Tapped resistor grid assembly
This invention relates to resistor assemblies used for dynamic braking of electric motors. It is more particular concerned with such assemblies having a simplified but rugged structure and having one or more taps between their end terminals. 
     BACKGROUND OF THE INVENTION 
     The resistor assembly disclosed hereinbelow is primarily intended for diesel electric locomotives and similar vehicles. The heat generated in braking and the vibration of the moving vehicle are detrimental to the service life of resistor units in that service. Resistor units presently intended for that service are described in U.S. Pat. No. 2,680,178 issued to C. W. Kuhn et al. on June 1, 1954; U.S. Pat. No. 2,874,257, also to Kuhn et al. on Feb. 17, 1959 and U.S. Pat. No. 4,316,172 issued to William R. Luy on Feb. 16, 1982. The structure of those articles may be described as relatively narrow flat strips of resistor material reflexed or fan-folded in a frame containing two or more units arranged side by side. Such a structure facilitates the provision of taps for fan or otherwise at the junction of such units. 
     The electrical resistance of flat-strip resistor material is a function both of the dimensions of the strip, its chemical composition ad its processing. It would be desirable from the manufacturing standpoint to make resistor assemblies or grids from a small number of units of wide strip and, in fact, such a structure is disclosed in Harness U.S. Pat. Nos. 4,651,125 and 4,654,627 of Mar. 17 and Mar. 31, 1987, respectively. The Harkness grid members occupy the full width of the frame but they have no taps. 
     SUMMARY OF THE INVENTION 
     Our grid resistor assembly may be formed of resistor strip of any width up to the full width of its frame although we prefer to employ two half-width units. It is fan-folded between the side members of its frame and is supported at its reflexes or loops through suitable insulating fixtures. To increase the stiffness of the strip, we emboss or indent its flights or stretches so as to form longitudinally extending offsets spaced from each other across the strip. The depth of those offsets is controlled so that the clear space between adjoining flights is just sufficient to admit a flat metal strip inserted between them crosswise. A portion of the inserted strip is allowed to project, thereby forming a tap which tap can be positioned anywhere between the parallel flights of the resistor ribbon after the assembly of the unit. Our invention includes a channel-shaped clip which fits over the projecting end of the tap and over the edges of the adjoining flights and may be welded to the tap and those flights. Our invnetion also includes certain structural improvements which will be described. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of a resistor grid of our invention. 
     FIG. 2 is a vertical section taken on the plane AA of FIG. 1. 
     FIG. 3 is a partial cross section taken on the plane BB of FIG. 2. 
     FIG. 4 is a cross section of the resistor ribbon taken on the plane CC of FIG. 1 showing a tap strip in place. 
     FIG. 5 is an elevation of the ribbon of FIG. 4. 
     FIG. 6 is a plan of a weld cap. 
     FIG. 7 is a detail of FIG. 4. 
     FIG. 8 is a detail of a modification of the resistor of FIG. 2. 
     FIG. 9 is a plan of a modifified form of tap. 
     FIG. 10 is a partial end elevation of the tap of FIG. 9. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Our resistor is assembled in a box-shaped frame having metal end pieces 11 and metal side frames 12. Affixed to the side frames 12 are blocks of insulating material 13, preferably of molded copolymer. Those blocks carry clips 14 which hld the fan-folded ribbons 15 of resistance material at their loops or fold 16. 
     The resistor element 15 is continuous and at each end is connected to a terminal 17, only one such terminal being illustrated in FIG. 1. We form our resistance element with longitudinal ribs or offsets 18 and 19 at each flight projecting on opposite sides of the ribbon as shown in FIGS. 2 and 3 in order to stiffen it. The spacing between indentations 18 of one flight and oppositely extending indentations 19 of the adjoining flight is controlled so that a flat rectangular strip of metal 20 fits closely between them as is shown in FIG. 4. Strip 20 extends beyond edges of resistor element 15 so as to form a tap 21 on the element 15, which tap may be inserted between any two adjoining flights of resistor element 15 after the resistor unit is assembled. Tap 21 may, of course, be removed from one location and inserted into another location between other adjoining flights of our assembled unit as may be desired without dismantling the resistor unit. 
     A tap which it is desired to fix in place may be so fixed by weld cap 22 shown in FIGS. 4, 6 and 7. weld cap 22 is a narrow channel of a length not greater than a length of a flight of resistor element 15 and a width between flanges 23 slightly greater than the spacing of resistor ribbon flights from each other. The web 25 of weld cap 22 is pierced by a longitudinal slot 24 of a length not less than the width of a tap 21 so that tap 21 projects through slot 24 when cap 22 is in place. Slot 24 is dimensioned to receive weld metal and tap 21, strip 20, adjoining flights of resistance element 15 and weld cap 22 are thereby all welded together. 
     The inverted T-shaped tap of FIGS. 9 and 10 is useful where heat and vibration are severe. The tap comprises a flat horizontal bar or member 30 with a narrower flat vertical stem of metal 31 projecting therefrom in the same plane midway between the ends of bar 30. Bar 30 is inserted between two adjoining flights of resistance element 15 in the same way as has been described hereinabove for tap 20, with stem 31 projecting therefrom. Bar 30 has turned-over projections 32 extending at right angles in opposite directions from its ends adjoining its edge nearer stem 31 so that when it is inserted between two flights, the projections act as stops preventing bar 30 from being pushed any further between the adjoining flights. The tap is thus automatically properly positioned by projections 32--32. Weld cap 22 may be used as described hereinabove to fix the tap in position. The length of weld cap 22 when so used is made equal or somewhat less than the distance between projections 32--32 on bar 30. 
     The frame side members 12, previously mentioned, are stiffened and braced to resist heat distortion by forming them with outwardly projecting V-shaped longitudinal corrugations 27 which extend over substantially the full length of the sides as shown in FIGS. 1 and 2. Side members 12 are further stiffened by a bracing member 28 which spans corrugations 27 and are welded to member 12 outside of both corrugations. Members 28 also extend substantially the full length of side members 12. Those side members may be formed from two longitudinal members, one for each of the fan-folded resistor ribbons 15 as shown in FIG. 2, each member having an edge flange 29, which flanges 29 are welded or otherwise affixed together as is also shown in FIG. 2. Alternatively, as shown in FIG. 8, side members 12 may be made in one piece with an internally projecting V-shaped corrugation 30 extending longitudinally. The V-corrugation also serves to separate the two fan-folded resistor strips 15 and to position them on side members 12.