US2340085A - Glass covered igniter - Google Patents

Description

Jan. 25, 1944. J. SLEPIAN ETAL GLASS COVERED IGNITER- Filed June 22, 1959 .ll l.

Wrmassas;

. INVENTORS Joseph Slepiam 8 John EJVIzite.

ATTORN Y Patented a... 25, 1944 UNITED: STATES PATENT OFFICE GLASS COVERED IGNITER Joseph Slepian, Pittsburgh, and John E. White,

Lilly, Pa., assignors to Westinghouse Electric 0. Manufacturing Company. East Pittsburgh, Pa., a corporation of Pennsylvania Application June 22, 1939, Serial No. 280,530

' 12 Claims. (Cl. 315-471) Our invention relates to electric discharge apparatus and has particular relation'to electric discharge valves of the pool type which are ignited by the application of a potential to an insulated ignition electrode.

In Figs. 2, 3 and 4 of the French Patent No. 453,113, published May 31, 1913, electric discharge devices of the insulated-ignition-electrode type constructed in accordance with the teachings of the prior, art of which we are aware are illustrated. In general, -the ignition electrode in a prior art discharge device comprises a metallic conductor such as a wire, a rod, or a column of mercury disposed in a relatively thick glass jacket. The electrode is immersed in the oathis particularly adapted for resistance welding and like purposes.

More generally stated, it is an object of our in- 'vention to provide adischarge device of the inodeof the discharge devicewith the glass serving as an insulator between the conductor and the cathode. When a potential of steep wave front is applied between the wire and the cathode, a discharge is initiated in the device.

Although the prior art discharge valves of the insulated-ignition-electrode type in which the electrodes are used for the occasional starting of an are have been known for many years, they have not been adopted for repeated periodic ig- When used nition at commercial frequencies. for repeated ignition, the prior art electrodes do not cause the arc to be ignited reliably every cycle, but miss occasionally, and the frequency of such missing increases with use so that in a short time the valves become inoperative. Higher voltage applied to the ignition electrode increases its reliability for repeated ignition,-but even at the highest potentials to which the insulating material can be safely subjected, the operation'is still faulty, and the life short.. High potentials are,-moreover, undesirable in an industrial plant because they are a source of danger to the employes and therefore require expensive protective apparatus. While the high potential requirement may be overlooked, in many cases the short life is fatal. Continuous power supply is indispensable to a modern industrial plant and a device which has a tendency to obstruct its how will not be used regardl ess of what it may offer to counteract the deficiency,

It is accordingly an' object of our. invention to provide a discharge device of the insulated-ignition-electrode type wherein the ignition potential required for reliablerepeated ignition shall be of relatively moderate magnitude.

Another object of our invention is to provide an electric discharge device of the insulated-igsulated-ignition-electrode type that shall be c'apable of withstanding the wear and tear of everyday'commercial use.

Our invention arises from the realization of the causes of the deficiencies in the prior art discharge devices, and may be understood in the light of the theory advanced in United States Patent No. 2,069,283 to Joseph Slepian and Leon R. Ludwig, dated February 2, 1937, and assigned to Westinghouse Electric & Manufacturing Company, in which an ignition electrode of a high resistance material is described. There,- it is shown (Fig. 11) that the conduction current flowing through the, igniter into the mercury takes a very high density at the junction of the igniter with the mercury, and that the high current density with the associated high local electric gradient causes the ignition of thearc. Now, a varying voltage applied to an insulator will also cause currents known in the art as displacement currents to flow through the insulator. Displacement currents also concentrate at the junction of the cathode and the ignition electrode making it possible that an arc may start there. The magnitude of the displacement currents will depend directly on the magnitude of the electric gradient, and the rate at which it is changed, i. e., on the frequency of the gradient. Therefore, it is desirable that the frequency be high. Also, it is found that as long aninterval as microseconds is required forthe arc which is formed as a result of ignition to reach to the main anode of the tube. Hence, it is desirable for the high frequency current to the igniter be maintained for that length of time for each repeated ignition. Ignition potential of large magnitude is required in the prior art arrangements be-,

cause of the thickness of the insulator, since a definite gradient is needed for a given displacement current at a given frequency.

