IMPROVEMENTS IN AND RELATING TO COILS

Abstract: The invention aims to provide coils for inductance applications or magnetic field generation suitable for use in high temperature coils. The invention provides the coil by a non-winding process, in which the conductor path is formed by depositing or removing material served to form a conductor path in its configuration of use relative to other parts of that conductor path. The coil may be formed of one or more elements formed in this way. The conductor paths in individual elements are connected to one another such that the current flows in a consistent sense between layers. A core provided in this way is less susceptible to damage to its insulating or anti-corrosion coatings than a coil produced by coiling a conductor into the coil configuration following insulation and/or anti-corrosion coating. Far higher levels of coil packing are also possible using this construction.

Description: IMPROVEMENTS IN AND RELATING TO COILS This invention concerns improvements in and relating to coils, particularly, but not exclusively, in relation to a form of coil construction. Such coils find application in high temperature coils for inductance applications or magnetic field generation, for instance in magnetic bearings, as well as in a variety of applications in which power to weight is important, such as aerospace. Magnetic bearings offer advantages over other bearing systems in terms of their long service life, elimination of the need for lubricants, low friction losses and low starting requirements . The use of magnetic bearings in high temperature applications is particularly desirable. Their provision as the back bearings of turbines, and as components of jet engines illustrate such uses. An essential part of any magnetic bearing is the provision of a long conductor physically deformed and wrapped around the soft magnetic material forming the poles of the magnet. The coils or windings must meet a number of requirements for successful operation and magnetic field provision. Conventional windings consist of a coated wire which is either wrapped around the soft magnetic material in situ , or wrapped on to a carrier / bobbin which is then inserted over the poles of the soft magnetic material. In high temperature environments, however, the windings must retain their structural integrity at temperatures in excess of 800 °C. Unfortunately, the most desirable materials for such windings, copper and silver, approach their melting points at such a temperature and suffer from structural deformation due to softening. An even greater problem is encountered with the insulating coating required for such materials. In conventional assemblies, the external insulation can readily be provided. However, conventional insulators are not designed to withstand, high temperature environments. Proposals have been made to use ceramic insulators in a variety of forms, but these are prone to chipping and cracking, both during use and more significantly during the winding process itself. Coating of the materials following winding is not possible as the packing density required for windings would restrict access of the insulator into the coil. Difficulties in maintaining the necessary clearance to avoid short circuits also arise if the material is wound uninsulated. The problems with winding are particularly apparent during the cross over between the various layers as the coil is made up. At the cross overs the stresses and strains imposed upon the wires are higher than elsewhere. The provision of insulating and / or corrosion protection coatings capable of withstanding the bending of winding is a very significant problem. The above mentioned problems represent serious hurdles to obtaining the degree of insulation and corrosion protection integrity demanded of the potential applications for such bearings and other electromagnetic devices, including inductance applications. Prior art coils are also limited in terms of the power to weight and / or volume ratio which can be achieved. The amount of conductor, commonly copper, which can be packed into an area is restricted by the shape of the conductor, the insulating material and the configuration of the winding. According to a first aspect of the invention we provide a coil comprising one or more elements, each element comprising one or more electrical conductor paths or portions thereof, in which at least a substantial part of the electrical conductor path(s) or portion (s) in an element is created in a fixed configuration relative to other electrical conductor path(s) or portion(s) within that substantial part of the element. A conductor formed in this way or formed out of a series of layers in which one or more is provided in this way is advantageous as the electrical conductors are not repositioned, deformed or moved relative to one another following their creation. As a consequence the overall structure is more rigid, more densely packed and is not prone to the damage to anti-corrosion and / or insulating materials which commonly arises in prior art techniques where winding is employed. The coil of the present invention is, therefore, produced by, or from one or more layers produced by, a non-winding process. The conducting path(s) may be created by the removal of material from an electrically conducting material. The voids produced, or material introduced into such voids created by the removal of such material may provide electrical insulation. Preferably it is formed by removing material from a unitary piece. The material may be removed by etching, spark eroding, cutting, stamping, punching, broaching or drilling, including laser, plasma or water-jet cutting or a combination of one or more of these techniques. The conducting path(s) may be produced by the application of electrically conducting material to a location. The locations to which electrically conducted material is not applied may define the electrical insulation, or provide voids into which such electrical insulation can be introduced. Preferably the coil is provided with a central aperture. The central aperture may be rectilinear in cross section. Preferably soft magnetic material is provided within this aperture in use. The soft magnetic material may be pure or substantially pure cobalt or an alloy thereof or pure or substantially pure iron or an alloy thereof, including cobalt iron alloys, silicon irons and nickel irons. The soft magnetic material may occupy the whole or only a portion of the central aperture. The coil may comprise one or more layers or slices. Preferably a plurality of substantially parallel configured layers are provided. Three or more and preferably eight or more layers may be provided. Between 8 and 60 layers may be provided, or