US2617082A - Device for igniting mercury arc mutators - Google Patents

Description

Nov. 4, 1952 H. KELLER DEVICE FOR IGNITING MERCURY ARC MUTATORS 2 SHEETS-SHEET 1 Filed Jan. 11, 1952 IN VENTOR BY M Jim,

ATTORNEYS Nov. 4, 1952 ,1. KELLER DEVICE FOR IGNITING MERCURI ARC MUTATORS Filed Jan. 11, 1952 2 SHEETS-SHEET 2 INVENTOR.'-

ATTORNEY Patented Nov. 4, 1952 DEVICE FOR IGNITING MERCURY ARC MUTATORS Hans Keller, Wettingen, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Application January 11, 1952, Serial No. 266,054 In Switzerland January 27, 1951 11 Claims. (Cl. 315330) This invention relates to devices for igniting mercury arc mutators of single or multiple anode type, and more particularly to devices or apparatus for varying the level of the cathode mercury with respect to one or more starting orignition electrodes in such manner as to establish one or more cathode spots to ionize the mercury cathode.

In mercury arc rectifiers or inverters, known generically as converters or mutators, it is customary to provide an ignition or starter electrode to develop a so-called cathode spot, and an exciter electrode or auxiliary anode which serves to maintain the mutator in a state of ionization. It has been proposed to ignite such mutators by imparting a rotating movement to the mercury cathode with the aid of a rotating magnetic field so that the mercury rises along the wall of the cathode container by centrifugal force to contact, or so closely to approach, an auxiliary or starting electrode that a luminous arc is initiated. This prior method and apparatus are not practical, however, with mercury cathode containers of large diameter as the peripheral velocity at the outer edge of the mercury becomes so small that no appreciable rise of the mercury level can be effected.

An object of the invention is to provide ignition devices .or apparatus for mercury arc mutators, and of rotating mercury cathode type, which impose no limitation upon the dimensions of the mercury cathode of the mutator. An object of the invention is to provide ignition devices or apparatus in which a sump of limited cross-section depends below the mercury cathode container, a field winding is associated with the sump to set the mercury therein into rotation, and one or more starting electrodes are positioned above the sump at such level as to dip below the mercury cathode when the mercury in the sump is not rotating, and to be spaced from the mercury when it is rotated. Another object is to provide starting devices or apparatus of the character stated in which an exciting electrode is associated or combined with each starting electrode.

These and other objects and the advantages of the invention will be apparent from the following specification when taken with the accompanying drawings, in which:

Fig. l is a somewhat schematic vertical section through a mutator provided with an "ignition device embodying the invention;

Fig. 2 is a fragmentary vertical section through the mercury sump and an igniter electrode which also serves as an exciter electrode; and also showing the circuits for energizing the electrode and 2 the field winding'which sets the mercuryinto rotation; H 1

Fig. 3 is a fragmentary vertical section through a three-phase ignition-excitation electrode assembly as seen on section line 3-3 of Fig. 4;

Fig. 4 is a plan view of the same;

Fig. 5 is a fragmentary vertical section through another multiple electrode assembly;

Fig. 6 is a schematic plan view of a further multiple electrode assembly; and j I :v

Fig. '7 is a schematic vertical section through a mutatorin which a plurality of igniter-exciter electrodes according to the invention are introduced through and supported by the top wall of the mutator casing.

In Fig. 1 of the drawing, a mutator is shown schematically as comprising a tank or housing I, a cathode pool 2 of mercury, an anode 3 and an exciter electrode 4. In accordance with the invention, an igniter electrode 5 passes upwardly through and axially of a cylindrical sump B of relatively small diameter which extends downwardly from the bottom wall of the tank, the electrode terminating below the normal level of the mercury cathode 2. A field winding Tsurrounds the sump 6 and, when energized by alternating current, establishes a rotating field which sets the mercury within the sump into rotation, thereby throwing the mercury outwardly by centrifugal force.

The negative terminal of a direct current source, which is indicated schematically as a battery 8, is connected to the mercury cathode 2 and the positive terminal is connected to the exciter electrode 4 and, through a series resistance 9, to the igniter electrode 5. The electrode 5 is of courseinsulated from the housing I by a sleeve II! which extends upwardly along the electrode 5 to a point adjacent the upper end.

