Abstract
The electromagnetic shielding of the inductor comprises a main field-concentrating shielding composed of vertical columns, and the columns are composed of ferromagnetic blocks separated by non-magnetic gaps which contribute to increasing the magnetic reluctance in order to strongly reduce the heat losses. The main shielding is supplemented by an outer conductive casing which confines the residual field that has escaped from the main shielding. The shielding is compact, the mass of ferromagnetic material to be used is modest, autonomous cooling of the main shielding is unnecessary and the electromagnetic coupling between the casing and the main shielding reduces or even eliminates the effects on the electrical characteristics of the equipment.
Claims
1. A shielded electromagnetic inductor, comprising an inductor arranged in front of a load to be heated by electromagnetic induction and composed of at least one conductive turn, where current flows in a turn length direction, and an electromagnetic shielding comprising a magnetic field concentrator arranged in front of the inductor, with the inductor between said field concentrator and the load, said concentrator comprising ferromagnetic columns, a main direction of elongation of which coincides with a direction of a main component of magnetic field lines propagated by the inductor on a side oriented to the field concentrator, the columns being separated from each other by an electrically insulating medium and each embracing a portion of turn length of the inductor, wherein the columns are composed of ferromagnetic elements succeeding each other in said direction of elongation and separated by electrically insulating gaps, having lower magnetic permeability and shorter length than the ferromagnetic elements in said direction of elongation, and wherein the electromagnetic shielding further comprises an electrically conductive casing, the field concentrator being located between said casing and the inductor.
2. The shielded electromagnetic inductor according to claim 1, wherein the ferromagnetic elements are of ferrite.
3. The shielded electromagnetic inductor according to claim 1, wherein the inductor has a rotational shape about the load and about an axis, the direction of elongation of the columns coincides with the direction of the axis, and the electrically conductive casing comprises a portion with a rotational shape about the axis, which surrounds the field concentrator.
4. The shielded electromagnetic inductor according to claim 1, wherein said gaps are dimensioned so as to increase the total reluctance of the shielding by a factor of 20 to 80.
5. The shielded electromagnetic inductor according to claim 1, wherein the inductor is a single turn inductor and the turn is composed of electrically parallel supplied strands.
6. The shielded electromagnetic inductor according to claim 3, wherein the inductor is a single turn inductor and the turn is composed of electrically parallel supplied strands and the strands are composed of portions tilted in the direction of the axis, alternately descending and ascending and distributed in two circular, concentric plies extending at identical heights along the axis, a first of the plies comprising all the ascending portions and a second of the plies all the descending portions.
7. The shielded electromagnetic inductor according to claim 5, wherein the ferromagnetic elements each extend in front of only one of the strands of the inductor, or only one of the descending or ascending portions of that one of the plies which is radially outermost if the turn is composed of said portions in said concentric plies.
8. The shielded electromagnetic inductor according to claim 1, wherein the inductor is cooled by an inner fluid circulation.
9. The shielded electromagnetic inductor according to claim 1, wherein the ferromagnetic elements are mounted to the inductor by support linkages.
10. The shielded electromagnetic inductor according to claim 8, wherein the ferromagnetic elements are mounted to the inductor by support linkages and the support linkages are overall thermally conductive and electrically insulating between the ferromagnetic elements and the inductor, and the ferromagnetic elements are devoid of a particular cooling fluid circuit.
11. The shielded electromagnetic inductor according to claim 10, wherein the support linkages each comprise an adaptive part, a first side of which has a curvature identical to the inductor and is connected to the inductor through a linking layer, and a second side, opposite to the first side, on which at least one ferromagnetic element is installed and which has a curvature identical to said at least one ferromagnetic element.
12. The shielded electromagnetic inductor according to claim 11, wherein said installed ferromagnetic element is linked to the adaptive parts through a second linking layer.
13. The shielded electromagnetic inductor according to claim 11, wherein the support linkages comprise screws for attaching the ferromagnetic elements to the adaptive part.
14. The shielded electromagnetic inductor according to claim 10, wherein the supports have less extension than the ferromagnetic elements in said direction of elongation, and the ferromagnetic elements have end edges in the direction of elongation that are clear of the supports.
15. The shielded electromagnetic inductor according to claim 1, wherein the ferromagnetic elements are blocks or tiles in the form of flat quadrangles, chamfered or provided with fillets at least at some corners of the quadrangle.
16. The shielded electromagnetic inductor according to claim 1, wherein the field concentrator is separated from the conductive casing through a layer of electrical insulator.
