US2078778A - Electrical resistor unit - Google Patents

Description

Aprl 27, 1937. J. A. sl-HEPARDV ELECTRICAL RES ISTOR UNIT Filed Sept. 27, 1954 2 Sheets-Sheet l INVENTO JWM/6 ATTORNEY 2 Sheets-Sheet 2 ATTORNEY April 27, '1937. J. A. sit-iEPARD ELECTRICAL RESISTOR UNIT Filed sept. 27, 1954 Wai Patented pr. 27, 937

UNITED STATES PATENT @FFICE ELECTRICAL RESISTOR UNIT James A.

Shepard, deceased, late of Montour,

Application September 27, 1934, Serial No. '745,719

7 claims.

The general object of the present invention is to provide an improved electrical resistor unit of the type comprising resistance wire and a support about which said resistance wire is wound. The improved unit was primarily devised and is especially adapted for use as a motor resistance in apparatus for controlling the operation of intermittently operating electric motors, such as crane and hoist motors, and specific objects of the invention are to provide a resistance unit of a form and construction well adapted to satisfy various hereinafter mentioned special requirements of said use.

A crane or hoist motor controller-resistance assembly must ordinarily be adapted to vary the motor circuit resistance in definite steps corresponding to different motor speeds required or desirable. In general, as the resistance in the motor circuit is diminished the current carrying capacity of the resistance must be increased, and it is therefore desirable that the controller assembly would include a plurality of resistor units with the resistor wire diameters and the number of resistor winding turns of one or more of the units, and consequently the total resistance of the resistors of such units different from the resistor diameters, number of winding turns, and resistance of one or more other units of the assembly. To permit of the formation of such assemblies for motors of different capacities or characteristics from stock parts, and to facilitate the adjustment of a particular assembly to better adapt it for particular conditions of use, each unit is desirably formed to facilitate the proper winding on the unit support of resistor Wires of different diameters or of materials differing in specific resistivity. Features of construction and arrangement adapting the units for interchangeable use and for their ready winding with different resistor wires as described also facilitate the making of repairs and replacements which become necessary or desirable from time to time.

As some of the unit resistor wire must be of the very considerable diameter of approximately three-sixteenths of an inch or so, and correspondingly stiff, the supporting frame of the unit about which the resistor wire is coiled must be suitably rigid so that it will not spring and allow the wire to slacken. Under normal conditions of use the resistor temperature may vary through a range of several hundred degrees F., so that the linear expansion of the resistor wire is considerable, and the unit construction should be such as to keep adjacent turns of the resistor windings from bending into contact with one another as the resistors expand. To limit the maximum temperatures attained, the individual resistance and their assemblage should be such as to insure effective ventilation for air cooling and dissipation of the heat evolved by the passage of electric current through the resistors. To permit and facilitate the making of proper circuit connections, the resistors should be provided with substantial and conveniently disposed terminals and intermediate taps without requiring the use of solder, as the working temperatures attained are higher than the melting point of solder. For crane and hoist use the mechanical construction of the units and assembly must be strong and rugged and the units should include mica or analogous insulating material not affected by moisture separating resistor portions between which appreciable differences of electric potential may occur.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a of this specification. For a better understanding of the invention, however, and the manner in which the previously mentioned and other objects of the invention are obtained, references should be made to the accompanying drawings and descriptive matter illustrating and describing a preferred embodiment of the present invention.

Of the drawings:

Fig. i is a sectional elevation of a motor controller resistance assembly;

Fig. 2 is an elevation partly broken away and in section of one of the grid resistance units of the assembly shown in Fig. 1;

Fig. 3 is an end View of the unit shown in Fig. 2;

4.- is a section on the line 4 4 of Fig. 2; and

Fig. 5 is a partial section taken similarly to Fig. 4 but on a larger scale.

The motor resistance assembly shown in Fig. 1 comprises a ventilated casing A with a removable cover A and a plate-like body or slab B of slate or other suitable insulating material mounted within the casing and providing a support for a plurality of electromagnetic switches or `contactors C and C", of which two only are shown in Fig. l, and a support for a plurality of grid resistance units D. The insulatingsupport B is shown as separating the interior of the casing into a rear chamber in which the resistance units i) are located, and a front chamber in which the contactors C and C' are located. While the diiferent resistance units D are preferably all similar in general form and construction and may be similar in all respects, usually some of the resistance units of a motor resistance assembly will comprise resistor wires of different diameters or of materials of diierent specic resistivities so that the different units may have different resistance values.

