US3064178A - Inert-gas arc rectifier - Google Patents

Description

J. A. PERSSON INERT-GAS ARC RECTIFIER Nov. 13, 1962 2 Sheets-Sheet 1 Filed May 19, 1958 .PZMEEDU Kurt-rummmwEEUmE Um EDUKM INVENTOR JOHN A. PERSSON Z? ATTORNEY Nov. 13, 1962 J. A. PERSSON 3,064,178

INERT-GAS ARC RECTIFIER Filed May 19, 1958 2 Sheets-Sheet 2 J6 W1 j k f E -Wafer INVENTOR JOHN A. PERSSON ATTORNEY United States Patent 6 3,064,178 lNERT-GAS ARC RECTIFIER John A. Persson, Kenmore, N.Y., assignor to Union Carbide Corporation, a corporation of New York Filed May 19, 1958, Ser. No. 736,256 8 Claims. (Cl. 321-8) The present invention relates to a novel gas-filled arc rectifier for converting alternating current to direct cur rent.

According to this invention, there is provided an inert gas are rectifier having an anode and cathode therein. The cathode is provided with an annular emissive line for laterally restricting the arc. The anode has an annular surface radially spaced equidistantly from the emissive line and forming therebetween an annular arc circulating path. Means are provided for establishing an are between the electrodes and for continuously circulating the arc around such path.

More specifically there is provided an arc rectifier body defining a chamber filled with an inert gas at substantially atmospheric pressure. Within the chamber there is provided an annular cathode surface having a narrow edge portion constructed of highly emissive material. A continuous water-cooled anode is located equidistant from said cathode and so disposed that the shortest arc path lies in the plane of the emissive edge of the cathode. Means are provided for establishing a magnetic field within the chamber substantially transverse to the direction of the arc perpendicular to the plane of the annular cathode surface for moving the arc along the arc path. The anode and cathode are connected to an alternating current source in series with a direct current load and also to a triggering circuit which functions to initiate the arc at a desired point on each alternating current cycle.

By means of the instant device an extremely simple and compact structure is achieved which can rectify currents in the power range. The power handling capacity can be easily varied by varying the size of the device. The simplicity of the design allows the removal and replacement of individual elements rather than replacement of the entire unit as with most presently used power are rectifiers. The device achieves high arc efficiency and extremely low arc-back due to the structure used. Of especial importance is the narrow emissive area provided on the periphery of the cathode. This functions both to limit are travel and also to facilitate heating of the area to achieve thermal emission in addition to emission resulting from the voltage gradient across the arc path. These efifects lower the work function of the oathode and thusimprove the efficiency greatly. The low ionization potential of the inert gas results in an unusually low voltage drop across arc. Voltages as low as 8 to 10 volts have been measured with the intant device. Arcback has been practically eliminated by means of the present design due to the great difficulty a cathode spot has in forming on the anode. The great area of anode utilized together with water cooling, high degree of polishing, and the arc rotating means prevents any great degree of heating of the anode even with very heavy current flow.

Any device that permits an electric current to flow- An alternating electric current at the end of each half cycle is reduced to zero and conducts at the beginning of the opposite half cycle in order to maintain a continuous current. If, by means of an arc, conduction is prevented on one half cycle and facilitated on the opposite half cycle, the current is said to have been rectified.

The most successful forms of arc rectifiers for power applications have been the various types of mercury-arc rectifiers. In these, the arc operates in mercury vapor at low pressure between a mercury pool cathode and a graphite anode. For most practical applications, the mercury-arc rectifier has proved superior to all others and is widely used in a variety of commercial applications. Certain disadvantages, however, are attendant with the use of mercury rectifiers. The presence of mercury, for instance, constitutes a problem in that splashing occurs if the instrument is subjected to a jolting action, as for instance on railroad locomotives. Because of the liquid pool of mercury, a mercury-arc rectifier must also be mounted upright. In addition, the mercury-arc rectifier is a relatively bulky, sealed unit. A breakdown of any important part, such as the arcinitiating mechanism, therefore, oftentimes makes it more expedient to discard the entire unit rather than undergo the high cost of repairs.

