Electrical Conductor For Superconductive Windings Or Switching Paths

Abstract

An electrical conductor formed with a plurality of electrically insulated d transposed superconductor filaments braided together along the axial length thereof. Each filament is a thin flexible glass capillary filled with superconductive material. These capillaries are formed by heating and drawing a glass tube containing the superconductive material which has a melting temperature below that of the softening temperature of the glass. During the braiding process, filaments, of other materials, can be worked in with the capillaries and a hardenable material, such as casting resin, can be applied to the intermixed filaments and capillaries. The other filaments can be removed from the mass by a suitable process to provide a number of interconnected cavities in close proximity to the capillaries for passage of a cooling medium. The electrical or multiple-core conductor can also be positioned within a sheath in such a manner that passageways for cooling medium extend alongside the outer surface of the electrical conductor. The electrical conductors can be wound to produce windings or switching paths in which passageways for a cooling medium are provided.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an electrical conductor for superconductive windings or switching paths comprising a number of electrically insulated and transposed superconductors.

It is known that losses occur in superconductors when a change takes place in the current flowing through the conductor or when a magnetic field acts on the conductor. Among these losses are the usual hysteresis losses which are proportional to the rate of change of the magnetic field (B) and the diameter of the superconductor. In conductors which also contain normally conductive metal, there are also eddy current losses which are proportional to B.sup.2 and the square of the entire conductor diameter.

To reduce the above-mentioned losses, electrical conductors which contain superconductive material are constructed as so-called multiple-core conductors when they are to be used for carrying currents which vary during the course of time or are influenced by magnetic fields. These multiple-core conductors contain a plurality of twisted super-conductor filaments (thin, thread-like wires) which are enclosed in a matrix of normally conductive metal. Such multiple-core conductors are substantially cheaper than conductor cables in which the individual conductors consist of thin superconductive wires, since it is very difficult to produce thin wires of superconductive material, e.g., by drawing.

Multiple-core conductors, however, exhibit additional hysteresis losses which are proportional to the rate of change of the electrical current (I), where I represents the current in the entire superconductive electric conductor. These losses, which are caused by the field of the multiple-core conductor itself, result from the entry of the current into the interior filaments and therefore they can not be reduced by twisting the multiple-core conductor. Furthermore, these losses can not be reduced to any substantial degree even if the electrical conductor is constructed of a plurality of multiple-core conductors which are twisted or braided together in the manner of a high frequency litz wire, as disclosed in the German laid-open patent application No. 1,908,885.

However, losses in multiple-core conductors can be reduced by "transposing" the individual filaments. Transposing means that the superconductive filaments along the conductor are arranged in such a manner that for a certain length they equally occupy each possible position within the conductor cross section. The transposing may be accomplished in several different ways as for example:

a. by braiding the filaments in the manner of a high frequency litz wire; e.g., three veins form a braid. Three such braids form a braid of the second order, etc.;

b. the conductor consists of a single-layer of filaments twisted in the form of a cable. The conductor can also be in the form of a flat single-layer of twisted filaments;

c. the conductor consists of a plurality of individual cables formed as in (b) and then twisted in the form of a cable.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electrical conductor for superconductive windings or switching paths in which the losses produced by variations in the currents or from magnetic fields are substantially less than with the known electrical conductors of the multiple-core type.

This is accomplished according to the present invention in that the insulated or electrically separated filaments of an electrical multiple-core conductor of the above-mentioned type each includes a thin and flexible glass capillary which is filled with a superconductive material whose melting temperature lies below the softening temperature of glass.

A preferred method for producing such a conductor includes filling a glass tube with a molten superconductive material, melting off at least one end of the tube, heating this end of the tube to the softening temperature of the glass and then drawing the tube into a fine filament. The thin conductive filaments thus produced are then further processed in a conventional manner to form an electrical conductor which may have, for example, a circular or rectangular cross section.

