US2116393A - Arc-back suppressor for electric discharge devices - Google Patents

Description

May 3, 1938. R. c. GRIFFITH 2,116,393

ARC-BACK SUPPRESSOR FOR ELECTRIC DISCHARGE DEVICES Filed Feb. 1, 1936 2 Sheets-Sheet l Inventor" Rol land C. Griffith,

i Attorney.

R. C. GRIFFITH May 3, 1938.

ARC-BACK SUPPRESSOR FOR ELECTRIC DISCHARGE DEVICES Filed Feb. 1, 1936 2 Sheets-Sheet 2 Inventor Rolland C. Griffith, b8 5.

His Attorneg.

Patented May 3, 1938 PATENT OFFICE ARC-BACK Holland 0. ennui.

sorraasson Foa EIECTRIC mscnrmcr: navrcas Schenectady, N. Y., assignor to General Electric Company, a corporation of New York A plication February 1, 19, Serial No. 61,938-

"Claims.

My invention relates to arc-back suppression ,rneans for electric discharge devices, and more particularly for such devices when used for the rectification of alternating currents.

Electric discharge devices employed as rectifiers are normally regarded as being unilaterally conducting; that is, they will permit a flow of current therethrough only at such times as the anode electrode is positive with respect to the cathode.

, It occasionally happens, however, that due to some transitory cause a back-fire will occur involving a flow of current from'the anode to the cathode during the period when the former is negative. While such back-fires are generally of very short duration, not exceeding one-half cycle, the back-fire may be transformed into a sustained arc-back which will endure through many successive cycles. This result is considered to be due to the factthat the single back-fire takes the form of a constricted arc stream fixed on a concentrated area of the anode surface. The intense heat thus generated may cause localized vaporization of the anode surface which then permits cathode emission to take place and causes the discharge device to lose its unilaterally conducting character. This effect is particularly objectionable iii-connection with metallic anodes, which have a relatively low vaporizing temperature, and has led to their general replacement by anodes consisting of graphite. consequence ofthis substitution has been the great difficulty encountered in properly degassing the graphite structure during the'manufacture of the tube.

It is an object of the present invention to provide back-fire suppressing means which will be effective to prevent a reverse current from enduring for more than a single half cycle.

It is a further object of the invention to prevent occurrence of a sustained arc-back due to heatingof the anode during any back-fire period.

It is a still/further object of the invention to prevent the anodesurface from becoming thermionically emirsive at any point by causing a continuous movement of the back-fire are over the surface of the anode whenever such an arc occurs.

In accordance with my invention these objects are attained by providing in connection with the anode structure'means, operative during a backfire period, which will act upon the are stream to insure a-continuous motion thereof over the anode surface. In a preferred form of the invention, this is accomplished by disposing a mag- An undesirable netic structure in such relation to the anode that the lines of magnetic flux are caused to pass transversely through the arc stream, thus producing motion of the latter in accordance with the Well-known right-hand rule of reaction between electric and magnetic fields.

The novel features which I consider to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and the method-of operation, together with further objects and advantages thereof will best be understood by reference to the following specification taken in connection with the accompanying drawings in which Fig. 1 shows in partial section a schematic drawing of an electric discharge device embodying my invention; Fig. 2 shows a fragmentary portion of a similar device having associated therewith an iron magnetic structure; Fig. 3 is a plan view of Fig. 2; Fig. 4 illustrates in section a modified embodiment of the invention as applied to a pool-type discharge device, Fig. 5 shows a detailed perspective view of the magnetic structure of Fig. 4; Figs. 6 and l are fragmentary sections of possible modifications of the invention, while Figs. 8 and 9 show in vertical and horizontal sections respectively the application thereof in connection with a special arc extinguishing structure. I Referring particularly to Fig. 1, 13 have shown a sealed envelope I of glass or other non-metallic insulating material. At its lower portion the envelope comprises a depending tubulation 2 of reduced cross-section while at the top it flares laterally into a hollow skirt structure, the outer portion of which is conformed to provide. an annular chamber 3. This is in communication throughout its circumference with the depending chamber 2 through the circumferentially extending passageway 4. Fixed to the base of the tube are bayonet contacts 5 which are in current-conducting relation with an enclosed cathode 1 by means of lead-in conductors sealed into a suitable vpress 8. While the nature of the cathode 1 is not essential to my invention, it will be understood that it may comprise a hollow cylinder enclosing extended emission surfaces having thereon a coating of an activating material such as an alkaline earth metal oxide. Such cathodes are conventionally heated indirectly by means of a resistance wire filament arranged coaxially with the outer shield and provided with heating current through two of the bayonet contacts 5. An example of-this type of cathode is disclosed in Patent No. 1,924,318 to A. W. Hull.