17. Use of the shielded electromagnetic inductor according to claim 1 in a furnace for vitrifying nuclear waste.
Description
[0045] A particular and purely illustrative embodiment of the invention will now be set out in detail, in order to grasp the various aspects, characteristics and advantages thereof, by means of the following figures:
[0046] FIG. 1: an overall view of the shielded inductor device;
[0047] FIG. 2: first embodiment of the ferromagnetic field concentrator;
[0048] FIG. 3: second embodiment;
[0049] FIG. 4: third embodiment;
[0050] FIG. 5: fourth embodiment;
[0051] FIG. 6: embodiment detail of the ferromagnetic elements;
[0052] FIG. 7: front view of the supports for the ferromagnetic elements;
[0053] FIG. 8: top view of one of the supports;
[0054] FIG. 9: partial vertical cross-section view of the inductor with the shielding;
[0055] FIG. 10: view of another possible embodiment of the shielded inductor;
[0056] FIG. 11: electrical parameter diagram;
[0057] FIG. 12: heat loss diagram.
[0058] FIG. 1 illustrates the shielded inductor seen from outside. Its general shape is rotationally symmetrical and cylindrical with a vertical axis X and upwardly open in this embodiment, and it is surrounded by an electrically conductive metal outer casing 1 which is used for the electromagnetic shielding. The outer casing 1 comprises an almost continuous circumferential wall, from which, however, inlet and outlet connections 2 emerge, through which an inductor 3 is supplied with electricity and cooling fluid. It may be supplemented by bottom and top walls to further isolate the inductor from outside. It may be made of metal, copper or aluminium alloy. The inductor 3 is better visible in the following FIGS. 2 to 5, which will be now discussed. It is formed by a single turn of complex shape which surrounds, for example, the load to be heated, which may be contained in a crucible, which may itself be a so-called cold crucible. Cold crucibles, used for the vitrification of nuclear waste, which is a contemplated application of the invention, are cooled by an inner liquid circulation and produce a surface solidification, capable of protecting them from corrosion, of the load in contact with them. The inlet and outlet connections 2 lead to two collectors 4 and 5 which are elongate conductive blades, close to each other and which are the ends of the inductor 3. The conductive turn is composed of conductive strands 6, all of which are connected to the collectors 4 and 5 and which are therefore electrically connected in parallel, making a single turn around the furnace. Each strand 6 is zigzag-shaped and essentially composed of so-called ascending portions 7 alternating with so-called descending portions 8 with opposite vertical tilts. All the ascending 7 and descending 8 portions extend from the lowest to the highest level of the inductor 3. The ascending portions 7 thus cross the descending portions 8, but without touching them, since they extend in separate concentric cylindrical plies, the (conventionally) ascending portions 7 in an inner ply 9 and the (conventionally) descending portions 8 in an outer ply 10. Indeed, the strands 6 still comprise short, radially directed connections 11, which connect each ascending or descending portion to the adjacent portions of the strand 6. This arrangement, already known from document WO 2007/031564 A1, is preferred here because it reduces induction heterogeneities in height and especially current concentrations at the upper and lower edges of the inductor 3, referred to here as edge effects. Finally, the collectors 4 and 5 as well as the strands 6 are hollow and have a cooling fluid flowing therethrough, similar to that described in this document.
[0059] The furnace comprises a main electromagnetic shielding which is a magnetic field concentrator 12 and which extends, with a generally annular shape, between the inductor 3, radially inwardly of it, and the outer casing 1, radially outwardly of it. It is composed of vertical columns 13 (erected in the direction of the axis X) each embracing a circumferential sector of the furnace but separated from each other and formed of ferromagnetic elements which are here parallelepipedic ferrite blocks 14 (also designated 14a, 14b, 14c or 14d in FIGS. 2 to 5), or quadrangular flat tiles, superimposed in the vertical direction, but separated by electrically insulating and non-magnetic gaps 15 interposed between them. The gaps 15 may be physical gaps maintaining continuity of the columns 13, or they may be empty spaces, which is permitted if the blocks 14 are supported independently of each other. The function of the field concentrator 12 is to channel magnetic field lines external to the inductor 3 to concentrate the magnetic field within a reduced perimeter, maintain or increase the currents induced in the load to be heated and maintain or enhance the thermal efficiency of the furnace, while partially limiting the inductive coupling with the peripheral casing 1 which finishes blocking outwardly the magnetic field leakages. Together with the casing 1, it forms the overall electromagnetic shielding of the equipment. The ferromagnetic blocks 14 can be up to a few centimetres high, a few degrees of angular extension (preferably less than 15°), and the gaps 15 a few tenths of a millimetre to a few millimetres high. The extension of the ferromagnetic blocks 14 in the radial direction is restricted to facilitate their cooling by the inductor 3 in a manner indicated below, whereas the ferromagnetic blocks 14 are devoid of a cooling fluid circuit specifically associated therewith. Their extension in angular direction is also restricted, in order to limit magnetic resonances. In addition, it is recommended that each ferromagnetic block 14 extends in front of a single strand 6, in order to limit risks of electrical conduction in the field concentrator 12 between adjacent strands 6; with the inductor 3 composed of intersecting zigzagging portions, this principle is applied by making each ferromagnetic block 14 extend in front of a single portion (here called the descending portion 8) of the external ply 10, which is contiguous thereto.
