US2106060A - Electric cable - Google Patents

Description

1938' i J. K. OSTRANDER 2,106,060

ELECTRIC CABLE Filed 001:. l, 1935 ImsuLa-Zn n06 bonded i0 cenier conducior 0 I Inventor:

John K. Ostrancler;

Patented Jan. 18, 1938 UNITED STATES PATENT OFFICE 7 Claims.

It is the practice to utilize distribution transformers to supply electric current to both urban and rural customers, the primary voltage of which may be of the order of from 2300 to 6600 volts. The conductors used to supply turrent to these transformers are rarely insulated and generally are composed of two bare copper wires for each single phase circuit. Bare or uninsulated conductors are used because they cost less than well insulated conductors. However, systems of distribution having bare conductors operating at line potential are subject to frequent interruptions due to external causes. This is a serious defect in any case but more especially so in a rural system of distribution where the distance to the fault may be considerable.

An object of my invention is the provision of an insulated cable of improved construction which may economically be used in place of bare conductors for systems of distribution, and while especially intended for such use is capable of wider application. My invention is also directed to an improved method of constructing conductors and cables.

For a consideration of what I believe to be novel and my invention, attention is directed to the accompanying description and the claims appended thereto.

In the accompanying drawing which is illustrative of my invention, Fig. 1 is a View in side elevation on an enlarged scale of a piece oi finished cable; Fig. 2 is an end view of the same; Fig. 3 is an end view illustrating the arrangement of the outer conductor prior to the step of compacting it about the insulation on the inner conductor; Fig. 4 is a diagrammatic illus tration of a machine for applying the outer con-- cluster, and Fig. 5 shows a separator between the center conductor and the insulation.

5 indicates the center conductor which may be of solid annealed copper, but which is desirably composed of arelatively large number of copper strands to increase the flexibility oi the finished cable. Surrounding the inner conductor is a body of insulation 0 which may he of any suitable material such as rubber or it may be made of synthetic material. It is desirable, regardless of the kind of insulation employed, that it be not securely bonded to the conductor since due totemperature changes there may be a difference in the ratios of expansion of the inner and outer conductors. To prevent the insulatlon from adhering to the conductor the latter may be suitably lubricated before the in sulation is applied by extrusion or other method, or a separator may be used.

The outer conductor is made of two different metals, one of which has high tensile strength and is hard and the other of high conductivity and soft. For example I may use galvanized steel wires and soft copper wires although other metals or combination of metals may be used. Of these outer wires, those of copper are intended to carry the current and for that reason their total cross section should be approximately the same as the center conductor. The steel wires may also carry some of the current since they are in direct electrical contact with the copper wires. Because of the heavy tension strain to which the steel wires are subjected when the cable is strung on poles, their total cross section is made greater than that of the copper wires. Both the steel and copper wires are grounded. This may conveniently be done at the poles. For a short length, one well established ground will .be suflicient, while for longer lengths, additional grounds may be provided. The wires made of hard metal are of round cross-section, while those of the soft metal are initially of square or rectangular section, the shape of which is subsequently changed. In the present illustration, 1 indicatesthe steel wires and 8 the copper wires, said wires being applied with a relatively long lay or spiral over the insulation. In the present illustration, ten wires of each kind are employed, but a greater or less number may be used for other applications of the invention. The soft wires as initially formed, have smooth flat side surfaces s and ill, Fig. 3. The outer and inner edges may be slightly rounded if desired to avoid sharp corners. The wires are arranged in alternation, first a steel wire and then a copper wire. As originally assembled about the insulation on the inner conductor they form a tube-like struccure or enclosure, the inside diameter of which is somewhat larger than the outside diameter of the insulation. This is due to the fact that at this stage in the manufacture, the sides of the copper wires are flat and smooth. For ex ample, with a center conductor of a size corresponding to a No. 8 A. W. Chanel an outside diameter for the insulation of .346 of an inch, the inside diameter of the tubular structure will be of the order oi one-eighth of an inch larger than that of the insulation. This is illustrated on an enlarged scale in Fig. In Fig. 5 is shown a separator it between the center conductor 5 and the surrounding insulation 6. As previously indicated, this separator may be in ill the form of a lubricant or other material, the purpose being to prevent bonding of the insulation to the conductor.