The conductors, according to the present invention, exhibit particularly low hysteresis losses when they are used for fluctuating currents (alternating currents, pulsating currents) and for magnetic fields. Their critical field strength is limited substantially by the superconductor material employed since, with the attainable capillary diameters, barrier layer effects do not yet play a significant part.

The conductors are extremely well suited for switching elements (cryotrons) which operate with stray field strengths below 20 kOe. The stray fields may be appropriately conducted in magnetic circuits with iron cores and may be used for heavy current cryotrons as they are required, for example, in the construction of a commutator-free d.c. machine according to Swiss Pat. No. 461,654. (In heavy current cryotrons the capillaries can be filled with pure lead or with a low-alloy lead to offer the advantage of particularly low stray field strengths.)

For use in superconductive rectifiers, protective switches and coil short-cicuiters, the superconductors according to the present invention are also of significance and are superior to the known conductors because of their low losses.

In a.c. cables the superconductor, according to the present invention, in the form of braided bands on carriers which form concentric hollow conductors, offers an advantage over soft superconductor layers because the magnetic surface field strengths and the area current density can be selected higher so that the cable diameter and the influx of heat from the outside are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the production of an insulated filament for an electrical conductor according to the present invention.

FIG. 2 is a side view of two layers of a cut-open multilayer coil or winding formed from an electrical conductor according to one embodiment of the present invention.

FIG. 3 is a cross-sectional view of a part of the coil taken along the line 3--3 of FIG. 2.

FIG. 4 is a longitudinal section of an electrical conductor enclosed in a sheath according to another embodiment of the present invention.

FIG. 5 is a cross-sectional view taken along the line 5--5 of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically indicates the production of a thin insulated wire which can be used for the filaments of an electrical multiple-core conductor according to the present invention. In order to produce such thin filaments, a glass tube 12 is used which is filled with a material 10 which is super conductive at low temperatures and whose melting point is lower than the softening temperature of the glass of which the tube is made. Suitable superconductive materials are, for example, metals such as lead which is a soft superconductor or alloys such as lead-bismuth alloys which constitute hard superconductors.

The glass tube 12, filled with the superconductive material 10, is heated until the glass softens and is drawn to form a filament which is as thin as possible. To achieve this, the filled glass tube 12 may be passed, by means of rollers 13 for example, through an electrical tubular furnace, the windings of which are shown generally at 14, or through a sequence of annular burners not shown.

The filled glass tube in a softened condition is then drawn down to a thin filament by any desired known means shown in box form at 15.

The drawn thin filament indicated by the dashed lines and the general reference numeral 15' in which the ratio of diameter of the metal core to the thickness of the glass wall is practically the same as in the original filled tube 10, 12, is wound onto a supply reel 16.

Many such thin filaments are than braided into a litz wire or processed to form transposed cables, or a braided tape. When using several apparatuses of the type shown in FIG. 1, the cable can be made without prior reeling.

The finished multiple-core conductor, in the shape of cables or tape, contain a relatively large number of interstices between the individual filaments which can be filled, for purposes of cooling, with a coolant, e.g., liquid or supercritical helium. In operation the coolant can be continuously exchanged or it can be recooled.

Since the danger exists that the individual filaments of the electrical conductor will move with respect to one another under the influence of the changing magnetic forces and thus produce frictional heat, it may be advisable, under certain circumstances, to fix the individual filaments with respect to their positions in the conductor. This can be done without any significant impairment of the cooling effect in the following described manner.

When the thin filaments are worked into a multiple-core conductor in the shape of a cable or a band, filaments or bands of a removable material are also worked in. The removable material may be, for example, easily meltable, thermally or chemically decomposable, or soluble in a solvent which does not attack the thin filaments of the conductor.

