the pentode tube 45 form a stage of resistance coupled amplification well known in the art.

The amplified alternating current pulse error signal passes through a rectifier 57 which, as shown, comprises a diode tube from the plate 57a of which there is emitted a negative direct current voltage corresponding to the error between the preset current flow and the actual current ow to the transformer 12.

This negative direct current voltage signal appears by way of a filter comprising resistor 56 and capacitor 56a at the grid 59 of a triode 61 and decreases the current flow through the plate circuit so as to cause a decrease in negative current flow to the electrical energy control device 20. Since the negative signal introduced on the grid 59 corresponds to the error between the preset and actual current fiow, the decrease in negative current flow, of course, also corresponds to the error.

The electrical energy control device 20 comprises an amplifying device in the form of a self-saturating magnetic amplifier 65 and a device for regulating the power input to the precipitator in the form of a saturable reactor 67 connected between the source and the precipitator.

The self-saturating magnetic amplifier 65 includes control windings 69 and 71, bias winding 73, and controlled windings 7-5 wound on a saturable core. Windings 75 provide controlled output for the saturable reactor 67 and include saturating rectifiers 77 and 79 connected to source 10 and direct current output rectiers 81 and 83 connected to the saturable reactor 67.

The bias Winding 73 is connected in series with a current adjusting rheostat 85 connected to a suitable source of direct current. In the embodiment illustrated, the current flowing through the control winding 69 produces negative ampere turns of control and the current fiowing through the control winding 71 and the bias winding 73 produces positive ampere turns of control. The ampere turns of control as shown on the characteristic saturation curve illustrated in Fig. 3 is the algebraic sum of the ampere turns in the control windings 69 and 71 and the bias winding 73. When zero current, i.e., zero ampere turns of control are in the bias winding 73 and the control winding 69, current flowing through control winding 71 produces positive ampere turns and causes the magnetic amplifier to be fully saturated at the point of the curve of Fig. 3. To achieve the desired control, sufficient current is caused to flow through a bias winding 73 and Winding 71 to add negative ampere turns of control such that during normal operation of the precipitator the magnetic amplifier operates on the point B of the curve.

Upon a flow of current in the primary of transformer 12 exceeding the preset value of current ow, the error signal corresponding to such an increase manifests itself as a decrease in current in the triode 61. This decrease in current in the winding 69 reflects itself in a decrease of positive ampere turns such that the negative turns predominate, resulting in a drop in output from the controlled windings 75 and a reduction in the current ow in the control winding S7 of the saturable reactor 67. The drop in output from the control winding corresponds to the error signal. Hence, the impedance of the controlled winding 89 is increased and a current drop corresponding to the error between the desired and actual current flow to the precipitator results.

As discussed hereinbefore, the conditions within the precipitator may be such that spark-over occurs. This spark-over manifests itself in pulses or surges of transient current flow having a magnitude and a rate of increase exceeding that of the normal or preset current flow to the discharge electrode.

In accordance with the present invention, there is provided the spark rate control circuit V which, in the embodiment shown in Figs. 1 and 2, is connected to the negative terminal 33 of the rectifier 29 and coupled to the positive terminal 31 by way of the capacitor 100 which also serves as a spark detector to beexplained below.

The spark rate control circuit V comprises generally a reference signal device 101 for developing a signal corresponding to the normal current flow through the line t0 the discharge electrode 1'5, the spark detector 100 for detecting and sensing the presence of transient surges resulting from spark-over at the electrode, a comparator 103 at which the reference signal is compared with the sensed detected transient current surges, an amplifier 105 for producing a spark indicating signal of constant amplitude, and a spark regulator 107 which is connected to the electrical energy control device 20 periodically lowering the voltage.

As heretofore explained, the current sensing device comprising the current transformer 2.1, potentiometer 25, transformer 27, and rectifier 29 develops a direct current signal voltage corresponding to the current through the primary of the precipitator transformer 12. In the presence of sparking, current transient surges exceeding the normal current flow are generated and these surges manifest themselves in a direct current voltage signal of short duration at the positive terminal 31. As heretofore pointed out, under normal current flow, or corona discharge conditions, the pulsations of direct current are known to include an alternating current value which is approximately proportional to the root mean square value of the alternating current fiowing to the transformer. When sparking occurs, the magnitude of the surge will be such that it greatly exceeds the root mean square value of the normal current flow. Accordingly, the normal current flow and the transient surges provide a convenient source for determining and controlling the sparking at the electrode. Y l y In accordance with the embodiment of Figs. 1 andi,

positive alternating current signal voltage from terminal 31 of rectifier 29 is applied to the comparator 103 which as shown constitutes the grid resistor of a pentode tube 105. The pentode tube serves as the amplifier, as will be more fully explained below. The positive alternating current signal generated by transient current surges passes through the capacitor 100 which also serves to block out the direct current values in the pulsating signal.

