US2821665A - Cutoff circuit for electrostatic devices - Google Patents

Description

W. A. BRASTAD 5' AL CUTOFF CIRCUIT FOR ELECTROSTATIC DEVICES Filed Feb. 24, 1956 Jan. 28, 1958 I v I Ag t A 7 V Y- r +0 V I ECBL E 6 I! ED. N wp Z .EM U h flspsT 6] 7 A United States Patent CUTOFF CIRCUIT FOR ELECTROSTATIC DEVICES William A. Brastad, Edward R. Van Krevelen, and Earl F. Diekholf, Minneapolis, Minn., assignors to General Mills, Inc., a corporation of Delaware Application February 24, 1956, Serial No. 567,510

6 Claims. (Cl. 317-9) This invention relates generally to a protective system for electrostatic devices and pertains more particularly to a system which will prevent ignition of explosive mixtures such as wheat flour and air in case an electrical fault occurs in the electrostatic processing machine.

A primary object of the invention is to provide a cutofi circuit for use with electrostatic separating equipment which is exceptionally fast acting so as to prevent ignition of those mixtures likely to form during the separating process. In this connection it may be pointed out that it has been determined by the U. S. Bureau of Mines and certain other organizations that a minimum energy of approximately 50 millijoules is required to ignite some mixtures. Of course, the rapid action above mentioned is exceedingly important in obviating the explosive happenings that have occurred in the past with sometimes disastrous consequences, for the contemplated system reduces the magnitude of arcing current so quickly that relatively little heat can be generated therefrom.

Another object of the invention is to produce a cutoff system that is very reliable in its operation, the invention envisaging several fail-safe features.

A further object of the invention resides in the provision of a system that is economical to operate.

Further, an object of the invention is to produce a system of the foregoing character that is of relatively low cost so that it is capable of use in a variety of installations, thereby encouraging more widespread adoption of the protective equipment and the concomitant avoidance of. taking chances that no ignition will occur.

Other objects will be in part obvious, and in part pointed out more in detail hereinafter.

The invention accordingly consists in the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereafter set forth and the scope of the application which will be indicated in the appended claims.

In the drawing, the single figure is a' schematic view showing the cutoff circuitry in association with a pair of vertically spaced electrode plates.

Referring in detail to the drawing, the invention has been exemplified in conjunction with electrode plates 10 and 12, these plates being disposed one above the other. It will be discerned that the lower electrode plate 10 is equipped with a double insulating trough 14 which may be fixedly disposed adjacent one end of this lower plate, the trough including a first recess 16 for receiving mate rial discharged directly from the plate 10 whereas another recess 18 is provided for receiving a discharge from the upper electrode plate 12. "It is to be noted that the upper electrode plate is formed with a number of apertures 20, several of which are visible in the diagram.

To present an idea as to the paths taken in the separation process, it is to be observed that those particles remaining on the lower electrode 10 traverse a path indicated by a sequence of arrows designated by the numeral 22. On the other hand, those particles levitated onto the ICC upper electrode 12 via the apertures 20 are denoted by arrows which bear the reference numeral 24. For instance, if we assume that a separation is desired with respect to comminuted wheat stock which contains pure bran particles, pure endosperm particles, and particles in which both bran and endosperm are attached in varying proportions, a relatively branny fraction thereof may follow the path denoted by the sequence of arrows 24. The remaining stock is of course discharged in the direction of the arrows 22. In presenting the electrode plates pictured in the drawing, it is important to appreciate that these electrode plates may assume a variety of configurations, the precise form that these plates may take being relatively unimportant to an understanding of the invention presently to be described.

in order to achieve the separation, not only is the electrode plate 10 jostled or vibrated but of course there is applied to the two electrodes a relatively high potential difference. To produce the desired potential difference between the lower and upper electrode plates 10 and 12 is an electrostatic power supply 26 having, say, its negative terminal electrically connected to the electrode plate 12 by means of a conductor 28. The positive terminal, however, leads directly to a vacuum tube such as 2053 tube 30 provided with a plate 32, grid 34, cathode 36, and a cathode heater 38, the power supply to the plate 32 being via a conductor 40. In serial relationship between the lower electrode 10 and the cathode 36 is a sensing resistor 42. In this way the D. C. power providing the difference in potential between the two electrode plates 10 and 12 is supplied via both the tube 30 and the sensing resistor 42. Use of the voltage drop across the sensing resistor 42 is utilized in a manner hereinafter made manifest.

Also included in the cutoff circuit is a gas tube such as a 2D2l thyratron tube 44-. The tube 44 comprises a plate 46, a shield grid 48, a control grid 50, a cathode 52, and a cathode heater 54. The control grid 50 is electrically connected to the sensing resistor 42 by way of a limiting resistor 56, a bias battery 58, and an adjustable tap 60 movable longitudinally along the sensing resistor 42. Thus the elements 42 and 6d constitute a potentiometer, the tap 60 of which permits the system to be adjusted for operation upon the flow of a predetermined amount of current through the resistor 42.