In the preferred form shown in 2-5 each resistance unit comprises a ilat rigid cast metal body or supporting member E of rectangular outline, and cut away to reduce its weight and to facilitate the ventilation and air cooling of the unit. As shown, the unit body E is formed with an aperture E' at each end to receive a supporting post. In the assembly shown in Fig. l, there are three side by side units D in an upper bank and three side by side units D in a lower bank, the three units of each bank being supported by a corresponding pair of bolts or posts b extending through the unit aperture E. The supporting bolts or posts b are each secured at one end to a frame member b which is bolted or otherwise rigidly secured to the insulation body support B. The diierent resistance units D secured to and supported by each post b are 'g spaced apart by bushings or spacers b2.

Preferably and as shown, each body or member E is formed with two thickened portions or ribs E2 parallel to and between and spaced away from the two elongated side edges E2 and each closer to the adjacent edge E2 than to the other rib portion E3. Each portion E3 is shown as thicker than the adjacent side edge E2, and is tapered in thickness so that the planes of its oppositely inclined sides extend in a general tangential manner away from the adjacent side edge E2 when the latter is rounded as shown. The shaping and relative disposition of each rib portion E3 and adjacent edge E2 described insures a support for trough shaped insulation parts F hereinafter described. To save weight and provide ventilation, portions of the unit body E at each side of each rib portion E3 are cut away, thus providing openings E4 between each rib E2 and the adjacent side rib E2, and larger openings E5 between the two rib portions E3.

In accordance with the present invention trough shaped insulating parts F are fitted over the side edges E2 of the member E with the sides of each part F bearing against and spaced apart by the rib portion E3 adjacent the side edge E2 over which the part F fits. As shown the inner edges of each part F project a short distance inwardly from the corresponding rib portion E3 but not suiiciently to substantially obstruct the adjacent ventilation openings E5. For the purposes of the present invention each insulation part F is formed of rather soft asbestos board or insulating material analogous thereto in that it may be rather deeply indented by the resistor wire wound under suitable initial tension about the member E and the insulation parts F mounted on its opposite edges. Each insulation part F may extend for substantially the full length of the edge E2 over which it ts, but when as is usual the resistor wire wound about the parts F is formed in a plurality of end to end ilat helices, the resistor ends of adjacent helices are advantageously connected to an intermediate terminal post secured to the member E adjacent one side edge of the latter, and in such case it is convenient to provide a separate end to end part F at opposite sides of each such intermediate terminal post.

In the particular form of construction shown there is an intermediate terminal post H2 secured to the body E midway between its ends adjacent one side edge E2, and a similar intermediate post h2 secured to the member E adjacent its opposite side edge E2, and there are two end to end parts F iitting over each side edge of the body E at opposite sides of the corresponding intermediate post. The intermediate post H2 serves as the common terminal for the adjacent ends of the resistor wires of two end to end helices G and GA, the opposite ends of the resistor wires of the two helices being connected to corresponding terminal posts H' and H carried by the member E at its opposite ends. The terminal posts H, H and H2 may be anchored in and insulated from the member E in any usual and suitable manner, and preferably and as shown, each post is threaded and provided with means including a clamping nut H3 for securing to the post the corresponding resistor wire and any lead conductor (not shown) extending away from the post.

As shown the post H2 extends transversely away from one flat side of the body E and the helices G and GA are oppositely wound so that the successive turns of each helix are at progressively greater distances from the intermediate terminal post H2. When the two helices G and GA are integrally connected portions of the same wire or are formed of different wires having their adjacent ends brazed together or otherwise connected, the connecting wire portion of the two helices may be looped about the terminal post H2 as shown, though the ends of the two helices connected to the post H2 may be separate and each clamped to the post H2.