Mounted within the annular chamber 3 and supported in any convenient manner I provide an anode I shown in this instance asbearing the shape of a toroid of generally rectangular cross-section and in conductive relation with the outside of the envelope l by means of a suitable lead-in conductor H. While it is preferred that the anode Ill be formed to have a substantially continuous re-entrant surface l2 for receiving a discharge, it is desirable for use in connection with my invention that it be provided at some portion of its circumference with a gap or insulating section which will minimize the free circulation of currents induced in the anode.

From the nature of the structure described in the foregoing it will be obvious that any discharge passing between the cathode I and the anode ill will be obliged to pass laterally through the passage 4. Since there may be some tendency for a tube so constructed to start with difllculty under certain conditions, I may provide an auxiliary electrode l3 mounted directly above the cathode and connected to the anode means by a resistance l4. With this arrangement, when potential is applied across the tube a discharge will be readily initiated between the cathode 1 and the auxiliary anode l3, and this in turn will transfer itself to the surface of the main anode l0.

As previously explained, tubes of this kind, particularly when used for 'the rectification of alternating currents, are occasionally subject to the occurrence of a reverse or back-fire are between the anode and the cathode. In accordance with my invention I propose to prevent such a back-fire from becoming a sustained are by assuring that throughout the period of its duration the concentrated back-fire arc stream shall be forced to move continuously around the inner surface l2 of the anode Ill. In this way it will be impossible for any small region of the anode surface to become so intensely heated as to be still capable of thermionic emission after the removal of the back-fire cause. In the present instance I accomplish this result by providing in the neighborhood of the anode a magnetic field producing means which will operate on the arc stream to give it rotary motion. As illustrated this comprises a coil l5 having its turns concentric with the single turn of the anode l0. Assuming "the direction of current flow in the coil to be that indicated by the small arrow appearing at the extreme left of the drawings, it is clear that a magnetic field will be created through the coil l 6 in the direction indicated by the aligned arrows.

appearing at the top of the figure. The portions of this field which traverse the passageway 4 will necessarily react with any convergent arc stream passing therethrough and in accordance with the right-hand rule of electromagnetic reaction will cause clockwise or counterclockwise rotation thereof with respect to the inner anode face depending on the direction of the discharge stream.

Since the motion of the arc stream will be very rapid, its velocity fortunately increasing with the intensity of the discharge, it is apparent that the anode body will be protected from any danger of localized heating. If the coil I6 is connected as shown, in series with the anode circuit, the rate of motion of the rotating arc will actually vary as the second power of the back-fire current since the force impelling it is determined by the prodv uct of this current and the magnetic flux which a negative potential. grid 25 is normally biased negatively with respect moves many of the limitations previously asso ciated with the selection of anode materials. In the past it has been 'necesary to avoid the use of inexpensive metals such as iron and nickel because of the relatively low melting points of these materials. As a consequence of their low melting characteristics a single stationary backfire arc may be sufficient to form an arc crater in the metal surface which will serve as an effective cathode. By the application of my present invention, however, it is possible to assure that the effect of the back-fire arc shall be distributed over such a large anode surface that vaporization of even very low melting point metals will be avoided, and their use as anodes made practical. This feature of the invention is of considerable importance since it may in many cases permit the elimination of graphite as an anode material, and do away with the extremely difficult and expensive task of degassing such structures in the course of tube manufacture.