vThe ignition of the are then is dependent on current should be of the order of 1 ampere per cm. to produce reliable ignition by its concen- .tration at the mercury junction in the usual case where the anode-cathode potential is of theorder of 100 volts. The displacement current for a given voltage is dependent on the thickness of the insulator between the conductor and the cathode. The greater the thickness of the insulator the smaller the displacement current produced for a predetermined potential. Because the insulators in the prior art apparatus are relatively thick, the potential required is relatively high. As one aspect of our invention, we accordingly contemplate the use of an insulator of small thickness. We have found that under proper circumstances, an insulator having a thickness of only .003 inch operates satisfactorily.

Further analysis of the operation of the prior art" discharge valves reveals that the briefness of the service which they afford may, at least in part, also be attributed to the use of insulators having a substantial thickness. To properly initiate a discharge it is desirable that the potential impressed between the ignition conductor and the cathode persist for a substantial time interval which is of the order of 100. micro-seconds in length. To properly apply the ignition potential for such an interval, it is desirable that an alternating potential of a frequency which may vary from several hundred cycles to several hundred-thousand cycles per second be impressed. The application of an alternating potential suflicient to produce a displacement current of one ampere per cm. to aninsulatorresults in the development of substantial energy losses in the insulator. Where the insulator is composed of relatively thick material, a substantial temperature elevation within the material arises therein when the ignition potential is repeatedly applied. Since the material used in accordance with the teachings of the prior art is glass, in which the dielectric losses increase with temperature, a thermal instability may arise. This leads to dielectric failure. The elevated temperature also induces crystallization in the glass with cracking and consequent failure.

Where the insulator is relatively thin as contemplated in accordance with our invention, it assumes the same temperature throughout and the difficulty is to a certain extent suppressed. While thin glass insulators are satisfactory, difficulty is involved in coating a conductor with a thin layer of this material. This difllculty may be avoided in accordance with a further aspect of our invention, by replacing the glass with a more suitable insulating material. Mica may be used but we prefer a ceramic substance such as vitreous enamel or porcelain. Not only do ceramic insulators facilitate the construction of a suitable ignition electrode but they are particularly propitious for use in an ignition electrode because of their mechanical properties.

In accordance with a further aspect of our invention, discharge devices of the insulated-ignition-electrode type are used to control the supply of power to an alternating-current load, such as a resistance welder. A pair of discharge valves are, in such a case, connected in antiparallel between the load and the source and when one of the valves is conductive, the potential across the other valve is relatively small and insuflicient for ignition. The ignition potential may here conveniently be applied continuously to both valves since the possibility of backfire during the reverse cycle, which usually arises by reason of the continuous supply of ignition potential is precluded by the small potential across the non-conductive valve. i

The novel features that we consider characteristic of our invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawing in which:

Figure 1 is a diagrammatic view showing a proferred embodiment of our invention; and

Fig. 2 is an enlarged view of a portion of a discharge device showing an essential feature of our invention.

Our invention is shown specifically as applied to a welding arrangement although it has general applicability; The apparatus shown in the drawing comprises a welding transformer I across the secondary 3 of which a pair of welding electrodes 5 are connected. The electrodes 5 engage the material I to be welded and when the transformer l is energized, welding current flows through the material- The primary 9 of the transformer l is supplied from the line conductors H and I3 of an alternating-current source (not shown) of the usualcommercial 60 cycle type through a pair of anti-parallel connected discharge valves 15 and ll of the insulated-ignition-electrode type.

