Resistor grid assembly
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is in the field of heat dissipating devices for use with electric motors. 
     2. Description of the Prior Art 
     The present invention relates in general to resistor units for the dynamic braking of large electric motors such as those used to power locomotives and specifically to a resistor grid design which reduces the heat transferred from the resistance material to the insulator mounting material. 
     During the braking of electric motors, the coasting motor continues to generate a large amount of excess electrical energy that must be dissipated. It has been customary in the art to divert such excess energy into banks of resistor grids. Such resistor grids have typically been arranged in the form of a plurality of parallel metal grids, with the grids being affixed to and positioned between a pair of insulator members. During braking, the high current values shunted into the resistor grids elevate grid temperatures to such a point that melting occurs unless the grids can be effectively cooled. Thus, it is known that it is necessary to preserve uniform air spaces between the parallel grids so that heat produced in the grids may be readily dissipated by convection to the surrounding air. Typically, a blower is used to aid the heat convection process. Furthermore, the stresses due to repeated expansion and contraction of the heated resistance grid members each time braking occurs must be taken into account to avoid failure of the grid members. Unless the heat expansion of the grids takes place in a controlled manner, the grids may warp out of their parallel alignment, thus resulting in a non-uniform cooling air flow and consequent unit failure. 
     In attempting to solve the aforementioned problems, various resistor grid structures have been devised as shown in U.S. Pat. Nos. 2,721,920 to Weide; 2,772,337 to DuBois; 2,858,402 to Griffes et al; 2,874,257 to Kuhn et al, and 3,543,213 to Weyenberg. Such grid structures have proven to be overly complex, therefore requiring expensive manufacturing and assembly procedures. For example, U.S. Pat. No. 2,721,920 issued to Weide teaches the use of a continuous one-piece formed grid supported between insulator members by means of brackets. Costly manufacturing equipment is required to fabricate such a one-piece grid and, once formed, such a grid is highly subject to damage during the assembly process. Further, the forming machines for such a one-piece grid structure are capable of making grids of but a single width and gauge thickness thus reducing unit component flexibility. The continuous grid structure also has the disadvantage of expanding in both vertical and horizontal directions upon heating. If the horizontal expansion is not precisely uniform throughout the structure, a warping of the parallel grids is caused thus reducing cooling air flow to the grid sections resulting in heat damage to the grids and consequent unit failure. U.S. Pat. No. 2,772,337 teaches the welding of resistance grids to circular pins which are then inserted into insulator members. The welding of planar grids to each side of a circular pin has proven to be costly and time consuming from a manufacturing aspect and, further, renders the structure susceptible to vibrational failures in a locomotive environment. U.S. Pat. No. 2,858,402 requires spreader plates along the length of the grid assemblies to prevent heat expansion warping of the parallel grids. In addition, the latter patent discloses welded grid portions extending entirely through open slots in the insulator members requiring costly means for mounting the insulator members to the outer casing. U.S. Pat. No. 2,874,257 uses a continuous one-piece grid structure and thus encounters problems similar to those of the Weide patent. U.S. Pat. No. 3,543,213 teaches the use of separate insulator blocks at each weld joint of a series of U-shaped grids strips. The grid strips extend through the separate insulator blocks and the construction is such that the grid strips must bend upon heat expansion. Thus, a complex assembly is required using a multiple insulator block construction. Unless the grid heat expansion bending is precisely uniform, a disadvantageous distribution of cooling air will result. 
     In Patent Application Ser. No. 892,887 filed Apr. 3, 1978, now abandoned, I suggested positioning welded grid panels between two slotted insulator members with a clearance between the grids and one insulator member to allow heat expansion of the grids solely in a one direction during operation. This structure addressed the resistor cooling and expansion problems, as well as the complexity and expense problems associated with the other improvements developed in the art. 