In Figs. 2 and 3 I have shown a modification embodying'a somewhat more effective magnetic structure. Since the tube parts shown in this figure correspond exactly to those described in connection with Fig. 1, corresponding elements have been similarly numbered. In this case the magnetic circuit includes an iron core having an extended flux gap paralleling the inner surface ofthe anode and including within its field the region adjacent the anode.- More specifically the core comprises a horseshoe portion I9 whose pole tips are provided with annular extensions 20 and 2i arranged on opposite sides of the transverse passageway The magnetic circuit is excited by means of a coil 22 wound on one leg thereof and acts to conserve and concentrate the lines of magnetic force in the region where they will be most effective in producing displacement of the arc stream. In order to provide a low reluctance path for the magnetic flux and, at the same time, to avoid circulating eddy currents the core pieces are comprised of laminations which lie insofar as possible parallel to the direction of magnetic flux.

In some cases, particularly in controlled tubes of considerable sensitivity where it is-desirable to avoid any possibility of interference with the normal functioning of the tube, it may be desirable to excite the arc suppressing means only during periods when a back-fire has actually sirable to provide a relay system automatically operable upon the initiation of a back-fire to a switch the exciting coil 22 into the anode circuit or into circuit with some other power source. In this way the field-producing means may be maintained inoperative during the normal operation of the discharge device, or more precisely during the forward conducting period of the discharge path comprising the particular anode involved in the back-fire occurrence.

As typical of the means which may be employed for this purpose, I have illustrated a simple circuit utilizing a controllable discharge device as a switching element. The energizing circuit for the "coil 22 includes a battery 23 of suitable capacity and a three-electrode thermionic discharge device 24 of a type which is non-conductive as long as its grid element is maintained at In the present instance the to. the tube cathode by being connected through the resistor 21 to the negative terminal of the biasing battery 26. A fixed potential relationship is maintained between the grid and cathode electrodes by means of the potentiometer connection 28.

In case of a back-fire occurrence the polarized relay 29, which has an energizing coil 29' in series with the main anode circuit and is of such nature as to be held open except on occasions of reverse current flow, is immediately picked up and complews a circuit through the contacts 30. It will be understood that the relay 29 should be adapted for extremely high speed operation and should therefore be provided with moving parts of very low inertia. For this reason instead of using a mechanical switch of the type illustrated, it may in some cases'be desirable to substitute a thermionic discharge device, such as a controlled vacuum tube.

The closure of the contacts 30 immediately connects the grid directly to the positive terminal of the biasing battery and renders the tube 24 instantaneously conductive. Under these conditions the exciting coil 22 is energized by current drawn from the battery 23 and sets up a magnetic fiux through the iron core l9. With a relay circuit such as that described it is somewhat difficult to cause the magnetic field to build up with sufficient rapidity to assure the desired rotation of the back-fire arc during the first half cycle of its occurrence. However, if the arc is repeated on subsequent half cycles it will be rotated in the manner described above and its prompt extinction ensured.

In order to permit resetting of the device described and automatically to restore the rectifying system to its normal operating condition, I provide in series with the coil 22 a delayed action relay comprising a coil 3| shunted by a resistance 32. The inductance of the coil 3| and the inertia of the mechanical parts associated therewith are sufllciently great so that operation of the switch arm 33 will be delayed for one or more half cycles. Thus, by the time the switch arm is finally lifted and current flow through the tube 24 stopped, the arc-back will have been extinguished and the contacts opened, again biasing the grid 25 negatively so as to prevent a renewal of current flow upon the ensuing reclosure of the switch 33.

In-Fig. 4 I have shown a. slightly modified embodiment of my invention as used in connection with a discharge tube having a pool-type cathode 35. In accordance with known practice this comprises a body of mercury or other readily vaporizable metal collected at the bottom of the envelope 36 and connected to the exterior thereof by means of a suitable lead-in connection 31. For causing initiation of a discharge at the beginning of each cycle of voltage, it is possible to use in connection with the pool cathode an auxiliary starting electrode such as that shown at 38. This may comprise, for example, a pointed body of some conducting material, such as boron carbide dispersed in clay, projecting into the mercury and supported and energized by means of a heavy lead-in conductor 39.

