FIELD OF THE INVENTION

The invention relates to a stator for a multi-phase electrical machine, in particular an electromagnetic multi-phase generator. The invention relates in particular to the manufacture of such stators for multi-phase generators used in aircraft.

BACKGROUND OF THE INVENTION

Multi-phase generators and their general principles of construction will be well known to the person skilled in multi-phase electrical machines. As will be well understood, a multi-phase generator generally comprises a stator having a large number of winding slots. These slots are provided for carrying conductors of the windings of the stator. In modern generators, it is common to find up to 100 such winding slots and each slot typically contains two conductors, one arranged radially inward of the other, and each extending longitudinally through the stator, parallel with an axis of rotation of the generator. These conductors must be connected to one another at each end of the stator. Typically, at a first end of the stator, the inner conductor of a first slot is connected to the outer conductor of a second slot. In a typical three phase machine, these first and second slots will be arranged around the circumference of the stator, separated by a radial angle from one another, for example by a circumferential offset angle of 60 degrees. A single winding will exit a first end of the stator at a first point, will extend circumferentially around the end face of the stator, outside of the stator core, and will re-enter a second slot in the stator core having travelled a certain angular distance around the end of the stator. As mentioned, in some examples this angle is around 60 degrees for certain three phase generators used in aircraft, but can be different for different generator configurations or numbers of phases.

Typically, this path of the windings outside of the stator core is achieved by bending the winding in the region of its exit and entrance points to the slots in the core of the stator. Typically, for electromagnetic reasons, the profile of the winding is such that its radial dimension in the stator is greater than its circumferential dimension and so the winding can only typically be bent in a direction perpendicular to the slot in the stator, and in a certain direction (i.e. the direction of the shorter, circumferential dimension of the conductor). The required form for routing the winding path around the stator core end face in a circumferential direction as described above, is sometimes therefore typically formed by either brazing separate conductor elements together, or by using a complicated set of tooling to produce a "hairpin" shaped conductor element with the appropriate geometry. These techniques add time, cost and complexity to the manufacturing process for the stator and the associated generator of which it may form a component. WO2015/198432 discloses bending U-shaped components for a stator winding into a U-shaped form, inserting the legs of the bent component into a stator core and connecting them to further electrical conductor elements at the distal ends of the legs.

However, a certain air gap must be maintained between the windings for electrical insulation purposes and for thermal cooling purposes. A further limitation is that the size of the conductor wires used, and hence their electrical resistance and the electrical losses experienced in the stator, is dictated by the size of the slot through which the conductors of the winding must be fitted. With all these restrictive factors in place, the space taken up outside the stator core to provide appropriate connections with necessary air gaps in between can contribute significantly to the overall length of the stator, and resultant overall length of the generator unit of which the stator forms a component. The bent end windings can take up a significant "overhang", measuring up to around 35% of the overall length of the stator assembly, including all of the windings. In certain designs of generators, the overhangs can take up as much as 60% of the total length. The space around the outside of this "overhang" can only be used at one end of the generator in current designs, and is in some cases otherwise redundant at the opposite end.

SUMMARY OF THE INVENTION

In addressing the drawbacks of the prior art, the present invention aims to provide a method of manufacture and an improved construction of a stator, which minimises the axial overhang length for at least one end of the stator. The invention further enables the winding cross-section in the overhang region to be greater than when it passes through the core, which can help with overall efficiency and thermal management.

According to a first aspect of the present invention, there is provided a method of manufacturing a stator for a multi-phase electrical machine, preferably for use in aircraft or the aerospace sector. The method comprises the steps of: providing a plurality of conductor elements in slots in a stator core of the stator such that the conductor elements extend between a first longitudinal end of the stator core and a second longitudinal end of the stator core; and forming a conductive bridge to electrically connect first and second conductor elements of a first winding at the first longitudinal end of the stator core, the conductive bridge extending between respective first ends of the first and second conductor elements in a direction substantially perpendicular to the first and second conductor elements, wherein the step of forming a conductive bridge comprises fabricating at least a portion of the conductive bridge using an additive manufacturing process to form one or more layers of an electrically conductive material at the first longitudinal end of the stator core.

The method of the invention enables the manufacture of windings with reduced axial overhang length at one or both ends of the stator compared to conventional methods. As the conductive bridge extends in a direction substantially perpendicular to the first and second conductor elements of the winding, the conductive bridge, or "end winding", can be made to lie flat, or close to flat, at the end of the stator. This can facilitate a reduction in the overall length, and thus weight, of the electrical machine with which the stator is used. The method allows the conductive bridge to be provided with a cross-section of differing shape or dimension or aspect ratios to the conductor elements, and/or to other conductive bridges, in order to increase electrical and/or magnetic performance of the stator. With the method of the invention, at least a portion of the conductive bridge is fabricated using an additive manufacturing process in situ at the first longitudinal end of the stator core. As used herein, the term "conductor element" is used to refer to a sub-section of the overall conductor which forms a winding of the stator core. The conductor element can form a portion of the conductor which extends between first and second axial ends of the core, through the core, and may be substantially straight.