The reference signal device is connected to the negative terminal 33 of the rectifier 29 and comprises the inductor 107, capacitor 109, and resistor 111, which serve to smooth out the negative pulsating signal into a substantially unidirectional direct current signal from the rectifier and are so designed as to have a very large time constant such that when sparking occurs and result-ant transient surges appear, no appreciable change of voltage takes place at the instant of the spark. The reference signal device, since it is connected to the negative terminal of the sensing circuit and since it has a long time constant which is not instantaneously responsive to the current surges, applies a negative direct current bias voltage on the resistor grid 103 of the pentode tube 10S such that bias voltage varies directly with the normal current ilow to the transformer 12.

To maintain the voltage control circuit inoperative in the absence of sparking, the values of the inductor 107, capacitor 109 and resistor 111 are selected such that the negative direct current bias applied to the comparator 103 during normal current flow is opposite to and greater than the peak amplitude of the positive alternating current voltage signal passing through the spark-detector 100 by an amount slightly greater than the cutoff voltage of the tube 105. In this manner, the grid of tube 105 is maintained suiciently `biased such that no error signal is emitted from the cathode 113 of the pentode 105 and no voltage control function is affected in the absence of sparking.

In the presence of sparking, the transient surge of current associated with the sparking results in an alternating current voltage signal at the

terminals

31 and 33 of the rectifier 29 `which greatly exceeds the normal current ow signal emitted therefrom. This increased transient current signal is applied to the comparator 103 by way of the spark detector 100 and reference signal devices 101. However, because of the long time constant in the reference signal device 101, the signal emitted therefrom and applied to the comparator 103 remains substantially the same and, accordingly, gorresponds to the current ow to the primary of the transformer prior to the surge of current or spark-over. As above described, this reference signal applies a bias on the pentode 105 slightly greater than the cutoff voltage of the tube 105. The tr-ansient current signal from the terminal 31 passes through the capacitor 100 forming the spark detector and is also applied to the comparator 103. Hence, -upon the occurrence of spark-over, the total sum of the negative bias signal from the reference signal device 101 and the positive Aalternating current voltage signal corresponding to the ow of transient current from the spark detector appears at the grit' of pentode tube 105 and exceeds the cutoff voltage of tue latter so as to cause a pulse signal to be emitted from cathode 113.

The pulse signal emitted from the cathode 113 is of substantially constant `amplitude regardless of the magnitude of the spark of the discharge electrode 15. This is accomplished by selecting the elements of the tube circuit in such a manner that upon the occurrence of spark-over, the :peak voltage on the grid of the pentode tube 105 always equals zero thereby to saturate the tube. The constant amplitude pulse appears Aat the spark regulator 107 by way of the `blocking diode tube 115 which serves to prevent the charged pulse from leaving the spark regulator 107 -by way of thecathode circuit.

As shown in Fig. 2, the spark rate regulator comprises a variable resistor 117, a fixed resistor 119, capacitor 120,

`a xed resistor 121, and variable resistor 123 connected in parallel to the capacitor 120, and a triode tube 127. The amplitude of the pulse applied to the grid 125 of the tube 127 is controlled by the resistor 119 and the variable resistor 117. Maximum amplitude is obtained by the resistor and lesser amplitude to -a selected desired magnitude by adjustment of the variable resistor 117. Adjustment of the amplitude of the pulse determines the magnitude of the voltage drop affected through the electrical energy control device.

The parallel connected, fixed and variable 121 and 123 are effective to control the rate at which the electrical energy device is effective to gradually increase the voltage as more fully to be explained hereafter.