The mutator is placed in operation by energizing the winding 1 to develop the rotating field which sets the mercury into rotation. The mercury above the sump forms an eddy or vortex, as indicated in broken line in Fig. 1, and the mercury recedes from the igniter electrode 5 to draw a luminous are when the mercury separates from the exposed lip of the igniter electrode 5. This establishes the cathode spot to initiate ionization of the mercury, and the are or discharge then transfers to the exciter electrode 4 since it is at a higher positive potential than the igniter electrode 5 in view of the voltage-dropping resistance 9. The ignitionis thus completed and themainanode 3 can take over the current fornormal;

operation of themutator. The field winding] is then de-energized and the cathode mercury drops back to its normal level surface, thus submerging the tip of the igniter electrode 5. A circult is completed from the source 8 through the cathode 2 and igniter electrode 5, but the series resistance 9 limits this current to a negligible value and it does not extinguish the are which plays between the excitation electrode 4 and the mercury.

The described separate exciter and ignition electrodes are not an essential feature of the invention as it is both convenient and practical to employ a combined igniter-exciter assembly. In a practical embodiment, as shown in Fig. 2, the exciter electrode 4' is an inverted cupor hoodshaped member of metal or graphite carried by a rod H which is insulated by a sleeve l2 and extends axially through the sump 6, the lower edge of the electrode 4 being somewhat above the normal level of the mercury cathode. The igniter electrode is a cylinder of high-resistance or semi-conducting material such as boron carbide or silicon carbide, the cylindrical electrode being secured to the lower face of the exciter electrode 4' and extending downwardly coaxial with the electrode 4' to dip below the normal level of the mercury cathode 2. The hood-shaped exciter electrode 4' has the advantage that it prevents a termination of the main cathode spot on the igniter electrode. A cover layer l3 of insulating material, for example quartz, over the top and well down the sides of the. exoiter electrode 4' prevents the main are from terminating on the exciter electrode since, if this should occur, the excitation would break down with the extinction of the main arc.

The energizing circuit for the igniter-excitation electrode assembly comprises a direct cur-- rent source in the form of a selenium rectifier bridge 8 supplied by a transformer l4 connected across one phase of a low voltage three-phase network. I5 to which the field winding '1 is connected by the normally closed. contacts 16 of a relay comprising a coil I1 and an armature IS. The relay coil I1 is in series in the ignition-exciter electrode circuit; being connected for example between the positive terminal of the rectifier bridge 8 and the terminal rod H by leads 19, I9, while the other bridge terminal is grounded on the tank l by lead IS".

The current rectifier or converter is brought into operation by closing the 3-pole switch 20 in the low voltage line [5, thus energizing the transformer l4 and the selenium rectifier bridge 8 to establish a direct current through the relay coil l'l, terminal rod l l, the ignition electrode 5', the cathode mercury 2, tank I, and lead 19" back to the rectifier 8. This current is relatively small in view of the high resistance of the ignition electrode 5', and is not sufiicient to lift the relay armature Hi to open the switch Hi. Th field winding 1 is therefore energized by the closing of the switch 20 and the mercury within the sump 6 is set into rotation. The level of the mercury above the sump is depressed by centrifugal action and an arc is drawn when the mercury separates from the high resistance ignition electrode. The arc shifts at once to the excitation electrode 4 since it has a higher positive potential than the terminal point of the are on the electrode 5. The current in the ignition-excitation circuit increases with this elimination of the high resistance of the ignition electrode 5', and relay arma' ture I8 is pulled in to open the-field coil switch it. The mercury cathode 2 falls back to its original level surface and again contacts the ignition electrode 5. The resistance of the electrode 5' is so high that the excitation are between electrode 4 and the mercury is not extinguished although shunted by the ignition electrode. In the event that the excitation arc is inadvertently extinguished, the resistance of the ignition electrode is again included in the circuit. and the current through the relay coil ll drops to a low value which is not sufficient to hold in the armature 18. The relay contacts l6 are thus closed to energize the field winding 1, and ignition is again efiected as above described.

The invention is also applicable to a threephase ignition and excitation of mutators through the use of three electrodes or electrode assemblies insulated from each other and in any desired grouping above a sump in which the mercury can be set in rotation by an alternating current field. As shown in Fig. 3, the resistance ignition electrodes are omitted and three excitation electrodes 4U, 4V and 4W are supported above a sump by terminal rods 1 l which extend through the bottom of the sump and are insulated from the tank I and the cathode 2 by sleeves 12'. The electrodes are energized from the star-connected secondary winding of a threephase transformer 2| through the respective coils 'IU, 1V and 'IW of the field winding which is employed to set the mercury in the sump 6 into rotation to depress the mercury above the sump to a level below the lower edges of the excitation electrodes. A luminous arc is drawn when the mercury separates from the electrode then at av positive potential with respect to the cathode mercury, and the arc shifts from one electrode to another in synchronism with the alternating current. The excitation current is relatively small but must, of course, be sufficient to maintain a continuous rotation of the mercury at such rate as to depress the level below the lower edges of the excitation electrodes. In the event that the excitation arc is extinguished, the field coil current drops to zero and the mercury level rises to contact the electrodes and again energize the field coils, thus effecting a re-ignition of the mutator. A triangular arrangement of the electrodes as shown in Fig. 4 is preferable since it permits use of a sump of minimum diameter, but is not essential as the electrodes are of relatively small size.