As used herein, the term "additive manufacturing process" refers to any process in which a three-dimensional object is formed one layer at a time by addition of material to the object. Example processes include: vat polymerisation; material jetting; binder jetting; material extrusion processes such as fused filament fabrication; sheet lamination processes such as ultrasonic additive manufacturing and laminated object manufacturing; directed energy deposition three-dimensional printing processes such as laser engineered net shaping; and powder bed fusion processes, such as direct metal laser sintering, electron beam melting, selective heat sintering, selective laser melting and selective laser sintering.

As used herein, the term "electrically conductive material" refers to a material having a conductivity at 20 degrees Celsius of at least 2 x 10 ⁵ S/m, preferably at least 1.4 x 10 ⁶ S/m. The electrically conductive material preferably has a conductivity at 20 degrees Celsius of at least 1.0 x 10 ⁷ S/m, more preferably at least 3.0 x 10 ⁷ ., most preferably at least 5.0 x 10 ⁷ S/m. The electrically conductive material may comprise any suitable electrically conductive material, for example a conductive polymer, or graphite. Preferably, the electrically conductive material is selected from the group consisting of copper, chromium, zirconium, nickel, aluminium, titanium, and alloys thereof, or any combination thereof. In certain embodiments, the electrically conductive material comprises a chromium zirconium copper alloy.

Preferably, the electrically conductive material is formed from an electrically conductive powder and the additive manufacturing process comprises a powder bed fusion process. The additive manufacturing process may comprise a powder bed fusion process in which an energy beam is used to fuse a first quantity of the electrically conductive powder into at least one layer. The powder bed fusion process may further comprise using an energy beam to fuse at least a second quantity of the electrically conductive powder into at least one additional layer on the at least one layer. The at least one additional layer is fused to the at least one layer. Example energy beams include an electron beam or electromagnetic radiation, such as a laser beam, which is used to sinter or melt a powder material. In certain embodiments, the electrically conductive material is formed at least in part from copper powder. For example, the electrically conductive material may be formed from an alloy of copper, chromium and zirconium.

The additive manufacturing process may comprise forming one or more layers of the electrically conductive material directly onto the end face of the stator core. In such embodiments, the stator core comprises an electrically insulative material. For example, the stator core may be formed from an electrically insulative material, and/or the stator core may comprise a layer or coating of electrically insulative material on one or both longitudinal end faces. This can prevent short circuiting of the end windings by the stator core. The method may further comprise forming the conductive bridge to comprise a first bridge portion and a second bridge portion both extending in a direction substantially perpendicular to the first and second conductor elements, wherein the first bridge portion and the second bridge portion are spaced apart in a longitudinal direction of the stator. By configuring the conductive bridge in this way, a the stator may be manufactured more quickly, the amount of "overhang" may be reduced, thereby resulting in a smaller overall length of stator unit, and manufacture may be relatively quick, cheap and low complexity. Furthermore, according to this design, each of the conductive bridges may be substantially the same, resulting in substantially the same electrical resistance for each conductive bridge.

Preferably, the method further comprises providing at least one base plate at the first longitudinal end of the stator core on which layers of the electrically conductive material are formed using the additive manufacturing process. The base plate thus provides a substrate for the additive manufacturing process onto which the conductive bridge can be fabricated. The base plate can be selected according to the additive manufacturing process desired. This can allow more flexibility in the type of additive manufacturing process employed without the need to modify the stator core to accommodate the additive manufacturing process. The provision of at least one base plate may also facilitate the fabrication of a conductive bridge comprising multiple parts which are spaced apart in the longitudinal direction. The provision of at least one base plate between the stator core and the conductive bridge can reduce the risk of damage to the stator core during fabrication of at least a portion of the conductive bridge using an additive manufacturing process.

Preferably, the at least one base plate comprises a heat resistant material. This can facilitate manufacture of the conductive bridge using a high temperature additive manufacturing process, such as a powder bed fusion process using a laser. The at least one base plate may comprise a layer or coating of heat resistant material on one or more outer surfaces. The at least one base plate may be formed from a heat resistant material. As used herein, the term "heat resistant material" refers to a thermally insulative material having a melting point of at least 800 degrees Celsius. Preferably, the heat resistant material has a melting point of at least 1000 degrees Celsius, more preferably at least 1100 degrees Celsius. Preferably, the heat resistant material has a thermal conductivity at 25 degrees Celsius of less than 50 W.m TK ^_1 , preferably less than 10 W.m TK ^_1 , and more preferably less than 2 W.m TK ^_1 .

The method may further comprise the step of removing the at least one base plate after forming the conductive bridge. For example, the at least one base plate may be removed by disassembly, destruction, or chemical erosion. In such methods, the at least one base plate may be a sacrificial component. The at least one base plate may be formed from an electrically conductive material. This can facilitate separation of the at least one base plate from the conductive bridge, for example using a spark erosion process. The at least one base plate may be formed from the same material as the conductive bridge.