The voltage signal from the capacitor 120 applied to the grid 125 of triode tube 127 is in the form of a pulse charge and causes the grid voltage to rapidly increase such that the tube current is accordingly rapidly increased a corresponding amount. The rapid increase in plate circuit current in tube 127 is applied to the winding 71 of the self-saturating magnetic amplifier `65 of the electrical energy control device 20. Accordingly, since as before described, the winding 71 is wound to provide negative ampere turns of control, the increased voltage causes additional negative turns of control in the winding 71 which, as shown in Fig. 3, results in a substantially instantaneous reduction in the output from the control Winding 75 of the magnetic amplifier 65. This decrease in voltage is reflected ina decrease in voltage in the control winding 87 ofthe saturable reactor 67 and, accordingly, increases the impedance in the controlled winding 39 so as to reduce the voltage at the transformer 12. The magnitude of the drop in Voltage at the transformer 12 will always be approximately equal for each spark-over. The amount of voltage reduction is dependent upon the amplitude of the pulse applied to the grid 125 of the tube 253. As before explained the magnitude of the pulse is determined by adjustment of the resistor 117. Preferably the pulse is adjusted to reduce the voltage a minimum amount capable of extinguishing spark-over at electrode 15. After the occurrence of sparking the

resistors

121, 123, and capacitor cause the voltage on the grid to dissipate gradually. The rate at which the grid voltage disspates is adjusted at the resistor 123. Since the. rate of dissipation of grid voltage controls the rate at which the negative turns of control are dissipated, it follows that the rate of voltage increase at the saturable reactor 67 is controlled by adjustment of the resistor 123. Hence, the time or rates between sparks is controlled by adjustment of the resistor 123.

From the foregoing, it is apparent that upon the occur rence of spark-over vat the electrodes the voltage in the transformer 12 is instantaneously lov/ered an amount determined by the resistance applied at 117 and the time between sparks or spark-over rate is determined by the resistance applied at 123.

Referring now to Figs. 4 and 5, there is illustrated a second embodiment for carrying out the invention and wherein like elements are designated the same.

As shown in Fig. 4, the electrical precipitator is represented schematically as comprising generally `a source 10 for supplying alternating current, a transformer 12 for raising the voltage level of the alternating current, a rectiiier 13 for changing the alternating current to direct current, a discharge electrode 15 for charging the particles, a collecting electrode 17 for collecting the particles, a current sensing device 225 for sensing the current at the discharge electrode, an electrical energy control device 220 for controlling the current flow and voltage to the precipitator, a current controlling device C for maintaining a constant level of current flow to the discharge electrode 15, and a spark rate circuit V for regulating the spark rate at the discharge electrode 15.

The alternating current is supplied from a suitable alternating current source to the transformer 12 and after suitable rectification by the full wave rectifier 13, the current is supplied to the electrode 15.

As hereinbefore discussed, the conditions within the precipitator may vary in respect of the corona discharge to cause a current flow from the source exceeding the current rating of the transformer 12 and rectifier 13 or a spark-over may result in currents exceeding the current rating of the previous mentioned units. In accordance with the present embodiment illustrated in Figs. 4 and 5, the flow of the current to the discharge electrode is maintained constant by the current control circuit C' or the voltage is regulated by the spark rate control V.

In accordance with the second embodiment the meassure of the flow of current to the discharge electrode 15 is obtained as the direct current value at the rectifier 13 rather than the alternating current value as in the embodiment of Figs. 1 and 2. The magnitude of the aver- -age positive direct current ow through the rectifier circuit is substantially proportional to the magnitude of the root mean square current through the secondary winding of the transformer 12. Since the root mean square secondary current is directly proportional to the root mean squa-re current ow through the primary winding, the average direct current flow through the rectifier 221 substantially corresponds to the root mean square value of the current flow from the source 10. Hence, the direct current at the rectifier is a substantially direct measure o-f ow to the electrode 15 and also a measure of fiow of alternating current to transformer 12. Connected to the rectifier 219 is the sensing device 225 which is in the form of a resistor 225 and produce a voltage signal corresponding to the fio-w of current to the discharge electrode 15 and of the alternating current flow in the transformer 12.

To maintain the current at the discharge electrode 15 substantially constant, the voltage signal from the sensing device 225 is applied to the current control device C', which comprises generally a filter for forcing a substantial smooth direct current signal corresponding to the actual current flow to the discharge electrode, a reference current device 239 for developing a signal corresponding to a desired preset current flow to the discharge electrode 15, a comparator for comparing the reference current signal and the actual current signal, an amplifying device for developing an amplified signal corresponding to the amount by which the current fiow exceeds the reference signal at the comparator, and an electrical energy control device responsive to the amplified signal for maintaining the current ow at said preset value.