Each of the discharge devices l5 and I1 is made up of a pair of coaxial metallic cylinders l9 and 2|. !The inner cylinder is is shorter than the outer one 2| but is symmetrically disposed relative thereto and the ends of the cylinders are joined by strips 23 of suitable construction to form an annular space through which a cooling liquid such as water may flow. The inner cylinder I9 is composed of stainless steel and a stainless steel ring 25 is sealed, preferably by welding, near its upper end. From the inner surface of the ring 25, a cylinder 21, composed of a cobaltiron-nickel alloy, known in the art as Kovar alloy, extends upwardly. The alloy cylinder 25 is,welded to the ring in such manner that the joint is gas-tight. A cylinder. 29 of glass having the proper thermal properties is sealed to the upper rim of the alloy cylinder 21 and a cobalt-nickeliron alloy cap 3| is sealed to the upper rim of the glass. A conducting rod 33 forming a gas-tight joint with the cap 3| projects through the cap to a point well below the ring 25 and carries at its lower end a block 35 composed of carbon or a metal which functions as anode for the device. The glass cylinder 29 insulates the anode 35 from the wall l9.

Near the lower end of the inner wall [9, a disc 31 of a metal such as stainless steel, is welded. The disc is provided with a plurality of cobalt-nickel-iron-alloy eyelets 39 and 4|. Through one of the eyelets 39, a tube 43 of glass of the type that seals to the alloy, extends. The tube is used for exhausting the valve, and after the device has been exhausted, it is sealed off. Through the other eyelet ll, another glass tube 45 is sealed. The latter extends well up into the interior of the cylinder l9 and a cobalt-nickeliron alloy wire I! is sealed through it. The wire 41 is bent at rightangles at two points to form a hairpin like structure with the free end extencbing towards the disc 31. The free end of the wire 41 is covered with a thin layer 49 of insulating material. The insulator 49 may be glass. but is preferably a ceramic substance such as vitreous enamel, porcelain, or the like. The thickness of the insulator is preferably of the order 011003 inch. However, we have foundthat glass having a thickness of'theorder of .020 inch may also be used with a certain degree of success. While the conductor within the insulator 48 is in the embodiment specifically shown composed of the cobalt-nickel-iron alloy, it maybe composed of other conductive materials. For example, a steel or copper wire may be welded to the wire sealed through the sleeve 45 and bent into the hairpin shape and covered with an insulator at itsfree end. Alternatively, a shortcylindrical bar covered-with the insulator may be suspended --from the free end of the wire sealed through the facilitated by the presence or a gaseous atmosphere at a pressure of several microns in the device. The desired gaseous medium may be obtained either by heating the mercury or by introducing a noble gas before the tube 48 is sealed oil.

The ignition potential required for rendering the valves I and I1 conductive is of the order of 150 volts and is supplied from an oscillation generator 53. The latter comprises a high-vacuum thermionic tube 55 or capacity sufliclent to transmit the necessary potential. The anode 51 and the cathode 59 of the tube are interconnected through a primarywinding 8| of a transformer 83 and the adjustable taps 85 and 81 of a volttage divider 69. The divider 89 is supplied from the line conductors II and I8 through-a suitable rectifier 1|. Across the primary 8| a variable capacitor 12 is connected. The capacitor may be set so that the frequency of the potent supplied by the oscillator 53 is of the desired value.

The transformer 83 is provided with a'pair'of secondary windings 13 and 14.- The windings 13 and 14 are each connected betweenthe' conductor 41 and the-cathode 5| of a corresponding valve I5 and I 1, respectively.

The primary 8| is coupled to a coil 15 which is connected between the grid 11 and the cathode 59 of the tube 55. When current is not to be supplied, to the load 1, the grid circuit for the tube 55 extends from the grid" through the coil 15, engages movable and fixed contacts 18 and 8|, respectively, of a timing relay 83, the closed movable contact 85 of a circuitcontroller 81, which may be a push button or a foot switch. another movable tap 89 of the voltage divider vider 89 isinterrupted when the closed contact 85 is opened. Another contact 9| of the controller 81 is, however, closed and the grid 11 of the tube 55 is connected to the cathode 58 directly through the coil 15 in a circuit which extends from the grid 11 through the coil 15, the closed contacts 19 and 8| 'of the timing relay 83, the closed contact 9| of the controller 81 to the cathode 59. The tube 55 in the oscillator circuit 53 is now rendered conductive and alternat ing potential of a frequency determined by the setting of the capacitor 12 is impressed between the ignition electrodes 41-49 and the cathodes 5| of the discharge devices I5 and I1.