even more. Preferably two or more of the layers, most preferably all bar the top and bottom layers and potentially all are formed from an integral block. The coil may be formed of a series of layers produced individually and subsequently assembled into the coil. The electrical connection between layers in such a coil may be an integral component of one or more of the layers or be provided separately. One or more layers may have a reduced width and / or length when compared with one or more other layers. Preferably the top and / or bottom layer has a lower width and / or length than each other layer. The coil may be provided with a plurality of layers of a length and / or width reduced compared with a first length and / or width. The reduced width and / or length layer adjacent a layer of first width and / or length may have a greater width and / or length that the next reduced width and / or length layer. Preferably the next reduced width and / or length layer has a still further reduced width and / or length. A stepwise reduction layer by layer from a layer of first width and / or length to the last layer may be provided. Preferably the layers are separated by cuts, from one another. The reference to cuts equates to a reference to gaps, voids or the like and should not be taken to imply the manner of formation unless explicitly stated. The layers may be each of equivalent thickness to one another. Preferably one or more layers are provided with a different thickness to one or more other layers. Layers near to and / or including the top layer may be thinner than intermediate layers and / or those near to and / or including the bottom layer. Layers near to and / or including the bottom layer may be thinner than intermediate layers and / or to those near to and / or including the top layer. Most preferably the top and bottom layers and / or adjacent layers are thinner than intermediate layers in the coil. Preferably adjacent layers are joined to one another by transition elements. The transition elements may be provided as a series of steps. The transition elements may be provided in alignment with one another. The alignment may be angled relative to the axis defining the layer thicknesses. Preferably the transition alignment is angled at between 10° and 60° to this axis. The transition elements may be provided integrally with one or more of the layers which it connects. Alternatively the transition element may be provided as a separate component to either layer which it connects. Preferably the transition elements extend from the outer surface of the coil to the inner aperture. Preferably one or more of the transition elements, and most preferably all, are provided in a unitary part of the block. Preferably the transition elements provide the only electrical contact between a layer and an adjacent layer. Preferably one or more of the layers are provided with one or more spirals. Preferably the spirals are defined by cuts through the layers. Preferably the spirals are concentric. Preferably the spirals continue across the transition elements and continue on the adjoining layer or layers. The coil may be produced from two or more layers, each layer having one or more spiral paths extending inwardly about itself. Preferably the hand of the spiral in one layer is the opposite of that in an adjacent layer. One layer may thus spiral inwardly clockwise, the adjacent layer spiralling inwardly anti-clockwise. Preferably the inner end of one spiral is connected to the inner end of one of its two adjacent layers. Preferably the outer end of the spiral is connected to the outer end of one of its two adjacent layers. Most preferably the inner connected and outer connected adjacent layers are different. The spirals may be of substantially equal width and / or depth to one another. Preferably the inner most and / or inner spirals of a layer, or alternatively the inner portion of a spiral in a layer, is of greater width and / or greater depth than the outer most and / or outer spirals or outer portions of that spiral in that layer. Preferably the cross sectional area is greater in inner spirals than in outer spirals or in the inner portion compared with the outer portion of a spiral. Preferably the cross sectional area of a given spiral is greater than the adjacent outer spiral and less than the adjacent inner spiral in the equivalent layer. Similarly the inner portion of a spiral in a layer may be of greater cross sectional area than the outer portion of a spiral in a layer. The cross sectional area of a spiral in a given layer may vary gradually from one spiral to the next. Alternatively a first set of spirals of one cross sectional area may be provided together with one or more further sets of spirals of different cross sectional areas. Similarly the cross sectional area of a spiral in a layer may increase gradually, or alternatively in a stepwise manner, as the spiral progresses inwardly. Preferably the coil is provided with a top layer, bottom layer and one or more intermediate layers. Preferably the top and / or bottom layers are an integral part of the block. Preferably one or more, most preferably all, of the intermediate layers are integral with the block. Preferably the spiral is continuous between the start of the upper intermediate layer at the end of the lowest intermediate layer. The upper layer may be provided with a series of start locations for the spirals. Preferably the start locations are provided at one edge of the coil. The upper layer may be provided as a separate component. Preferably the bottom layer is provided with a series of tail portion for the spirals. Preferably the tail portions are provided at one edge of the coil and most preferably on the same edge as the start portions. The bottom layer may be provided as a separate component. Preferably an electrical conductor leading to the start portion of one spiral is provided leading either to the top or bottom layer. Preferably a further electrical conductor is provided leading to a separate spiral on either the top or bottom layer. Preferably both connectors are connected to either the top layer or bottom layer. Preferably the only continuous electrical path between the two electrical conductors extends throughout all the spirals and layers of the coil. Preferably the start portion of one or more of the coils on the top layer is connected to a tail portion on a spiral of the bottom layer. Preferably all the tail portions on the bottom layer are connected to start portions on the top layer. Most preferably the two input connectors are also connected to start portions on the top layer. Preferably the tail and start portions are connected to one another by transition paths. Most preferably the transaction paths are provided