GB2034107A - Horizontally operable high intensity discharge lamp - Google Patents

Abstract

A high intensity arc discharge lamp includes a coil 31 to provide a substantially vertically uniform magnetic field transverse to the arc discharge and oriented so as to exert a downward force on the arc discharge, thus preventing the relatively high temperature plasma discharge from contacting and eroding the envelope containing the discharge. Because the magnetic field provided is substantially uniform, unnecessary constriction of the arc does not result and lamp efficiency is not decreased. <IMAGE>

Description

SPECIFICATION Horizontally operable high intensity discharge lamp This invention relates to high intensity arc discharge lamps and in particular to lamps exhibiting an unconstricted arc.

High intensity arc discharge lamps of the kind discussed herein are typically found in street lighting and parking lot lighting applications and other applications requiring a high intensity, efficient light source. These lamps conventionally operate by passing electrical current through an ionizable vapour typically contained in a transparent quartz envelope. At either end of the quartz envelope there is an electrode for carrying the ionizing electric current. The operation of these lamps is similar to the operation of a conventional fluorescent lamp except that the electrodes herein need not be heated filaments as in fluorescent lamps and a phosphor coating is not required for the production of light output; in these lamps the excitation of the ionizable vapour itself is responsible for the emission of visible wavelength photons.Efficiency in these high intensity discharge lamps is promoted when the surface area of the plasma discharge is relatively large, particularly as compared with the plasma volume. It is therefore highly desirable that the discharge arc be unconstricted and that it fill a relatively large portion of the envelope, without actually touching the envelope.

Vertical operation of such lamps typically poses few problems with respect to the arc location within the envelope. However during horizontal operation, there is a tendency for the arc discharge to bow upwards often contacting the top wall of the arc tube due to convection and buoyancy of the less dense, hot plasma in the region dissipating the greatest amount of energy. This effect results in a loss of efficiency due to cooling of the plasma by conduction through the top wall of the envelope and also results in shortened life and even catastrophic lamp failure due to overheating of the top wall of the quartz envelope.

Since a quartz envelope typically operates at a temperature of approximately 7500C, it is typically contained within an outer envelope with an inner gas such as argon or nitrogen disposed betweeen the outer envelope and the inner quartz envelope, both of which are transparent.

Various proposals to the solution of this problem have included the use of an arcuate inner quartz discharge envelope conforming to the natural arch of the discharge. Another approach to this problem, for the case of constricted arc discharges is found in U.S. Patent No. 2,027,383 issued January 14, 1 936 to C.E. Kenty in which there appears to be a coil and core structure disposed along the entire length of the arc discharge and located exterior to the outer envelope. However, the apparatus shown produces a magnetic field with a very steep vertical magnetic gradient which causes the arc to be constrained to a relatively small portion of the discharge tube, concomitantly reducing its surface area and its efficiency.Moreover, a structure similar to that apparently disclosed in Kenty exhibits a mass in excess of twenty-four grams and is not sufficiently thermally protected to be disposed within the outer envelope, adjacent to the inner quartz envelope. The mass of the structure is significant since the lamps of the present invention must typically be able to withstand certain drop tests onto a hard surface and it is critical that the momentum (massvelocity product) acquired by the internal lamp structure be as small as possible. Additionally, the structure of Kenty is such a large structure that undesirable light shadows are cast.

Another approach to the discharge arc bowing problem is found in U.S. Patent No. 4,064,418 issues December 20, 1977 to J.P. Kearney in which a single conductor is disposed adjacent the top and bottom of the arc discharge tube and is serially connected with the arc discharge and connected to produce a magnetic field tending to exert a downward force on the discharge arc.

However, the resultant magnetic field also possesses a gradient similar to the gradient in Kenty (here decreasing as 1/r, where r is the distance from the conductor), thereby tending to confine the arc to a smalier volume than is required, thus resuiting in a smaller plasma surface area and lower than necessary efficiency.

