Patent Publication Number: US-2015084501-A1

Title: Electrode design in a ceramic metal halide (cmh) lamp

Description:
I. FIELD OF INVENTION  
     The present invention is related to extending the life of a lamp. More particularly, the present invention relates to the middle electrode component in a CMH electrode coil for reducing heat conduction around the seal of the lamp. 
     II. BACKGROUND OF THE INVENTION  
     In general, a CMH lamp electrode assembly consists of three welded parts: a tungsten electrode tip, a middle electrode portion that is usually made of molybdenum, and a niobium lead-wire. 
     The middle electrode portion is usually also a combination of at least two components: a mandrel wire and a coil overwind. A small portion of the middle electrode assembly, close to the niobium lead-wire weld, is covered by a seal glass since niobium cannot withstand the chemical reaction associated with a highly corrosive discharge atmosphere. Consequently, the role of the middle electrode portion is to isolate the niobium lead-wire from the inside volume of its related arc tube. Because of a thermal expansion disparity between molybdenum and seal glass, coiling of the middle electrode portion occurs to compensate for the disparity. A related overwind coil also plays an important role in heat conduction from the electrode tip towards the niobium weld. 
     Prior attempts to redesign the coil structures of a CMH lamp electrode assembly to reduce thermal conductivity and increase interstitial space between windings, has long been devised. Such attempts include coil overwinds of varying sizes and diameters; doubling the number of coil overwinds around a mandrel; and the counter winding of coil overwinds, where two coil overwinds are wrapped around the mandrel in opposite directions. 
     However, the prior attempts to redesign the coil structures do not focus on the ratio of the mandrel wire to the radius of the overwind coil(s). Nor do these prior attempts ponder the use of a coil overwind assembly consisting of a mandrel nested inside an overwind coil where the coil overwind assembly is used to surround the mandrel. 
     III. SUMMARY OF EMBODIMENTS OF THE INVENTION  
     Given the aforementioned deficiencies, a need exists for a CMH lamp electrode that reduces thermal conductivity through the use of a primary mandrel surrounded by a secondary mandrel nested inside a coil overwind. 
     Under some conditions, the embodiments provide an electrode assembly. The electrode assembly includes an overwind assembly including a secondary mandrel wire and a coil wire. The coil wire is configured to receive the secondary mandrel wire, locating the secondary mandrel inside and proximal to a cylinder created by the coil wire helix and along a longitudinal axis of the secondary mandrel wire. The electrode assembly includes a primary mandrel wire configured to be received by the overwind assembly, the overwind assembly being received around the diameter of the primary mandrel. 
     Embodiments of the present invention provide a nested overwind assembly construction. An advantage of the proposed assembly construction is that its use enables heat conduction towards the seal to remain as low as possible to extend life of the lamp. Lower heat conduction of the middle electrode portion results in a lowered seal temperature, which is one of the major life-limiting factors of CMH lamps. Some CMH lamps suffer from this problem, which is determined by their electrode and ceramic leg designs. Higher seal temperatures translate into faster corrosion rates of the seal material due to their direct contact with the liquid phase of the chemically corrosive halide dose. 
     Another advantage of the embodiments is the middle electrode portion has lower axial heat conductivity than a single coil overwind structure, virtually the same overall diameter of the mandrel plus the overwind structure. Since heat conduction of a wire is proportional to wire diameter and inversely proportional to its length, a nested coil overwind assembly makes it possible to reduce coil wire diameters and increase their length in the same overall volume. 
     Yet another advantage of the embodiments is that the seal glass material used to surround the middle portion of the electrode assembly can easier fill the gaps, known as seal voids, that occur in a coil&#39;s interstitial spacing. The reduction of seal voids reduces the probability of failure of the electrode assembly due to a failure of the seal created by the seal glass. 
     A further advantage of embodiments is that the distance between the middle electrode portion and the inner surface of the leg can potentially be smaller. The reduction of this inner surface space reduces the probability of dose bubbling, which occurs when air bubbles created from the seal glass become trapped around the inner surface leg of the wall leg. Dose bubbling can ultimately lead to seal voids which can lead to failure of the electrode assembly. Conversely, reduction of dose bubbling lamp an electrode assembly more stable with time. 
     The embodiments also have commercial advantages including the reduction in electrode assembly cost and replacement electrode assembly costs. The proposed nested coil overwind assembly makes minimal changes in electrode component cost, since the components used in the proposed assembly are similar to those already being used in current CMH lamps. Additionally, the proposed coil overwind assembly allows the opportunity to replace more expensive cermet (ceramic metal) electrode assembly components in future designs with the more efficient coil overwind assembly construction. 
     Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
    
    
     
       IV. BRIEF DESCRIPTION OF THE DRAWINGS  
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. 
         FIG. 1  is a perspective view of an electrode assembly with a nested coil overwind constructed in accordance with embodiments of the present invention. 
         FIG. 2A  is a cross sectional view of the middle portion of an electrode assembly overwind assembly using a nested coiled overwind in accordance with the embodiments. 
         FIG. 2B  is a cross sectional view of an alternate embodiment of the nested coil overwind depicted in  FIG. 2A . 
         FIG. 3A  is a parallel cross sectional view of the middle portion of an electrode assembly using a nested coil overwind within a ceramic housing in accordance with the embodiments. 
         FIG. 3B  is a perpendicular cross sectional view of the middle portion of an electrode assembly using a nested coil overwind within a ceramic housing in accordance with the embodiments. 
     
    
    
     V. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     While the present invention is described herein with illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility. 
       FIG. 1  is an illustration depicting a CMH electrode assembly containing. The CMH electrode assembly includes a molybdenum primary mandrel  100 , a tungsten electrode  106 , and a niobium lead wire  108 . Coiled around the length of the primary mandrel  100  is a molybdenum coil overwind assembly  115 . The coil overwind assembly  115  is secured to the mandrel at a predetermined position. 
     The tungsten electrode  106  is joined to the molybdenum primary mandrel  100  through weld knot  102 . The weld knot  102  is not a true weld with intermingling of metals, but an overlapping of the tungsten by the molybdenum which softens at a lower temperature. The weld knot  102  is made, for example, by passing welding current through the molybdenum and tungsten parts while pressing them axially together. The molybdenum softens more than tungsten and overlaps the tungsten producing an enlargement or weld knot. Typically, the weld knot is larger in diameter or cross-section than the tungsten knot, or shank. 
     The molybdenum primary mandrel  100  is also connected to the niobium lead wire  108  through weld knot  104 . Niobium is used to form the weld given its resistance against many chemicals and it can be easily formed, even at low temperatures. The diameter of the niobium lead wire component typically has uniform cross section at about 0.025 inches, but can vary depending on the lamp in which the electrode assembly is mounted. Similar to the weld knot  102 , weld knot  104 , the interface between the molybdenum and niobium components, occurs by passing welding current through each metal while pressing them together axially. During, or after, the welding process, a cover gas, typically argon, nitrogen, hydrogen, or a mixture thereof, is applied to cool the weld and prevent oxidation. 
     It can be appreciated by one of skill in the art that materials with similar properties to molybdenum, niobium, and tungsten may be used and would be within the spirit and scope of the present invention. 
     The coil overwind assembly  115  is fitted loosely onto the primary mandrel  100  and retained in place by frictional engagement with the weld knots  102  and  104 . The coil overwind assembly  115  consists of a secondary mandrel  110  and a coil overwind  120 , in which the secondary mandrel  110  (depicted in  FIG. 1  as a hidden line) is one diameter and the overwind  120  is of a different diameter. These different diameters allow the coil overwind assembly  115  diameter to be larger than it would be using a traditional a single coil overwind construction. Additionally, different diameters of the coil overwind assembly  115  are used because there is a limit on the ratio of the overwind diameter to the diameter of the helix that can be formed by winding it on the primary mandrel  100 , specifically the diameter of primary mandrel  100 . 
     The coil overwind assembly  115  becomes easier to manufacture as the ratio between the secondary mandrel  110  diameter and the coil overwind  120  diameter decreases. Thus, the secondary mandrel  110  diameter may be smaller than the coil overwind  120  diameter since it is winding about the combined diameter of the secondary mandrel  110  and the coil overwind  120 . A spring-back in the coil overwind  120  assures a loose fit on the electrode shank while the enlargement at the weld knot provides frictional engagement adequate to retain the coil overwind assembly  115  in place. 
     The secondary mandrel  110  wraps around the primary mandrel  100  in a coil-like fashion, similar to the way a coil wraps around a mandrel in a traditional CMH lamp electrode assembly. However, where the present invention differs is that wrapped around the secondary mandrel  110 , which is wrapped around the primary mandrel  100 , is the coil overwind  120 . The coil overwind  120  is wrapped around secondary mandrel also in a fashion similar to a traditional CMH lamp. 
     However, the coil overwind assembly  115 , creates a helical pattern about the primary mandrel  100  which creates channels between the turns, instead of the traditional interstitial spacing created by having one coil or multiple coils adjacently aligned, as seen in prior art. In essence, both the formation of the coil overwind assembly  115  (i.e., secondary mandrel  110  and coil overwind  120 ) and the formation of the overall middle electrode assembly (i.e., primary mandrel  100  and coil overwind assembly  115 ) join to form a “nested” construction of an electrode assembly. 