Telescoping or nesting with each of the insulation parts F and bearing against the outer sides of the corresponding portions of the resistance wire G is a trough shaped part of lnsulating material f which in respect to its shape and the material of which it is formed, may be a counterpart of the part F except that the part f is shown as slightly larger than the part F. Resistor wire helices g and gu surrounding the helices, G and GA, respectively, are wound about the parts f as the helices G and GA are wound about the parts F and the ends of the helices g and ga are secured to terminals h, h and h2 as the ends of the helices G and GA are secured to the posts H, H and H2. The posts H, H', H2, h, h and h2 may all be similar in form and mounting. Conveniently and as shown, the terminals h and h are adjacent the ends of the side edge portion of the frame E remote from the aperture E', and the terminals H and H are located intermediate, and at a substantial distance from each of the two side edges E2 of the member E. Advantageously and as shown the intermediate terminals H2 and h2 are adjacent the opposite side edges E2 of the member E, and the portions of the resistors connected to and adjacent the posts H2 and h2, respectively, are at opposite sides of the member E.

While the various resistor helices G, GA, g and ga may all have the same resistance values, it is frequently, if not usually desirable that they should have different resistance values. A variation in the length or diameter of the helix resistor wire, or in the specific resistance of the wire material will obviously produce a corresponding variation in the resistance value of the resistor. Ordinarily resistor wires of different diameters are employd in forming helices having such different resistance values, as may be required for the desired changes in resistance in a motor circuit to be produced by varying the number of the helices connected in the circuit, or the manner of their connection in the circuit.

Advantageously each trough shaped part f is lined by a trough shaped layer or sheet I of mica, which is impervious to moisture and prevents undue reduction in the insulation between the inner helix G or GA and the corresponding helix g or ga, respectively, at times when the insulation value of the part f is reduced by moisture.

To increase the spacing between adjacent intermediate portions of the flattened sides of the resistor helices separated from one another by parts f and I, a piece of asbestos rope or cord is placed alongside of each rib portion E3 .at each side of the latter and between the cuter side of the adjacent portions of the corresponding parts f, and helices g and ya..

As previously indicated, each helix forming resistor should be wound in place on the unit v supporting member under a substantial initial tension to produce `adequately deep resistance receiving indentations in the corresponding parts F and f, whereby the resistor portions received in said indentations are held against objectionable displacement when the resistors heat up and elongate. It is obviously desirable that the helical resistor turns should be accurately formed, and in general that the helical pitch of each resistor helix should be substantially uniform. It is possible, of course, to employ a special winding machine for winding the resistors on a resistance unit body, but the winding operation may readily be effected with the use of an ordinary screw cutting lathe, and by the use of the screw thread feed adjusting provisions of the lathe, the resistor wire can be wound with any desired pitch. ln consequence for unit production or for unit repairs, only ordinary machine shop facilities are required, and the only differences in stock materials required for the production or repair of units of dilferent resistance capacity, may be confined to the sizes and/or metallic compositions of the resistor wire stock.

While units constructed in accordance with the present invention may have their resistor portions insulated and anchored in place, in

lwhole or in part, by cement, one practical advantage of the present invention is that for most purposes, at least, it is unnecessary to imhed the resistors in cement, so that the use of the invention may thus avoid the expense of the ceinenting operation, and the delay involved in the application of cement and the drying of the latter by exposure to the air or by baking in an oven or furnace. Moreover, the features of the present invention may be employed in the construction of units in which the resistor wire diameter is too large to make it practically feasible to secure the resistor turns in place by imbedding them in cement without risk of having the cement fractured by the expansion of the resistor wire. In the commercial use of the invention, excellent results have been obtained with units constructed in accordance with the present invention of relatively large capacity, i. e., great motor horse power capacity, and operated with relatively high working temperatures.