At the instant when the .circuit controller 91 is operated and the ignition potential is first supplied, the anode-cathode potential impressed on one of the devices I5 or I1 is positive while that impressed on the other is negative. For the purpose of explaining the operation, wev may assume that initially the left-hand discharge device I5 is supplied with positive anode-cathode potential. Initially, therefore, the left-hand device IE is rendered conductive and current flows from the upper line conductor II through the anode and cathode 5| of the device, the primary 9 of the welding transformer I, to the lower line conductor I3. The flow of current through the primary of the welding transformer induces corresponding current flow in the secondary 3, and the material to be welded is supplied with the necessary current. When the po- -tential of the source reverses,.the conductive discharge device I5 is rendered non-conductive and the other device I1 is thereafter rendered conductive. The current of the opposite polarity now flows through the "welding'transfori'ner I.

Since the anode 35 and the cathode 5| of the conductive discharge device are connected respectively to the cathode and the anode of the non-conductive device, the potential'impressed across the latter is equal to the potential drop across the former, and is, therefore, relatively small. The ignition potential supplied in the control circuit of the non-conductive device, therefore, has no tendency to produce backfire.

The energizing coil 93 of the timing relay 83 is connected across the primary 9 of the welding 89, the central tap 81 of the divider to the cath-- ode 59. The voltage divider 69 in the circuit Just traced supplies a biasing potential tb the tube 55 which is of such magnitude-that the current flow through the tube is zero and the oscillator is therefore deenergized.

To initiate a welding operation, the circuit con transformer in a circuit which extends from the right-hand terminal of the primary 9 through a resistor 95, the coil 93 of'the relay, a conductor 91 to theleit-hand terminal of the primary.-

When the transformer I is energized, current flows through the coil 93 of-the timing relay and the timing lever 99 is raised. The levermoves for an interval of time predetermined by the setting ofa cam.|0 on which it is supported, and thereafter engages the lower one .of a pair of levers I03 and I05 pivotally suspended together from one leg of the armature I01 of the relay. The lower pivotal lever I03 carries the movable, contact 19 which is initially engaged with the fixed contact 8| and another movable contact I09 which engages a cooperative fixed contact I when the suspended levers are pivoted by the timing lever. The direct connection between the coil 15 in circuit with the grid 11 of the tube 55 and the cathode 59 is now opened and another circuitis completed through the closed contacts I09 and III and the taps 61 and 89 of the voltage divider 81. 'The biasing potential of the'voltage divider 61- is thus reapplied to the oscillator tube 55 and it isrendered' nonconductive. The application of alternating pothe biasing potential provided by the voltage ditential to the ignition electrodes 4149 is, therefore, interrupted and the discharge device which is last conductive continues to conduct until the end of the half-period, and thereafter the current flow through the welding transformer l ceases.

The upper pivotal lever I05 of the timing relay 83 also carries a movable contact H3 which engages a fixed contact H5 when the lever is pivoted by the timing lever 99. Through the latter contacts a circuit is now closed which extends from the upper line conductor II, through a resistor H1, the contacts H5 and H3 of the timing relay 83, a conductor H9, the energizing coil 93 of the timing relay 83, and the conductor 91 to the lower line conductor l3. The circuit just traced replaces the circuit through the primary 9 of the transformer l in providing the necessary current through the coil 93 to maintain the relay 83 energized and the system remains in a condition such that welding current flow cannot be reinitiated until it is reset by the release of the circuit controller 81. The latter is provided with a third contact [2| which closes when prior art and by the spirit of the appended claims.

We claim as our invention:

1. In combination, an electric discharge device including a cathode of the pool type and an ignition electrode comprising a conductor covered with a layer of insulating material of the order of several thousa'ndths of an inch in thickness in contact with said cathode and means for applying a potential having a frequency of the order of from several hundred to several hundred thousand cycles per second between said a conductor and said cathode to render said device conductive.