from the unitary piece from which the coil is formed. The transition paths may be provided on a separate component. Preferably the transition paths are inclined relative to the axis defining the thickness of the layers. Preferably the tail portion of the outermost spiral of the coil is connected to the start portion of the innermost spiral of the top layer. Preferably the innermost tail portion of the bottom layer is connected to the next but outermost start portion of the top layer. Preferably an alternating sequence is provided. Most preferably the connecting elements are provided in parallel arrangement. Alternatively, the outlet may be provided on an opposing layer to the inlet. Preferably in this embodiment the connecting strips are provided in two sets, the strips in each set being parallel to one another but inclined relative to each other. Preferably a substantial part of, and most preferably all of the surface of the coil is provided with a corrosion resistant coating, such as a plating. Nickel or alloys thereof may be used. Preferably the cuts / gaps defining the coil are coated with, and most preferably filled with an electrically insulating material. A ceramic insulator may be used. Preferably all external surfaces are also insulated. According to a second aspect of the invention we provide a method of forming a coil, the coil comprising one or more elements, each element comprising one or more electrical conductor paths or portions thereof, in which at least a substantial part of the electrical conductor path(s) or portion (s) in an element are created in a fixed configuration relative to each other within that element. In this way the conductor path(s) in an element are not bent, deformed, stressed or manoeuvred in any way following their creation into a further configuration prior to use. The normal damage arising from such further processing is therefore avoided. The conducting path(s) may be created by the removal of material from an electrically conducting component. The material removed may produce an electrically insulating void around the conductor path(s). The conducting paths are thus created out of a larger piece of conducting material in their position of use. The material may be removed by cutting, stamping, etching, eroding, spark eroding, drilling, boring, broaching, including laser, plasma and water-jet cutting, or one or more of these techniques. The conducting path(s) may be produced by the application of electrically conducting material to a location. The locations to which electrically conducting material are not applied may define the electrical insulation. The conducting paths are thus created by the build up of conducting material at the desired location, the conductor subsequently being used in that position. Alternatively and / or additionally material may be built up by a deposition process, for instance electrodeposition, or printing or from a filling or replacement process, such as casting or investment casting. Preferably a central aperture is provided in the coil or layers thereof. This may be formed by removal from the unitary piece of material or by selective application. The aperture may be symmetrical. Preferably the aperture is rectilinear in cross section. The coil may be provided with a soft magnetic core. The core may be produced or applied simultaneously or stepwise with the conductor paths / layers. The coil may be formed of or made up from a series of layers. Two or more and preferably eight, twenty or more layers may be provided. The coil may be formed of a series of layers formed from an integral piece of conductor. Alternatively or additionally the coil may be made up from two or more separate layers or combinations of layers subsequently electrically connected together. One or more layers may be formed substantially from an integral piece of conductor. Other individual layers may be attached thereto. Preferably the layers are defined or formed by a series of substantially parallel, and most preferably, parallel cuts. The layers may be each of equivalent thickness. Preferably one or more layers are provided with a different thickness to one or more other layers. Layers near to and / or including the top layer may be thinner than intermediate layers and / or those near to and / or including the bottom layer. Layers near to and / or including the bottom layer may be thinner than intermediate layers and / or those near to and / or including the top layer. Most preferably the top and bottom layers and / or adjacent layers are thinner than intermediate layers in the coil. Where the coil or two or more layers thereof are formed from a unitary piece preferably the cuts leave one or more portions of the block intact. Preferably intact portions connecting each layer to the others are left intact. Most preferably an intact portion is provided on the exterior of the coil. Most preferably an intact portion is provided on one of the sides of the block. This allows a greater portion of the cuts to be made through the entire thickness of the block. Preferably an intact portion extends across the plane of the various cuts made. In this way the layers remain connected to one another at that stage. Preferably an intact strip / wedge spans the slices and passes from the exterior of the block to the central aperture. Preferably this intact strip is angled relative to the plane of the layers. The layers may be formed by insertion of a tool cutting means or cutting beam from one or more sides of the block, most preferably externally. Preferably a transition element between adjacent layers is provided. Preferably this is formed in this intact strip. Preferably the transition between layers is provided by material integral to the block or to one or both of the layers. Preferably the transition elements are defined or formed by the connection of the cut defining the upper surface of one layer with the cut defining the upper surface of an adjacent layer. The transition elements may be formed by removing material, for instance by drilling, cutting or broaching, most preferably from the outside of the block. A transition element for a layer may be deposited integrally with the deposition of that layer. The transition element between a layer and an adjacent layer may be provided separately from those layers. The transition element may be provided after the assembly or manufacture of the layers. Preferably the transition elements extend the full width from the exterior of the block to the central aperture. The transition elements may provide a step transition between adjacent layers. Preferably one or more layers are divided into a plurality of spirals or part spirals defined by cuts through the said one or more layers. Preferably the cuts are made