The sensed voltage signal from the resistor 225 is applied to filter 226 comprising an inductor 227, capacitor 229, and resistor 231 which serve tosubstantially smooth out the ripple of the pulsating direct current signal from the rectifier measure of current. Located in the line connecting the filter 226 with the sensing device 225 is a diode tube 228 which serves to prevent the resistance at 225 from determinin-g the time constant at which the currentv signal is introduced into the current control circuit C. In this manner, the signal from the filter 226 is representative of and substantially corresponds to the current flowing the rectifier 13 and accordingly at the ydischarge electrode 15.

The direct current voltage signal from the filter 226 is then applied to the comparator 233 which as shown is the grid resistor of an amplifier tube 237. Also applied to the comparato-r 235 is the voltage from the reference device 239. The reference device includes a source of negative biasing voltage 241 which is effective by way of adjustment of potentiometer 243 to provide a voltage signal proportional to a preset current flow. The negative bias voltage provided by the reference device 239 is set to equal the sum of the signal from the filter 226 and the cutoff voltage of the tube 237. In this manner, during the desired or preset level of current flow to the discharge electrode 15, the algebraic sum of the signal at the comparator 235 is such that the amplifier tube' 237 is at or below its cutofvoltage so that no error signal is emitted on the plate 245 of the tube during the preset current ow or below. However, should the signals at the comparator 235 show that the signal voltage from the current sensing device 225 is greater than the preset level such that the algebraic sum of the signal voltage and the constant negative reference voltage result in a value greater than the cutoff voltage on the grid resistor 235, the tube 237 conducts -an amplified voltage error signal which serves to operate the electrical energy adjusting device 220. The signal emitted at the plate 245 corresponds to the difference between the actual current flow and the desired current flow to the discharge electrode 15.

The electrical energizing device 220, ias in the embodiment illustrated in Figs. l and 2, compris a self-saturating magnetic amplifier 65 and a saturable reactor 67.

The self-saturating magnetic amplifier 65 includes control windings 247 land 248, bias winding 249 and control-led windings 75 wound on a saturable core. The windings provide controlled output for the saturable reactor 67 and saturating rectifiers 77 and 79 connected to source 10 at X and output rectifiers 81 and 83 connected to the saturable reactor 67 at Y. The bias winding 249 is connected in series with a current adjusting rheostat 251 connected to a suitable source of direct current. In the embodiment illustrated in Fig. 5, the current flowing through the control windings 247 and 248 produces negative ampere turns of control and the current fiowing through the bias winding 249 produces positive ampere turns of contro-l. The ampere turns of control, as shown on the curve illustrated in Fig. 3, is the algebraic sum of the ampere turns in control windings 69 and 71 and the bias winding 73. The tube 253 normally carries a quiescent current which results in producing negative ampere turns of control in the winding 248. The bias winding which is wound to produce positive ampere turns is adjusted so as to cancel the quiescent ampere turns resulting from the current flow through the tube 253. In the presence of lan error signal when the tube 237 conducts and emits a signal corresponding to the condition where primary current exceeds the preset value, additional negative ampere turns of control are supplied to the winding 247 of magnetic amplifier and the output current from the control windings 75 is reduced. This reduction in output of the winding 69 reects itself in a `decrease in current to the control winding 87 of the saturable reactor 67. Hence, the impedance of the controlled winding 89 is increased and a current drop corresponding to the error between the desired and .actual current flow to the precipitator results. In this manner, the current flow to the precipitator is maintained substantially constant.

For the purpose of controlling the voltage and the spark rate, there is connected to the sensing device 221 the spark or voltage circuit V.

As previously discussed, when a spark-over results, the power supply is effectively short-circuited such that the electrical energy stored in the precipitator capacitance is discharged. The electrical energy is recharged on the capacitance fro-m the power supply when the spark-over is extinguished. This recharging results in a surge of current of a duration of one-half of one cycle of a line frequency cycle and the surges are not necessarily of a constant magnitude and may only be slightly larger than the normal desired flow of current to the precipitator.`

Heretofore, these recharged pulses of current have been employed to indicate when spark-over occurs. However, because the surges are not always constant and are occasionally too small to be distinguished from the normal current pulses, diiculties heretofore have been encountered.

