US3453585A

US3453585A - Embossed zigzag resistor grid

Info

Publication number: US3453585A
Application number: US624042A
Authority: US
Inventors: Arthur E Croft; Ernest W Moodie
Original assignee: Square D Co
Current assignee: Schneider Electric USA Inc
Priority date: 1967-03-17
Filing date: 1967-03-17
Publication date: 1969-07-01
Anticipated expiration: 1986-07-01
Also published as: GB1178361A

Description

July 1, 1969 CROFT ET AL 3,453,585

EMBOSSED ZIGZAG RESISTOR GRID Filed March 17, 1967 INVENTORS.

ARTHUR E. CROFT kl ERNEST I4. MOOD/E F/M 7% M FL-H55 United States Patent 3,453,585 EMBOSSED ZIGZAG RESISTOR GRID Arthur E. Croft, Weston, Ontario, and Ernest W. Moodie, Downsview, Ontario, Canada, assignors to Square D Company, Park Ridge, lll., a corporation of Michigan Filed Mar. 17, 1967, Ser. No. 624,042 Int. Cl. H01c 3/00 US. Cl. 338-284 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention A Sheet-metal electrical resistance element having a zigzag shape and composed of a plurality of elongated resistive portions extending side by side, the alternate adjacent ends being electrically joined by integral resistive connecting portions to provide a series path through the elongated portions and wherein the elongated portions of the zigzag element include integral channellike embossments to provide additional strength and rigidity to the grid structure.

Description of the prior art Resistor banks in common use for the control of electric jmotors comprise an assembly of stamped sheetmetal resistive elements or grids held in parallel-spaced relationship by a metal framework. Examples of such a resistor bank are disclosed in Wright Patent No. 2,378,056 issued June 12, 1945, Widdows Patent No. 2,639,360 issued May 19, 1953, and Dyer Patent No. 2,647,978 issued Aug. 4, 1953. One of the disadvantages of such prior resistor banks is the lack of rigidity of the grids, particularly when they are made of relatively thin sheet metal so as to have a relatively high resistance. The lack of rigidity permits beinding of the grids resulting in contact therebetween under vibrational forces, thermal stresses, or a combination of the two.

A previous attempt to overcome the problem of lack of rigidity of resistor grids is disclosed in British Patent No. 26 ,252 AD. 1908 to Allen West. In this prior art structure, a zigzag or sinuous grid is provided with a continuous corrugation which runs throughout the entire path of the resistance element and which is not oppositely directed in adjacent legs.

In other prior structures, such as illustrated in US. Patent No. 2,665,358 to Robert E. DuBois, the adjacent legs of a resistor grid are of opposite curvature to provide increased rigidity and electrical separation of adjacent legs, but the curvature extends the full width of each leg portion and the opposite curvatures gradually merge at the bridging sections so that there are no flat areas at any place in the grid.

SUMMARY OF THE INVENTION A zigzag sheet metal resistor grid in accordance with this invention has embossments which, in adjacent leg portions of the grid, are oppositely directed with respect to the plane of the grid, each leg portion having a separate and distinct embossment which extends throughout the length of the leg portion but terminates short of the resistive section connecting adjacent leg portions so that the connecting sections .are fiat and unembossed. This structure provides a sheet-metal resistor grid which has adequate rigidity and in which any flexure or expansion of the grid upon heating is controlled so that no substantial deformation or buckling occurs, thereby to prevent contact between closely spaced grids in a resistor bank. The control of flexure is facilitated by the fiat unembossed connecting section between adjacent legs which provides a break in the embossed pattern to effect a hinge point for opposing flexural forces of adjacent legs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of a resistor grid in accordance with this invention;

FIG. 2'is a right end view of the grid of FIG. 1;

FIG. 3 is a sectional view taken generally along the line 3-3 of FIG. 1; and

FIG. 4 is a perspective view of a portion of the resistor grid of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, a sheet-metal resistor grid 10 is shown as having terminal lug portions 11 and 12 at its opposite ends. The lug portions 11 and 12 have respective openings 14 for receiving respective mounting bolts (not shown) of the metal framework of a resistor bank. The grid 10 is stamped from a sheet of metal of relatively high resistance, such as stainless steel sheet, to define a zigzag formation having a plurality of mutually adjacent intermediate leg portions 15 and 16 interconnected by connecting sections 18. The zigzag formation is connected at its ends to the lug portions 11 and 12 thereby providing a sinuous conductive current path of considerable length extending between the lug portions 11 and 12. A central one of the connecting sections 18, identified as 18a, has an opening 14a for receiving a further mounting bolt (not shown). The next to last leg portions, identified as 15a and 16a, at opposite ends of the grid 10 are connected to the respective lug portions 11 and 12 by respective terminal leg portions 19 and 20 which are offset in opposite directions at 21 and 22, as best illustrated in FIG. 2, from their associated lug portions to effect the desired spacing of the grids as explained in the aforementioned Patent No. 2,378,056 to D. C. Wright.