     Recent governmental restrictions on the use of asbestos as an insulating material have created a need for a resistor grid design which will allow the use of lower temperature insulating materials. The heat from the current carrying resistor grid is transferred to the insulator by conduction, due to the method of mounting the grid to the insulator. Heat transfer by radiation and convection also occurs due to the proximity of the current carrying portion of the resistor grid to the insulator and the fact that forced air is used to aid in transferring heat away from the grid. 
     Improvements in resistor grid structures have helped solve problems associated with grid warping and cooling, and have made the manufacturing of grid structures easier and less expensive. The present invention is addressed to continuing to solve the aforementioned problems, and further allowing for the use of lower temperature insulating materials in resistor grid structures. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention is a resistor grid comprising a plurality of first planar grid members of resistance material, each of the grid members having first and second offset planar ends, the ends of adjacent grid members being welded together to form a tight first serpentine path for the flow of electric current, the grid members lying in substantially parallel relationship to each other, the grid members positioned outermost in the serpentine path having first terminal means connected thereto to provide electrical current input and discharge paths for the resistor grid, the ends of the grid members welded together across their widths and each having a first leg extending only partially across said widths and projecting outwardly therefrom in a direction generally parallel to the longitudinal axis of the grid members with said grid members including said leg being of a one piece sheet metal construction, and first and second insulator members each having a plurality of leg-receiving holes located on one side thereof with the holes lying in parallel relation to each other and extending only partially through the first and second insulator members, the insulating members being spaced apart and in parallel relation to each other with the one side of the members being inwardly opposed to one another, the leg of the members being positioned in the leg-receiving holes of the first and second insulator members. 
     Another embodiment of the present invention is a resistor grid assembly comprising a pair of parallel mounting members of electrical non-conductive material having mutually opposed faces each with rows of generally round spaced apart holes formed therein, a plurality of individual electrically-conductive heat transfer strips with mutually opposed faces, each strip having a sheet metal main body with a first end portion and a second end portion offset equal distances but on opposite sides therefrom and being integrally connected to the main body, the strips are arranged in alternating fashion to position each first end portion into first pairs and each second end portion into second pairs offset from the first pairs, the first pairs and the second pairs each have width equal to the main body with the first pairs welded together and the second pairs welded together across the width arranging the strips into an integrally connected and unitary serpentine grid providing a current path extending from one main body to an adjacent main body through the entire width of the first pairs and the second pairs, each first end portion and each second end portion located in the first pairs and the second pairs include a first projection extending across only a portion of the width, each projection mating with another projection cooperatively forming legs extending from the first pairs and second pairs into each of the holes mounting the grid to the mounting members, the legs extend only partially across the width to minimize heat transfer by conduction through the legs into the members, the legs have sufficient length to position the first pairs and second pairs away from the members locating the current path away from the members and minimizing heat transfer by conduction and radiation in the members allowing air flow around the first pairs and second pairs as well as the body to remove heat therefrom. 
     It is an object of the present invention to provide a new and improved heat dissipating device for use with electric motors. 
     Another object of the present invention is to provide improved structure for securing heat dissipating grids between insulators. 
     A further object of the present invention is to provide a new and improved resistor grid assembly having mounting legs to minimize heat dissipation to the adjacent insulators. 
     In addition, it is an object of the present invention to provide a resistor grid assembly having different sizes of grids within an overall assembly. 
     Further, it is an object of the present invention to provide a resistor grid assembly having significantly lower temperature insulating brackets. 
     In conjunction with the above objects, it is an object of the present invention to provide a resistor grid assembly for dissipating heat from an electric motor wherein the assembly includes glass insulators for the mounting of the current carrying grids. 
     Related objects and advantages of the present invention will be apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of the resistor grid assembly. 
     FIG. 2 is an enlarged prospective view of two adjacent grid plates. 
     FIG. 3 is an end view of the grid plates of FIG. 2. 
     FIG. 4 is a fragmentary enlarged side view taken along line 4--4, FIG. 1 and viewed in the direction of the arrows. 
     FIG. 5 is an enlarged fragmentary end view of two adjacent grid plates illustrating current flow and spacing from the mounting insulator. 
