US2971175A

US2971175A - Tubular electric resistors and rheostats

Info

Publication number: US2971175A
Application number: US823582A
Authority: US
Inventors: Abrahamson Edmund
Original assignee: Individual
Current assignee: Individual
Priority date: 1959-06-29
Filing date: 1959-06-29
Publication date: 1961-02-07
Anticipated expiration: 1978-02-07

Description

Feb. 7, 1961 E. ABRAHAMSON TUBULAR ELECTRIC RESISTORS AND RHEOSTATS Filed June 29, 1959 FIG.

FIG. 3

24 *J I M FIG 43 FIG. 9

/ EDMUN D ABRAHAMSON 5 FIG. 7

k PATENT AGENT tats TUBULAR ELECTREC RESETQRQ AND RHEGSTATS Filed June 29, 195?, Ser. No. 823,582

8 Claims. (Ci. 533-27*9} The present invention relates to electrical resistances, such as resistors and rheostats and, more particularly, to tubular resistors and rheostats comprising an insulating tube, on which the actual electric resistance means ishelically wound in form of a wire or ribbon. The resistance values of these resistors or rheostats can be adjusted or set by means or" a slidable brush or an adjustable endband.

In the operation of such electrical resistors or rheostats in which heretofore the insulating tubes have had substantially uniform cross sections over their lengths, it has been found that the turns of the resistance wires tend to burn out close to or toward one end of the winding because either the movable slide brush or endband is frequently set in these resistors or rheostats in such a manner, that only a few turns of the winding are inserted in the electric circuit, while no current will flow through the remaining turns. As a result of this kind of operation, the few resistance wire turns in the circuit are relatively highly heated, because the total resistance of the circuit is low when the larger part of the resistors or rheostats is not inserted in the circuit and the amount of heat generated in these few resistance turns is a function of the square of the current intensity times the resistance value inserted in the circuit.

It is an object of the present invention to improve the thermal condition or heat dissipation in a resistor or rheostat of the tubular type, i.e., to distribute the heat developed in the end turns of such resistance more evenly throughout the entire apparatus when on y these end resistance turns are inserted in the electric circuit.

It is another object of this invention to provide as supporting insulating tube for the turns or winding of the resistance wire a tube having an inner diameter which is larger at that end of the tube where the turns of the resistance wire are Wound, said turns being inserted either permanently or for a longer period of the operation in the circuit to be controlled by the resistance.

It is a further object of the invention to provide an insulating tube with a tapering inner cross section, whereby the turns of the wire winding remaining permanently or for a longer period in the circuit are at the end with the smaller internal opening of the tube.

It is a sti l further object of the invention to provide a tube as support for the wire winding of the resistor or rheostat, said tube having at least two different inner diameters, i. e., comprising sections with at least two diiferent diameters, of which the smallest one is at the end carrying the turns of the wire winding remaining permanently or for a longer period of the operation in the electric circuit.

It is another object of the invention to divide the insulating tubes along their lengths into two parts which can be manufactured more economically and then put them together prior to the winding of the wire on the tube.

Still further objects and the entire scope of applicabi ity of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific atent examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

in the drawings:

Figure 1 shows schematically a circuit including a rheostat according to the present invention;

Figure 2 is a longitudinal section through an insulating tube according to the present invention taken along the line 2-2 of Figure 3 and looking in the direction of the arrows;

Figure 3 is a top view of the tube of Figure 2;

Figure 4 shows schematically a circuit including a potentiometer in form of a rheostat or resistor according to the present invention;

Figure 5 shows a top view of a modified insulating tube which is divided into two halves along its length;

Fi ure 6 shows a perspective view of the tube of Fi ure 5 with endbands assembled on their ends ready to re wound with resistance wire;

Figure 7 shows a top view of a modified insulating u e similar to that shown in Figures 5 and 6;

i are 8 shows a longitudinal section through another modified tube according to the invention;

i "gure 9 illustrates a longitudinal section through an insulating tube according to the present invention, said tube being a modification of that of Figure 8.

Referring in detail to the drawings, Figure 1 shows a rheostat it inserted in an electric circuit comprising a battery 12 and a load 12. The rheostat 1.0 has a helical resistance winding 13 supported on an insulating tube 14, for example of porcelain. While one terminal of the load 12 is connected to the right end of the resistance wire, the one terminal of the battery 11 is connected to a slide arm or brush 15 frictionally engaging the one or the other turn of the resistance wire 13 on the tube 14 of the rheostat ill and the other terminal of the battery H is connected to the other terminal of the load 12, whereby the desired current intensity value in the load circuit can be adjusted by changing the position of the slide brush 15 along the tube 14. Assuming that during operation this slide brush 15 remains closer to the right end of the rheostat 1%, i.e., that only a part of the rheostat is inserted in the circuit, as it is very often true in such load circuits, then the resistance wire turns on the right end of the rheostat 10 will be subjected to a greater wear than the remaining turns of the rheostat, due to the unfavorable heat dissipation and distribution in the rheostat. According to this invention, the insulating tube on which the resistance wire is wound is conically tapering, i.e., the interior cross section is increasing towards the end of greater thermal stress, so that the heat produced in the turns of the resistance wire can be better dissipated by the insulating tube 14, i.e., better absorbed by the larger cross section of the same at the right portion thereof.

