US2619571A - Resistive device - Google Patents

Description

Nov. 25, 1952 c, 1, GANcl RESISTIVE DEVICE 2 SHEETS-SHEET l Filed Dec. 6, 1949 mf v KQ", Mg

Patented Nov. 25, 1952 RESISTIVE DEVICE Charles J. Ganci, Bellerose Manor,- N. Y., assignor to Ward Leonard Electric Company, a corporation of New York Application December 6, 1949, Serial No. 131,474

7 Claims. 1

This invention relates to resistive devices and is particularly applica-ble to rheostats and the like wherein the resistive conductor or conductors are embedded in insulating material on a flat support which may be of circular or polygonal form.

The main object of this invention is to increase the dissipation of power of resistive devices and thereby greatly increase their capacity; or in other words, to decrease the size of the resistive device necessary for a given power dissipation.

In variable resistive devices, such as rheostats, one of the difficulties is that the power dissipation or watts capacity is limited by the hot spot temperature of 250 C. or 300 C., although the rest of the device may be considerably under that temperature. One reason for the occurrence of hot spots is due to only part of the total resistance being utilized at times with a comparatively large current which localizes excessive generation of heat in the used portion. Another reason is due to the use of tapered resistors which results in some portions of the total resistors having a much greater power dissipation than other portions, a common ratio of current taper being five to one. A common form of rheostat has an external iron plate as its main support and although this aids in reducing the hot spot temperatures somewhat, it is of comparatively little help in that respect.

The main feature of this invention is to introduce within the resistive device an eflicient heat distributor properly positioned with reference to the resistive conductors and insulated therefrom. The heat distributor should preferably be made of metal of good heat conductivity, such as copper, aluminum, and various alloys, the particular metal or alloy selected being dependent upon the maximum temperature to which the device is subjected in the making and in its use. The distributor may assume various forms and shapes, such as a perforated or solid plate in the form of a disc or otherwise, or a wire mesh or expanded metal material may be used with advantageous results.

The accompanying drawings illustrate preferred embodiments of the invention. Fig. 1 is a plan View of a circular rheostat plate, partly broken away to show the ground coat and resistors applied thereon; Fig. 2 is a vertical section on the line 2-2 of Fig. 1; Figs. 3, 4, 5, 6 and 7 are similar vertical sections showing various modiiications; Fig. 8 is a plan view of a rectangular resistive device with projecting taps and partly broken away to show the ground coat and resistors connected to the taps; Fig. 9 is a vertical section on the line 9 9 of Fig. 8; and Figs. 10 and 11 are vertical sections similar to Fig. 9 showing modifications.

The rheostat shown in Figs. 1 and 2 is provided with the usual circular iron plate I having an upturned edge la. A central opening in the plate is occupied by a metal tube 2 welded or otherwise secured to the plate l and serves as a bearing for the mounting of the adjustable contact arm.

A heat distributor 3 in the form of a disc of good heat conducting metal is placed over the base plate and extends over nearly all of its inner surface and is spaced therefrom by small posts or blocks of insulating material, such as porcelain or glass cord, during the process of making. These spacing insulators may be cemented or otherwise secured to the base plate or to one side of the heat distributor 3. The distributor is perforated as shown in Fig. 2 with numerous openings 3a distributed over its full extent, as indicated by dotted lines in Fig. l. A ground coat 4 of insulating material is then applied over the heat distributor and over the base plate completely embedding and insulating the distributor from the base plate. The perforations in the heat distributor permit the insulating material to pass through and under the distributor so that no open spaces remain. This ground coat may be of vitreous enamel material or of various forms of cements or other mixtures and completely embeds and insulates the heat distributor and it provides an insulating covering over the distributor of desirable depth for insulating the resistive conductors therefrom. The ground coat is then hardened by ring in a furnace or by drying in accordance with the character of the insulating material used. The resistors 5, together with their contact buttons 6 to which they are electrically connected, are next placed in proper relative positions over the top of the matured ground coat. A further application of insulating material is then applied over the resistive conductors and lower portions of the contact buttons and this top layer is then matured by ring or drying according to the nature of the insulating material covering the resistors. This results in the resistive elements and the heat distributor being embedded in a solid adherent mass of insulating material 4 as indicated in Fig. 2.

The heat distributor is preferably made of copper which has high heat conductivity and it can also withstand high heating during the making of the rheostat and during its use without being distorted or melted, although other metal substances may be used in particular cases according to the method of making and use of the device. It serves to rapidly and efiiciently transfer the heat developed in any spot or portion of the resistive device to other portions having lower temperatures. It thereby prevents portions of the device from attaining the high temperatures which would otherwise be reached and gives a far more general dissipation of heat from the entire surface of the rheostat than could otherwise be obtained.

The preferred embedded insulating material is vitreous enamel in the ground and cover coats due to its Well known advantages. Where inorganic cements are used for the insulating material, they are treated in a known manner to produce a hard adherent mass by reacting with water, as in the case of hydraulic cements, or treated with chemicals as in the case of chemically reacting cements. In some cases where the temperature of ultimate use is less than 250 C., the insulating'ground coat on the metal base embedding the heat distributor may be formed of granules of organic or semi-organic resinous material, such as organo-silicone materials, and subjected to heat and pressure. After applying the resistors and contacts in proper positionthereon, more of such material is applied and subjected to heat and pressure to form the cover coat.