In forming the grid 10, suitable separate or combined blanking and forming dies may be utilized to cut away portions of the sheet metal thereby to define the lug portions 11 and 12, the leg portions intermediate leg portions 15 and 16, the terminal leg portions 19 and 20, and the connecting sections 18; to form the offsets 21 and 22; and to form reverse embossments in the 'leg portions 15, 16, 19 and 20 in accordance with this invention.

The intermediate leg portions 15

bossments

24 and 25, respectively, being convex and the embossment 25 being concave with respect to the side of the grid 10 seen in FIG. 1. The

embossments

24 and 25 .are centrally disposed along the longitudinal axes of their respective intermediate leg portions 15 and 16 and are narrow and shallow relative to the width of the leg portions 15 and 16.

The sections 18 connecting the adjacent leg portions 15 and 16 are not provided with embossments. Thus the

embossments

24 and 25 do not extend throughout the length of the sinuous path but angle slightly toward the connecting sections as at 26 and terminate at areas and 16 have emthe embossment 24 27 and 28, respectively, so as to leave the connecting sections 18 totally fiat. Because the

embossments

24 and 25 face in opposite directions relative to the plane of the grid, any tendency for the grid to distort rmaterially in one direction under thermal stress is neutralized by the tendency of the adjacent intermediate leg portions to bend in opposite directions. The control of the distortion appears to be facilitated by the flat unembossed connecting section between adjacent legs which provides a break in the embossed pattern to effect a hinge point for opposing fiexural forces in adjacent legs.

The terminal leg portions 19 and 20 are similarly embossed at 29 and 30, respectively, the embossment 29 being concave and the embossment 30 being convex with respect to the side of the grid seen in FIG. 1.

It has been found that the relatively shallow and

narrow embossments

24, 25, 29, and 30 impart adequate rigidity to the grid to prevent contact between closely spaced grids under high thermal stresses and vibrational forces. Because the vibration is controlled by the em- Ibossments and thermal flexure is controlled by the cooperation of the oppositely directed embossments .and the fiat sections connecting adjacent leg portions, grids in accordance with this invention have significantly better operating characteristics than those of the prior art.

We claim:

1. In .a generally flat zigzag sheet-metal resistor grid having a plurality of elongated generally-flat intermediate leg portions extending generally side-by-side in spaced-apart edgewise relationship, the alternate adjacent ends of said intermediate leg portions being joined by integral connecting sections to provide a continuous current path therethrough, a plurality of embossments respective to the intermediate leg portions, each embossment being disposed along the longitudinal axis of its respective intermediate leg portion and being narrow and shallow relative to the width of its intermediate leg portion, the improvement comprising the embossments of adjacent intermediate leg portions being oppositely directed with respect to the plane of the grid, and each embossment extending throughout the length of its respective intermediate leg portion and terminating short of said connecting section so as to leave said connecting section essentially fiat and unembossed.

2. A resistor grid according to claim 1 wherein the end portions of each embossment curve toward .a connecting section joining adjacent intermediate leg portions and wherein the intermediate portion of eachembossment is essentially straight throughout its length.

3. A generally fiat zigzag sheet-metal resistor grid comprising .a pair of spaced-apart lug portions and a plurality of spaced adjacent intermediate leg portions disposed in a common plane intermediate said lug portions, said intermediate leg portions being elongated and generally flat, connecting sections integrally conmeeting the alternate adjacent ends of said intermediate leg portions, a first terminal leg portion joining one of said lug portions to the intermediate leg portion next adjacent said one lug portion, a second terminal leg portion joining the other of said lug portions to the intermediate leg portion next adjacent said other lug portion, at least one of said terminal leg portions having means offsetting its associated lug portion from said common plane, said intermediate leg portions, .said connecting sections and said terminal leg portions providing a continuous resistive path through the grid between said lug portions, a plurality of embossments respective to the intermediate leg portions, each embossment being disposed along the longitudinal axis of its respective intermediate leg portion and being narrow and shallow relative to the width of its intermediate leg portion, the embossments of adjacent intermediate leg portions being oppositely directed with respect to said common plane, and each embossment extending throughout the length of its respective intermediate leg portion and terminating short of said connecting section so as to leave said connecting section essentially flat and unenrbossed.

4. A resistor grid according to claim 3 wherein each of said terminal leg portions has means olfsetting its associated lug portion from said common plane and said lug portions are oflset in opposite directions relative to said common plane.

References Cited UNITED STATES PATENTS 1,233,191 7/1917 Collins 338-284 2,665,358 1/1954 DuBois 338284 X FOREIGN PATENTS Ad. 26,252 1908 Great Britain.

E. A. GOLDBERG, Primary Examiner.

L. H. MYERS, Assistant Examiner.

US. Cl. X.R. 338287