perpendicular to the plane of the layers. Preferably the cuts extend through a plurality of the layers, most preferably in a direction perpendicular to the plane of the plurality of the layers. Preferably the spirals formed in a layer are concentric with one another. Preferably the spirals in two or more adjacent layers follow one another. Preferably the spirals are initially formed around two or more, preferably three complete sides of the layer. Preferably a portion of the layer is left without spiral defining cuts. Most preferably a portion of the layer integral with an intact portion of the block spanning the layers is left without spiral defining cuts. Preferably equivalent cuts are provided in each layer. The coil may be produced from two or more layers, each layer having one or more spiral paths extending inwardly about itself, the hand of a spiral in one layer being the opposite of that in an adjacent layer. One layer may thus spiral inwardly clockwise, the adjacent layer spiralling inwardly anticlockwise. Preferably the coil of this embodiment is formed from a series of two or more equivalently spiralling layers, adjacent layers being rotated about their long axis through 180° to one another prior to assembly. Flipping one layer relative to the next in this way ensures an additive magnetic force results. Preferably the inner end of one spiral is connected to the inner end of one of its adjacent layers. Preferably the outer end of the one spiral is connected to the outer end of one of its adjacent layers. Most preferably the inner connected and outer connected adjacent layers are different. The adjacent layers may be interconnected by welding or other means, most preferably after assembly. The spirals may be of substantially equal width and / or depth to one another. Preferably the inner most and / or inner spirals of a layer or alternatively the inner portion of a spiral in a layer, is of greater width and / or greater depth than the outermost and / or outer spirals or outer portions of that spiral. Preferably the cross sectional area is greater in inner spirals than in outer spirals or in the inner portion compared with the outer portion. This has the advantage that the inner coils from which heat dissipation is the hardest can be provided with a greater cross-section. In this way the copper loss is varied throughout the coil. This is also true for a spiral within a given layer where the inner part is thicker than the outer. A lower temperature rise for an equivalent overall power loss may thus be provided and undesirable "hot spots" in the coil provided. The cross-section area of a given spiral may be greater than the adjacent outer spiral and less than the adjacent inner spiral in the equivalent layer. Similarly for the inner portion against the outer portions of the spiral in a layer. This may be true for one or more or even all the spirals. The cross-section of spirals in a given layer may vary gradually from one spiral to the next. Alternatively a first set of spirals of one cross-section may be provided together with one or more further sets of spirals of a different cross sections. Similarly for a spiral extending inward within a given layer. In a particularly preferred embodiment the width of one or more inner spirals, or the inner portion of a spiral, on a plurality of layers is greater than for one or more outer spirals, or the outer portion of a spiral, and the thickness of one or more intermediate layers is greater than the thickness of the top and / or layers adjacent to it and / or the bottom and / or layers adjacent to it. In this way intermediate layer spirals, or the portion, near the inside of the coil have a greater cross-sectional area than outer spirals, or the portion of a spiral, in intermediate layers and than the same inner spirals, or portion, thereof in top and / or bottom layers. The outer spirals or portion thereof for bottom and / or top layers have the smallest cross-section in such an embodiment. Preferably the spirals are of square or rectangular cross section, relative to the direction of current flow in use. Preferably the spiral defining cuts are provided by cutting, for instance by application or a plasma or laser beam or a spark eroder or inserting a tool, from one side of the coil. Alternatively the spiral defining cuts may be provided by not depositing material or not depositing electrically conductive material at these locations. Preferably cuts in a plurality of layers are provided by cutting on through a plurality of layers, most preferably perpendicular to the plane of those layers. Preferably the locations of non-electrical conducting deposition in a plurality of layers are equivalent to one another. Preferably intermediate layers of the block are provided with spiral defining cuts extending around the entire layer. Preferably the spirals on a given intermediate layers are connected to equivalent spirals on an adjacent intermediate layer. Preferably the spiral defining cuts are substantially provided by removal of equivalent material in each layer. The cuts to complete the spirals in one or more intermediate layers may be formed by tools or cutting means inserted or provided through existing cuts, most preferably existing spiral defining cuts in other layers. The inserted tools, cutting beam or cutting means may be inserted or retracted to the required degree and / or pivoted / rotated / arced about the plane of one or more of the layers. In this way intermediate layers can be machined without affecting the top and bottom layers with undesired cuts. Preferably the top and bottom layers are provided with spiral defining cuts extending into the intact portion of the layer spanning portion. Preferably portions are provided on the top and bottom layers defining start and tail portions respectively, most preferably in the intact layer spanning portion, most preferably one for each spiral. These cuts may be formed by inserting the tool, cutting means or cutting beam from a given side of the block, the side for the start portions being opposing to that for the tail portions. Alternatively selective deposition may be employed. Most preferably external electrical contacts leading to the coil are connected to two start or tail portions or one of each. Preferably the top and / or bottom layer are formed integrally from the block. Preferably transition paths are provided connecting one or more of the tail portions to one or more start portions. Preferably the start and tail portions are provided at an edge of the coil. Preferably the transition paths are formed by a series of cuts in an intact portion, most preferably by cuts extending between cuts defining the tail portions and cuts defining the start portions. The transition paths