In accordance with the embodiment of Figs. 4 and 5, the sensing device` 225 is arranged so that the discharge of distributed capacitance in the transformer is employed as a spark indicating signal. As shown in Fig. 5, the sensing device employs the circuit of the rectifier 219. When a spark occurs, thus discharging the precipitator and short-circuiting the power supply, the distributed capacitance in the secondary winding of the transformer 12 is also discharged. This discharge current from the secondary winding passes through the rectifier circuit 219 and reflects itself in transient current surges which are substantially ten or more times the normal current ow. These transient current surges have been found to be damped rapid oscillations of about microseconds duration and appear at the sensing resistor 225 as an oscillating voltage signal.

The resulting oscillating voltage signal also appears at the current control circuit C' but because of the short duration of the surge, the filter 226 including

elements

227, 229, and 231 do not respond thereto. In this manner, the spark signal does not affect the current control circuit C but is only applied to the spark rate control circuit V.

The spark rate control circuit C comprises generally a pulse forming device 261 for converting the rapidly oscillating spark voltage signal to a unidirectional, nonoscillating signal of somewhat longer duration, a pulse control device 263 for forming a pulse of constant magnitude and duration, and an amplifier 295 for amplifying the constant magnitude pulse, and a spark rate device 267 for regulating the voltage in accordance with a selected spark rate, and the electrical energy control device 226 which is common to both the current control circuit and voltage control for reducing the voltage at the discharge electrode 15.

The pulse forming device 261 comprises diode 269, capacitor 271 and resistor 273 which are arranged to rectify and lter the oscillating voltage signal resulting from transient current surges or spark-over and form a substantially unidirectional pulse signal of somewhat longer duration than the oscillating signal.

This unidirectional signal is applied by way of the coupling capacitor 275 to the constant pulse magnitude device 263 which, as shown, constitutes a one shot multivibrator of substantially conventional circuitry. As shown, the multivibrator 263 comprises a dual triode tube 277 including

resistors

279, 281, capacitor 283, resistors 285, 287, and 289. The output of the multivibrator 263 is rectangular voltage pulse of which the duration is controlled by the values of the capacitor 283 and resistor 285. These values are selected such that the pulse width is less than one-half the period of the alternator 10.

The rectangular pulse signal emitted from the multivibrator 263 appears by way of capacitor 291 on the grid 293 of the amplifier 295. Also applied to the grid is a bias signal obtained from a suitable source of direct current 297 and resistor 299. This bias signal is of a xed value and arranged so as to maintain the amplifier 295 below its cutol voltage in the absence of a pulse from the multivibrator 263. Upon the occurrence of a pulse, iov/ever, the tube 295 emits an amplified signal from the cathode 301. The grid resistor 303 is selected to limit grid current when the voltage swings positive upon the occurrence of a pulse signal indicating spark-over.

The signal emitted from the cathode 301 is then applied to the spark rate circuit 267 which serves to control the voltage and spark rate and includes a variable resistor 305, fixed resistor 307, capacitor 309, a fixed and variable resistance 310 and 311, respectively, connected in parallel to the capacitor 309, and a triode tube 253. Connected in the line between the amplier 295 and the spark rate circuit 267 is a blocking diode 313 which serves to prevent the pulses applied to the capacitor 309 from feeding back into the amplifier 295. The amplitude of the pulse applied to the grid 317 of the tube 253 is controlled by the resistor 307 and the variable resistor 305. Maximum amplitude is achieved by the fixed resistor 307 and a selected lesser amplitude by way of adjustment of the variable resistor 305. Adjustment of these two resistors determines the magnitude of the voltage drop elected. The fixed and variable resistors 310 and 311 are eiective to control the rate at which the voltage gradually increases at the transformer 12. The manner in which the

resistors

305 and 307 and resistors 310 and 311 are effective to control the magnitude and voltage increase respectively will be more fully explained below.