A cathode resistor 62 is placed in circuit with the cathode 52 of the tube 44, a conductor 64 leading from said resistor to the grid 34 of the vacuum tube 30. In this way, when the thyratron tube 44 is fired by causing the bias voltage applied to the grid 50 thereof to become sufiiciently more positive, there is a biasing potential produced by way of the cathode resistor 62 which modifies the biasing potential of the vacuum tube 30, causing it to go more negative so that any current passing through the tube 30 to the electrodes 10 and 12 is substantially reduced to zero owing to the fact that the change in biasing voltage causes the resistance of this hard tube 30 to become very large.

In order to supply power to the thyratron tube 44 there is a rectifier tube 66 such as a 6 x 5 tube having electrical connection with a secondary portion 68 of a power supply transformer 70. Also included between the thyratron 44 and the rectifier tube 66 is a filter circuit 72 conventionally composed of resistance and capacitance as presented in the schematic diagram. The primary 74 of the power supply transformer is suitably connected to a source of A. C. voltage as by way of a switch 76. As shown, the previously mentioned electrostatic power supply 26 may also be connected to this same A. C. line voltage, the switch 76 serving to energize both the supply 26 having a suitable rectifier therein and the transformer 70.

Supplying the heating elements 38 and 54 with current is another secondary portion 78 of the transformer 79. The secondary portion 78, however, is provided with a center tap 80 which permits only the heating element 54 to be energized during a preliminary period, the reason for which will soon be explained. Incorporated into the circuit is a relay 32 having a coil 84 and an armature 86, the armature being equipped with a bridging contact 88. In the deenergized condition of the relay 82 the bridging contact '88 rests upon a first set of contacts labelled 90 and at this time only the heating element 54 receives heating current from the secondary portion 78. Inasmuch as the coil 84 of the relay 82 is connected across the output from the rectifier 66, the relay will not pick up its bridging contact 88 until the rectifier 66, together with the filter circuitry 72, is producing an adequate output voltage. However, once the relay 82 is energized then the bridging contact 88 is attracted upwardly so as to bridge across a second set of contacts 92. It is the bridging of these contacts 92 that places the heating elements 38 and 54 in series.

By reason of the circuit arrangement described in the above paragraph, one fail-safe feature is incorporated into the over-all system for if the heating element 33 should fail for any reason to heat properly the cathode 36, then the tube 30 is automatically cut off so that there is no voltage applied to the electrode plates it) and 12. Further, by disposing the heating elements 38 and 54 in series should the heating element 54 fail it, too, will assure that the vacuum tube 36 is deactivated so that no potential difierence is applied to the electrode plates 10 and 12. In other words, failure of either element 38 and 54, owing to their serial connection, will remove the operating potential from the electrodes 10 and 12. Still further, if the rectifier tube 66, the filter circuit 72, or the transformer 70 should fail for any reason, the voltage impressed on the coil 84 of the relay 82 will be lowered or removed so that the heating element 38 is deenergized by virtue of the bridging contact 38 dropping down from its bridging position with respect to the contacts 92 so that it bridges only the contacts 90, having the effect of deenergizing the element 38. Yet another refinement resides in the fact that the warm-up time of the thyratron tube 44 might very well be longer than that of the vacuum tube 30. Owing to the utilization of the two sets of contacts 90 and 92, it will be appreciated that there is a preheating of the heating element 54 via the contacts 99 during the period that the rectifier tube 6-5 is warming up, the function of the rectifier tube, of course, being to supply a B voltage to the thyratron tube 44. Thus when the proper B voltage is obtained, then this voltage is sufiicient to energize the relay 82 and cause the bridging contact 83 to i move upwardly into engagement with the second set of contacts 92 to energize then the element 38 so that voltage to the electrodes 10 and 12 is not applied before the protective circuit is activated.

While the foregoing description is believed sufiicient to afford a ready appreciation of the merits involved in the practicing of the instant invention, nonetheless a brief outline of the operation that takes place will undoubtedly be of added help. Therefore in describing a typical operational sequence, We will assume that a fault occurs at the situs designated by the numeral 94. A number of reasons can cause such a fault. For ex ample, the material being separated may very well pile up in the vicinity of the situs 94 so that the relatively high voltage to which the electrode plates It and 12 are subjected is sufiicient to cause an arcing. In this regard, it may be pointed out that in the separation of comminuted wheat stock, as previously mentioned, a potential difference between the electrode plates of from approximately 1500 to 5,000 volts per inch is employed.

Of course the particular voltage and the spacing of the electrodes depend upon the type of particulate material undergoing separation and the above presented voltage values are only exemplary. However, the cited figures will demonstrate the electrical stress to which the illustrated separating machine is subjected.

