GB2311197A - Induction heating - Google Patents

Abstract

A susceptor (2) is provided for location inside an induction coil whereby induction heating of the susceptor (2) generates infra red radiation that is transmitted to a component, for example a splice between conductors that is enclosed within a heat-shrinkable polymeric sleeve containing solder, that is disposed within the susceptor (2). Heating of the component is limited and applied uniformly therearound by providing the susceptor (2) with a helical slit (4) extending completely along its length. The slit (4) also prevents the generation of eddy currents. An alternative construction has a straight slit parallel to the axis of the susceptor. The slit may be filled with electrically insulating material.

Description

INDUCTION HEATING This invention relates to induction heating, and in particular to a susceptor for use in apparatus for the induction heating of an article, and to the apparatus itself.

It is well known to use electric induction for heating articles. Usually, a wire is coiled several times around the article, or otherwise in close proximity therewith, and a high frequency electric potential is applied to the coil to generate heat in the article. Such a technique is effective when the article consists of or comprises electrically conductive material. For heating nonconductive materials, such as polymeric materials for example, it is known to employ an intermediate ferromagnetic component such as a cylindrical tube, known as a susceptor. In such apparatus, the susceptor is heated directly by induction heating arising from generation of eddy currents therein, and the polymeric article, located within the susceptor, is heated by infra-red radiation emitted by the susceptor. This technique is useful, for example, where the polymeric article is a heat recoverable article, for example a heat-shrinkable tubular sleeve for enclosing a splice between two electrical wires. Such a closure can be obtained significantly more rapidly and under better controlled conditions than is the case with the application of a hot air gun to the sleeve.

The application of a high frequency electric field to the cylindrical susceptor generates heat by two methods. Eddy currents flow in a completed electrical circuit and generate heat by the Joule effect, the amount being a function of, amongst other things, the thermal properties of the material from which the susceptor is formed. Additionally, the magnetic permeability of the material causes it to increase its temperature, up to a limit determined by its Curie point. In some instances, however, the temperature that is reached in a very short time of a few seconds can be so high as to cause burning or scorching of the sleeve.

This problem arises with traditional susceptors that are of a closed configuration. that is to say are formed, for example, from a tube with a complete cylindrical configuration, where temperatures as high 10000C are easily achievable, by eddy currents induced within the susceptor that give rise to Joule heating, and can damage small diameter, say 2 to 10 mm, crosslinked polymeric tubing. This problem has been overcome, and consequently the temperature generated reduced, by slitting the cylindrical susceptor along its length parallel to its axis. The interruption of circumferential current paths prevents the generation of eddy currents in the susceptor, such that heating is by the magnetic effect only and can thus be limited by the Curie point of the material chosen. However, there are disadvantages with heating an article enclosed within such a longitudinally-slit susceptor. The infra-red radiation generated by the slit susceptor and directed inwardly on to the article is not uniform around its circumference - a "cold" line arises along its length.

It is one object of the present invention to provide a susceptor that overcomes, or at least alleviates, the above-discussed problems of known susceptors.

Thus, in accordance with one aspect of the present invention, there is provided a susceptor for use in apparatus for the induction heating of an article, the susceptor being of generally cylindrical configuration for insertion into an induction coil, which may be helically-wound, and for receiving the article, which may be substantially axially aligned therewithin, wherein the susceptor extends substantially 3600 of arc around the article when received therein, and is arranged not to sustain the flow of eddy current therethrough.

The susceptor of the invention thus avoids the temperature generated therewithin being too high by preventing the generation of eddy currents, and by limiting the temperature achieved by the susceptor by means of its Curie point. Furthermore, by extending substantially 3600 of arc around the article disposed therein in use, substantially uniform heating of the article can be achieved circumferentially..

The slit is preferably open, or alternatively it may be closed, or filled, by electrically insulating material. In either case, the interruption of the cylindrical metallic circumference prevents the occurrence of eddy currents, thus limiting the temperature to which the susceptor is heated.

In one embodiment of the invention, the susceptor has at least one slit in (ie.

completely through) the cylindrical wall thereof extending from one end to the other thereof. The slit preferably extends helically over the length of the tube, ie. forming at least one substantially complete turn, so as more evenly to generate heat circumferentially thereof. Provided that the susceptor extends substantially completely, ie. 360" of arc, around an article enclosed therein, that is to say provided there is substantially at least one full pitch of the helix, the value of the pitch, the number of turns it comprises, and the spacing apart of the turns may be selected for convenience. However, since it is the surface area of the susceptor that is a determining factor of the amount of heat it generates, the larger this area the more efficient will be the susceptor. Thus, it is preferred to minimise the size of the slit, for example the gap between successive turns of the helical configuration of the cylindrical susceptor, such that the slit forms only a minor part of the total surface area of the susceptor. It will be appreciated that in order to avoid Joule heating, and thus ensure that heating is by magnetic effects only, the slit is required simply to interrupt the electrical circuit that would carry eddy currents - the slit may then advantageously be as small as practicable. The width of the slit is preferably less than 1 mm, compared with a typical susceptor diameter of about 2 to 5 cms.

It is envisaged, for example, that the susceptor could be formed from a cylindrical coil spring. The wire forming the spring, or other shaped split susceptor, may be of circular or rectilinear cross-section.