Another electrode arrangement which may be employed with a three-phase. excitation system is shown in Fig. 5. The several excitation electrodes 40 are coaxially arranged, the outer electrode being supported by a terminal rod 4!, and the inner electrodes being supported on terminal tubes 42 and 43 respectively. The terminal members are insulated from each other by sleeves or tubes 44 of insulating material. The circuit connections to the electrodes may be as above described with reference to Fig. 3.

Another small diameter grouping of electrodes for three-phase excitation in accordance with the invention is schematically shown in Fig. 6. The electrodes iflU, 40V and WW are each of sector shape in plan and complement each other to form a circular array.

It is not essential that the ignition and/or excitation electrodes be suppported by terminal members extend through the lower wall of the sump 6. When convenient for structural or other reasons, the electrodes may be supported from the top wall of the tank I. As shown in. Fig. '7, a triangular array of three 4'5 electrodes, each of the composite ignitionexcitation type shown in Fig. 2, is supported from the tank cover by terminal rods 5|. The terminal rods are insulated from the tank by ceramic or quartz coatings 52 which extend along the rods 5! and merge into insulating coatings 53 which cover and extend down the sides of the excitation electrodes 4'. Any of the other types of electrodes may of course be supported from the top wall of the mutator tank.

It is to be understood that the invention is not limited to the particular embodiments herein illustrated and described since various modifications which may occur to those familiar with the art fall within the spirit and scope of the invention as set forth in the following claims.

I claim:

1. In a single or multiple anode mutator of mercury cathode type, a closed tank having a cylindrical sump depending below the bottom wall thereof, a cathode pool of mercury filling said sump and extending above the same, ignition electrode means and means rigidly supporting the same substantially axially of the sump and with a part below the normal cathode pool level, and means for altering the cathode pool level to break contact between the mercury and said part of the ignition electrode means, said level-altering means comprising a field winding surrounding said cylindrical sump and adapted when energized to set the mercury within and above the sump into rotation.

2. The invention as recited in claim 1, wherein said supporting means passes through and is insulated from the top wall of said tank.

3. The invention as recited in claim 1, wherein said supporting means passes through and is insulated from the bottom wall of said sump.

4. The invention as recited in claim 1, in combination with an excitation electrode, and an energizing circuit connecting said ignition electrode means and said excitation electrode in series with a current source through the mercury cathode pool, and wherein a potentialdropping resistance is included between the current source and said part of the ignition electrode means to make the potential thereof less than the potential established on said excitation electrode by said current source, whereby the positive terminal of an ignition arc initially drawn by the separation of the mercury from the ignition electrode means shifts to said excitation electrode and increases in magnitude.

5. The invention as recited in claim 4, wherein said part of the ignition electrode means which is below the normal cathode pool level is of semi-conducting material and constitutes said potential-dropping resistance.

6. The invention as recited in claim 4, wherein said potential-dropping resistance is of a magnitude in excess of that value which can extinguish an excitation arc playing between said excitation electrode and the cathode pool on return of the cathode pool to normal level on de-energization of said field winding.

'7. The invention as recited in claim 4, in combination with a three-phase supply circuit for said field winding, a relay with normally closed contacts in said supply circuit and with its operating coil in series in said energizing circuit for said ignition electrode means and said excitation electrode; the pull-in current value for said relay being in excess of the current established in said energizing circuit by an ignition arc and less than the current established by an excitation arc, whereby said relay pulls in to open the supply circuit to the field winding upon the development of an excitation arc.

8. The invention as recited in claim '7, wherein said current source of said energizing circuit comprises a rectifier bridge energized from a phase of said three-phase supply circuit.

9. The invention as recited in claim 1, wherein said ignition electrode means comprises a terminal rod supporting an excitation electrode of inverted cup shape, the lower edge of the excitation electrode being above the normal cathode pool level, and said part comprises an ignition electrode of high resistance carried by said excitation electrodes and extending downwardly to below the normal cathode pool level.

10. The invention as recited in claim 1, wherein said ignition electrode means comprises three electrodes insulated from each other and from said tank, in combination with a source of threephase alternating current, and circuit elements,

connecting the respective phases of said source to the several electrodes to energize the same.

11. The invention as recited in claim 10, wherein said field winding includes three coils, said current source comprises a star-connected transformer secondary having its center point connected' to said tank, and the respective coils of said field winding are series circuit elements between the several electrodes and the respective phases of the transformer secondary.

HANS KELLER.

No references cited.

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