High-pressure arcs between solid electrodes for current conversion have also been devised. In rectifiers of this type, the arc conducts in streaming air at atmospheric pressure between water-cooled copper electrodes. A high-voltage spark is employed to initiate the arc and an air blast is utilized to quench the arc at the end of the conducting half cycle. A magnetic field keeps the arc in constant motion over the electrode surfaces. Rectifiers of this type are characterized by high are voltage, of the order of about volts; accordingly, the efiiciency is low except for circuits employing very high operating voltages (10,000 to 100,000 volts).

The primary object of the present invention is to provide a novel and efficient arc rectifier that is capable of operating at relatively low voltages, characterized by a low arc drop and a high efficiency.

A further object of the present invention is to provide an arc rectifier that is rugged, simple, contains no liquid pool, and can be mounted in any position.

Another object of the invention is the provision of a rectifier, all parts of which can be easily replaced.

The two most important properties that characterize the performance of an arc rectifier are the arc-voltage drop and the arc-back. The arc-voltage drop is an expression for the efiiciency of the rectifier, since practically all the power consumed within the apparatus is in the arc. Therefore, the power loss can be expressed as are current times are voltage. The arc-back rate defines the tendency of the arc to reignite on the reverse half cycle. When an arc-back occurs, rectification ceases and control of the rectifier is lost. It is, therefore, important to keep the arc-back rate low. Measures taken to prevent arc-back often lead to an increase in arc voltage. A compromise which gives the most economical rectifier is largely a matter of design.

The inert-gas arc rectifier of the invention is a controlled rectifier in which the arc is maintained in an inert gas of atmospheric pressure between a relatively small electrode of refractory metal such as tungsten (pure or thoriated) and a larger electrode of water-cooled copper. If an alternating E.M.F. is applied across the arc gap, conduction of current is favored when the tungsten is the cathode. The higher temperature of the tungsten and its better emission properties compared with the emission from the cold copper, makes this result possible. To facilitate deionization and prevent reignition on the reverse half cycle, the arc is kept moving over the electrode surfaces by means of a magnetic field. -In this way, calized over-heating and the formation of a cathode spot on the copper anode are prevented. Since current flows only during alternate half cycles, the gas will become deionized during the period of no conduction, and a relatively high voltage will be required to reignite the are. This voltage is supplied from a separate circuit as a short pulse, which can be controlled in time with respect to the impressed voltage.

By the term controlled rectifier, it is meant that it is possible with this device to delay the point of reignition within the conduction period. This has the effect of varying the average value of the recified current and thereby the output of the rectifier. This type of control is very important in control circuits for welding and motor speed regulation. Tungsten, either pure or thoriated, the latter being preferable, has been found to be exceptionally useful as the cathode, with copper as the anode. A specific tungsten, thoriated tungsten, tungsten sand- 'wich electrode which is particularly suitable as a cathode in this device is disclosed in copending United States application Serial No. 698,762, filed November 25, 1957, of Butler et al. Another important feature of the instant arc rectifier is the use of commercially pure inert gases especially argon, xenon and krypton which are characterized by low ionization potentials, as the atomsphere in which the arc is maintained. The various features of the rectifier combine to facilitate deionization and prevent reignition on the reverse half cycle of the alternating current, and in every way provide optimum rectification characteristics of the device of the invention.

Other objects and advantages will be apparent from the specification and drawings in which:

FIGURE 1 is a partial cross-sectional view of the arc rectifier, and 7 FIGURE 2 is a chart showing comparative voltage and current waveforms of a mercury-arc rectifier and the arc rectifier of this invention.

FIGURE 3 is a partial cross-sectional view of another embodiment of the are rectifier.

FIGURE 4 is a schematic diagram of a typical arc initiating pulse circuit.

Referring specifically to FIGURE 1, rectifier R consists of a disk-shaped cathode 10 of any suitable material,

such as tungsten, pure or thoriated, mounted coaxially inside a copper ring 12, the latter serving as the anode. The anode ring fits tightly inside a water-cooled copper cylinder 14 which forms a chamber 15 therein. One end of the chamber is closed by an insulating bushing 16 through which the cathode electrical connection 18 is made; the other end is sealed with a sight glass 20 which permits observation of the arc. The cylinder may be evacuated through any suitable opening, as at 22, and subsequently filled with inert gas. wound on a spool form surrounding the cylinder 14. Direct currentrthrough the coil produces an axial magnetic field which causes the arc to rotate around the cathode 10 in the space between the electrodes. The spacing between anode and cathode can be varied by inserting anode rings 12'of various inner diameters. The cathode 10 is also readily interchangeable. The anode may be connected to a current source in any suitable manner through the anode connection 13.

