US2776391A - Induction lamp - Google Patents

Description

Jan. 1, 1957 s. c. PEEK, JRQ, ETL

INDUCTION LAMP Filed Dect. 30, 1955 5 3 /6 I l In lv 9 i@ -/6 8 /4 Q I@ Q I@ Bysmw fsw,

United States Patent O iNDUcTIoN LAMP ASandford Christopher Peek, Jr., Hamilton, Ohio, and John F. 'Waymouth, Marblehead, Mass., assignors to Sylvania ElectricProducts Inc., Salem, Mass., acorporation of Massachusetts Application December 30, 1955, Serial No. 556,550

1 Claim. (Cl. 313-161) This invention relates Vto `increasing the useful lifeof induction lamps, and particularly to such lamps having a held concentrator around a light-en1issive target.

Such lamps are shown for example in a patent application, Serial No. `520,718 of Sanford Christopher Peek, `tiled in the UnitedStates Patent Office on July 8, 1955. The target is heated to incandescence by the action of a high frequency coil outside the concentrator, the latter generally comprising a vslotted yannular' metal disc, inside the central opening of which-the target is set.

During operation of the lamp, the target material evaporates, condensing on the metal concentrator t-o form crystals of the target material, generally tantalum carbide. The crystals grow across the gap between the inside of the annular disc and the outside of the target, eventually bridging it and thereby increasing the heat loss from the target. This decreases the temperature of the target vfor a -given input, and also causes uneven temperatures over the target surface. `It also has the effect of limiting the useful life of `the lamp to about tiftyhours.

We have discovered that a lamp which has reached the end of its life becauseof such crystal growth can be made operable againby applying a high voltage across thegap, the application of the voltage causing the crystals to vaporize and Aremove themselves from the gap. The high voltage may be a direct current voltage or an alternating voltage, but we have found Vthe `use of a condenser discharge across the gap to be particularly effective.

The crystals can be removed as they form, without waiting for them to bridge the bap, by applying a suf-v ciently high voltage across said gap at desired intervals -of lamp operation. In that way, the lamp Vlife can be greatly'extended because the crystal growth vis no longer the limiting factor.

In order to facilitate the removal of the crystals at various intervals `during the life of the lamp, it is best to include a condenser, and apush-buttonfor discharging it across the lamp, in the equipment for operating the lamp. The condenser can then be discharged across the gap hy the operator after or before each use of the lamp.

In order to permit the application of a voltage across the gap, `electrical connections must be brought from the concentrator and the target to the outside of the lamp. If the concentrat-or is connected to a water jacket, as in the device of the previously-mentioned patent application by Peek, the tubes which bring the water through fthe lamp envelope can be used as one connection, and a lead-in wire brought from the target through the glass envelope, for the other electrical connection.

In some cases, it may be desirable to apply an alternating or direct voltage across said gap continually while the lamp is in operation. To prevent continuous arcing or sparking across the gap, the voltage would preferably be below the value necessary to arc or spark across said gap when the latter is free of crystals, yet high enough to spark or arc across when the gap is partially bridged by crystals. Then as soon as a predetermined amount of 2,776,391 Patented Jan. 1, :1957

crystal structure appears, -it will be removed by the voltage. In order to keep the arcing or sparking from continuing after the short period necessary to remove the crystals, a relay could be used in serieswith the voltage source and the gap, the relay beingiof a type which cuts ofi the Voltage as soon as a predetermined current passes and keep it off for a period suicient to allow the vapors formed `by the dischargeto be dispersed and for the concentrator to cool.