     This “nested” coil construction increases thermal resistance by allowing the dissipation of heat through the two intertwining coil formations, specifically the secondary mandrel  110 /coil overwind  120  formation and primary mandrel  100 /coil overwind assembly  115  formation. The dissipation of heat through two nested coil formations is unlike the prior art which only describes dissipation through one coil formation or multiple adjacent coil formations. Dissipation through this additional nested formation can increase thermal resistance of the secondary mandrel  110  and coil overwind  120 . 
     The spacing between each turn of a coil overwind, known as interstitial spacing, is determined by the desired change in thermal resistance. Prior art teaches that adjacent turns of a coil overwind are intended to be tight (i.e. no space between the overwind coils) to allow a more elongated path, which allows for increased thermal resistance instead of an increase in the coil overwind diameter. However, these tight overwinds create seal voids, when the electrode assembly is filled with seal glass during the manufacturing process. 
     In embodiments of the present invention, the interstitial spacing is also tightly wound, to keep the increased thermal resistance. However, the approach of the embodiments reduces the amount of seal voids. The addition of the secondary mandrel  110  and coil overwind  120  create additional resistance and provide an axial structure conducive for reducing seal voids, which is discussed further in relation to  FIG. 3B . The helical overwind of the secondary mandrel  110 , preferably has an interstitial space  140 , which is the distance from the secondary mandrel  110  on one helix to the adjacent helix. 
     In addition to interstitial space  142  (i.e. space between the turns of the secondary mandrel  110 ), there will also be interstitial space between the turns of coil overwind  120 , denoted as  142 . The interstitial space  142  will be smaller than interstitial space  140 , but can ranges depending on the application of the electrode assembly. 
     Finally depicted in  FIG. 1  is interstitial space  144 , which measures the adjacent turns between the coil overwind  220 . This interstitial space  144  should be as close to zero as possible, meaning the turns on coil overwind  220  should be tight or closed. 
       FIG. 2A  is an illustration depicting a cross sectional view of an electrode assembly embodiment where there is a primary mandrel  200  in accordance with the embodiments. The primary mandrel  200  is surrounded by a coil overwind assembly  225 . Similar to the coil overwind assembly  115  in  FIG. 1 , the coil overwind assembly  225  in  FIG. 2A  includes a secondary mandrel  210  and a coil overwind  220 , in which the secondary mandrel  210  is one diameter and the coil overwind  220  is of a different diameter. The primary mandrel  200 , the secondary mandrel  210 , and the coil overwind  220  can be constructed of molybdenum, or a similar material of thermal resistance capability. 
     In this embodiment of  FIG. 2A , the primary mandrel  200  will have a diameter  230 , having a value D. The secondary mandrel  210  will also have a diameter  235 , and the coil overwind  230 , will have a diameter  237 . The ratio for the primary mandrel  200  to secondary mandrel  210 , as well as the ratio for the secondary mandrel  210  to coil overwind  220 , is approximately 1:1. The distance from the centerline of the secondary mandrel  210  on one helix to the centerline of the secondary mandrel  210  on the adjacent helix is the secondary mandrel spacing, denoted as  240 . The value of the spacing  240  is approximately three times the diameter of the primary mandrel  200 . For example, the diameter of the secondary mandrel spacing is 3 D. 
     Additionally, the length of one helix of the secondary mandrel  210  is denoted as  250 . This length  250  has a value of 4 D, i.e., four times the diameter  230  on the primary mandrel  200 . Finally, in this embodiment of  FIG. 2A , when the coil overwind assembly  215  is placed around the primary mandrel  200 , there is a coil overwind assembly diameter  270 , which is equivalent to 7 D, i.e. seven times the primary mandrel  200  diameter  230 . 
     The illustrious embodiment of  FIG. 2A  increases thermal resistance of the electrode assembly by lowering the temperature change between the niobium and molybdenum, by approximately 10° C. This reduction in temperature leads to an increased thermal resistance of the electrode assembly by approximately 500% over a single coil overwind construction, which drastically increases the life of the assembly. However, manufacturability of such an electrode assembly is difficult. 