In some embodiments, the method may further comprise the step of fabricating an intermediate support structure on the at least one base plate using the additive manufacturing process. The conductive bridge may then be formed at least in part on the intermediate support structure. With this arrangement, the intermediate support structure is disposed between the at least one base plate and at least part of the conductive bridge. This can reduce the thermal stresses experienced in the base plate during fabrication of the conductive bridge. Preferably, the intermediate support structure is porous. For example, the intermediate support structure may comprise a lattice structure. This can further reduce the thermal stresses in the base plate.

Where the method further comprises the step of removing the at least one base plate after forming the conductive bridge, the intermediate support structure can facilitate removal of the at least one base plate by weakening the link between the conductive bridge and the at least one base plate. Preferably, the intermediate support structure is porous, for example comprising a lattice structure. This can facilitate the removal of the intermediate support structure from the conductive bridge and the at least one base plate.

Preferably, the at least one base plate comprises an electrically insulative material. This avoids short circuiting of the windings by the base plate. This can facilitate manufacture of the stator, since it allows the at least one base plate to remain in the stator assembly after forming the end windings. The at least one base plate may comprise a layer or coating of electrically insulative material on one or more outer surfaces. The at least one base plate may be formed from an electrically insulative material.

The at least one base plate may comprise any suitable material. For example, the at least one base plate may be formed from a plastics material. Preferably, the at least one base plate comprises a ceramic material. For example, zirconia or alumina. The at least one base plate may comprise a layer of ceramic material on one or more outer surfaces. The at least one base plate may be formed from a ceramic material. The at least one base plate may comprise mica, fluorphlogopite mica, and/or borosilicate glass. One example suitable material is Macor (RTM) machinable glass ceramic, available from Corning Inc. of Corning, New York, which comprises 55 percent fluorphlogopite mica and 45 percent borosilicate glass.

Methods according to the invention comprise fabricating at least a portion of the conductive bridge of a winding using an additive manufacturing process in situ at the first longitudinal end of the stator core. The conductive bridge may further comprise one or more portions which are pre-formed, for example cast or extruded. Preferably, substantially all of the conductive bridge is fabricated using an additive manufacturing process in situ at the first longitudinal end of the stator core.

The conductive bridge may be configured to provide an electrical path between the first end of the first conductor element and the first end of the second conductor element when the first conductor element is disposed in a radially outer position in a first slot of the stator core and the second conductor element is disposed in a radially inner position in a second slot of the stator core.

The at least one base plate may comprise a single base plate. The single base plate may abut or be longitudinally spaced from the first longitudinal end of the stator core. In certain embodiments, the step of providing at least one base plate comprises providing a first base plate at the first longitudinal end of the stator core, and providing a second base plate over the first base plate. The first and second base plates may be spaced apart in the longitudinal direction of the stator. With this arrangement, the first and second base plates can provide support to first and second portions of the conductive bridge which are spaced apart in the longitudinal direction. For example, conductive bridge may comprise a first bridge portion which is pre-formed, for example cast or extruded, and supported on the first base plate, and a second bridge portion which is fabricated in situ on the other of base plate. Preferably, the step of forming a conductive bridge comprises fabricating a first bridge portion using the additive manufacturing process to form one or more layers of the electrically conductive material on the first base plate, and fabricating a second bridge portion using the additive manufacturing process to form one or more layers of the electrically conductive material on the second base plate. In such embodiments, the first and second bridge portions are both fabricated in situ on the respective base plates such that they are spaced apart in the longitudinal direction. The first and second bridge portions may be electrically connected to each other and/or to the first and second conductor elements by one or more pre- formed bridge portions. The first and second bridge portions may be connected to each other directly and/or to the first and second conductor elements, by one or more further bridge portions which are fabricated in situ on one or more base plates.

Preferably the first bridge portion extends from the first end of the first conductor element in a first plane substantially perpendicular to the first and second conductor elements and, wherein the second bridge portion extends from the first end of the second conductor element in a second plane substantially perpendicular to the first and second conductor elements. The first and second bridge portions may be electrically connected by a step portion of the conductive bridge which extends longitudinally relative to the stator core between the first bridge portion and the second bridge portion. The step portion may be pre-formed. Preferably, the step portion is fabricated in situ using the additive manufacturing process to form one or more layers of the step portion on the first bridge portion and/or on the first base plate.

The longitudinally extending step portion allows a longitudinal step to be created in the conductive bridge, preferably at a radially outward extremity of the conductive bridge. This also enables a space-efficient change in longitudinal direction of the path of the winding, since in order to pass from an inner position on a first slot, to an outer position on a second slot of the core, it is necessary for the winding to pass around an adjacent winding assembled on the stator. The first bridge portion of the conductive bridge may be configured to provide a first conductive path extending from the first conductor element through a first radial distance and a first circumferential distance relative to the stator core, while the second bridge portion of the conductive bridge may be configured to provide a second conductive path over a second radial distance and a second circumferential distance to connect the second conductor element to the first bridge portion. The second radial distance may be greater than the first radial distance.