While the principles of the present invention obviously do not depend upon the precise dimensions of the parts employed, it is noted by way of example and illustration that commercial forms of resistance units embodying the present invention and all substantially similar to the construction shown in Figs. 2 5 have been designed with resistance frame members which vary from a length of 8 inches and a width of 3% inches to a length of 171A; inches and a width of 71A; inches; and that the thickness of the asbestos boards parts F and f in the smaller of such units is 3-32- of an inch and in the larger units is 1/8 of an inch; and that the diameter of the asbestos cord parts J varies from e of an inch to 1A of an inch; and that the resistor wire employed in said units is a commercial coppernickel alloy, and varies in diameter from .01 of an inch to about .162 of an inch. It is noted also that in general the indentation of the asbestos board parts F and f is much more pronounced with a resistor wire of large diameter than with a resistor wire of smaller diameter. While the indentation can be controlled within the elastic limit of the resistor wire, by the tension applied in winding, the indentation practically obtainable with a very small diameter resistor Wire is sufiiciently smaller in comparison with the indentation obtainable with a wire of appreciably larger diameter. rl."he resistor turns in a unit including the small diameter resistor wire must be spaced relatively further apart, or the unit must be worked at a lower maximum temperature, than is the case with a unit including resistor wire of larger diameter.

With the supporting member E of the resistance unit shaped and cut away as described, sufficient mechanical strength and rigidity is obtainable With the member formed of cast iron, without giving the unit such heat absorbing and storage capacity or so interfering with the ventilation of the unit, as to prevent a suitably rapid dissipation of the heat generated by the circuit flowing through the resistor wires of the unit.

An advantage of the double wound resistance unit construction disclosed is the elimination or reduction in resistor induction which is made possible by passing the current in opposite directions through the inner and outer resistance wires. When the current passing in opposite directions through the inner and outer wires is of the same value, and the inner and outer wires comprise the same number of turns about the supporting grid, the inductive effect of one coil is substantially neutralized by the inductive effect of the other. Perfect neutralization is not obtained when the ampere turns are not the saine for the inner and outer wires, but with current flowing in opposite directions through the two windings, inductive difficulties are reduced even though the ampere turn relation is not that required for substantially complete neutralization of the inductive effect of one coil by that of the other.

Resistor induction effects are especially objectionalole when the current passing through the resistance wires is alternating current. In such case, for one thing, the eddy current flowing in the support about which the resistance wire is wound, when the support is made of iron and steel, is especially objectionable because of the resultant support heating eect, as well as because of the circuit reactance effect. This has led heretofore to the use of non-metallic wire supports in some cases, and in other cases to the use of supports formed of steel and cast iron which are smaller than otherwise would be desired. The use of cast iron is of itself advantageous because of the lower magnetic properties of cast iron as compared with steel, but with the support made of cast iron, the reduction in the size of the support is limited by the necessity for keeping the dimensions great enough to insure proper support, strength and rigidity. Even when the current flowing through the resistance wires is direct current, resistor inductance is objectionable because of its tendency to increase arcing at the controller switch contacts when the current is interrupted.

With the elimination or reduction made possible by the use of the present invention, the grid supports, preferably made of cast iron, may be of ample size and weight to insure the proper mechanical strength and rigidity and a suitable open spacing of the resistance wires to insure a desirable rapid dissipation of heat from the latter.

While in accordance with the provisions of the statutes the best known form of the invention has been illustrated and described in detail herein, those skilled in the art will understand that changes may be made in the form of the invention disclosed without departing from the spirit of the invention as set forth in the appended claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

Having thus described said invention, what is claimed and desired to be secured by Letters Patent is:

i. An integral resistor unit comprising a flat rigid supporting grid with opposed side edge portions, an inner flattened resistor Wire helix the convolutions of which surround said grid, an outer flattened resistor wire helix also surrounding said grid and having larger convolutions than the first mentioned helix, indentable insulating material covering said side edge portions and indented by the inner helix convolutions, and indentable insulating material covering the first mentioned insulating material and the adjacent portions of the first mentioned helix and spacing apart the convolutions ofthe helices and indented by the convolutions of said outer helix, said convolutions being generally transverse to said side edge portions and each of the latter being extended in the direction of the convolutions to provide a support for and between opposed flattened sides of the inner helix at a distance from the outer edge of said side edge portion greater than the distance between the opposite flattened sides of the convolutions of the inner helix.

2. A resistor unit comprising a flat rigid supporting grid-like cast metal body, an inner fiattened resistor wire helix surrounding said body, an outer attened resistor wire helix surrounding said body and having larger turns than the first mentioned helix, each helix having adjacent turns laterally displaced from one another, insulating material interposed between the two helices and spacing apart the turns thereof and comprising a layer of relatively dense insulating material impervious to moisture, and a layer of less dense material pervious to moisture and indented by the turns of the outer helix.