may be provided by one or more units attached to the assembled coil. The transition path defining cuts may be provided in parallel arrangement, or alternatively in two sets of cuts, the cuts in each set being parallel with one another, but inclined relative to the other set. In the second alternative one contact may be provided at the tail portion and the other contact provided at the start portion of the coil. Preferably the cuts are formed by a tool, cutting means or cutting beam inserted externally from the end, alternatively selective deposition may be used. Preferably one or more, most preferably all, of the transition paths are provided in an integral portion most preferably of the initial block. Preferably the transition paths are separated from the intermediate layers by a cut extending past all the intermediate layers. Preferably the cut is formed by inserting a tool, cutting means or cutting beam in the plane of the cut. Preferably the outermost spiral on the top layer is connected via a transition path extending from the tail portion of the bottom layer, to the start portion of the innermost spiral of the top layer. Preferably the innermost spiral of the top layer is connected via a transition surface extending from the tail portion of the bottom layer to the next but outermost spiral of the top layer. A layer or part thereof may be formed by depositing or provided the electrically conducting material on a support. The support may comprise an electrically insulating material and is most preferably incorporated into the assembled coil. The support layer may be provided with one or more gaps or apertures into which electrically conducting material is introduced. Such a gap or aperture may define a transition element for connecting the layer to an adjacent layer in use. An alternative or additional gap(s) or aperture (s) into which electrically conducting material is introduced may define a transition path or paths. Each layer may be provided with an electrically conducting portion in this way which in the assembled coil combine to produce the transition paths from top to bottom layers. The electrically conducting material and / or initial support and / or electrical insulating may be introduced by printing, screen printing, casting, moulding or inversion casting. A further detachable support layer may be provided under the support to close off apertures or gaps in the initial support. A substantial portion of an element may be taken to be the spiral or spiral portions therein. The transition elements between layers and / or the transition paths and / or the start and / or the end portions may be produced separately. Preferably the method provides for coating and / or plating a part or all of the surfaces of the coil with a corrosion resistant coating. Nickel or alloys thereof may be used. Preferably the method provides for coating all of the surfaces of the coil with insulating material. Preferably all, or a substantial part of the cuts are filled with insulating material. A ferromagnetic plating may be applied to the coil or part thereof. Such a plating would be used to aid attenuation of high frequency electromagnetic interference, this has particular relevance in applications such as inductors. According to a third aspect of the invention we provide an device incorporating one or more coils according to the first aspect and / or produced according to the second aspect of the invention. Preferably the device is an electromagnetic device. According to a fourth aspect of the invention we provide a magnetic bearing system incorporating one or more coils according to the first aspect and / or produced according to the second aspect of the invention. Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which :- Figure 1 illustrates a single layer spiral coil; Figure 2 illustrates an initial stage in the preparation of one embodiment of the invention; Figure 3 shows an end view of the layer transition end of the first embodiment of Figure 2; Figure 4 shows the end view of Figure 2 following the next stage in production; Figure 5 is a perspective view of the invention following a further production stage; Figure 6 is a top plan view of Figure 5; Figure 7 is a top plan view of the top layer only following the next stage of the invention; Figure 8 illustrates the intermediate layers of the coils ; Figure 9 is a top plan view of the bottom layer only of the coil; Figure 10 is a partial side view of the layer connecting end after a further stage; Figure 11 is an end view of the layer connecting the end of the coil following a still further stage of the production; Figure 12 shows a partial view of the flow sequence in the start portion of the top layer; Figure 13 shows a partial view of the flow sequence in the start portion of the top layer of an alternative embodiment; Figure 14 illustrates an alternative embodiment of a coil according to the invention; Figure 15 illustrates the coil of Figure 14 in a pole assembly; Figure 16a and 16b illustrate adjacent layers of a coil according to a further embodiment of the invention; and Figure 17 illustrates an assembly stage in a still further embodiment of the invention. Figure 1 illustrates a simple single layer spiral. Such a structure is relatively attractive from a manufacturing point of view, but due to the relatively low number of series turns involved it is only suitable for low voltage, high current drives. Single layer spirals, for instance formed by stamping, can be used as a building block for more extensive coils, however . Figures 2 to 11 illustrate the stages involved in producing a vertically and horizontally sub-divided block with a far larger number of series turns. In a solid machine coil, as in other coils, it is important that the relative direction of current flow in each layer and each spiral of the layer must be consistent. Otherwise the magnetic fields will subtract from one another rather than add. Thus a coil in which the current in one layer flowed from the inside spiralling outwards to the outside and then on the next layer down spiralled inwards from the outside to the inside would result in a severely weakened or entirely cancelled magnetic field. Many easy configurations to manufacture are thus excluded as they would not function. The present invention manufactures the coil from a single block 1 of conductor, such as copper. The block is provided with the desired external periphery and a central aperture 3 is removed. This aperture 3 is occupied by a core of soft magnetic material in use. This basic configuration, as illustrated in Figure 2 is then cut by a series of parallel slits 5. The slits are cut through to the central aperture 3 in most directions. However, an end portion 7 is left as a unitary block, so