GB2114382A - Apparatus including electric current transfer - Google Patents

Abstract

Heavy electric current is transferred between two relatively movable conductive members, for example a moving wire and stations 11 or 12, by way of a bed of carbon spheres of 1 DIVIDED 4 mm diameter or of carbon fibres 1/8 mm x 3 mm. The bed is maintained under pressure, for example by a lid 15, and is in contact with both the conductive members. <IMAGE>

Description

SPECIFICATION Apparatus including electric current transfer This invention relates to apparatus having two relatively movable conductive members between which electric current is to be transferred.

Examples of types of apparatus which are envisaged are electro-chemical plant, arc furnaces and induction heaters, where very heavy currents are handled and where a certain freedom of movement of current-carrying members during use is needed.

Another example is a device for direct resistance heating of moving wire, to and from which wire heavy current must be passed.

An economic soiution would be desirable to the problems of providing a high current sliding contact.

These problems arise out of the difficulty in ensuring uniform current distribution between the elements of the contact (so avoiding progressive overheating and failure) at reasonable first cost, maintenance cost and space requirement. They have been virtually insuperable at currents approaching 100 000 amperes - hence the use of inefficient, cumbersome and expensive flexible cables on such apparatus as arc furnaces, induction heaters and electrochemical plant.

According to the present invention, apparatus has two relatively movable conductive members between which electric current is to be transferred, characterised in that between (and contacting) the two members there is provided a mass of conductive carbon particles under compression, displaying spatial flexibility as hereinafter defined.

"Spatial flexibility" is the property of a mass of particles that, when compressed, the particles react and move in directions other than that of the applied pressure. The particles thus move with respect to each other (not as a body), and therefore present large numbers of points of contact with both of the said members.

The particles may be substantially equiaxed (such as spherical) or may be elongated.

Elongated particles are preferred, since, if randomly oriented, they will tend to make mainly 'tangential' contacts (each having a finite area) with the said members; equiaxed particles will tend to make mainly point contacts, a small current through each of which could still amount to a significant current per unit area. Furthermore, elongated particles tend to form an interlocking mat which, when compressed, tends to remain as a coherent body and does not ooze out of every small gap in its container; with equiaxed particles, which may flow under pressure, a higher standard of seal is desirable.

With elongated particles, a greater proportion of the total current path is of solid conductor, the contact interfaces being corespondirigly fewer. Thus the effective overall resistance of the medium tends to be less.

Conveniently, the elongated particles may be very short lengths of carbon fibre, for example 1/8 mm in diameter and 3 mm long. For the substantially equiaxed particles, such materials as screened, crushed graphite electrodes may be used.

Carbon particles have the important advantage that they do not require to be de-oxidised before use or protected from re-oxidation, since carbon forms only volatile oxides. They therefore make good electrical contact at all times.

However, carbon particles, if left to settle under their own weight and then caused to conduct heavy currents, tend to form routes of preferred conduction. The negative temperature co-efficient of resistance of carbon ensures that any such route, when it becomes warmed up, becomes even more preferred. Moreover, the 'pinch effect' (heavy current drawing matter radially inwards) strengthens such preferred routes. For these reasons, carbon was excluded as a material that could be used in the electric contact device of European Patent 1118.

By compressing the mass of carbon particles, the formation and strengthening of such preferred routes can be inhibited. The pressure required to do this and to effect the desired uniform multi-point contact with the surfaces of the conducting members may be applied directly by the members themselves or by pressure applicators acting in another direction -the spatial flexibility of the mass of carbon particles then transmitting the pressure to the contact surfaces.

This latter method is particularly applicable to a very convenient embodiment of the invention wherein the conducting members are concentric tubes. Axial pressure may then be applied between end rings sealing that part of the annular gap between the members which contains the carbon particles.

A pressure of 2 kg cm-2 has been found effective on chopped carbon fibres, bringing the resistivity of the mass of particles down to levels comparable with that of elongated copper wires (of nominally lower resistivity), showing that, in the latter, interparticle resistance must be significant.

The invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 shows schematically part of an apparatus according to the invention, such as for supplying an arc furnace, and Figure 2 shows a different embodiment of an apparatus according to the invention, namely an apparatusfordirect resistance heating of wire.

Turning to Figure 1, a first member lisa copper column having a rigid radial arm 1a leading to a fixed supply of electricity.

A second member 2 is mounted concentrically on the column 1 with clearance and able to move axially and rotationally with respect to the first member 1.

The annular gap between the members 1 and 2 is determined, and bounded top and bottom, by nonconductive guide blocks (e.g. of nylon or PTFE), of which the upper is shown as 3. The space bounded by the guide blocks and the members 1 and 2 is filled with carbon fibres of 1/10 to 1/8 mm diameter chopped into 2 to 3 mm lengths. The distance across the gap between the members 1 and 2 is 10 mm. A force of 70 kg was applied via spring-loaded jack screws in the upper guide blocks 3 to a pressure pad immediately beneath it.

Current of 20 kiloamps was passed between the members 1 and 2 while they were in relative motion, with negligible voltage drop.

Turning to Figure 2, a coil 10 of 1 mm diameter copper-plated steel wire requiring to be heat treated for use in tyre carcases is unwound and passes first through an electric current in-feed station 11,then through an electric current abstracting station 12, and is then re-coiled onto a drum 13. The heattreatment takes place between the stations 11 and 12, between which a potential drop of about 200V is maintained; a heavy current thus passes along the wire between these stations and its passage imparts the required heat-treatment by direct resistance heating. The stations are 4 m apart and the wire is run at 15m/s.

The stations 11 and 12 are of identical construe.

tion, and in each, the wire may be considered as a member which is relatively movable with respect to the fixed structure of the station. The station 11 is shown with part cut away, for clarity.

Each station consists of a sheet metal box 60 mm high, 200 mm long in the direction of the wire and 15 mm wide transversely to the wire. A loose-fitting piston or lid 15 can slide up and down within the box. The box is filled with graphite particles of about 41 mm diameter. The dimensions of these particles are preferably 0.1 to 1.0 times the wire diameter With very small spheres, particles can leak round the edges of the lid 15, while with very large spheres, the number of contacts made with the wire itself becomes too smal to ensure that only a negligible current passes through each.

To avoid contamination ofthe spheres, the wire is preferably cleaned to remove die lubricant before it reaches the first station 11.

A downwards force is applied to each lid 15 by a screw mechanism (not shown) to compress the particles. The force can be quite high before the particles 'brake' the wire, the graphite being of course a lubricant, and must be high enough to resist the tendency of the moving wire to bore itself a tunnel through the bed of particles. In the present case, a pressure on the lid of 2 kg/cm2 was used.

The wire enters and leaves the stations 11 and 12 through electrically insulating heat resistant, nonscratching seals about 10 mm from the floor of the box. The seals are just sufficiently tight to retain the particles. Being loose enough to allow free passage of the wire, the seals perforce also permit ingress of air, but since modest oxidation of the particles does not affect their ability to transfer current, that does not matter; protection of the particles from gross dirt is however desirable. After leaving the station 12, the wire (which has reached 400"C) passes through a reverse flow quench tube (not shown) about 0.5 m long, before being coiled onto the drum 13.

Under these circumstances, the wire heating apparatus consumes 21 kW and the heat-treated wire is unmarked and visually indistinguishable from untreated wire. The graphite particles remain as good as new.

The layout of the apparatus in Figure 2 is for illustration only. In practice, to avoid the need to isolate the 'live' side of the equipment, i.e. the side represented by the station 11, two stations 12 would be provided, one on each side of the station 11 and both at an earth potential.

Claims (3)

1. Apparatus having two relatively movable conductive members between which electric current is to be transferred, characterised in that between, and contacting, the two members there is provided a mass of conductive carbon particles under compression, the particles being such as to react and move in directions other than that of the applied pressure.

2. Apparatus according to Claim 1, wherein the particles are substantially equiaxed.

3. Apparatus according to Claim 1, wherein the particles are elongated.