In another embodiment of the rectifier shown in FIG- URE 3 the magnet coil 24 is replaced by two permanent magnets 26 and 28 of annular shape, mounted on both sides of the anode ring 12. This reduces the overall size of the apparatus and makes a separate direct current source unnecessary.

As stated previously, the cathode may be a pure tungsten disk as shown in FIGURES 1 and 3 or coated with thoriated tungsten. Alternatively, it may be a tungsten-thoriated tungsten-tungsten sandwich electrode as disclosed in copending application 698,762 filed No- A magnet coil 24 is vember 25, 1957 by Butler et a1. Another possible form is that a narrow annular line of emissive material might be sprayed or otherwise deposited on the peripheral edge of the cathode. However, in all of these possible embodiments the cathode must present a relatively narrow peripheral edge or line, which will heat up and keep the are from wandering over the surface of the cathode. It is possible that the anode and cathode positions could be reversed providing the cathode had a narrow peripheral edge, in this case on the inside of the annulus, adjacent the anode as stated above. Also, the water cooling of the anode and the provision of the transverse magnetic field would have to be maintained.

The are rectifier of the invention may be controlled by a pulse circuit to initiate the are by ionizing the gap to supply a source of electrons and form a cathode spot. The circuit may be devised so that a high-voltage pulse is produced at the desired instant of the conducting half cycle that is strong enough to break down the arc gas and is of sufliciently short duration to cause a minimum of radio interference.

As stated previously, the rectifier is controlled by a pulse circuit which initiates the are by ionizing the gap to supply a source of electrons and form a cathode spot. The circuit produces a high-voltage pulse at the desired instant of the conducting half cycle, strong'enough to break down the arc gas and of short duration to cause a minimum of radio interference.

Referring to FIGURE 4, the circuit functions in the following way: the condenser C is charged from a DC. source through a high resistor R When the thyratron V is triggered, C is rapidly discharged through the primary of the pulse transformer T The high voltage output from the pulse transformer is charged into condenser C through resistor R When the voltage across C reaches the breakdown potential of the spark gap S, a spark leaps the gap and starts a high-frequency oscillation in the circuit consisting of S, C and the primary of T T is an aircore (Tesla) transformer of aratio of 111, the secondary of which is connected in series with the cathode of the rectifier. The high-frequency, shortduration discharge induced in the secondary coil breaks down the arcgap of the rectifier and starts conduction of the main current. C is a by-pass condenser which protects the power supply and the load. The frequency and the duration of the pulse discharge are determined by the circuit constants. In a typical operating embodiment, the circuit elements are chosen sothat the frequency is 3 megacycles per second, the total duration of the pulse is '4 microseconds.

The thyratron V can be triggered from any external trigger source to give single or periodically repeated pulses. In normal practice the trigger pulse Would be derived from the alternating current being rectified by means of a suitable rectifying, phase-shifting and pulse-forming circuit.

The inert-gas arc rectifier of the invention was operated in conjunction with a mercury-arc rectifier to compare the performance of the two instruments under the same operating conditions. The circuit employed was ar- 7 ranged in such a manner that the'two devices would conduct on alternate half cycles. 'The voltage and current were recorded'with an oscillograph, and a typical oscillogram is shown in FIGURE 2 to indicate the efiicient rectifying properties of the device of the invention.

Care must .be exercised to minimize arc-backs, which may be defined in connection with the present rectifier as the tendency of the arc to reignite on the reverse half cycle. Arc-backs may result by the are sticking to irregularities on the anode surface, thereby causing localized heating. This condition, in turn, facilitates the formation of a cathode spot on the reverse half cycle. A highly polished anode surface and a magnetic field sufficiently strong to drive the are around the electrodes would prevent the sticking effect and reduce the danger of arcbacks. Eflicient cooling of the anode is important.

It has thus been experimentally established that the inert-gas arc unit of the invention is capable of rectifying alternating currents successfully. Needless to say, the cathode and anode of this instrument may be constructed of a wide variety of materials, and inert gases such as argon, xenon, krypton, or others may be successfully utilized. The principal requirement of the cathode is that it readily supply electrons, and of the anode that it be able to dissipate heat readily thereby making it difficult to initiate a cathode spot thereon.