3. A resistor unit comprising a iiattened rigid supporting body having opposed side edge portions and inner and outer attened resistor wire helices surrounding said body and having adjacent turns laterally displaced from one another, trough shaped pieces of indentable insulating material fitting over said side edge portions and interposed between the latter and each inner helix and indented by the turns of the latter,

trough shaped pieces of indentable insulating material fitting over the first mentioned pieces and adjacent inner helix portions and interposed between the latter and the adjacent outer helix turns and indented by the latter and cords of indentable insulating material extending longitudinally of said side edge portions at each side of said body and interposed between the sides of the last mentioned trough shaped parts and the adjacent outer helix turns.

4. A resistor unit comprising a fiattenedrigid supporting grid formed with ribs at each side one adjacent but displaced inwardly from each of two opposed side edges of said grid, trough shaped bodies of insulating material extending over said side edges and each having its sides separated by the two adjacent ribs at the opposite sides of the grid, resistor Wire helically wound about said body and insulating parts to form one or more flattened helices, the turns of which indent said part, and other trough shaped insulating parts of indentable material extending over the rst mentioned insulating parts and adjacent portions of said turns and resistor wire wound about said body and parts to form one or more flattened helices with turns spaced from the first mentioned turns by and indenting the last mentioned insulating parts.

5. A resistor unit comprising a flattened rigid supporting grid formed with ribs at each si e one adjacent but displaced inwardly from eac of two opposed side edges of said grid, trough shaped bodies of insulating material extending over said side edges and each having its sides separated by the two adjacent ribs at the opposite sides of the grid, resistor wire helically wound about said body and insulating parts to form one or more flattened helices, the turns of which indent said part, other trough shaped insulating parts of indentable material extending over the first mentioned insulating parts and adjacent portions of said turns and resistor wire wound about said body and parts to form one or more flattened helices with turns spaced from the first mentioned turns by and indenting the last mentioned insulating parts, and cords of indentable insulating material parallel to and in front of each of said ribs and interposed between the last mentioned insulating parts and the adjacent outer helix portions.

6. A resistance unit comprising a cast metal grid with parallel opposing side edge portions, two terminal posts rigidly secured to said grid each intermediate the ends of the latter and one at one side and one at the opposite side of the grid and each intermediate the ends of the grid, trough shaped parts of indentable insulating material fitting over the side edge portion of the grid, resistor wire Wound about said grid and parts to form two oppositely wound flattened inner helices one at each side of said posts with the turns of the helices indenting said material, the adjacent end turns of the two helices being secured to one of said posts, other trough shaped parts of indentable insulating material fitting over the first mentioned parts and the adjacent portions of said turns, resistor wire wound about said body to form two oppositely wound attened outer helices the turns of which are spaced away from the turns of the first mentioned helices by and indent the last mentioned insulating material, the adjacent end turns of the outer helices being connected to the second of said posts, and other terminal posts rigidly secured to said grid to which the end turns of the helices remote from the rst mentioned posts are connected.

7. A resistance unit comprising a cast metal grid with parallel opposing side edges, two terminal posts secured to said grid each intermediate the ends of the latter and one at one side of and adjacent one side edge of the grid and the other at the opposite side of and adjacent the opposite side edge of the grid and each intermediate the ends of the grid, trough shaped parts of indentable insulating material fitting over the side edge of the grid, resistor wire wound about said grid and parts to form two oppositely Wound flattened inner helices one at each side of said posts, the turns of the two helices indenting said material and the adjacent end turns of the two helices being secured to one of said posts,

other trough shaped parts of indentable insulating material fitting over the rst mentioned parts and the adjacent portions of said turns, resistor wire wound about said body to form two oppositely wound flattened outer helices the turns of which are spaced away from the turns of the rst mentioned helices by and indent the last mentioned insulating material, and the adjacent end turns of the outer helices being Connected to the second of said posts, and other terminal posts secured to the opposite end portions of said grid to which the end turns of the helices remote from the first mentioned posts are connected.

FRANCES H. SHEPARD, Eecutrz'a: of the Last Will and Testament of James A. Shepard, Deceased.

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