By use of the modified .magnetic structure shown it is possible to form the envelope as a smooth glass cylinder without the laterally projecting aprons described in connection with Figs. 1 and 2. For proper co operation with the iron core the envelope is provided at the top thereof with a re-entrant depending portion 40 which is encircled by an annular anode 4|. The magnetic structure, which is illustrated more completely in Fig. 5, comprises a frame 42 including a stack of three-legged laminations of which the central element 43 terminates in a solid cylindrical portion 44, the outer legs 45 and 46 being symmetrically connected to a stack of annular laminations 41. When this structure is mounted adjacent the anode 4| in the relation illustrated in Fig. 4 and excited by means of the coil 48, it will be apparent that the flux passing through the gap between the concentric surfaces of the magnetic poles 44 and 41 passes transversely across the lower face of the anode. Since this face is the one nearest the cathode surface and, consequently, will receive any discharge therefrom, the relation of parts is such as to produce the rotary motion of the are over the anode surface which characterizes my invention.

In Fig. 6 I have illustrated a construction by means of which my invention may be applied to a rectifier comprising a perfectly smooth'cylindrical envelope 49. The electrode structure illustrated comprises a thermionic cathode 50 and an inverted cup-shaped anode 5| which is shown as being sectioned at the center plane thereof. In order to assure thepassage of magnetic flux across the face of this anode, I have provided a pair of superposed differentially wound coils 52 and 53 so spaced that the region bounded by planes lying in their adjacent faces will coincide substantially with the region Just below the anode. Due to the fact that the coils 52 and 53 are differentially connected so that the flux produced in the area enclosed by one is in an upward direction and that included by the other in a downward direction, the magnetic field produced by the reaction at their neutral plane will necessarily cut across the. discharge stream. The nature of this effect is shown by the dotted lines 54 which indicate generally the directions taken by the magnetic flux of the two coils. In the presence of such a fiux field, it will be apparent that in accordance with the principles already explained a back-fire arc will be impelled to travel rapidly along the face of the depending lip of the anode 5|.

Fig. '7 shows a further simplified construction in which means are provided for assuring that the direction of flow of the discharge current in the region closest to the anode will be transverse to the magnetic field created by the coil 54'. In this embodiment the envelope 55 is shown as enclosing a thermionic cathode 56 and a solid cylindrical anode 51 supported from the top of the envelope. Arranged below the anode and dimensioned to extend laterally beyond it in all directions, I provide a conical baifle member 58 of suitable refractory insulating material such as magnesium oxide or lava. The operation of this last-named element is to cause the discharge stream proceeding from the cathode 56 to be diverted first outwardly toward the walls of the envelope 55 and, after passing the upper edges of the baffle 53, inwardly to the circumferential surface of the anode 51. Since the magnetic field of the coil 54 is necessarily in a vertical direction, it will be clear that the required electromagnetic reaction will be produced to assure the desired movement of the are stream.

In Figs. 8 and 9 I have illustrated the use of my invention in connection with a special structure adapted to insure the immediate extinction cf backfire arc during the first half cycle of its duration. Apart from certain special features to be described presently this construction is similar to that already described in connection with Fig. 2. It comprises an envelope 6| enclosing a thermionic cathode 62 and a toroidal anode 63 having a short insulating section 69 therein to prevent circulation of induced currents. Outside the envelope and adjacent the inner face of the anode is provided an iron magnetic structure comprising a core 64 terminating'in a pair of. extended annular pole pieces 55 and 6B. These are arranged on opposite sides of the laterally extending passageway 61 and when excited by v passage of current through the core 68 tend to cause rotation of any discharge stream flowing through the passageway. Within the passage 61 and suitably secured therein by attachment to its upper and lower walls I provide a plurality of spaced annular baflies 'Il successively interposed between the anode and cathode. These bafiles are of conducting material and are each provided with a gap to permit the passage of a discharge therethrough. The gaps may be variously associated but are shown in this particular instance as being dimensioned so that as one proceeds from the anode to the cathode each gap subtends a radial angle greater than that subtended by the preceding bafiie. In order to prevent the baffle members from accumulating electrostatic charges which might interfere with the normal functioning of the tube, I have diagrammatically illustrated them as being maintained at fixed potential with respect to the anode by means of a sectionalized resistance 10, to which they are each connected. The various sections of. the resistance may be given an extremely high value as long as they are effective to permit continual leakage of charges which tend to accumulate on the baffle surfaces. With this arrangement, if it be assumed that during the occurrence of a backfire the effect of the magnet 64 is to drive the arc stream in a counterclockwise direction as indicated by the arrow A, then it will be clear that the body of an arc initially extending through the open gaps will be moved into position to be sectionalized by the baffles Ii. As is well known sectionalization of an arc in this manner has a very strong tendency to extinguish the same both by elongation of the arc stream and by the socalled dc-ion effect. Consequently the construction described operates not only to prevent the are from becoming stationary on a particular portion of the anode surface at any time during its existence but also acts to shorten appreciably the actual are life.