In certain embodiments, at least one of the first or second bridge portions has a first sub-part which has a tighter radius of curvature in a plane perpendicular to the longitudinal axis of the stator than a second sub-part, the first sub-part being located radially inward of the second sub-part.

In any of the above embodiments, the conductive bridge may have a greater cross- sectional area than at least one of the first and second conductor elements. This allows electrical resistance in the conductive bridge to be reduced, thereby reducing heat losses from the conductive bridge. It can also reduce the overall electrical resistance of the windings. This may permit the use of lighter, less conductive materials, such as aluminium windings in place of copper windings. This could facilitate a significant weight reduction, which is particularly beneficial in aerospace applications, where weight is at a premium.

The first and second conductor elements may have substantially the same cross- sectional area. The cross-section of the second conductor element may have a different, preferably greater, dimension in a radial direction of the core stator, than that of the first conductor element. The cross section of the first conductor element may have a different, preferably greater, dimension in a circumferential direction of the stator core than that of the second conductor element. Providing different cross sections in the first and second conductor elements can allow a more efficient use of space in the magnetic core, so that electrical and/or magnetic performance can be improved. It can also allow the outer diameter of the stator core to be reduced by reducing the radial extent of the outer conductor element. This allows the provision of a more compact and lighter stator.

Where the method comprises providing at least one base plate at the first longitudinal end of the stator core on which at least a portion of the conductive bridge is fabricated using the additive manufacturing process, the first and second conductor elements may extend through apertures in the at least one base plate, and/or through slits around the inner and/or outer circumference of the at least one base plate. The at least one base plate may be planar. The at least one base plate may have a solid disc shape. The at least one base plate may be annular. Where the at least one base plate is annular, one or both of the first and second conductor elements may extend through the central aperture in the annular base plate, preferably immediately adjacent to the radially inner surface of the annular base plate.

Methods according to the invention preferably further comprise forming a plurality of conductive bridges, each of which is configured to electrically connect first and second conductor elements of one of a plurality of windings at the first longitudinal end of the stator. The plurality of conductive bridges may be formed such that the first bridge portion of a first conductive bridge passes between the second bridge portion of a second conductive bridge and the stator core, and the second bridge portion of the first conductive bridge overlays the first bridge portion of a third conductive bridge passes. Each of the plurality of conductive bridges may be manufactured according to any of the methods described above.

The method may further comprise simultaneously inserting first and second conductor elements of a plurality of windings into a plurality of corresponding slots in the stator core. The conductor elements may be pre-formed or pre-assembled as a pair of conductor elements connected at their second ends by a second conductive bridge to form a conductor. This allows pre-assembled conductors, sometimes referred to as 'hair-pins' in view of their shape, to be pre-assembled into an array outside of the core and then inserted simultaneously into the core. Once inserted into the stator core, a conductive bridge is fabricated according to the present invention to electrically connect the first end of a first conductor element of a first conductor to the first end of one of a second conductor element of a second conductor. The connected conductor elements thereby form first and second conductor elements of a winding. In such embodiments, the conductors may be cast in a mould. For example, each conductor may be cast having first and second substantially parallel conductor elements, or "legs", and a second conductive bridge extending, in a direction substantially perpendicular to the conductor elements, between respective second ends of the first and second legs, to electrically connect the first and second legs. The pair of conductor elements may then be inserted into the slots in the stator core such that at least a portion of the second conductive bridge lies in a plane substantially parallel with an end face of the core. The stator core may be any suitable hollow shape having a cavity within which a rotor of the electrical machine may be received. Preferably, the stator core is substantially cylindrical, for example having a circular cylindrical shape.

The method may further comprise the step of forming one or more electrical connections at an end point of one or more of the plurality of windings by which the windings may be connected to an electrical load. The step of forming the one or more electrical connections may comprise fabricating at least a portion of the one or more electrical connections using an additive manufacturing process to form one or more layers of an electrically conductive material. Preferably, the one or more electrical connections extends in a direction substantially perpendicular to the longitudinal axis of the stator core. The method may further comprise providing one or more further base plates at the first longitudinal end of the stator core and on top of the conductive bridge. The one or more layers of the electrically conductive material may be formed on the one or more further base plates using the additive manufacturing process. This can allow the phase connections to be made at the end points of the windings without the need to bend and/or braze the windings to form the connections. This can facilitate manufacture of the stator and reduce the amount of overhang at the end of the stator.

This novel method of manufacture allows the previously-used bending or brazing operations to be avoided, and allows a more efficient structure of the stator to be provided in which the "overhang" of the windings of the stator outside an end of the stator core in minimised. This can reduce the overall length and weight of a multi-phase electrical machine with which the stator is used. This is particularly beneficial in aircraft applications, where both space and weight are at a premium.