In the manufacture of the cable the inner conductor is first insulated in any suitable way, as for example, by extruding or otherwise applying a covering of rubber or equivalent material over it. The wire so insulated is then inserted in a machine, for example, a stranding machine, for applying the outer conductor and pulled longitudinally through it by a capstan ll or other means. The machine is provided with wire carrying bobbins l2 which are mounted as usual on a rotatable support and as the insulated wire is pulled through the machine, the rotating bobbins supply the steel and copper wires. To properly guide the individual wires a. tubular conical guide [3 is provided through which the insulated wire moves. The guide has as many grooves M as there are steel and copper wires and as the wires leave the guide they are in contact. Due to the fact that the insulated wire moves longitudinally and the outer wires rotate about it, the steel and copper wires are given the necessary lay or spiral. The pitch of the spiral depends of course on the relative speeds of rotation of the bobbins and the longitudinal speed of the insulated wire. In this part of the operation the steel wires are stranded without any twist while the copper or conducting wires are stranded with a twist, the reason being that for the wires to properly assemble into a structure of tubular shape the rectangular copper wires must always occupy positions which are substantially radial to the axis of the center conductor.' The particular means for removing the twist which would naturally take place in the steel wires and for twisting the copper wires is not illustrated since machines capable of performing these functions are well known to cable manufacturers.

Having formed the tubular structure the next step is to make it into a self-supporting str'ucture and one in which the alternate wires are interlocked. This is done by pulling the assembly through a die l5 having a well rounded entrance and an exit portion of a diameter sufficiently smaller than the entrance to force the round steel wires into the sides of the rectangular copper wires and finish the cable to size. Other forms of dies may be used if desired. The effect of the foregoing is that seats are formed in the opposite side walls of each of the rectangular soft wires and in which the hard wires are located in firm engagement with the walls of the seats, the said hard wires and the soft wires being interlocked.

Stated briefly, the hard round wires deform the soft copper wires and hold them in their deformed condition, the effect of which is to securely interlock them and because the compression pressure exerted by the hard wires is so great, access of foreign matter to the insulation is fully prevented. The compression or deformation of the copper wires reduces the inside diameter of the annulus formed by the steel and copper wire enclosure to a point where it corresponds to the outside dimeter of the insulation. These wires should be in sumciently firm contact with the insulation to prevent void formation for such voids would give rise to corona effect if the potential of the current conveyed was sufficiently high.

A cable constructed as herein described having thedimensions heretofore given and an outside diameter ofone-half of an inch remarkable strength. A piece of such cable approximately 36 inches long was firmly heldby clamps afiixed at its ends. From the center of the cable by means of a support or shoe covering four inches of the cable was suspended a weight of 1500 pounds. Upon dissecting the cable it was found that notwithstanding the great weight applied, there was no visible indication of injury to the insulation.

An outstanding advantage of my improved cable resides in the fact that it insures continuity of service. This is due to the concentric arrangement of the inner and outer wires, the latter rendering it practically impossible for a short circuit to occur due to external disturbances unless the cable is actually broken which on account of the steel wires is not liable to happen. Because the outer conductor is at ground potential, there is no danger to life in case of accidental contact, and suspension insulators are unnecessary. Being at ground potential, the cable may be strung quite close to the ground and thus shorter supporting poles may be used, and even if the cable should fall from its supports to the ground, the service would not be interrupted.

For urban lines, the conductors required may be considerably larger than for rural lines. According to prior practice, if bare and larger than No. 4 wire is used, the reactance of the circuit will be quite high compared to the resistance thereof. With my concentric cable on the other hand, the reactance is kept low and therefore the voltage regulation of the system remains good. Also, it is possible due to the lower reactance to eliminate some of the expensive voltage regulating and circuit recloslng devices now required for bare wire circuits. The concentric arrangement of the wires of the cable is also effective in eliminating inductive interference with adjacent telephone wires, thus making it possible to support both cable and wires on the same poles. It also reduces radio interference. The spiral arrangement of the wires forming the armor in addition to forming a tight enclosure for the inner wire and its insulation affords the necessary flexibility without impairing the tensile strength of the cable as a whole with the result that widely spaced poles may be employed for supporting it without employing messenger cables. Although intended primarily for overhead work, the cable may be located underground. In such case, if the character of the soil is of a. nature adversely to affect the wires of the outside conductor, the cable may be wrapped with materials commonly used as protective means in such case.

An advantage of my improved construction from a manufacturing cost standpoint resides in the fact that both the steel and copper wires are of conventional shapes and therefore do not require any special dies in their manufacture. By using hard round wires and relatively thin edgewise positioned soft wires of rectangular section as disclosed, it is apparent that neither of them requires any preforming which would appreciably increase their costs. A further advantage of my improved construction resides in the fact that by simultaneously pressing all of the wires radially inward, ssby passing the cable through a. properly sized die, the armoring enclosure can be made to exactly fit theinsulation, or if desired, it may be caused to make a. tight or a loose fit therewith, which ever is desired for a particular purpose. It will also be observed that with myimproved construction both the hard round wires and the rectangular wires directly engage the intherefor. By exerting sufficient external pressure on the wires as they pass through the die, the insulation will be caused to flow slightly and thus completely fill any small spaces which otherwise might exist,

A further advantage resulting from the use of hard round wires and soft rectangular wires from a manufacturing standpoint resides in the fact that the flat sides of the softer wires temporarily act as guides for the round wires until the assembly enters the die, and upon entering they assist in preventing one or more of the round wires from being forced radially inward to a greater extent than the others. In brief, the combination is such that all of the wires are simultaneously moved radially inward and by equal amounts.