The tube 253 is connected to the control winding 248 and is self-biased by way of the resistor 319 such that the negative ampere controls provided by way of the winding 248 are in opposition to the positive ampere turns of control provided by the bias winding 249 of the magnitude amplifier 265. The bias winding, as previously described, is sufficient to overcome the negative ampere turns of control provided by the winding 248 during normal current ow, that is in the absence of sparking, so that maximum output is obtained from the controlled windings 75 thereby to assure an optimum voltage in the winding 89 of the saturable reactor 67. However, in the presence of sparking, the magnitude of the pulse on the grid 317 of the triode tube 253 is such as to increase the flow of current in the plate circuit 321 whereby the negative ampere turns of control are increased so that, as shown in Fig. 3, the output of the controlled windings 75 is accordingly reduced thereby increasing the irnpedance in the controlled winding S9 of the saturable reactor 67 as previously explained. The amount of reduction achieved by the saturable reactor 67 is dependent upon the amplitude of the pulse applied to the grid 317 of the tube 253. Accordingly, since the total values of the resistance from the

resistors

305 and 307 determines the magnitude of the pulse applied to the grid 253, adjustment of the variable resistor 305 controls the amount of voltage reduction upon the occurrence of spark-over. Preferably the resistor 305 is set to result in a voltage drop which does not materially reduce precipitator efficiency. After the occurrence of sparking the resistors 311, 310, and capacitor 315 cause the grid voltage and also the negative ampere turns in the control windings to dissipate gradually whereby a gradual decrease in voltage in the controlled winding 89 of the saturable reactor 67 results. The rate at which the voltage to the precipitator is increased is determined by the value of the resistances of resistor 310 and 311 since the latter control the decay rate from the plate of the capacitor 309. By adjustment of the variable resistor 311, it is possible to control the rate at which capacitor 309 discharges and in this manner to control the rate of increase of voltage in the controlled windings 89. As described in connection with the embodiment of Figs. l and 2, the rate of increase of voltage determines the sparking rate at the discharge electrode 15. Hence, adjustment of the variable resistor 310 is effective to control the sparking rate.

What is claimed is:

1. A precipitator for removing foreign particles from a gas comprising a discharge electrode for charging said particles, a collecting electrode for receiving said charged particles, means for supplying voltage and a substantially constant level current flow to said discharge electrode, means for sensing said current flow during corona discharge and sparking at said electrode, and for developing a signal corresponding to said sensed current ow, means for developing a reference signal corresponding to said constant level current flow during corona discharge, means responsive to said sensing means when said current flow during corona discharge exceeds said reference signal for maintaining said current flow at said constant level, and means responsive to said sensing means being operative solely upon sparking resulting in transient current surges for lowering said voltage to a value at which spark-over is diminished.

2. A precipitator for removing foreign particles from a gas comprising a discharge electrode for charging said particles, a collecting electrode for receiving said charged particles, means for supplying voltage and a substantially constant level current flow to said discharge electrode, means for sensing said current ow during corona discharge and sparking at said electrode, means responsive 'to said sensing means when said current flow during corona discharge exceeds said constant level current oW for maintaining said current flow at said constant level, and means responsive to said sensing means being operative solely upon sparking resulting in transient current surges for reducing said voltage to a value at which spark-over is diminished and including means for adjusting the magnitude and duration of said voltage reduction.

3. In an electrostatic precipitator a power supply system comprising means for supplying a voltage and a substantially constant level current flow to said precipitator, means for sensing the ow of current to said precipitator, means for controlling the ow of current and voltage to the precipitator including saturable reactor means, means connected between said sensing means and said saturable reactor means responsive to normal current fiow exceeding said constant level current flow to vary the impedance of the saturable reactor means so as to reduce the current flow to said precipitator to the constant level fiow, and means connected between said sensing means responsive to transient surges of current resulting from sparking including means for rendering said responsive means inoperative during normal current flow and operative upon transient surges for varying the impedance of said saturable reactor means to instantaneously lower said voltage.

4. An electrical precipitator system comprising a transformer supplied from an alternating current supply source providing a preset constant level of current fiow, rectifying means for the output of said transformer, a discharge electrode connected to said rectifying means, an electrical energy control system for controlling the flow of current and voltage to the discharge electrode comprising means for sensing the ow of current to said precipitator, saturable reactor means connected between said source and said transformer, a current control circuit connected to said sensing means and said saturable reactor including means for developing a signal corresponding to the actual current ilow to said discharge electrode, and means for developing a reference signal corresponding to the preset constant level current flow, means for generating a signal corresponding to the dierence between said actual and preset constant level only when said actual current flow exceeds said preset level of current iiow so as to vary the impedance of said saturable reactor means and reduce said current flow to said discharge electrodes to said preset value.