Currents as high as 1000 amperes have been successfully rectified by a prototype of the instant device operating from a 440 volt A.C. supply. The said prototype was only about six inches in diameter, a larger unit would undoubtedly be able to handle correspondingly larger currents.

In addition, electrical circuits other than those described herein may be employed, depending on the specific utility to which the rectifying instrument is applied. The rectifier of the invention has the advantage over an ignitron mercury-arc rectifier in that it can withstand rougher treat ment, and is simpler in design and lighter in weight. It has the additional advantage of being capable of being mounted in any position for a wide variety of service conditions. Furthermore, worn or otherwise disabled parts may be replaced with comparati e ease at little cost.

What is claimed is:

1. A power are rectifier which comprises a body defining a chamber tilled with an inert monatomic gas at substantially atmospheric pressure, a member having an annular rim provided with a relatively narrow emissive cathode surface located within said chamber, a fluid cooled anode in the shape of a ring having an annular inner surface disposed equidistant radially from said cathode and shaped so that the shortest arc path therebetween lies in the plane of said member which includes such narrow annular cathode surface, the annular space between said annular surfaces being open except for such gas, magnetic means for creating a field in said are path transverse to said plane to annularly circulate in such annular open space a power are, means connecting said anode and cathode in an arc rectifier circuit with an AC. power source and a direct current load, and means connected to said anode and cathode for providing a high voltage are initiating pulse at a selected point on each cycle of such A.C. power.

2. A power are rectifier as defined by claim 1, wherein said magnetic means comprises at least one circular permanent magnet concentric with the axis of such annular path in which the arc rotates.

3. A power arc rectifier which comprises a body having an inert-gas filled chamber therein containing at substantially atmospheric pressure commercially pure gas selected from the class consisting of argon, xenon, and krypton, a disk shaped cathode located in said chamber, an

also extremely annular fluid-cooled anode surrounding the rim of said cathode and spaced equidistantly therefrom to form an unobstructed annular path therebetween, said anode being shaped so that the shortest arc path to said cathode lies substantialiy in the plane of the disk, magnetic means for establishing a field in the arc path transverse to the direction of said are to cause said are to move freely around said unobstructed annular arc path, means connecting said rectifier in a rectifier circuit with an alternating current power source to be rectified, and means connecting the rectifier to an arc initiating pulse source.

4. A power are rectifier as set forth in claim 3, wherein the disk shaped cathode is composed of tungsten.

5. A power arc rectifier as set forth in claim 3, wherein the rim of said disk shaped cathode comprises a thoriated tungsten edge.

6. A power arc rectifier as set forth in claim 2, wherein the disk shaped cathode is a sandwich of thoriated tungsten between tungsten.

7. A power are rectifier as set forth in claim 3, wherein the peripheral edge of the disk shaped cathode consists of a thin line of material more highly emissive than that of the adjacent material.

8. A power are rectifier for rectifying AC. from a suitable AC. power supply circuit and supplying the so rectified DC. to a DC. output circuit, said rectifier comprising a disk shaped cathode composed of tungsten having a thoriated tungsten rim, an anode in the form of copper ring surrounding said rim in unobstructed spaced concentric relation, means insulating said anode and cathode from each other, and annular magnetic means arranged in spaced concentric relation with said anode ring and cathode disk that provides an axial-magnetic field for causing a power are energized therebetween to rotate freely about the axis of such ring in the open annular space provided therefor between the rim of said anode and the inside of said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,039,449 Radclitf et al. May 5, 1936 2,058,950 Bruijnes Oct. 27, 1936 2,069,283 Slepian et al. Feb. 2, 1937 2,259,451 Bennett Oct. 21, 1941 2,491,234 Veenemans et al Dec. 13, 1949 2,559,769 Hullegard July 10, 1951 2,577,066 Arnold Dec. 4, 1951 2,585,439 Coleman Feb. 12, 1952 2,660,687 Coleman Nov. 24, 1953 2,683,851 Parsons July 13, 1954 2,780,759 Boyer et al. Feb. 5, 1957 2,813,992 Linder Nov. 19, 1957 2,944,179 Lafierty July 5, 1960 FOREIGN PATENTS 147,856 Great Britain Jan. 9, 1922

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