According to a second aspect of the invention, there is provided a stator for a multi- phase electrical machine, the stator comprising a stator core having a plurality of slots extending longitudinally in a direction of a rotation axis of the stator; a plurality of windings each having first and second conductor elements received in the plurality of slots and a conductive bridge at the first longitudinal end of the stator core to electrically connect the first and second conductor elements. The conductive bridge may extend between respective first ends of the first and second conductor elements in a direction substantially perpendicular to the first and second conductor elements. At least a portion of the conductive bridge has a microstructure indicative of an additive manufacturing process. As used herein, the phrase "a microstructure indicative of an additive manufacturing process" refers to a microstructure indicative of any process in which a three-dimensional object is fabricated one layer at a time. Example processes indicated by the microstructure include: vat polymerisation; material jetting; binder jetting; material extrusion processes such as fused filament fabrication; sheet lamination processes such as ultrasonic additive manufacturing and laminated object manufacturing; directed energy deposition three-dimensional printing processes such as laser engineered net shaping; and powder bed fusion processes, such as direct metal laser sintering, electron beam melting, selective heat sintering, selective laser melting and selective laser sintering. Preferably, at least a portion of the conductive bridge has a microstructure indicative of a powder bed fusion process. The skilled person will understand how to determine whether the microstructure of a portion of the conductive bridge is indicative of an additive manufacturing process. As a skilled person will appreciate, for some additive manufacturing processes, it may also be possible to determine whether a component, or part of a component, has been formed using that additive manufacturing process through visual inspection of a cross-section of the component, or through non-destructive visual inspection of surface features of the component.

The conductive bridge is electrically conductive and may be formed from any suitable material. Preferably, the conductive bridge is formed from a material selected from the group consisting of copper, nickel, aluminium, titanium, and alloys thereof, or any combination thereof. In certain embodiments, the conductive bridge is formed from copper and has a microstructure indicative of a powder bed fusion process in which a copper powder has been fused into one or more layers by an energy beam.

The conductive bridge may be arranged directly on the end face of the stator core. The conductive bridge may be spaced apart from the end face of the stator core by an air gap. In such embodiments, the conductive bridge may be fabricated by forming one or more layers of an electrically conductive material onto a base plate positioned between the end face of the stator core and the conductive bridge, after which the base plate is removed or destroyed. The conductive bridge may comprise a first bridge portion and a second bridge portion both extending in a direction substantially perpendicular to to the first and second conductor elements, wherein the first bridge portion and the second bridge portion are spaced apart in a longitudinal direction of the stator. This configuration may reduce the amount of "overhang", thereby resulting in a smaller overall length of the stator. Furthermore, each of the conductive bridges may be substantially the same, resulting in substantially the same electrical resistance for each conductive bridge.

Preferably, the stator further comprises at least one base plate between at least a portion of the conductive bridge and the first longitudinal end of the stator core. The at least one base plate comprises an electrically insulating material. Preferably at least a portion of the conductive bridge has a microstructure indicative of an additive manufacturing process and is fixed directly onto the at least one base plate. Such a base plate can simplify manufacture of the windings by providing a substrate onto which at least a portion of the conductive bridge may be fabricated using an additive manufacturing process. The at least one base plate can reduce the risk of damage to the stator core during the additive manufacturing process. Further, no additional process steps are required to remove the base plate following fabrication of the conductive bridge since the electrically insulative material prevents short circuiting of the windings by the at least one base plate. The at least one base plate may comprise a layer or coating of electrically insulative material on one or more outer surfaces. The at least one base plate may be formed from an electrically insulative material.

Preferably, the at least one base plate comprises a heat resistant material. This can facilitate manufacture of the conductive bridge using a high temperature additive manufacturing process, such as a powder bed fusion process. The at least one base plate may comprise a layer or coating of heat resistant material on one or more outer surfaces. The at least one base plate may be formed from a heat resistant material.

The at least one base plate may be held in place at the first longitudinal end of the stator core by the windings, for example by the conductive bridges of the windings. The at least one base plate may be fixed to the first longitudinal end of the stator core independently of the windings. For example, the base plate may be fixed to the first longitudinal end of the stator core by a mechanical connection or using an adhesive. This can result in a more robust assembly.

The conductive bridge comprises at least one portion having a microstructure indicative of an additive manufacturing process. The conductive bridge may further comprise one or more portions which are pre-formed, for example cast or extruded. Preferably, all, or substantially all, of the conductive bridge has a microstructure indicative of an additive manufacturing process.

The conductive bridge may comprise a first bridge portion extending from the first end of the first conductor element in a first plane substantially perpendicular to the first and second conductor elements. The conductive bridge may comprise a second bridge portion extending from the first end of the second conductor element in a second plane substantially perpendicular to the first and second conductor elements. The conductive bridge may further comprise a step portion extending longitudinally to the stator core between the first and second bridge portions by which the first and second bridge portions are electrically connected.

One or more of the first bridge portion, the second bridge portion and the step portion may comprise a microstructure indicative of an additive manufacturing process. In preferred embodiments, the first bridge portion, the second bridge portion and the step portion all have a microstructure indicative of an additive manufacturing process.