Moreover, Kearney requires the use of a relatively expensive refractory metal wire such as tungsten or tantalum because of its proximity to the inner quartz discharge envelope.

In accordance with the present invention a conventional high intensity arc discharge lamp is made particularly suitable for operation in a horizontal position through the use of means providing a substantially vertically uniform magnetic field transverse to the arc discharge and oriented so as to exert a downward force on the arc discharge in at least one locale along the arc.

The vertical substantial uniformity of the magnetic field produced in accordance with the present invention does not unnecessarily constrict the discharge arc, so as to reduce its area and the ultimate efficiency of the lamp. In accordance wití one embodiment of the present invention, the relatively localized substantially vertically uniform magnetic field is produced by means of an electromagnet which is energised directly through serial connection with the arc discharge current.

The electromagnet is provided with giass fibre e.g.

Fiberglas (D insulation, permitting the use of inexpensive copper or nickel-coated copper wire and allowing the electromagnet to be placed in close proximity to the inner quartz discharge envelope. If desired, the elecromagnet may include a core consisting of iron or a iron silicon alloy exhibiting a Curie temperature in excess of 5000C. In accordance with a preferred embodiment of the present invention, an electromagnet exhibits a mass of less than three grams and therefore has a neglible impact on the stresses produced during standard drop test of the lamp. The present invention will be further described, by way of example only with reference to the accompanying drawings, in which : FIG. 1 is a perspective view illustrating a conventional high intensity arc discharge lamp.

FIG. 2 is a perspective view illustrating a high intensity arc discharge lamp of the present invention employing means to produce a vertically uniform magnetic field transverse to the arc.

FIG. 1 illustrates a conventional high intensity discharge lamp. While shown with a conventional Edison screw-in base, these lamps are typically not suitable for insertion into the standard size incandescent lamp socket but rather are designed to be screwed into large size Edison type sockets.

Additionally, special ballasting circuits are required for the typicai lamp of the present invention. Supported within outer tranpsarent envelope 10 which typically comprises quartz or a transparent or translucent (that is , light transmissive) ceramic material capable of withstanding temperatures in excess of 500cm.

Disposed within the arc discharge envelope 11 are electrodes 1 2 and 1 3 at opposite ends of said envelope 11 which typically possesses an elongate cylindrical shape flattened at either end.

Also disposed at one end of said envelope 11 is starting electrode 14 disposed a short distance from one of the electrodes so as to facilitate an initial ionization of the vapour 15 contained within the envelope 11. The vapour 1 5 comprises mercury vapor and in some cases a metal halide.

The excitation of this vapor 1 5 by the passage of electric current through it, between electrodes 12 and 13, is responsible for the production of the output of visible wavelength photons.

The lamp is usually started through ionization of vapor 1 5 between electrode 1 3 and starting electrode 1 4. This initial ionization facilitates ionization of a substantial portion of the vapour 1 5 and leads to an increasing discharge throughout the entire interior volume of envelope 11 and the passage of a discharge current between electrodes 12 and 13. The resultant discharge heats the tube and, eventually bimetallic strip 1 6 acts to electrically connect electrodes 13 and 14 so as to reduce electrolysis of the halides.

Strip members 1 9 connected to support members 20 and 22 have deposited on them gettering material which acts to remove gases such as oxygen from the volume between the inner envelope 11 and the outer envelope 1 0.

Support member 20 is typically connected to one side of the power supply and has spring members 21 spot-welded thereto for increased support for the inner lamp structure consisting essentially of transparent quartz envelope 1 The quartz envelope 11 is also supported from the other end of the outer envelope 10 by means of dimple 24 in envelope 10, said dimple 24 providing an attachment structure for hexagonal band 23 to which support structure for hexagonal band 23 to which support structure 22 is attached. Support structures 20 and 22 are typically attached by spot-welding to straps snuggly fixed to flattened ends of envelope 11. A support structure such as that shown in FIG. 1 is typical and provides a sufficiently rigid support for discharge envelope 11 and its associated structures.Such a structural configuration is well adapted to withstand prescribed standard drop tests.