Other objects, features and advantages of the invention will be apparent from the following specication, ltaken in connection with the accompanying drawings, in which:

Fig. l is a view'of a lamp according to the invention;

Fig. 2 is a schematic View of apparatus according to the invention; and Fig. 3 is a top view of the lamp of Fig. l.

ln Figure l, the glass envelope 1 includes the plane glass `disc 2, of a :type vgenerally called an optical flat, through which light from the cylindrical target 3 can emerge when the latter is 'heated by the passage -of a high frequency current in coil 4, which is external to the lamp and not an integral part thereof. The target 3 is set in the central opening 5 of an annular metal Vdisc 6, and is of smaller diameter ythan said opening. The disc 6 has the radial slot 7 `so that there will not be a complete conductive turn yin :which current can ow hy induction. The disc 6 acts as a closure member 'for one end of the cylindrical metal shell `8, which extends longitudinally back in the envelope 1, preferably for a distance equal t-o the length of the coil 4 and in register therewith. The slot 7 extends from disc `6 longitudinally `down the wall of cylinder 8, to prevent the cylinders acting as a complete conductive turn, .that is, as a so-called shorted turn. The disc y6 and she1li8 act to concentrate the magnetic cld in the vicinity of the target.

The tubes V9, 9 should not extend across the slot 7 at any point, for if they did, they would short circuit it. In cases where liquid cooling is not desired, the concentrator disc 6 and shell 8 can be supportedin some other manner than by tubes 9, 9, for example by being supported from the glass walls of envelope 1.

The cylinder 3 is supported by the hollow metal tubes 9, 9 which are attached to the copper thimbles 1t), 10 at the external ends 11, 11 thereof, said copper thimbles being sealed to the bottom wall 12 of the glass envelope 1 in a manner customary in the art of such seals, which are generally called Housekeeper seals.

The hollow tubes 9, 9 terminate in the customary type of metal connectors 13, 13 used 'for sealing copper tubing together. Inside the envelope 1, the tubes 9, 9 extend around and across the inner wall 14 of cylinder 7, being attached thereto by soldering or in some other convenient manner.

The hollow tubes 9, 9 are 4not electrical lead-in conductors, but are merely Aconductors for the circulation of a cooling liquid such as water. vIf the water supply is grounded, however, the tubes 9, 9 and the cylinder 8 and disc 6 to which they are thermally connected, will also be at ground potential.

The thickness of target 3 is greater than that of the disc 6, and the target 3 is placed so that part of it extends above the plane of the top of said disc as shown in the co-pending application Serial No. 520,767 of Frithjof N. Hansen, filed July 8, 1955. In one example, the target thickness was about 3732 inch and the disc thickness about 1/6 inch. The disc was about 2 inches in diameter, and beyond the 3/32 inch thickness of the target itself, the target piece was machined down to a diameter of about 1A@ inch to provide a projecting support 15 for the main portion of the target 3, said support extending about inch back from said target itself. The diameter of the target 3 itself was 5/16 inch.

The bottom end of projecting support is tted into a zirconia supporting tube 16 at a considerable distance, about M: inch in one example, from the main target 3, in order to reduce heat losses. The zirconia tube extends downward along the axis of cylinder 7, and its lower end 17 is held in collar 18 by set screws 19, supported from the lower end of cylinder 7 in the bracket 20, held to the wall of said cylinder by the screw 5, as shown in Figure 3, although it can be held in other suitable manner if desired. Other refractory materials than zirconia can be used, the zirconia being given as an example.

The whole internal unit or mount is thus supported from the Housekeeper seal thimbles 10, 10, set in the glass base of header 20. The seals to the thimbles 10, 10 can therefore be made before the glass header 21 is sealed to envelope 1, the sealing being accomplished in a manner well-known in the art.

Before sealing header 21 to envelope 1, a lead-in wire 24 to target 3 is sealed through said header 21. A glass bead 25 is formed around the wire 24 where it goes through the header 21. It is generally convenient to seal a glass tube to the header as if it were an exhaust tube, and then seal the wire 24 through said tube to form the bead 25. Because of the high voltage to be used, the wire 24 can merely be pressed against target 3 for good contact; or a small gap can even exist between the end of wire 24 and target 3.