As previously stated, the round hard wires are so applied that they are free of all torsional or twisting stresses, the advantage of which resides in the fact that the ends of the wires when out have no tendency to open up or splay. As previously stated, the wires of rectangular section are twisted as they are applied to cause-them to occupy definite radial positions but because they are initially soft, there is little or no tendency for them to open up when the wires are cut. Such tendency as may exist is overcome by the pressure exerted by the adjacent hard wires which effect removal of internal stresses. In brief, the construction is such that when the wires are out, they all remain in firm contact with each other;

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric cable comprising an inner conductor, a body of insulation therefor, and a second conductor surrounding the insulation and forming a protective armor therefor, said conductor comprising sets of bare conductors, the metal of one set being hard, untwisted and of circular crosssection, and that of the other set being soft and of rectangular cross-section, the total cross section of all of the hard wires being greater than that of the soft Wires, all of said wires being spirally wrapped around the insulation with the soft wires so twisted as to occupy radial positions between each two hard wires, the side walls of the soft wires having shallow seats formed therein by sidewise pressure on the hard wires and in which they are permanently located, the hard and soft wires being spirally wrapped and interlocked with the round wires making tight fits with the seats throughout their length.

, 2. An electric 'cable comprising an inner conductor, a body of insulation surrounding the conductor, and a second conductor surrounding the insulation and forming a protective armor therefor which engages the insulation to prevent corona effect, said armor comprising hard untwisted round wires of high tensile strength and soft twisted wires of rectangular section and low tensile strength and high conductivity which have seats in the side walls thereof formed by exerting inward radial pressure simultaneously on all of said wires, the wires being spirally arranged in alternation, each pair of round wires being located in the seats on opposite sides of each of the rectangular wires, said hard wires holding the soft wires under compression.

3. The method of making cable which comprises insulating a conductor, forming a tubular enclosure around the so insulated conductor of spirally disposed hard wires of round section and soft wires of initially rectangular section arranged in alternation, the insulation and the inner wall of the enclosure being in spaced relation, and applying sufficient external radial pressure simultaneously and uniformly to cause the hard wires to deform the sides of the soft wires and hold them permanently in the deformed condition and also to cause the wires to contact with the insulation on the inner conductor.

4. The method of making cable which comprises insulating a conductor, enclosing the insulated conductor in a tubular enclosure comprising spirally arranged hard round wires and soft current carrying wires having fiat sides and radially disposed, said enclosure initially having an internal diameter greater than the outside diameter of the insulation, passing the insulated conductor and the enclosure through a die to simultaneously press the hard round wires into the flat sides of the soft wires to form curved recesses therein of a shape and size corresponding to the adjacent surfaces of the round wires and also to reduce the internal diameter of the tubular enclosure and cause the inner surfaces of both hard and soft wires to-engage the insulation.

5. The method of making a conductor whic comprises spirally disposing untwisted hard wires of round section and radially disposed soft twisted wires of initially rectangular section in alternation to form an annular body with the soft wires occupying radial positions, and applying sufficient external radial pressure simultaneously and uniformly on all of the wires to decrease the diameter of the annular body and cause the hard wires to deform the sides of the soft wires and hold them permanently in the deformed condition.

6. An electric cable-comprising an inner conductor, a covering of insulation therefor, a second conductor surrounding the first and forming a protective armor therefor, said second conductor comprising hard metal destressed wires of circular section and bare soft wires of rectangular section occupying radial positions between the hard wires and having shallow seats in the sides thereof formed by sidewise pressure of the hard wires and conforming in shape and size to the portions of the hard wires seated therein, said rectangular wires being free of internal mechanical stresses tending to alter their relative positions with'respect to the hard round wires, to the end that the hard and soft wires will not separate when the cable is bent or when out, the hard and soft wires being spirally arranged and forming an annulus.

7. An electrical conductor of annular form comprising spirally arranged hard non-twisted wires of circular cross-section having a smooth peripheral surface and high tensile strength and soft wires of low tensile strength and rectangular cross-section twisted from end to end so as to occupy radial positions throughout their length, the hard and soft wires being arranged in alternation to form a hollow annulus, the soft wires having shallow seats in their sides formed therein by sidewise pressure exerted by the hard wires and in which the hard wires are firmly seated throughout their length, said conductor being characterized by the fact that when the conductor is out, the hard and soft wires preserve the shape and diameter of the annulus and remain JOHN K. OSTRANDER.

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