Additionally shown in FIG. 1 is return lead 25 which is replaced in the lamp of the present invention by an appropriate electromagnet structure as is shown in FIG. 2. The return lead 25 is connected to the other terminal of the power supply for the iamp.

FIG. 2 illustrates a preferred embodiment of the present invention in which a high intensity arc discharge lamp includes an electromagnet structure disposed adjacent to the upper surface of the arc discharge tube thus better enabling the lamp to be operated in a horizontal position. The electromagnet structure of the present invention provides a magnetic field which is vertically substantially uniform, that is, possesses a zero or small gradient in a top to bottom direction. The relative uniformity of the magnetic field does not unnecessarily constrict the arc discharge but does operate to exert a locally downward force on the discharge current so as to prevent excessive heating of the discharge envelope 11 and to prevent colling of the plasma discharge itself. The structure shown is powered directly from the discharge current itself and requires no additional energy source.Additionally, the structure shown in FIG. 2 exhibits minimal interference with the light output of FIG. 1 except for the position of the arc discharge. While there is a slight increase in temperatuI-e at the lower surface of the discharge envelope 11, as the result of the use of the electromagnet of the present invention, this results in no degradation of performance and, in fact, aids in the revaporization of metal halides which have a tendency to become deposited, by thermal and gravimetric forces, on the lower surface of the discharge envelope 11.

The electromagnet structure of the present invention comprises conductive wire 32 which preferably comprises copper or nickel-coated copper wire. The conductor 32 is insulated both thermally and electrically by means of Fiberglas(g) sleeve 33. The conductive wire 32 is fashioned into a C-shaped arcuate coil 31 of between approximately 20 and 80 turns as shown in FIG. 2 This C-shaped coil is disposed adjacent to the upper surface of the discharge envelope 11 and oriented in a plane substantially perpendicular to the discharge path between electrodes 12 and 13.

The coil is wound in a direction determinable by the well known right-hand rule of elementary physics so as to exert a downward force on the arc discharge which constitutes a flow of current between electrodes 12 and 14, said discharge occupying a substantial portion of the interior volume of discharge envelope 11. The winding direction of coil 31 as shown in FIG. 2 is such a proper direction.

While the conductive wire of the electromagnet of the present invention may be sufficiently rigid to provide a self-supporting structure, additional support may be provided for the electromagnet by means of wire support 34 attached to support member 20, said wire support 34 being both thermally and electrically insulated by means of Fiberglas (8) sleeve 35 as shown in FIG. 2.

While the electromagnet of the present invention is marginally functional without core member 30, it is preferred that an arcuate core coincident with the axis of coil 31 be provided. Such a core advantageously comprises a material exhibiting a Curie temperature in excess of 5000C and may be comprised of a number of materials. However, any of the materials used are preferably formed into a core having a cross section of between approximately 4 and approximately 6.5 mm2 (0.006 - 0.0010 in2). Each of the core lengths are preferably between approximately 3.5 and approximately 4.5 cm. The core itself may be comprised of 0.010 inch thick stips of iron or an alloy of iron and silicon containing approximately 2 to 3 percent silicon.Sheet iron strips having a cross section approximately 0.125 inches by approximately 0.030 inches may also be employed as a core material. Moreover, also usable are 0.100 inch coat hanger wire and 0.075 to 0.090 inch diameter plain iron wire. All of these materials exhibit sufficiently high permeability along with a Curie temperature sufficiently greater than the operating temperatures in the vicinity of the inner discharge envelope 11. All the forementioned materials suitable for core 30 are light weight, inexpensive and easily manufactured.

The conductive wire 32, in addition to being comprised of copper wire or nickel-coated copper wire may comprise anodized aluminum, in which case the Fiberglas (E) insulation sleeve 33 is no longer required.