connecting all of the slices 9 defined by the cuts 5 together. A further unitary block 11, outlined by the dotted line in Figure 2, is also left intact. This inclined wedge 11 also, therefore, joins each of the slices 9 together at this stage. The inclined wedge 11 is illustrated more clearly in Figure 3. The result of the next stage in the production process is shown in Figure 4. In this stage holes are drilled, broached or otherwise introduced into the wedge 11 so as to connect each cut 5 in the right hand part, as illustrated, with the cut 5 below in the left hand part. The apertures 13 so created result in a transition surface being formed in the wedge 11 between the slices 9 in descending order. Thus each slice in the right hand portion is connected with the one below it in the left hand portion through a step. If the general area 15, shown in Figure 2, is treated as the "start of the spirals" the portion 17 of the top slice and the portion 19 of the bottom slice are in effect redundant and can be removed. A perspective view of the coil with these portions removed is illustrated in Figure 5. Figure 5 also illustrates the next stage in the production process. In this stage each slice 9 is divided into a series of concentric spirals 21 by vertical cuts which pass down through the full depth of the coil. These vertical cuts pass down through the wedge 11, but at the other end of the coil the cuts are only made in selected positions. The selective nature of these cuts is best illustrated in Figure 6. Figure 6 provides a top plan view of the coil of Figure 5 and clearly illustrates the concentric spirals 21 provided by the vertical cuts. The transition surface 23 formed by the top hole drilled into the wedge 11 is also clearly shown. At the other end of the coil the portion 7 is still left intact. A number of cuts are made into the area inward of portion 7, vertically at different locations. Each of the concentric spirals 21 is provided with an in-turned portion 25. A series of full depth vertical cuts 27 are also provided through a central strip. Each slice 9 is therefore left with a triangular shaped solid area 29 with the portion 7 still connecting each of these portions 29, and as a consequence all the slices 9 together. The next stages affect differing slices in differing ways. The top slice 9 is provided with a series of start portions 31 for each of the concentric spirals 21. The top layer 9 alone, in plan view, is illustrated in Figure 7 from the start 15 up to the transition surface 23 where it descends to the next slice 9. The start portions 31 are formed by a series of vertical cuts 33 which only pass through the top slice 9. The slices between the top slice of Figure 7 and the bottom slice of Figure 9 are represented by the layout in Figure 8. This illustrates a single slice through the coil consisting on the right hand side of slice 9 and a lower left hand slice 9, the two being connected together by the transition surface 23 as illustrated in Figure 4. Thus any given spiral 21 passes along right hand slice 9 and then down through the material forming the transition surface 23 to continue as the same spiral 21 but at a lower level. The left hand slice 9 continues round and passes under the right hand slice 9 to form the next slice down and so on. The intervening slices 9 therefore consist of a series of concentric coils which spiral downwards in a relatively straightforward manner. The continuous nature of the spirals 21 is provided by a series of vertical cuts 35 which are provided to join the cuts 27 to the cuts defining the turnover portions 25 on each side. These cuts 35 are provided through each of the slices bar the top slices of Figure 7 and the bottom slice of Figure 9. Access to form these cuts is achieved through the cuts forming the turnover portions 25 and the central cuts 27. These allow spark eroders or the like to be inserted and moved to the desired degree. Slices of the type illustrated in Figure 8 join the bottom slice of Figure 9 at a further transition surface 23 of the type previously discussed. From the transition surface 23 each of the concentric spirals 21 continues round to the turnover portions 25 previously formed. As with the top layer a series of cuts 37 are introduced to form connected tail portions 38 to the end portion 7. These cuts 37 do not therefore interfere with the continuous spiral in the layers above. At this stage the end portion 7 still links all of the slices 9 together in a continuous manner. As illustrated in figure 10, therefore, a full width vertical cut 39 is introduced to separate all the intermediate slices 9 from the end portion 7. The top slice of Figure 7 and the bottom slice of Figure 9, however, are still connected to the portion 7. Following this stage, therefore, the intermediate slices 9 are connected to the next slice above and / or below only at transition surface 23. The bottom slice is however still connected to portion 7 and hence to the top slice. Portion 7 is, however, now cut further to provide discrete connections between the end of one spiral 21 in the bottom slice of Figure 9 to the beginning of a different spiral 21 in the top slice of Figure 7. The cuts to achieve this are illustrated in Figure 11 which represents an end view of this face. A series of parallel inclined cuts 41 are provided between the end surface of portion 7 and the previously introduced cut 39. A series of inclined strips 43 result, each strip connecting one tail portion 38 in Figure 9 to a start portion 31 on the top slice. The cuts 41 may be inclined throughout, as shown, and thus at their top and bottom assist in defining the start / tail portions 31, 38. Alternatively the inclined butts 41 may extend between the ends of the vertical cuts defining the start / tail portions 31, 38 in portion 7. The outer most spiral 21 on the top slice starts at area 15, start portion 31, the inlet, and proceeds along the outside of the top slice of the coil to transition surface 23 where it descends to the outside of the next slice down. This continues until it reaches the end of the outside spiral on the bottom slice illustrated in Figure 9. This portion corresponds to point 49 on the right hand most inclined strip 43 of Figure 11. This strip leads back up to the top slice of Figure 7 where it connects to the start of the innermost of the spirals 21. This spiral then proceeds through the various spirals by means of the transition surfaces 23 until it too reaches the bottom slice of Figure 9 where it ends at the end of the innermost spiral. From here it is transferred back up to the top slice once more. The sequence of progress of the current through the start portions 31 of the top slice is illustrated in Figure 12. Here a current is