While I have described my invention as being particularly useful in connection with discharge devices operating in an ionizable medium, it should be understood that the principles set forth are equally applicable in connection with highly evacuated devices wherever the problem of excessive anode heating arises. Furthermore, although the constructions illustrated show the magnetic field-producing means as being mounted out-side the discharge envelope, my invention contemplates other arrangements including those in which the field coil is mounted inside the tube and even on the anode structure itself.

Although I have chosen-to exemplify the invcntion by reference to specific structures, it is not limited thereto, and I contemplate by the appended claims to cover all such equivalent modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In an electric discharge device subject to back-fire, a cathode, an anode having a substantially continuous re-entrant surface for receiving a discharge, and means operable during the occurrence of a back-fire arc to produce a magnetic field parallel to said surface, said means comprising a pair of. superposed diiferentially connected coils concentric with said anode and having their intermediate plane substantially in the plane of said surface.

2. In an electric discharge device subject to back-fire, a cathode, an anode having a substantially continuous re-entrant surface adapted to receive a discharge from said cathode, means comprising a plurality of spaced conducting baffles successively interposed between said cathode and anode, a magnetic structure adjacent to said anode having a major flux path parallel to the re-entrant surface thereof, and means operable to excite said magnetic structure during the occurrence of a backfire are thereby to move said are into position to be sectionalized by said bafiles.

3. In an electron discharge device subject to back-fire. a cathode, a toroidal anode adapted to receive a discharge from said anode, a plurality of spaced annular baffles successively interposed between said anode and cathode, but each having a gap therein for passage of a discharge, a magnetic structure adjacent to said anode and having a major flux path transverse to the direction of. discharge flow, and means operable to excite said magnetic structure during the occurrence of a back-fire are through said gaps thereby to move said are into position to'be sectionalized by said bailies.

4. In a discharge rectifier subject to back-fire,

a discharge path comprising a cathode and an anode, and means effective during a back-fire occurrence to produce continuous motion of the back-fire arc over the anode surface, said means being inoperative during the forward conducting period of said discharge path.

5. In a discharge rectifier subject to back-fire. a discharge path comprising a cathode and anode, and means including a magnetic field-producing element adjacent said anode effective during a back-fire occurrence to cause continuous motion of the back-fire arc'over the surface of said anode, said means being inoperative during the forward conducting period of said discharge path.

6. In a discharge rectifier subject to back-fire, a cathode, an anode, and a magnetic structure adjacent said anode for producing continuous rotary motion of a back-fire are over the surface of said anode,'said structure being energized only during the occurrence of a back-fire arc.

'7. An electric rectifier comprising arc-supporting electrodes, one of said electrodes being intended to function normally as anode but being subject to temporary undesired functioning as cathode for a back-fire are by the formation of a cathode spot thereon, and means for setting up by the current supporting such reverse are a magnetic field effective to move the position of said cathode spot over the surface of the anode. said means being inoperative during the normal forward conducting period of said rectifier.

8. In a discharge rectifier subject to back-fire, a cathode, an anode, and an electromagnetic structure adjacent said anode and operative only during the passage of a back-fire arc to said anode for producing a magnetic field transverse to the direction of said are, thereby to cause continuous motion of the are over the anode surface.

9. In an electric discharge device subject to 75 hack-fire. an envelope enclosing a cathode and a major axis of the envelope, said anode having a substantially continuous reentrant surface for receiving a discharge proceeding in a direction transverse to said axis, means forconflning a discharge to said surface, a magnetic structure mounted adjacent said anode and having a flux path parallel to said surface and an exciting winding for said magnetic structure adapted to be energized during the occurrence of a back-fire are thereby to produce a continuous motion of the m arc over said surface.

ROLL-AND C. GRIFFITH.

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