Electromagnet structures produced in accordance with the present invention typically exhibit a mass no greater than 3 grams, require no external power source, and exhibit minimum interference with the light output of the lamp.

The position of the electromagnet above the discharge envelope 11 is preferred both to minimize light blockage and for stable arc positioning. While the magnetic field decreases somewhat with increased distance from the poles of the C-shaped magnet core 30, the magnetic field between the poles of the core is substantially uniform in a vertical direction. Additionally, the use of an electromagnet structure of the present invention acts to localize and concentrate the magnetic field in one position along the discharge arc. Thus, it is desirable that the electromagnet structure, with or without core 30, be positioned centrally along the discharge arc since this is typically the position experiencing the greatest upward tendency to contact the upper surface of the discharge envelope 11.Thus, with a substantially localized magnetic field such as produced in the present invention, any undesirable constriction in the arc discharge occurs only at one location so as not to unnecessarily reduce the surface area associated with the discharge plasma volume. Therefore, minimal reduction in efficiency occurs.

Because of the preferential position of the electromagnet above the discharge envelope 11, lamps of the present invention are advantageously provided with a bayonet type base rather than the Edison type base, so that the positioning of the lamp in its socket assures proper alignment of the electromagnet. FIG. 2 shows such a base.

During normal operation of a 400 watt mercury-metal halide lamp, such as lamp MV 400/BUH as manufactured by the General Electric Company, Nela Park, Cleveland Ohio, a discharge current of approximately 3.5 amps is employed.

Such operating conditions result in a localized substantially uniform vertical magnetic field approximately 1 50 gauss for lamps produced in accordance with the present invention.

While FIG. 2 depicts only a single electromagnet of the present invention, a plurality of said electromagnets, similarly serially connected, may be employed for a longer discharge envelope 11.

From the above, it may be appreciated that the apparatus of the present invention provides a high intensity arc discharge lamp operable in a horizontal position without loss of efficiency and with minimal interference with the light output of the lamp. Additionally, the structure disclosed has a relatively small mass which has a negligible impact on the results of standardized drop tests for such lamps.

Claims (12)

1. A high intensity arc discharge lamp suitable for operation in a horizontal position, comprising (a) an inner evacuable, light transmissive envelope having opposed ends and positioned inside an outer bulb; (b) a plurality of electrodes disposed through the ends of said envelope at least two of said electrodes being disposed at opposed ends of said envelope; (c) an ionizable vapor contained with said envelope; (d) means to initiate and sustain an arc discharge through said vapour between electrodes disposed at opposited ends of said envelope; and (e) means to provide a substantially, vertically uniform magnetic field transverse to said arc discharge and oriented to exert a downward force on said arc discharge at at least one position on said arc.

2. A lamp as claimed in claim 1 in which said magnetic field providing means is an arcuate electromagnet disposed adjacent an upper surface of said envelope, and disposed in a plane oriented perpendicular to the arc discharge.

3. A lamp as claimed in claim 2 in which said electromagnet comprises an arcuate core surrounded by a plurality of turns of conductive wire.

4. A lamp as claimed in claim 3 in which said core comprises material exhibiting a Curie temperature in excess of 5000C.

5. A lamp as claimed in claim 3 or claim 4 in which said core comprises iron or iron silicon alloy.

6. A lamp as claimed in any one of claims 3 to 5 in which said wire is insulated with glass fibre.

7. A lamp as claimed in any one of claims 3 to 6 in which said wire comprises copper, nickel coated copper, or anodized aluminium.

8. A lamp as calimed in any one of claims 2 to 7 in which said electromagnet comprises a coil of between 20 and 80 turns.

9. A lamp as claimed in any one of claims 2 to 8 in which said electromagnet possesses a mass of less than approximately 3 grams.

10. A lamp as claimed in any one of claims 2 to 9 in which said electromagnet produces a magnetic field of approximately 1 50 gauss.

1 A lamp as claimed in any one of the preceding claims in which the magnetic field is substantially localized at one position along the arc.

12. A lamp as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.