At any rate, once a fault occurs in the region designated by the numeral 94, it will be understood that current then flows from the electrostatic power supply 26 via the vacuum tube 30* and the sensing resistor 42 in circuit therewith, for these elements are in serial relationship with the electrodes 10 and 12. The flow of a predetermined amount of current, for example, in the neighborhood of 0.4 milliampere may constitute a critical value of current above which the system should not operate in order to avoid having ignition take place. The potential drop across the sensing resistor 42 is, however, instrumental in producing sufiicient bias on the control grid 50 of the thyratron tube 44 which makes the biasing of this tube more positive and the tube fires immediately. The firing of the tube, in turn, causes cur rent to flow through the cathode resistor 62 and the potential thus applied across the cathode resistor 62 is responsible for driving the grid 34 of the vacuum tube 30 sufiiciently negative so as to cause the current flowing through the tube 30 to drop to zero or to a substantially zero value. This reduction in current, which need not necessarily be total but preferably so, is caused by having the internal resistance of the tube 30 substantially increased. The foregoing action happens extremely rapidly, approximating only 64 microseconds in time duration so that the heat from any spark produced at 94 is inadequate to cause the damaging ignition that is to be avoided. Once the fault has been eliminated, then, of course, the operating potential can be restored to the electrode plates 10 and 12. To do this, the switch 76 should be opened and left open for an interval long enough to be certain that the thyratron tube 44 is no longer conducting. In this respect it will be understood that some residual energy will remain and that it takes a moment or .two for the plate voltage applied to the thyratron tube 44 to decay. After this, the switch 76 can be reclosed and another operating cycle inaugurated.

-It has already been mentioned that the system is equipped with a number of fail-safe features. In this regard it will be recalled that the heating elements 38 and 54 of the tubes 30 and 44 are in a serial relationship and should either one of them fail for any reason, then there is no potential applied to the electrodes 10 and 12. Also, as was pointed out during the starting up period, the heating element 38 is not energized until sufiicient voltage is impressed on the relay 82. Furthermore, any elimination or serious reduction of the B voltage coming from the rectifier tube 66 will cause the relay 82 to become deenergized and thus the safety feature introduced during the starting up procedure is preserved throughout the operation of the cutoff circuitry.

As many changes could be made in the above construction and many apparently widely difierent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the language used in the following claims is intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

What is claimed:

1. A cutoff circuit for electrostatic devices having a pair of spatially disposed electrodes between which a difference in electrical potential is normally maintained, the

circuit comprising an electrostatic power supply for said electrodes, a vacuum tube provided with a plate, cathode and grid, a sensing resistor, said plate, cathode and resistor being in series with said power supply, a gaseous tube provided with a plate, cathode and grid, circuit means connecting the grid of said gaseous tube to said sensing resistor so that said grid is triggered by the voltage across said resistor to fire said gaseous tube upon passage of a predetermined amount of current between said electrodes and through said sensing resistor, a cathode resistor, a source of power in circuit with said cathode resistor and the plate and cathode of said gaseous tube, circuit means connecting the grid of said vacuum tube to said cathode resistor so that said vacuum tube is biased by the voltage across said cathode resistor to increase the tube resistance of said vacuum tube when said gaseous tube is fired, thereby to limit the passage of current through the vacuum tube and hence between said electrodes to a value no greater than said predetermined amount of current.

2. A circuit in accordance with claim 1 in which said sensing resistor constitutes a potentiometer having an adjustable tap whereby the voltage applied to the grid of said gaseous tube may be varied.

3. A cutoff circuit for electrostatic devices having a pair of spatially disposed electrodes between which a difference in electrical potential is normally to be maintained, the circuit comprising an electrostatic power supply for said electrodes, a vacuum tube provided with a plate, cathode and grid, a sensing resistor, said plate, cathode and resistor being in series with said power supply, a gaseous tube provided with a plate, cathode and grid, biasing means connected between said sensing resistor and the grid of said gaseous tube for biasing said gaseous tube to fire when a predetermined potential is developed across said sensing resistor by reason of the passage of a predetermined amount of current therethrough, a cathode resistor, and a source of power in circuit with said cathode resistor and the plate and cathode of said gaseous tube, circuit means connecting said cathode resistor to the grid of the vacuum tube for applying a more negative potential to said vacuum tube grid upon the firing of said gaseous tube to thereby sufliciently increase the tube resistance of said vacuum tube to such an extent that the current therethrough is reduced substantially to zero.

4. A circuit in accordance with claim 3 including respective electrical heating means for each of said cathodes, said respective heating means being in a serial relationship with each other so that the potential applied to said electrodes via said vacuum tube is automatically cut ofi should the heating means associated with said gaseous tube become inetfectual.

5. A circuit in accordance with claim 4 including a relay energized by the power source for said gaseous tube, said relay having contact means in circuit with the heating means associated wtih said vacuum tube for inactivating said vacuum tube and thereby removing the potential applied to said electrodes upon a sufficient decrease or failure in voltage from said gaseous tube power source.

6. A circuit in accordance with claim 5 in which said contact means includes a first set of contacts for energizing only the heating means associated with said gaseous tube when said relay is deenergized and a second set of contacts for energizing both of said heating means when said relay is energized, whereby the heating means associated with said gaseous tube is preheated during the warming up of said gaseous tube power source.

No references cited.

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