Preferably the at least one base plate comprises a first base plate between the first bridge portion and the first longitudinal end of the stator core. Preferably, the at least one base plate further comprises a second base plate between the first bridge portion and the second bridge portion, the first and second base plates being spaced apart in the longitudinal direction. The first and second base plates may be spaced apart in the longitudinal direction by the first bridge portion. The stator may further comprise one or more spacer elements between the first and second base plates by which the first and second base plates are spaced apart in the longitudinal direction.

The at least one base plate may comprise a plurality of apertures and/or slits around its circumference through which the first and second conductor elements extend. The at least one base plate may be planar. The at least one base plate may be annular. Where the at least one base plate is annular, one or both of the first and second conductor elements may extend through the central aperture in the annular base plate, preferably immediately adjacent to the radially inner surface of the annular base plate.

The stator may further comprise one or more electrical connections at an end point of one or more of the plurality of windings by which the windings may be connected to an electrical load. Preferably, at least a portion of the one or more electrical connections has a microstructure indicative of an additive manufacturing process. Preferably, the one or more electrical connections extends in a direction substantially perpendicular to the longitudinal axis of the stator core. The stator may further comprise one or more further base plates at the first longitudinal end of the stator core and on top of the conductive bridge. The one or more electrical connections may be supported on the one or more further base plates. This can allow the phase connections to be made at the end points of the windings without the need to bend and/or braze the windings to form the connections. This can facilitate manufacture of the stator and reduce the amount of overhang at the end of the stator.

A third aspect of the present invention provides an electrical generator arranged to be driven by an aircraft engine, the electrical generator comprising a stator according to any of the embodiments discussed above. A fourth aspect of the invention provides an aircraft propulsion system comprising an electrical generator according to the third aspect and an aircraft engine configured to drive the electrical generator.

A firth aspect of the present invention provides an aircraft comprising an aircraft propulsion system according to the fourth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will become apparent from the following description of an embodiment thereof, presented by way of example only, and by reference to the drawings, wherein like reference numerals refer to like parts, and wherein: Figure 1 shows a view of the first end of a stator according to the present invention;

Figure 2 shows a partial cross-section of the first end of the stator of Figure 1;

Figure 3 shows detail of the first end of the stator of Figures 1-2, in which the first and second base plates are omitted for clarity;

Figure 4 illustrates an enlarged perspective view of the first end of the stator of Figures 1-3, in which the first and second base plates and are omitted for clarity and a sub-set of windings are shown;

Figure 5 shows a cross-section through a portion of the stator of Figures 1-4; Figures 6 and 7 illustrate a schematic cross-sectional view of the stator of Figures 1-5, in which a single winding is shown at different stages of a manufacturing method according to the invention; and

Figure 8 shows a view of a second end of the stator of Figures 1-7.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a stator 10 for a multi-phase electrical machine in accordance with aspects of the present invention. Stator assembly 10 includes a stator core 100 and a plurality of windings 200. As can be seen and as is typical for known electrical machines, the core 100 has a substantially cylindrical outer surface 101 and a substantially cylindrical inner surface 102. As will be appreciated by the skilled reader, in a fully assembled electrical machine, such as a motor or a generator, a rotor will be present within a void 103 provided in the stator core 100, and will be arranged to rotate relative to the stator core 100 about a rotation axis X. Magnetic flux created in the rotor interacts with the stator windings 200 to generate a voltage and/or current in the stator windings in a conventional manner. The axis X extends in a longitudinal direction relative to the axis of rotation of the stator assembly 10. As will also be understood by the skilled reader, the stator 10 comprises a plurality of slots 104 which extend longitudinally along the length of the stator core 100. In the illustrated example, the slots 104 extend substantially parallel with the rotation axis X. The slots 104 are arranged in a substantially circular array and are arranged with a longitudinal opening toward the inner surface 102 of the core 100. This longitudinal opening preferably extends along substantially all of the length of the core 100. As will be appreciated by a skilled reader, each slot has disposed in it two conductor elements or conductor element portions of the windings, each defining a separate electrical path. The two conductor elements each extend longitudinally along the slot and through the stator core, parallel with the rotation axis X, with one of the conductor elements arranged radially inward of the other. The two conductor elements in a slot preferably occupy substantially the same circumferential position relative to the core, i.e. they are at the same angular position around the circumference of the core in the same slot.

At the first longitudinal end 110 of the stator core 100, the radially outer conductor element of a first slot is connected to the radially inner conductor element of a second slot by a conductive bridge to form a winding. These first and second slots are arranged around the circumference of the stator and are separated by a radial angle from one another. Thus, a single winding will exit the first end of the stator core at a first point, will extend circumferentially around the end face of the stator core outside of the stator core, and will re-enter a second slot in the stator core having travelled a certain angular distance around the end of the stator. In some examples this angle is around 60 degrees for certain three phase generators used in aircraft, but can be different for different generator configurations or numbers of phases. These features will be further illustrated in greater detail in later figures.

The present invention relates principally to the arrangement of the windings at the first end 110 of the core assembly 10, where the conductor elements of the windings are connected to one another in a relatively uniform manner. One or more, a majority, or substantially all, of the windings provided in the stator core may have substantially the same connection path between respective first ends of the conductor elements of the winding, i.e. the assembly may comprise a plurality of pairs of conductor elements, with an interconnection between the conductor elements in each pair being repeated for any or all of the pairs of conductor elements provided in the stator core.