5. An electrical precipitator system comprising a transformer supplied from an alternating current supply source providing a preset substantially constant level of current ow, rectifying means for the output of said transformer, a discharge electrode connected to said rectifying means, an electrical energy control system for controlling the fiow of current and voltage to the discharge electrode comprising means for sensing the flow of current to said precipitator, saturable reactor means connected between said source and said transformer, a voltage control circuit connected to said sensing means and said saturable reactor means including means being inoperative during normal current iiow and being operative only upon the occurrence of transient current surges resulting from spark-over for developing a pulse signal, a Voltage regulating circuit including means for selectively adjusting 7 the magnitude and the rate of the pulse so as to vary the impedance of said saturable reactor means and reduce said voltage to said discharge electrode in accordance with a selected spark rate.

6. The invention as defined in claim 5 in which said signal developing means comprises a multivibrator.

7. The invention as defined in claim 5 in which said pulse magnitude and discharge rate means comprise a variable resistor'and a condensor.

8.,Ihe invention as defined in claim 7 in which said signal developing means ,comprises a multivibrator and -saidpulse magnitude and rate means comprise a variable resistor and a condensor.

9. An electrical precipitator system comprising a transformer supplied from an alternating current supply source providing a preset constant level of current flow, rectifying means for the output of said transformer, a discharge electrode connected to said rectifying means, an electrical energy control system for controlling the tlow of current and voltage to the discharge electrode comprising means for sensing the flow of current to said precipitator, saturable reactor means connected between said source and said transformer, a current control circuit connected to said sensing means and said saturable reactor including means for developing a signal corresponding to the actual current flow to said discharge electrode, means for developing a reference signal corresponding to the preset constant level current flow, means for generating a signal corresponding to the difference between said actual and preset constant level solely when said actual current ow exceeds said preset level of current flow so as to vary the impedance of said saturable reactor means and reduce said current flow to said discharge electrodes to said preset value, a voltage control circuit connected to said sensing means and said saturable reactor means including means being inoperative during normal current iiow and being operative only upon the occurrence of transient current surges resulting from spark-over for developing a pulse signal, a voltage regulating circuit including means for selectively adjusting the magnitude and the rate of the pulse so as to vary the impedance of said saturable reactor means and reduce said voltage to said discharge electrode in accordance with a predetermined spark rate.

10. The invention as defined in claim 9 in which said saturable reactor means comprises a first saturable reactor having a control winding and a controlled winding, and a second saturable reactor having controlled winding means connected to said control Winding and control windings arranged to be responsive to said error signal and said pulse signals to increase the impedance :in said first saturable reactor control windings.

l1. The invention as defined in claim l0 in which said second saturable means comprises a magnetic amplifier.

l2. The invention as defined in claim 10 in which said signal developing means comprises a multivibrator.

13. The invention as defined in claim 10 in which vsaid pulse magnitude and rate means comprises a variable resistor and a condensor.

14. The invention as defined in claim ll in which said signal developing means comprises a multivibrator and said pulse magnitude and rate means comprises a Variable resistor and a condensor.

l5. A power control for an electrostatic precipitator having a transformer connected to an alternating current source, a rectifier for the output of said transformer, a discharge electrode connected to said rectifier, means connected to said rectitier for sensing the current fiow to said electrode and developing a signal corresponding to said current flow, a current control means connected between said sensing means and said course responsive to variations in current flow relative to a reference current flow to maintain said current to said discharge electrode constant when said current flow exceeds said reference fiow.

16. A power control for an electrostatic precipitator having a transformer connected to an alternating current source, a rectifier for the output of said transformer, a discharge electrode connected to said rectier, means connected to said rectifier for sensing the current ow to said electrode, and a voltage control means connected between said sensing means andsaid source -responsive upon sparking at said electrode to reduce the voltage at said discharge electrode, said voltage control means including means for forming a square wave of substantially constant amplitude and wave length for each spark.

17. The invention as defined in claim 16 in which said voltage control means includes selectively adjustable References Cited in the file of this patent UNITED STATES PATENTS Hildebrand May 7, 1940 Stenitz Sept. 24, 1957 Notice of Adverse Decision in Interference In Interference No. `9 H. T. Williams and J. W.

adverse to the patentees W Gazette April 30, 1.963.]

2,443 nvo Drennng,

lvng Patent No. 2,961,577, Electrostatic preciptators,

as rendered Dee. 13, 1962, as to claims J. B. Thomas, nal

judgment

1, 9, 10 and 13.