       FIG. 2B  is an illustration depicting a cross sectional view of another electrode assembly embodiment where, similar to  FIG. 2A . In  FIG. 2B , a primary mandrel  202  is surrounded by a secondary mandrel  212  nested inside of a coil overwind  222 . The secondary mandrel  212  and the coil overwind  222  make up a coil overwind assembly  217 . This embodiment is beneficial due to ease of manufacturability, which is a result of an increased ratio between the primary mandrel  200  and the secondary mandrel  210 . This is also a result of the ratio between the secondary mandrel  210  and the coil overwind  220 . The approach of the present embodiment also leads to an increased thermal resistance of approximately 115% over a single coil formation. Such an increased thermal resistance can correspondingly increase the life of an electrode assembly by up to 10,000 hours. 
     In the embodiment of  FIG. 2A , the ratio between the primary mandrel  202  and the secondary mandrel  212  is approximately 3:1, but may increase to 5:1, whereas the ratio between the secondary mandrel  212  and coil overwind  222  is approximately 1:1. The primary mandrel  202  has a diameter  232  and a value D′. The embodiment also has a secondary mandrel spacing  242 , i.e., the space between adjacent helixes on the secondary mandrel, with a value of diameter  232 , specifically D′. 
     Additionally, a length  252 , which describes the length of one helix on the secondary mandrel  212 . The length  252  has a value of 2 D′, i.e., twice the diameter  232  on the primary mandrel  202 . Finally, in this embodiment, when the coil overwind assembly  217  is placed around the primary mandrel  202 , there is a coil overwind assembly diameter  272 , which is equivalent to 3 D′, i.e. three times the primary mandrel  202  diameter  232 . 
       FIG. 3A  is an illustration of a parallel cross sectional view of a primary mandrel  300  surrounded by a secondary mandrel  310  nested inside of a coil overwind  320 . The assembly of the primary mandrel  300 , the secondary mandrel  310 , and the coil overwind  320  are located within a ceramic body having a discharge chamber and an opening defined on either side by a wall leg  330 . The defined area within each wall leg  300 , close to the niobium weld knot  104  described in  FIG. 1 , creates an area that is filled with a seal glass  340 . The location where the wall leg  330  abuts the outside diameter of the coil overwind  320  is known as the inner leg surface, denoted as  350 . This abutment of the wall leg  330  and coil overwind  320  can create seal voids  360  once the electrode assembly is filled with seal glass  340 . 
     Since niobium cannot withstand a discharge atmosphere, as described above, the seal glass  340  is protects the elements the electrode assembly. Approximately 1-2 millimeters (mm) of the molybdenum portion of the electrode assembly (i.e., the primary mandrel  300 , the secondary mandrel  310 , and the coil overwind  320 ), adjacent the niobium lead wire will be covered by the seal glass  340 . 
     The reason for formation of seal voids during the sealing process is that seal glass may not fully enter into the turns of the overwind structure(s), due to the high viscosity of the seal glass and the small entry spaces of the seal voids. As discussed in  FIG. 1 , the interstitial spacing of an overwind can greatly affect the thermal resistance of the electrode. Thus, by increasing the interstitial spacing between the overwind turns, the probability of having seal voids is reduced and likewise the amounts of such voids are decreased. Unfortunately, the increase in interstitial spacing reduces the length of molybdenum over which the heat conduction must occur prior to reaching the niobium. 
     However, a nested coil overwind structure enables the increase of the molybdenum in the same volume within the electrode assembly, reduces coil wire diameters, and thus increases thermal resistance. Electrode assemblies having this nested coil overwind configuration eliminates seal voids both for high wattage (150 W to 400 W), as well as low wattage (39 W to 70 W) CMH lamps. 
     Embodiments of the present invention allow the seal glass  340  to penetrate the seal voids  360 , similar to a slightly open coil overwind configuration, but without the loss of thermal resistance. The embodiments enable the coil overwind  320  to touch the inner leg surface  350  of the electrode assembly without blocking the seal glass  340  penetration. This occurrence is due to the axial channels, described in  FIG. 3B , created by the nested coil overwind assembly of the primary mandrel  300 , the secondary mandrel  310 , and the coil overwind  320 . 
       FIG. 3B  is an illustration depicting a perpendicular cross section of an electrode assembly in accordance with embodiments of the present invention. In the electrode assembly of  FIG. 3B , the primary mandrel  302 , the secondary mandrel  312 , and the coil overwind  322  are located within a ceramic housing defined on either side by a wall leg  332 . Similar to  FIG. 3A , the defined area within each wall leg  302  is filled with a seal glass  342 .  FIG. 3B  also illustrates axial channels  370 , which are created through the nested coil overwind assembly construction. In the embodiment, the axial channels  370  allow seal glass  342  to flow more easily, due to the creation of space between the secondary mandrel  312  and coil overwind  322 . This increased flow reduces the number of seal voids, specifically at or near inner leg surface  352 . 
     It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.