2. In combination an electric discharge device including a cathode of the pool type and an ignition electrode comprising a conductor covered with a layer of insulating material of the order of .003 inch in thickness in contact with said cathode ,and means for applying an alternating" potential'for a time interval of the order of 100 micro-seconds between said conductor and said cathode to render said device conductive.

3. In combination an electric discharge device including a cathode of the pool type and an ignition electrode comprising a conductor covered a load the combination comprising a plurality of electric discharge valve means so interposed between said source and said load that when one of said valve means conducts current it substantially absorbs the potential across the other valve means, each said valve means including a cathode of the pool type and an ignition electrode comprising a conductor covered-with insulating material in contact with said cathode and means for supplying an alternating potential ccntinuously between the conductors and cathodes of all i said valve means.

6. For use in supplying power from a source to a load the combination comprising a plurality of electric discharge valve means so interposed between said source and said load that when one of said valve means conducts current it substantially absorbs the potential across the other valve means, each said valve means including a cathodeof the pool type and an ignition electrode comprising a conductor covered with insulating material in contact with said cathode and means for supplying an alternating potential having a frequency of the order of several hundred to several hundred-thousand cycles per second continuously between the conductors and cathodes of all said valve means.

7. For use in supplying power from a source to a load the combination comprising a plurality of electric discharge valve means so interposed between said source and said load thatwhen one of said valve means conducts current it substantially absorbs the potential across the other valve means, each said valve means including a cathode of the pool type and an ignition electrode comprising a conductor covered with insulating material having a thickness of the order of .003 inch in contact with said cathode and means for supplying an alternating current having a frequency of the order of several hundred to several hundred-thousand cycles per second continuously between the-conductors and cathodes of all said valve means.

8. For use in supplying power from a source to a load the combination comprising a plurality of electric discharge valve means so interposed between said source and said load that when one of said valve means conducts current it substantially absorbs the potential across the other valve means, each said valve means including a cathode of the pool type and an ignition electrode comprising a conductor covered with enamel having a thickness of the order of .003 inch in thickwith a layer of insulating material in contact with said cathode and means for applying an alternating potential for a time interval of the order of 100 micro-seconds between said conductor and said cathode to render said device conductive.

4. In combination an electric dischargedevice including a cathode of the pool type and an ignition electrode comprising a cathode covered with a layer of insulating material in contact with said cathode and means for applying an alternating potential having a frequency of the order of from several hundred to several hundred-thousand cycles per second continuously between said conductor and said cathode to render said device conductive.

5. For use in supplying power from a source to ness in contact with said cathode, and means for supplying an alternating potential having a frequency of the order of several hundred to several hundred-thousand cycles continuously between the conductors and the cathodes of all said valve means.

9. For use in supplying power from a source of alternating current to a load the combination comprising a pair of electric discharge valves interposed between said source and said load to conduct alternate half-cycles from said source to said load, each of said valves including a,cathode of the pool type and an ignition electrode comprising a conductor covered with insulating material in contact with said cathode and means for supplying alternating potential continuously between the conductors and the cathodes of both said valves.

10. For use in supplying power from a )UI'CG of alternating current to a load the combination comprising a pair of electric discharge valves interposed between said source and said load to conduct alternate half-cycles from said source to said load, each oi said valves including a cathode of the pool type and an ignition electrode comprising aconductor covered with insulating materialin contact with said cathode and means for supplying alternating potential having a frequency of the order of several hundred to several hundred-thousand cycles continuously between the conductors and the cathodes of both said valves. 4 l

11. For use in supplying power from a source tween the conductors and the cathodes of all said valves.

12. For use in supplying power from a source of alternating current-to a load the combination comprising a pair of electric discharge valves connected in anti-parallel between said source to 7 said load, each said valves including a cathode of of alternating current to a load the combination comprising a pair of electric discharge valves connected in anti-parallel between said source to said load, each said'valves including a cathode of the pool type and an ignition electrode comprising a conductor covered with insulating material in contact with said cathode and means for supplying alternating potentialcontinuously be- I

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