To allow use of the stator in a multi-phase electrical machine, the windings are separated into a number of winding groups, as will be understood by the skilled reader. The winding groups each comprises a plurality of electrically connected windings forming an electrical path through the winding group and two end connectors 205 at either end of the electrical path by which the winding group may be connected to a power source (when used in a motor) or an electrical load (when used in a generator).

As can be appreciated from the overall assembly shown in Figure 1, the longitudinal extent of the overhang portion outside of the core 100, where the conductor elements of each winding are connected to one another by the conductive bridge is relatively small, when compared to prior arrangements of stator windings, where bent conductor elements and/or brazed connections have been used.

Figure 2 shows a partial cross-section view of the first end 110 of the stator 10. As can be seen, the stator includes first and second base plates 310 and 320 arranged on the first end face of the stator core 100 on which the windings 200 are supported. The first base plate 310 is fixed on the end face of the stator core 100 and extends in a direction substantially perpendicular to longitudinal axis of the stator core 100 and substantially parallel to the first end face. The second base plate 320 is substantially parallel to the first base plate 310 and is spaced apart from the first base plate 310 in the longitudinal direction. The first and second base plates 310 and 320 are preferably planar. The first and second base plates 310 and 320 are preferably annular. The first and second base plates 310 and 320 preferably include a plurality of apertures which are aligned with the slots 104 in the stator core 100 and through which the conductor elements of each winding extend. Alternatively, the first and second base plates may be annular and positioned radially outward of the first and second conductor elements so that the first and second conductor elements extend through the central aperture of each base plate 310, 320. Preferably, the base plates 310 and 320 are formed from a heat resistant material. Preferably, the base plates 310 and 320 are formed from an electrically and thermally insulative material. Preferably, the base plates 310 and 320 are formed from a ceramic material.

Figure 3 illustrates in more detail an example of a path of a selected winding 200 of the plurality of windings provided in the stator assembly 100. In Figure 3, the first and second base plates are omitted for clarity. As an illustrative example, the winding 200 has a first conductor element 210 extending longitudinally through the stator core 100 in the radially outer position in a first slot 104A and a second conductor element 220 extending longitudinally through the stator core 100 in the radially inner position in a second slot 104B. The first and second slots 104 and 104B are separated from one another by a radial angle around the circumference of the stator. A conductive bridge 230 is formed to connect the first conductor element 210 to the second conductor element 220. As can be appreciated from the figures, the conductive bridge 230 extends in one or more planes which are preferably substantially perpendicular to the direction of extension of the conductor elements 210 and 220 through the core, and preferably substantially perpendicular to the rotational axis X. In order for this electrical path to be created by the conductive bridge 230 it must pass along a path which passes over a first set of conductive bridges and then under a second set of conductive bridges. Further detail of how this may be achieved will be described in relation to later figures.

Figure 4 illustrates an enlarged perspective view of the first end of the stator showing the winding 200 of Figure 3 in more detail. The base plates and several of the adjacent windings have been omitted from Figure 3 for clarity. As can be seen, the conductive bridge 230 comprises a first bridge portion 231 extending from the first end of the first conductor element 210, and a second bridge portion 232 extending from the first end of the second conductor element 220. The conductive bridge 230 also has a longitudinally extending portion 240 in the form of a step portion, which connects the first bridge portion 231 to the second bridge portion 232. The first bridge portion 231 extends in a first radially outward direction relative to, and in a first circumferential direction about, the core 100. The second bridge portion 232 extends in a radially inward direction, and also in the first circumferential direction around the core 100. Preferably, the second bridge portion 232 extends further in a radial direction than the first bridge portion 231 such that the first conductor element 210 can be located in a radially outer location it its respective slot 104A, while the second conductor element 220 can be located in a radially inner position in its respective slot 104B. The first bridge portion 231 of the conductive bridge 230 passes under a first array of conductive bridges of windings of the assembly, such that the first bridge portion 231 is disposed between the stator core 100 and the second array of conductive bridges, while the second portion 232 of the conductive bridge 230 passes over a second array of conductive bridges of the assembly such that the first array of conductive bridges passes between the second bridge portion 232 and the core 100. As can be appreciated, one or more of the windings 200 can have a substantially similar or identical form or describe a substantially identical path, but a path which is displaced around the core by one slot distance relative to an adjacent winding. Preferably, each or substantially all of the windings in the array have a substantially identical form. The longitudinally extending portion 240 is preferably provided at an outer-most radial position of the conductive bridge 230. In this way, the longitudinal change in path of the conductive bridge 230 occurs at or adjacent to its radially outermost point. It will be appreciated that this is necessary in order for the conductive bridge 230 to pass over a sufficient number of further conductive bridges, and then under a sufficient number of further conductive bridges in order to reach the radially inner position of the conductor element 220 in slot 104B. One or both of the first and second bridge portions 231 and 232 may comprise a first curved sub-part 233, 234 which has a tighter radius of curvature than a second sub-part 235, 236. The first sub-part may be located adjacent the first conductor element 210 and/or between a second sub-part and the first conductor element 210. The second sub-part may be located away from the first conductor element 210. In this manner, the conductive bridge can follow a first path away from the first end of the first conductor element 210 that has a first relatively tight curve 233, which connects to a straighter and less curved portion 235 extending to a radially outward extent of the conductive bridge at a longitudinally extending portion 240. The longitudinally extending portion 240 of the conductive bridge 230 can be connected to a curved sub-part 236, which has a lesser radius of curvature than a further curved sub-part 234 located adjacent to the second conductor element 220 and which preferably connects to the first end of the second conductor element 220. As will be appreciated, any or all of the above features can help in facilitating the reduced extent of the overhang portion of the windings outside of the core 100, as illustrated in the assembly of the earlier figures.