[0060] Several alternative embodiments can then be contemplated, considering that the cross-section and the number of columns 13 can be varied, in order to optimise channelling of the flux and the magnetic resonance effects and that the ends of the ferromagnetic blocks 14 can be adjusted in different ways, depending on the desired ease of assembly and optimisation of the ferrite mass. In FIG. 2, the ferromagnetic blocks 14a are shaped like parallelepipeds, or rectangular tiles, tilted like the strands 6, thereby producing columns 13a with irregular side edges in broken lines; in FIG. 3, the ferromagnetic blocks 14b are still rectangular tiles, but with their vertical side, a smaller angular extension than in the previous case, but a greater vertical extension; each of the columns 13b is then composed of several juxtaposed stacks of ferromagnetic blocks 14b, and the upper edges of these columns 13b are stepped. The arrangement of FIG. 4 is similar to that of FIG. 2, except that the rectangular tiles are replaced by trapezoidal tiles, so that the magnetic blocks 14c result in columns 13c whose lateral sides are vertical and regular. Finally, the arrangement of FIG. 5 is similar to that of FIG. 3, except that here too the rectangular tiles are replaced by trapezoidal tiles whose upper and lower edges are tilted similarly to those of the strands 6 and at right angles to the lateral sides at the ends of the columns 13d, giving rectilinear edges to the columns 13d. In each of these arrangements, it may be contemplated that the sharp corners of the ferromagnetic blocks 14 may be broken by chamfering or filleting between sides, as spike effects, resulting in high magnetic field concentrations and localised losses, are likely to occur at these places. One possible embodiment is represented in FIG. 6, where the chamfers are marked as 16.
[0061] It is contemplated that the ferromagnetic blocks will be supported by the inductor. The support may possibly be direct, if the inductor strands 6 and the ferromagnetic blocks 14 have complementary shaped faces allowing them to be joined directly by adhesive or otherwise. However, this can cause significant problems, because of the difficulty of shaping ferrites to complex shapes or of replacing conventional inductors of simple, regular rotational shape with others. If a direct support is excluded, the strands 6 could support the ferromagnetic blocks 14 by means of supports 17 (not represented in the previous figures) summarily represented in FIG. 7, in the form of plates, which, according to FIG. 8, will be shape-adaptive parts between the strands 6 and the ferromagnetic blocks 14 and will have an irregularly shaped cross-section, an internal side 18 having a curvature similar to that of the strands 6 and an external side 19 being flat if the ferromagnetic blocks 14 are flat tiles, or, more generally, having a curvature identical to that of the ferromagnetic blocks 14. Such an arrangement allows the strands 6 and the ferromagnetic blocks 14 to be pressed against the supports 17 and thus facilitates cohesion of the whole. FIG. 7 shows that the vertical extension of the supports 17 is advantageously less than that of the ferromagnetic blocks 14 so that the ferromagnetic blocks 14 project from the supports 17 at their upper and lower ends and so that couplings of the magnetic field with the supports are avoided in the place of the gaps 15 and at the ends of the columns. The supports 17 therefore advantageously have tilted upper and lower edges, like the corresponding and adjacent edges of the ferromagnetic blocks 14.
[0062] The supports 17, as well as the other components of the assemblies for supporting ferromagnetic blocks 14 by the strands 6, are preferably designed to allow thermal conduction between the strands 6 and the ferromagnetic blocks 14, but instead electrical insulation, in order both to promote cooling of the ferromagnetic blocks 14 by the cooling fluid circulating in the strands 6 and to avoid additional Joule losses in the ferromagnetic blocks 14 through the flow of electric current with the strands 6 or any other electrically conductive element subjected to electromotive forces. A schematic device, represented in FIG. 9, may further comprise a solder 20, or another thermal and mechanical binder, between the internal side 18 of the supports 17 and the strands 6, an epoxy adhesive 21 or another thermal and thus preferably electrically insulating binder between the external side 19 of the support 17 and the ferromagnetic blocks 14, and screws 22, bearing on the external face of the ferromagnetic blocks 14, extending along their lateral faces and bolted into bosses 23a of the supports 17 or nuts 23b located behind them, on the inductor 12 side. Holding ferromagnetic blocks 14 in place is thereby achieved, the retention in the vertical position being ensured mainly by the adhesion of the linking layer 21. Alternatively, the screws 22 could pass through the ferromagnetic blocks 14 to ensure this function too. Other types of support could be considered to perform these functions of holding in place. Alternatively, the device could include supports not represented provided with lower or lateral edges allowing the ferromagnetic blocks 14 to be placed on them and thus replacing at least some of the screws 22 or dispensing with the adhesive property of the linking layer 21. However, the device of FIG. 9 has the advantage of reducing the area facing the supports 17 and the ferromagnetic blocks 14 and of constructing more easily supports 17 of metal, thus avoiding electrical contact with the ferromagnetic blocks 14 through the connecting layer 21 easily laid.