When producing a winding or coil, from the multiple-core conductor, filaments or bands of such removable material may again be worked in. The finished arrangement, e.g., a coil or winding is then saturated with casting resin, e.g., an epoxy resin and subsequently the removable material is removed in a manner which does not impair the cast resin and the winding, e.g., by heating or dissolving. This then produces interconnected cavities through which the coolant can penetrate. The entire coil, however, forms a solid block in which the thin filaments of the multiple-core conductor are held in position with respect to one another.

FIGS. 2 and 3 show a schematic representation of a longidudinal section or cross-sectional view of a part of two turns of a coil which is made according to this principle. The electrical multiple-core conductors, according to the present invention, are generally indicated at 18, 19, 20 and 21. They are substantially square in cross section and consist of a plurality of braided thin filaments 15' in the form of a glass capillary which is filled with a superconductive material and produced according to the method previously described. Threads of a removable material are braided into the multiple-core conductors, which threads are shown as the black circles in FIG. 3 and indicated by the reference numeral 23. The individual conductors 18, 19, 20 and 21 are enclosed in tapes 22 of a removable material. The insulation layer 24 is correspondingly constructed. When a coil of the type shown in FIGS. 2 and 3 is encased in a cast resin (not shown) and the removable threads and tapes are removed, cavities remain in the conductor cables, at the points shown as black circles in FIG. 3, through which the coolant can flow.

Another way to substantially prevent mutual movement of the fine filaments of the electrical conductor, according to the invention, involves filling the conductor formed from the fine filaments, at axial intervals, with a kneadable or very viscous hardenable material, i.e., an adhesive or a viscous casting resin, while in the interstices therebetween, the original cavities between the filaments of the generally rather loose conductor remain free. An arrangement of this type is shown in FIGS. 4 and 5. Preferably the multiple-core conductor 25 is encased in a sheath 27 which is, for example, square in cross section, and in which the spaces for the passage of a coolant are left free.

As seen in FIG. 4, the sections of the multiple-core conductor 25 which are filled with adhesive are marked 26, the conductor being accommodated in a sheath 27 which is square or rectangular in cross section. In the free corners 30 the coolant can flow through the sheath 27 and in the areas not filled with adhesive 26 it can radially penetrate into the multiple-core conductor 25. The adhesive in areas 26 of course only fills the multiple-core conductor in the shape of a cable but not the corners 30.

If desired, the areas of the multiple-core conductor 25 provided with adhesive 26 may be made somewhat permeable by the use of removable threads as described in connection with FIGS. 2 and 3. This is shown in FIG. 5 by means of the black circles that indicate cavities.

The above-described measures for fixing the position of the thin filaments within a multiple-core conductor may also be applied with advantage in multiple-core conductors which contain other types of filaments than those described here.

The connection of the electrical multiple-core conductors according to the present invention with identical conductors, other types of superconductors, or normally conductive leads can be effected by dividing the multiple-core conductor in the shape of a cable or band, over a certain length, into the individual capillaries. After this has been done the glass covers on the capillaries can be smashed, e.g., ultrasonically, and removed. The remaining superconductive elements are then soldered to corresponding elements. This operation should be done in a room or area which is shielded from magnetic fields and is also well cooled.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Claims

I claim:

1. An electrical conductor for superconductive windings or switching paths comprising, in combination:

a. a plurality of electrically insulated and transposed filaments each of said filaments consisting of a thin and flexible glass capillary which is filled with a superconductive material whose melting temperature lies below the softening temperature of the glass;

b. a hardenable material which encases said plurality of filaments at a plurality of positions axially spaced along the length thereof; and

c. a sheath enclosing said plurality of filaments and said hardenable material and in partial contact with the outer circumferential surface of said plurality of filaments and said hardenable material along the axial lengths thereof so as to define channels for the free passage of a coolant axially along said plurality of filaments.

2. An electrical conductor as defined in claim 1, wherein said hardenable material is an adhesive.

3. An electrical conductor as defined in claim 1, wherein said hardenable material is a viscous casting resin.