fed into the top slice and proceeds via spiral A. From the bottom slice it is returned to spiral B; then on to spiral C and spirals D to J in sequence. On reaching the bottom slice following its descent on spiral J the current is led away from the spiral once more along outlet 50. In an alternative embodiment illustrated in Figure 13, which minimises and simplifies the strip structure, the final strip returns to an outlet I provided in the top layer. A redundant zone 60 separates the outlet I from the other spirals. The arrangements resulting from this cutting process result in a large number of spiral coils all connected in the correct sequence to provide adding magnetic forces. The various cuts separating the components of the coil can be provided with insulation by introducing a fluid form material into the various cuts. Various techniques can be provided to ensure that the coating is continuous throughout all of the cuts produced. The insulating material can then be dried or cured in its final form to provide a high degree of insulation. Pre- coating the finished assembly with an anti-corrosion plating, prior to the ceramic based insulation may be desirable. The fully formed coil can then be positioned on a soft magnetic core which occupies all or part of aperture 3 and connected up for use. The coil form illustrated in Figure 14 provides a particularly advantageous coil for use in a pole arrangement for a magnetic bearing in which adjacent poles are provided with non- constant gaps. As shown in Figure 15 the gap 100 between poles 102 reduces considerably towards the shaft area 104. The coil 106 of Figure 14 accommodates such a structure but provides the maximum turn numbers and packing factor. The coil 106 is formed of a series of layers 108 corresponding to the larger constant gap portion between the poles 102. Subsequent layers 110 closer to the shaft 104 are of step wise reduced width to correspond to the decreasing gap between poles 102. Instead of forming layers with a portion of a number of spirals thereon it is possible to form individual layers as an individual spiral. Figure 16a illustrates one such layer where a spiral conductor from the outside of the coil to the inside is illustrated. In practice a central aperture for soft magnetic material is provided but this does not affect the manner in which this embodiment performs. According to the orientation of the conductor path in the layer of 16a the magnetic force generated thereby is illustrated by the arrows, anti-clockwise spiral. If the next layer in the coil was connected to this one with the coil in the same arrangement then the magnetic force would be generated in an opposing direction with a cancelling effect. However, if the next layer represents an eguivalent version to the first layer in layout, but rotated through 180°, by flipping it over, then the central part of the layer of Figure 16a can be connected by a weld or other electrical conductor 200 which extends between that layer to the adjacent layer of Figure 16b. Thus current is able to pass from the central portion of one layer to the central portion of the adjacent layer. Current is fed into the layer of Figure 16a by means of a weld 202 passing down the outside of the coil to the layer below it for instance. Equally the outside of the coil of Figure 16b is connected to the layer above it by means of a further weld 204. A series of layers alternately arranged in this way and connected at their centre to their layer above and at the outside to the layer below can give rise to a composite structure having many spiral turns in it, yet maintaining all the advantages of the solid nature of each layer. As an alternative to manufacturing the coil or individual layers thereof by removing material from a solid block or layer it is possible to form individual layers by a deposition process. In this way a layer consisting of a significant number of part spirals, the other parts of the spirals being in adjacent layers, or alternatively of a series of layers of a spiral each, as illustrated in Figure 16, can be produced. The material may be electro deposited, investment casted, provided by a rapid prototyping process or screen printed for instance. The production process may be assisted by application of ultrasound, for instance to promote even take up of the mould / cast by the material or by the insulator. In a rapid prototyping process the resin can be caused to harden at the relevant locations. Ceramic or metallic inks can be used to form the constituent parts. The soft magnetic or other core material can be simultaneously provided or deposited. The use of screen printing is particularly preferred. The conducting material can be deposited in the necessary configuration for a given layer to provide the discrete conductor path and gaps / cuts. In the embodiment illustrated in Figure 17 as a side view a layer 302 of insulating material is provided. This may also be formed by screen printing onto a support substrate 300. The layer of insulating material 302 is provided in the desired configuration but with an omitted strip 304. To this structure the electrically conducting layer 306 can then be applied by screen printing. Careful control of the application of the printing ensures that as well as providing a layer 306 of conducting material on the insulating material 302, a portion 308 of the conducting material 306 also extends into the gap 304 in the insulating layer 302. This slot element 308 of conducting material in effect provides the transition from one layer down to the next as illustrated for the solid machined coil in Figure 4. The strip 304 extends from the outside of the coil to the central aperture which contains a soft magnetic material in use. A further gap 310 in the electrical conducting material 306 ensures the current path. If desired a further stage can be applied to this structure to introduce insulating material into the cuts, 310 for instance, between different spiral portions in the layer of electrically conducting material. In this way full insulation is provided, potentially also by screen printing. A series of layers of this type, with the portion 308 staggered in the manner outlined above and the support 300 removed can then be stacked one on top of each other to provide an overall coil in which the conductor path leads around the outside of one layer and then down and around the outside of each layer below in order. A return path from the end of the outside conductor in the bottom layer to the inner most spiral start on the top layer can be provided by attaching a block of transition paths of the type described above to one end or side of the coil. A continuous spiral of a very large number of turns is thus provided. Other techniques