     FIG. 6 is a side view of an alternate embodiment of the resistor grid assembly having different lengths of grid plates. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring now more particularly to FIG. 1, there is shown my new and improved resistor grid assembly 10. In the preferred embodiment shown in FIG. 1, assembly 10 includes two resistor grid banks 11 and 12 connected together to form a continuous serpentine current path in order to dissipate heat as the excess current is applied by an electric motor to the grid assembly. Assembly 10 includes four upright insulating members 13, 14, 15 and 16 connected to four end insulating strips 17, 18, 19 and 20. In the particular embodiment shown in FIG. 1, strips 13 through 16 are parallel and are perpendicularly arranged with respect to the four parallel end strips 17 through 20. The resistor grid assembly disclosed herein is particularly advantageous in that the insulating strips may be sized and oriented to receive a particularly shaped grid thereby enabling the overall assembly to fit into any type of desired reception area. Further, the size of the overall assembly may be varied depending upon the amount of heat to be dissipated. 
     The insulating strips of the assembly shown in the drawings are produced from glass which are able to withstand considerably lower temperatures, in the neighborhood of 130° C., as compared to the prior asbestos insulating strips which would withstand temperatures in the magnitude of 200° C. With the prohibition of asbestos and the substitution therefor of glass, particular care has been taken in designing the mounting of the legs of the grid plates to the insulating strips in order to protect the insulating strips from heat destruction. Such a mounting arrangement allows increased current carrying capacity of the grid plates from approximately 600 amps to 1,000 amps. 
     The insulating strips may be secured together by conventional fastening means with a pair of mounting bars 21 and 22 affixedly mounted exteriorally to the outer insulating strips 13 and 16. 
     A grid bank 11 will now be described, it being understood that a similar description applies to grid bank 12. Grid bank 11 is composed of a plurality of grid plates or members arranged in parallel fashion with the adjacent ends of the plates welded together forming a continuous serpentine current path. The resistor plates or members are arranged in pairs with one such pair being shown in FIG. 2. Resistor plates 25 and 26 are produced entirely from sheet metal having parallel main bodies with converging end portions 27 and 28 secured together by seam weld 29. The opposite end portions 30 and 31 diverge outwardly and are not secured together and instead are secured to the respective plates of adjacent pairs. Thus, resistor plate 25 and 26 have a relatively thin sheet metal main body 32 and 33 each with a longitudinally upraised strengthening rib 34 and 35. End portions 27 and 28 of the two main bodies converge together until the main bodies contact along parallel flat ends 36 and 37 with seam weld 29 extending across the width thereof. Integrally formed on end 36 are two radius or semi-circular outwardly extending legs 38 and 39 which are aligned and in contact with a pair of radiused or semi-circular legs 40 and 41 integrally attached to end 37. Thus, legs 38 and 40, along with legs 39 and 41 from a pair of cylindrically hollow mounting legs. The legs are located in the outer one third portions of width of each grid member. 
     The opposite end portions 30 and 31 of main bodies 32 and 33 extend divergingly outward to parallel spaced apart flat ends 42 and 43 which are integrally joined by diverging portions 30 and 31 to the main bodies 32 and 33. Flat portions 42 and 43 are seam welded to flat end portions and adjacent grid members. For example, flat end portion 43 is integrally connected by seam weld 44 to flat end portion 45, in turn integrally connected to portion 46 diverging from portion 31 and integrally connected to the main body 47 of an adjacent grid member. Similarly, flat end portion 42 is integrally attached by a seam weld to the flat end portion of the adjacent grid member positioned above grid member 25. In such a manner, a serpentine path is provided, such as shown in FIG. 1, which winds from the bottom or one end of the resistor grid assembly to the opposite or top end of the assembly. Flat end portions 42 and 43 have outwardly extending pairs of legs 48 and 49 which are identical in configuration to legs 38 and 39 being aligned with adjacent legs of the adjacent grid members to form cylindrical outwardly extending legs. Legs 48 are concave as viewed from atop portion 42 whereas legs 49 are convex as viewed from atop end portion 43. All grid members are identical and are simply reversed 180° in order to provide the serpentine path. For example, grid member 25 is identical to grid member 26 except that grid member 25 is oriented 180° with respect to grid member 26. In all cases, the strengthening ribs 34 and 35 are convex as viewed from atop the grid members such as shown in FIG. 2. 