After sealing the header 21, to envelope 1, the lamp can be exhausted in a manner usual in the art, through the metal exhaust tube 22 which is brazed to the side of one of the thimbles 10. The tube can then be lled with argon at about lVz atmospheres absolute pressure, for example, and the exhaust tube 22 then sealed off to complete the lamp. The exhaust tube 22 being of metal, for example copper, can be flattened together at one part of its length to seal the lamp from the exhaust system, and then cut off at the flattened part. Such seals are wellknown in the art.

In operation, one of the tubes 9 is connected to a water inlet by connector 13, and the other tube 9 is connected to a water outlet by its connector. Water, or other cooling fluid, is then circulated through the tubes 9, thereby keeping the disc 6 and shell 8 cool. A high frequency source of say 4 megacycles although other frequencies can be used, is connected to coil 4, through which high frequency current will accordingly flow. The magnetic field inside said coil is concentrated, by disc 6 and shell 8, around the refractory target 3, which is accordingly heated by the currents induced in it. The target 3 becomes very hot, its temperature rising to about 3600 K. if suflicient energy is supplied.

The light from the front surface of target 3 then travels through the optical at 2 to the plane in which it is to be used.

During operation, the material of target 3 will be vaporized, depositing on the concentrator disc 6 around the opening 5 therein, and tending to reduce or bridge the gap between the concentrator disc 6 and the target 3 by formation of crystals. A circuit for removing such crystals is given in Figure 2. The concentrator disc 6 is connected to one terminal 27 of a condenser 28 and the target 3 to the other terminal 29, through resistor 25 and switch 26. Connection to the concentrator disc 6 is made through cooling tube 9, and to the target 3 through the lead-in Wire 24.

The positive end of condenser 28 generally goes to the target 3, because the concentrator disc 6, being ordinarily connected to a water-cooling system, is thereby grounded, and the negative side of the oscillator circuit for feeding coil 4 is generally also grounded. The condenser 28 can then be charged from the voltage used to supply the plate circuit of the oscillator. The condenser 28 can be permanently connected across the direct current supply, if desired, preferably through a Voltage divider 30, 31, and then discharged across the gap at various intervals by closing switch 26. A resistor 25 can be used to limit the discharge current from the condenser. Such a current limitation keeps the relay contacts from sticking.

A condenser voltage of about 1000 volts can be used with the lamp described, with a condenser 28 of about 30 microfarads and a resistor 26 of about 10 ohms. The switch 27 can be of the relay-actuated type because of the high voltage. These values are not critical, however, and other values can be used with good results. The voltage across the input terminals 32, 33 to the voltage divider 30, 31 can be about 4000 volts, for example, in the embodiment described.

lf it is desired to keep voltage across the gap at nearly all times, so that a discharge will pass as soon as the crystals narrow the gap to a predetermined value, the voltage will have to be disconnected almost as soon as it is applied, to prevent the resultant arc or spark from continuing for a period sufficient to wear away the target. The arc being stopped, and the gap cleared, the eventual reapplication of voltage will not produce another arc; at least not until crystals again narrow the gap.

To achieve this result, the switch 26 can be a normally closed relay, with its actuating coil connected in series with the gap, that is its actuating coil could be in the same circuit as resistor 25, or in series therewith, and the switch 26 should preferably be of a type which recloses slowly, giving the arc time to de-ionize.

The resistance 31 of the voltage divider 30, 31, will prevent shorting of the power line 32, 33 by the discharge across gap 5. It can be made of high enough resistance to prevent feeding the arc across gap 5 directly from the line. In the example described, the resistance 30 is about 200,000 ohms and 31 is about 500,000 ohms.

What we claim is:

An induction lamp comprising a light-transmitting envelope, an annular concentrator disc within said envelope, a target of refractory material inside but spaced from the inside of said disc, and electrical connections from said concentrator and said disc sealed through said envelope.

References Cited in the le of this patent UNITED STATES PATENTS 2,628,330 Williams Feb. l0, 1953

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