[0063] FIG. 9 further shows that a layered electrical insulating material 34 can be interposed between the field concentrator 12 and the outer casing 1, if they are close to each other, since sufficient channelling of the magnetic field into the ferromagnetic blocks 14 can allow such a close proximity, advantageous for reducing the overall size of the device, provided that electrical insulation is maintained between the two shielding components.
[0064] The invention could be implemented in other ways, especially with non-cylindrical inductors. FIG. 10 represents such an alternative, where a flat inductor replaces the previous inductor and is located under the load to be heated 26, placed as before in a cylindrical crucible 27 which may be a cold crucible. The strands 28 of the inductor 25 can be parallel to each other or can be placed as in the previously contemplated embodiment, developing the plies on a plane. The field concentrator 29 is made up of columns 30 perpendicular to the overall direction of the strands 28, and they are composed, as before, of ferromagnetic blocks 31, each of which can be associated with one of the strands 28 facing it, and separated by gaps 32. The principles of construction of the field concentrator 29 are therefore applicable to such inductors at will. The outer casing 33 extends under the inductor 25 and the field concentrator 29, preferably all around the device and over the crucible 27. Magnetic field lines have been represented which remain enclosed within the casing 33 or are channelled through the ferrite columns 30.
[0065] Some benefits of the invention are as follows: [0066] improved confinement of the magnetic field, reducing the field amplitude outside the shielding by up to a factor of 40 with respect to an arrangement with ferromagnetic columns of the same mass and arrangement but continuous in the alignment of the magnetic field lines, free of the gaps 15, and in the absence of an outer casing; [0067] total shielding mass reduced by a factor of up to 4 with respect to a shielding exclusively composed of ferromagnetic material with equal attenuation of magnetic field leakage outwardly; [0068] radial overall size divided by a factor of 2 with respect to a shielding with only a metal casing without any particular cooling device; [0069] less than ±10% shift in electrical parameters observed from the inductor power supply with respect to an unshielded configuration; [0070] an imbalance of about ±20% in the currents flowing through the individual strands of the inductor provided with the shielding, instead of about ±97% for a single turn arrangement with parallel, non-interlaced strands.
[0071] This will be illustrated in the final figures. FIG. 11 illustrates the effect of a plausible embodiment of the invention on the electrical parameters of resistance and inductance of the device as observed from the power supply to the inductor (operating, for example, at 3000 V and 300 kHz, for an active power of 300 kW in an inductor 0.8 m in diameter and 0.6 m high, composed of fourteen interwoven strands 6 in parallel supply and a ferrite arrangement as described hereinabove having a relative magnetic permeability of 4000), in percentage deviations from the parameters valid for the same embodiment without shielding, based on the height of the separations between the ferromagnetic blocks 14, that is the thickness of the gaps 15, expressed in millimetres in the abscissa. A range of values of interest for applying the invention based on the criterion of maintaining the electrical parameters of the equipment extends from about 1 mm to 4 mm, between which the resistance R varies between ±8% maximum, and the inductance L varies from +2% to −4% maximum. The result can be compared to the one that can be inferred from FIG. 12, which indicates the course of the heat losses in the ferrite assembly in watts based on the same thickness parameter of the gaps 15. The decrease in these losses is immediate as soon as gaps 15 are installed, and greater the greater their thickness; however, the thickness increments have less and less effect as the thicknesses to which they are added are greater. In this embodiment, a loss reduction of about 40% is already achieved for thicknesses of 1 mm, and a reduction of about 70% for thicknesses of 4 mm. The range of values stated above can be considered satisfactory on the basis of this second criterion. The physical phenomenon controlled and adjusted by virtue of the presence of the gaps 15 is a variation in the magnetic reluctance of the ferrite columns 13, here vertical, that is their capacity to capture magnetic flux. An approximation of the increase in reluctance in a column can be expressed in absolute values (units in the international system) as: deltaR=1/μ*e/S*nEsp where: deltaR, increase in reluctance; μ, magnetic permeability in the gap 15; e: dimension of the gap 15 in the direction of the field lines; S, cross-sectional area of the field lines in the column 13; nEsp, number of gaps 15 along the column 13. The overall reluctance of the shielding can also be estimated using an expression comprising the unit reluctances of each component through which the magnetic field lines pass, formulated according to the usual laws of calculating reluctances in magnetic circuits.