Fig. 3 is similar to Fig. 2 with the reference characters indicating corresponding parts; but in Fig. 3 the perforated metal heat distributor 3 is embedded in the insulating material above the resistive elements 5. In this case the copper distributor serves to very emciently distribute the heat. The metal base plate I also aids in such distribution when of the customary iron or steel. However, I have found "that'by making the base plate of copper, a much more efficient distribution of the heat is obtained; and when combined with a perforated copper distributor embedded in the insulation above the resistive conductors, the most advantageous result is obtained as regards the greatly increased capacity of the device.y

Experiments and test data have shown that the use of a copper base plate and copper heat distributor embedded within a vitreous enamel insulating material and having a 5 to l taper has resulted in doubling the watt capacity for the same temperature rise. Thus an S-inch diameter rheostat plate may-replace a l3-inch diameter plate for the same temperature rise or watts capacity. Tests also have shown that when only a portion of the resistive device is incircuit, the load imposed thereon may be very great because the heat generated in that portion is conducted rapidly by the heat distributor to other portions of the device not in circuit. Although the increase in capacity by this improvement is very marked in rheostats having a 5 to 1 current taper, it is of very considerable advantage in 1 to l current tapers, or equal ohms per step, with all resistance in circuit; and, of course, when only part of the resistors are in circuit they can be quite heavily loaded because the heat is rapidly conducted to cooler portions of the rheostat not in circuit. Copper has proved to be the most desirable metal for the heat distributor, but other metals, such as aluminum, alloys and other metals of high heat conductivity may be used. The greater the mass, the greater the heat conductivity of the distributor. The perforated plate is desirable not only for allowing the insulating material to pass through it and 'fill all spaces 4 during manufacture but because it is more yieldable under high heating during manufacture or use and less likely to crack or ake olf the insulating material due to diierences in the coe'icients of expansion, especially when copper is used with vitreous enamel insulation. Where the insulating material is of cements, or organic or semi-inorganic resinous material and the maximum temperatures. during manufacture or use are comparatively low, the metal distributor need not be perforated as after the resistors are placed on the ground coat, more insulating material may be applied, then the solid heat distributor placed in position and then the cover coat applied.

Fig. 4 is similar to Figs. 2 and 3 but shows the perforated heat distributors 3 both above and below the resistive conductors 5. This, of course, secures a more rapid and eiiicient distribution of heat than obtained in the case of either Figs. 2 or 3, although the advantage is not proportionally increased for the samesize and mass of the distributors. However, when it is desired to use distributors of small thickness and low mass, the structure of Fig. 4 is highly advantageous.

Fig. 5 is similar to Fig. i except the distributors 7 above and below the resistors 5 are shown in theform of a wire mesh of metal. Fig. 6 is likewise similar except the distributors 8 are shown in the form of expanded metal. These figures illustrate the wire range of choice in the character of the distributors according to particular requirements and conditions.

Fig. 7 shows a structure of a strong and sturdy character adapted to withstand pronounced mechanical shocks in any direction and also having high watt capacity for any given temperature rise. Fig. 7 is similar to Fig. 3 except in addition to the distributor 3 located above the resistive conductors, a copper plate 9 forms a lining for the inside of the iron plate i. The copper plate is rmly secured to the iron plate Vas by rivets indicated in Fig. 7 in the flat portion and in the upturned edges. The copper plate 9 being a much betterheat conductor than the iro-n plate if, serves to act e'iiciently as a heat distributor in cooperation with the plate 3. Good results are obtained even when the distributor 3 is omitted from this structure. Also in Figs. 2, 4, 5 and 6, the distributor below the resistors may sometimes be placed directly against the main base plate I andY good results obtained. In some cases the base plate itself may be made of copper or of other good heat conducting metal with advantageous results over the use of iron or sheet steel.

VResistive devices instead of being formed on a metal base are sometimes made with a base ofV insulating material such as of porcelain or of various organic or inorganic materials. Some forms of resistive devices instead of having a movable contact element mounted thereon for changing the amount of resistance in circuit are merely provided with taps from which variable connections are made for changing the amount of resistance in circuit. Figs. 8 and 9 show my improvement applied to devices embodying both of these features. These gures show a main base support It having a fiat portion of rectangular form with an upwardly extending edge lila, The insulating material embedding the resistive conductors 5 may be of any of the kinds already described depending on the process of making, the character of the insulating base and the intended use of the device. The taps I I connected to the resistive conductors 5 project above the insulating material for connection to the control circuits, a double row of such taps and resistors being shown. Fig. 9 shows the resistors 5 placed directly on the inside bottom of the insulating base I9 and shows the heat distributor 3 embedded in the insulating material 4 above and spaced somewhat from the resistors.