such as investment casting are also envisaged for the production of coils according to the present invention. In an investment casting technique the coil structure is first defined in a wax or plastics material with the necessary gaps desired for the insulating material. The insulating ceramic can then be introduced into this structure so as to fully take up its position. It is then possible to replace the wax / plastic by melting out using the molten electrically conducting material which is introduced. The solid machine coil of the present invention provides significant advantages over wire wound coils, particularly for high temperature applications and applications where weight to power is important. High temperature applications involve significant increases in copper losses, principly due to the increase in resistivity arising in the copper due to the elevated temperatures. The stresses and strains involved in winding wires preclude the use of plating materials such as nickel. Existing wire wound structures, therefore need nickel cladding to be deployed around the copper wire to maintain its integrity during the forming process and resist corrosion / copper losses in use. This not only results in a significantly increased cost due to the raw material and manufacturing costs involved in the cladded products, but also causes a significant reduction in the volume of copper present in the resulting coil volume. Solid machine coils on the other hand present a fully finished product to which a thin layer of protective plating can readily be introduced. No subsequent processing is required and the levels of stress and strain associated with winding in the prior art are therefore avoided. Lower cost corrosion protection and higher volumes of copper in the product result. Similar factors apply to the provision of the insulating coating. Suitable insulating coatings can be provided by ceramic materials, but the stresses and strains imposed in the winding process again reduce the turn radius which can be achieved and in a great number of cases result in chipping or other damage to the insulation during winding. The reliability of the inner structure is therefore significantly reduced. Solid machine coils, however, once again present a chance to introduce the insulation to a finished structure which in subsequent handling will be relatively stress and strain free. A higher degree of insulation is therefore rendered possible. The level of insulation which can be applied and the choice of suitable materials is also significantly increased by the present invention. Winding techniques require a ceramic coating which is sufficiently flexible to withstand the winding process. No such requirements are imposed by the present invention. Once again enhanced performance arises. Ceramic and other non-polymeric insulators are also advantageous in radioactive environments. Polymers decay relatively rapidly in such locations due to damage to the polymer strands. Ceramics offer higher levels of insulation and longer service life. Soft magnetic material or other contributory materials to the magnetic field may be provided in or dispersed in the insulating material. This may be achieved without disrupting the insulation. The packing factor applied to a coil is a significant feature in controlling the copper loss and / or in the overall mass required of the coil to produce the desired force. Typical wound coils in prior art machines achieve packing factors in the order of 40 to 50%. This factor is, potentially reduced where ceramic coated wires and / or cladding are required for high temperature applications due to the difficulties in coiling such materials. Solid machine coils on the other hand offer very significant improvements in packing. Packing factors approaching 80% are rendered feasible by the present invention. Although the coil and its applications have been principally discussed in relation to the provision of a magnetic field, the coil is equally suited to other applications including inductive uses.

Claims: CLAIMS ; 1. A method of forming a coil, the coil comprising one or more elements, each element comprising one or more electrical conductor paths or portions thereof, in which at least a substantial part of the electrical conductor path(s) or portion (s) in an element are created in a fixed configuration relative to each other within that element. 2. A method according to claim 1 in which the conducting path(s) is created by the removal of material from an electrically conducting component. 3. A method according to claim 1 in which the conducting path(s) is produced by the application of electrically conducting material to a location. 4. A method according to any of claims 1 to 3 in which the coil is formed of or made up from a series of layers. 5. A method according to claim 4 in which one or more layers are provided with a different thickness to one or more other layers. 6. A method according to claim 4 or claim 5 in which the coil or two or more layers thereof are formed from a unitary piece. 7. A coil comprising one or more elements, each element comprising one or more electrical conductor paths or portions thereof, in which at least a substantial part of the electrical conductor path(s) or portion (s) in an element is created in a fixed configuration relative to other electrical conductor path(s) or portion (s) within that substantial part of the element. 8. A coil according to claim 7 in which the electrical connection between one or more layers in the coil is an integral component of one or more of the layers. 9. A coil according to claim 7 or claim 8 in which one or more layers has a reduced width and / or length when compared with one or more other layers. 10. A coil according to any of claims 7 to 9 in which one or more layers are provided with a different thickness to one or more other layers. 11. A coil according to any of claims 7 to 10 in which the coil has two or more layers, each layer having one or more spiral paths extending inwardly about itself, the hand of the spiral in one layer being the opposite of that in an adjacent layer. 12. A coil according to any of claims 7 to 11 in which the cross sectional area is greater in inner spirals than in outer spirals or in the inner portion compared with the outer portion of a spiral. 13. A coil according to any of claims 7 to 12 in which a start portion of one or more of the spirals on the top layer is connected to a tail portion on a spiral of the bottom layer.

Inventors: HOWE DAVID, JEWELL GERAINT WYN, HOWE, DAVID, JEWELL, GERAINT, WYN
Applicants: BRITISH NUCLEAR FUELS PLC, HOWE DAVID, JEWELL GERAINT WYN, HOWE, DAVID, JEWELL, GERAINT, WYN
Application year: 1997
Main classification: H01F41
IPC codes: H01F17, H01F27, H01F41