     A pair of insulator members are provided at the opposite ends of the grid members, each having a plurality of leg receiving holes located on the mutually facing sides thereof with the holes lined in parallel relationship to each other to receive the outwardly extending legs of the grid members. One such insulator member 13 is partially shown in FIG. 4 and will now be described it being understood that a similar description applies to insulator members 14, 15 and 16. The insulator member 13 is produced from a material to withstand at least 130° C. temperature. In one such embodiment, the insulator was produced from polyester glass. Two rows 50 and 51 of generally round holes 52 and 53 are provided on the inwardly facing surface 54 of the insulator member, with holes 52 aligned with holes 53 in order to receive the outwardly extending legs of the grid members. Holes 52 and 53 extend only partially through the insulator member, but have sufficient length to mountingly receive the legs. The depth of each hole may be dimensioned to be less than the length of the corresponding receiving leg so that the leg will bottom out in the hole thereby spacing apart the edge of the flat end portion from the insulator inwardly facing surface. In other words, a gap may be provided between surface 54 and the edge of flat end portions 36 and 37 limiting the contact between the grid members and the insulator members except at the location of the legs. In one particular embodiment, each leg had a length 55 (FIG. 5) of 1/2 inch whereas the flat end portions 36 and 37 had a length 56 from the legs to the converging end portions 27 and 28 of approximately 3/8 inches. Thus, if each hole 52 and 53 has a depth of for example 3/8 inches, then the legs will bottom out and the edge of each flat portion will be spaced apart from the insulator at least 1/8 inch and the location of divergence 58 of main bodies 32 and 33 will be a distance 57 of at least 1/2 inch maximizing the distance between the main bodies of the grid members and the insulator members. The current flow path 59 extends through main body 32 to the point of divergence 58 and then through the main body 33 of the adjacent grid member and is directed away from the legs and the insulator member. As a result, the distance 57 of the current flow path from the insulator is maximized decreasing the amount of heat transfer either by convection or radiation from the grid members to the insulator members. Forced air means are provided to direct pressurized air through the grid members thereby transferring the heat from the grid members and away from the grid assembly. 
     The grid assembly disclosed herein is particularly advantageous over the prior grid assemblies in that grid members and insulators of different lengths may be mixed together to obtain an optimum configured and sized grid assembly. For example, a grid assembly 60 (FIG. 6) includes a plurality of grid members identical to those previously described which are assembled into a single serpentine path with the lower grid member 61 through 65 having equal lengths and longer than the top seven grid members 66 through 72. An intermediate length grid member 73 integrally connects together grid members 65 and 66. Thus, the two upper insulator members 74 and 75 are spaced apart a distance less than the spacing of the two lower insulator members 76 and 77. In such a manner, the overall grid assembly may be designed to fit into a particular space or to handle a specific heat carrying capacity. 
     In all cases such as the design shown in FIGS. 1 or 6, the grid assembly must include input and output terminal means and additional means for interconnecting separate banks together. For example, bank 11 includes an input terminal 80 electrically connected to the lower grid member 81 and with an output terminal 82 being electrically connected to the lower grid member 83 of bank 12. Banks 11 and 12 are electrically connected by the uppermost grid member 84 of bank 11 to the uppermost grid member 85 of bank 12. The banks may be electrically connected together by extending connection means such as a plate from one end of grid member 84 to an opposite end of grid member 85. Thus, a continuous serpentine path is provided from terminal 80 through bank 11 and then through bank 12 to output terminal means 82. Terminal means 80 and 82 extend respectively either through end insulator members 19 and 20 or side insulator members 13 and 16 in order to be electrically connected to the lower most grid members 81 and 83. The input and output terminals are not shown in FIG. 6. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.