Fig. 10 is similar to Fig. 9 except it shows the heat distributor 3 placed directly against the inside botom of the base I and the resistive conductors 5 are positioned above the distributor and spaced therefrom and embedded in the insulating material 4. Fig. 11 is similar to Fig. 10 except a heat distributor 1, shown in the form of a metal screen, is embedded in the insulating material 4 and positioned above and spaced from the resistors. The structures of Figs. 8 to 11 may be made in diierent ways according to the character of the insulating material used, the temperatures in the process of making and maximum temperatures of the intended use, as described with reference to Figs. l to 7. Likewise the heat distributors may be formed in diierent ways, as already described, of good heat conducting metals such as copper, aluminum, brass and the like depending on the particular requirements.

It is evident that this invention may be embodied in various forms of structure and using resistive conductors connected to contact buttons, taps or other forms of terminals. Also various kinds of embedding insulating material may be used; and the embedded layers of metal which form the distributors may be of various forms and be positioned in various relationships to other parts of the resistive device and spaced from the resistors and covering an area approximately co-extensive in extent with that occupied by the resistors.

I claim:

l. A rheostat comprising a plurality of resistive conductors connected to a plurality of terminals respectively, a supporting base, a heat distributor of good heat conductivity in the form of a layer of metal spaced from said resistive conductors below said conductors and extending parallel thereto and approximately co-extensive therewith in extent of area, and insulating material embedding said resistive conductors and portions of said terminals and also embedding and serving as the sole insulation between said heat distributor and said conductors and terminals.

2. A rheostat comprising a supporting base, a heat distributor in the form of a layer of metal on said base, a plurality of resistive conductors connected to a plurality of terminals respectively and positioned above said heat distributor and spaced therefrom and approximately co-extensive therewith in extent of area, and insulating material embedding said resistive conductors and portions of said terminals and also covering and serving as the sole insulation between said heat distributor and said conductor and terminals.

3. A rheostat comprising a supporting base, a heat distributor in the form of a layer of metal on said base, a plurality of resistive conductors connected to a plurality of terminals respectively and positioned above said heat distributor and spaced therefrom and approximately coextensive therewith in extent of area, a second heat distributor in the form of a layer of metal above and spaced from said resistive conductors and extending parallel thereto and approximately co-extensive therewith in extent of area, and

insulating material embedding said resistive conductors and portions of said terminals and also covering and embedding said heat distributors.

4. A rheostat comprising an outer cup-shaped casing, a hard rigid disk-shaped insulating material in said casing, a resistor having resistive segments positioned side by side and spaced apart a distance substantially less than their length to form a, compact resistance unit embedded within said insulating material, terminals in electrical contact with said resistor and projecting outside of said material to provide external contacts to said resistor, and a metallic heat distributor of good heat conductivity in the form of a layer embedded in said insulating material and substantially parallel to and electrically spaced from said segments, said distributor being approximately coextensive in extent of area with said segments to receive heat therefrom and to distribute concentrated heat at one portion of said resistor throughout the insulating material.

5. A rheostat comprising a rigid insulating member, a resistor having resistive segments positioned side by side and spaced apart a distance substantially less than their length to form a compact resistance unit embedded within said insulating member, end and intermediate terminals in electrical contact with segments of said resistor and projecting outside of said insulating member to provide external contacts to said resistor, and a metallic heat distributor of good heat conductivity in the form of a layer embedded in said insulating member and substantially parallel to and electrically spaced from said segments, said distributor being approximately coextensive with and having a high thermal coupling with said segments to receive heat therefrom and for distributing heat concentrated at one portion of said resistor.

6. A rheostat comprising a rigid insulating member, a resistor having resistive segments positioned side by side and spaced apart a distance substantially less than their length to form a compact resistance unit embedded within said insulating member, end and intermediate terminals in electrical contact with segments of said resistor and projecting outside of said insulating member to provide external contacts to said resistor, and a metallic heat distributor of good heat conductivity in the form of a layer embedded in said insulating member and electrically spaced from said segments, said distributor extending substantially parallel to said segments and in a heat exchange relationship with said resistor to receive heat concentrated at tapped segments for increasing the rate of dissipation from said tapped segments and spreading the heat in said insulating member for wider dissipation.

7. A rheostat comprising a rigid insulating member, a resistor having resistive segments positioned side by side and spaced apart a distance substantially less than their length to form a compact resistance unit embedded within said insulating member, end and intermediate terminals in electrical contact with segments of said resistor and projecting outside of said insulating member on the same side of and at an angle to said resistor to provide external contacts to said resistor, and a metallic heat distributor of good heat conductivity in the form of a layer embedded in said insulating member on the projecting side of said terminals and electrically spaced from said segments, said distributor extending substantially parallel to said segments and in a heat exchange relationship with said resistor to receive heat concentrated at tapped segments for increasing the rate of dissipation from said tapped segments and spreading the heat in said insulating memberifor Wider dissipation.

CHARLES J. GANCI.

' REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Number Name Date Wiegand July 25, 1916 Howard Sept. 17, 1940 Mann Nov. 1, 1949 FOREIGN PATENTS Country Date Germany Mar. 3, 1938

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