Figures 2 and 3 show the actual design of such rheostat tube it having at the end of the smallest inner diameter, i.e., the largest cross section, a flare 16 necessary or desirable in view of the brittleness of ceramic material, such as porcelain, mostly used for such tubes.

Fi ure 4 shows a similar circuit as that illustrated in Figure 1. In this case, a rheostat 2G is inserted in a circuit and used as potentiometer or voltage divider, the helical resistance winding 21 of which with its two ends 22. and 23 being permanently connected to the terminals of a battery 24. Potentiometer taps 25 and 26 along the resistance winding 21 are connected to a low current load 27 and a high current load 28, respectively, the other terminals of said loads being connected to one of the battery terminals. This kind of potentiometer used for tapping ofi currents of controlled intensities has been known per se.

As shown in Figure4, the insulating tube 14 is provided as in Figure 1 with an interiorly tapering cross section to improve the heat dissipation, i.e., to provide the largest cross section of the tube at the place of highest heat development, i.e., on the right side thereof.

In view of difliculties in the manufacture of integral tubes having varying. cross sections over their lengths, these tubes are made, according to the invention, of two equal parts or

halves

30 and 31, as shown in Figures and 6, with the dividing lines along the length of these tubes. Each of the engaging surfaces of the halves 3t) and 31 of these tubes has a projection 32 on one side and a notch or recess 33 on the other side, whereby opposite projections and notches engage one another, so that the tubes after being assembled are securely held together when clamped by endbands 34 for the resistance wire at the ends of the tubes, as shown in Figure 6. The resistance wire a (not shown) is then wound on the tube thus assembled and clamped together by the endbands 34, as shown in Figure 6. As the two

halves

30 and 31 are identical in-shape, they can be formed or molded in the same mold. In other words, only a single half-shell mold is necessary for the manufacture of the two halves.

Figure 7 shows a'somewhat different configuration of the engaging surfaces of the parts or

halves

36 and 37 of the tube, said parts or halves fitting together. In this embodiment, the interengaging surfaces of the

tube halves

36 and 37 form steps 38.

Instead of providing a tapering cross section, it is possible to design the tubes according to the invention with different cross sections along their lengths, for example, as shown in Figure 8. In this case, the right end 39 has a larger cross section than the remaining portion 40 of the tube. The turns of the resistance winding more frequently used will be located on the right end 39 of the tube, as shown in Figure 8. Thus, the heat produced in the resistance wire turns at this end will be more readily dissipated.

According to the embodiment of Figure 9, the insulating tube has three different cross sections starting from a largest 41 on the right side to a smallest 43 on the left side with an intermediate 42 therebetween. Such tube will be advantageously used in case where the turns of the resistance wire mostly used in the circuit are extending from the right end to the center of the tube.

The heat distribution will be further improved by mounting the resistor or rheostat with the porcelain tube substantially arranged in vertical position, as shown in Figure 2, whereby the end of the tube with the larger cross section is facing downwardly. As a result of this, heat is removed by the air current flowing through the interior of the tube in a superior manner, due to the chimney efiect.

I claim:

1. An electric resistance comprising an insulating tubular supporting member, a helical winding of resistance wire on said tubular supporting member, means to adjustably connect the turns of said resistance wire to an electric circuit, the external diameter of said tubular member remaining substantially constant throughout the length of said member and the internal diameter of said tubular member varying in cross section throughout the length of said tubular member to impart to said tubular members a greater wall thickness at one end than at the other.

2. An electric resistance according to claim 1, wherein said tubular member is internally tapered towards said one end.

3. An electric resistance according to claim 1, wherein said insulating tubular member has at least two different internal diameters defining sections each of which has substantially the same wall thickness throughout its respective length.

4. An electric resistance according to claim 1, wherein said insulating tubular member is divided along its length in two halves fitting together.

5. An electric resistance according to claim 4, wherein the engaging surfaces of said two halves are respectively provided with projections and recesses interengaging one another.

6. An electric resistance according to claim 5, wherein one of said surfaces of each half has a projection and the other surface has a recess.

7. An electric resistance according to claim 4, wherein steps are formed on the engaging surfaces of said two halves, said steps respectively interengaging one another.

8. In an electric load circuit including a source of current and a load, an electric resistance comprising a tubular supporting member, a helical winding of resistance wire on said members, means to adjustably connect the turns of said resistance wire into said load circuit,

the external diameter of said tubular member remaining substantially constant throughout the length of said member and the internal diameter of said tubular supporting member varying in cross section throughout the length of said tubular member to impart to said tubular member a greater wall thickness at one end than at the other, the portion of said tubular member with the greatest wall thickness supporting the resistance wire turns which are inserted in said load circuit during the longest period of operation.

References Cited in the file of this patent UNITED STATES PATENTS