When electrically insulating material extends continuously from one end of the susceptor to the other so as to prevent current circulating circumferentially around an enclosed article, this should form only a minor part of the cylindrical wall, so as to maximise the metallic surface area available for generating infra-red radiation. On the other hand, and especially when the insulating material extends helically along the susceptor, it is envisaged that its surface area in relation to that of the metallic surface area of the susceptor may be selected so as to control the amount of heat generated by the susceptor.

Although a susceptor that is closed, as defined above, is envisaged as part of the present invention, it is still preferred that the slit extend around its periphery, so as better to distribute the heat generated around the circumference of an enclosed article.

The metal forming the susceptor comprises a ferromagnetic material, and may be selected from, nickel, iron, or steel, which may be plated with nickel or chromium to nmise corrosion.

The invention also provides induction heating apparatus comprising a susceptor as defined above with reference to the one aspect of the present invention, and a coil for inducing current therein. The apparatus preferably also comprises a mounting arrangement for locating the susceptor within the induction coil. Such a mounting arrangement needs to be non-magnetic, and also resistant to the temperature, of the order of 10000C, reached by the susceptor (the induction coil itself is usually cooled by the flow of water therethrough).

A preferred holder for the susceptor is a cylindrical tube made from a ceramic material.

Induction heating susceptors, each in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an isometric view of one embodiment, Figure 2 is an isometric view of another embodiment of susceptor adjacent an induction heating coil, and Figure 3 is an isometric view of a holder for the susceptor.

Referring to Figure 1, a susceptor 2 is of generally cylindrical configuration formed from nickel-plated steel. The susceptor 2 was initially formed as a closed tube and a helical slit 4 was then cut in its cylindrical wall to extend over several turns completely from one end to the other.

Referring to Figure 2, a susceptor 6 is of generally cylindrical configuration formed from nickel-plated steel. The susceptor 6 was initially formed as a closed tube. A slit 8 was then cut in its cylindrical wall to extend completely from one end to the other substantially in a straight line parallel to its axis. The slit 8 is filled with electrically insulating material 10.

The susceptors 2, 6 are of a diameter such that they can be supported (by means not shown) within the space defined by a helically-wound water-cooled coil 12 of induction heating apparatus (not shown).

The susceptor 2 or 6 is located within a closed cylindrical mount 14 of ceramic material that is apertured at various points over its surface to allow circulation of cooling air therethrough.

In operation, the susceptor 2, 6 is disposed within the induction coil 12, and the article to be heated (not shown), being for example a heat-shrinkable polymeric tubular sleeve containing a solder ring and enclosing a splice between two insulated conductors, is disposed within the susceptor. The coil 12 is energised with a suitable high-frequency power supply, the susceptor 2, 6 becomes hot due to the magnetic heating effect, and the infra-red radiation generated heats the solder causing it to melt and form an electrical connection between the two conductors, and also heats the polymeric sleeve causing it to shrink sealingly on to the conductor insulation. Since the susceptor 2, 6 encloses the article substantially completely around its circumference along the whole of its length, ie. except for a minor area directly beneath the narrow slit 4, 8, the article will be substantially uniformly heated circumferentially and longitudinally.

It is envisaged that the susceptor, and the entire induction heating apparatus, may be used repetitively in cyclical operations. It is possible with such usage that the susceptor and/or its support tube may not cool down between cycles to their original temperature.

Over a period of time therefore, the support in particular may gradually increase in temperature, to the detriment of the heating of the enclosed article. To avoid this problem, a flow of cooling air is directed along the susceptor and its support between the heating phases of operation of the apparatus. The apertures in the support member assist in this cooling step. Additionally, when the heating operation involves the emission of gases, such as those given off when a fluxed solder is heated for example, it is advantageous to provide a low pressure flow of air through the apparatus to effect their removal during the heating step.

Claims (10)

CLAIMS:

1. A susceptor for use in apparatus for the induction heating of an article, the susceptor being of generally cylindrical configuration for insertion into an induction coil and for receiving the article wherein the susceptor extends substantially 3600 of arc around the article when received therein, and is arranged not to sustain the flow of eddy currents therethrough.

2. A susceptor according to claim 1, having at least one slit in the cylindrical wall thereof extending from one end to the other end thereof.

3. A susceptor according to claim 2, wherein the slit is of substantially helical configuration, and preferably is an open slit.

4. A susceptor according to claim 3, wherein the slit in the cylindrical wall of the susceptor is filled with electrically insulating material extending from one end to the other end thereof.

5. A susceptor according to claim 2, wherein the slit extends substantially in a straight line parallel to the axis of the cylinder, and is substantially filled by electrically insulating material.

6. A susceptor according to any preceding claim, comprising a slit cylindrical tube.

7. A susceptor according to any one of claims 1 to 5 formed from wire, preferably of rectangular or of circular cross-section.

8. A susceptor according to any one of the preceding claims, wherein the width of the slit is less than about 1 mm, and the diameter of the susceptor is between about 2 cms and about 5 cms.

9. A susceptor according to any one of the preceding claims, made from a ferromagnetic material, including, nickel, iron, and nickel-and/or chromium-plated steel.

10. Induction heating apparatus comprising a susceptor in accordance with any one of the preceding claims, and a coil for inducing current therein.