Figure 5 shows a cross-section through a portion of the stator 10 showing the arrangement of the slots 104. The required size of the slots 104 is defined by electromagnetic considerations and so variations to the cross-sectional dimensions of the conductor elements within the slots are limited. To make the most efficient use of the slots to create electrical currents and the desired resulting electromagnetic fluxes in the core, an optimised design of the windings may provide a greater cross-section in the end sections of the windings, or "conductive bridges", present in the "overhang" section and a smaller cross-section in the conductor elements passing through the core. This is to reduce electrical resistance and heat losses due to resistance in the conductive bridges outside of the slots of the core 100. To date, the small slot-size in the core has typically made it impractical to use materials other than copper. Aluminium has been considered, but is not typically suitable for high-efficiency generator stators due its reduced electrical conductivity compared to copper and the higher temperatures generated during use for conductors of comparable size. However, an increase in cross-section of the end windings in the "overhang" section outside of the slots in the core 100 could reduce the electrical resistance of the overall windings. This can permit the use of aluminium in place of copper in the conductive bridge and/or in the conductor elements extending through the stator core. This could achieve a significant weight reduction, which is particularly beneficial in aerospace applications, where weight is at a premium. As can be seen in Figure 5, it is possible to form one or more of the radially outer conductor elements 210 in the outer position in the slots 104 of the core 100 with a first aspect ratio and to form one or more of the radially inner conductor elements 220 in the inner position in the slots 104 of the core 100 with a second aspect ratio which is different to the first aspect ratio. As illustrated, one or more of the conductor elements 210 located in an outer radial position in the slot 104 may have a relatively wider circumferential dimension, and/or may have a shorter radial dimension, when compared to the conductor elements 220 in the inner radial positions. Conversely, one or more of the radially inner conductor elements 220 located at the radially inner position in the slot 104 may have a greater radial dimension relative to the outer conductor element 210 and/or may have a smaller circumferential dimension than the outer conductor element 210. The provision of this radially outer array of legs being wider in the circumferential direction than the radially inner array can allow a reduced outer diameter of the slot while still allowing a sufficient amount of current to pass through the outer conductor element 210. This can allow the overall outer diameter of the core to be reduced. This in turn can reduce the overall weight of the stator assembly, since the outer diameter of the core can be reduced by the same amount as the reduction in radial dimension of the outer conductor element 210 provided by the described change in aspect ratio. Even greater differences in aspect ratio than that shown can be provided. However, there is a limit to the benefit which may be obtained, since it is preferable to avoid reducing the circumferential gap G between adjacent slots. This is because reducing the gap G can limit the magnetic flux which can pass through the core through the gap G between adjacent conductor elements, and this can limit the power output of the generator. Therefore, it can be beneficial to provide one or more windings in the assembly, which has a first conductor element having a first aspect ratio, and a second conductor element having a second aspect ratio. The differences between the aspect ratios of the first and second conductor elements can be provided as described above for at least one, preferably, a plurality and optionally for all windings of the stator assembly 10. Figures 6 and 7 illustrate a schematic cross-sectional view of the stator in which a single winding is shown at different stages of an example manufacturing method according to the invention. In the example manufacturing method, the first conductor element 210 of each winding is provided in the radially outer position of a first slot while the second conductor element 220 is provided in the radially inner position of a second slot. A first base plate 310 is positioned over the first end 110 of the stator core 100 such that the first and second conductor elements extend through apertures in the first base plate 310. The first portion 231 of the conductive bridge 230 is then fabricated directly on the first base plate 310 using an additive manufacturing process to form one or more layers of an electrically conductive material until the required height has been achieved, as indicated by the dashed line in Figure 6. In one example, the additive manufacturing process is a powder bed fusion process in which a thin layer of electrically conductive powder is spread across the first base plate 310 and fused together into the required shape using a laser, thereby forming one or more layers of the first bridge portion 231. This process is repeated to form further layers of the first bridge portion 231, with each layer adhering to the previous layer. Once the required height for the first bridge portion 231 has been achieved, a second base plate 320 is positioned over the first base plate 310 and the first bridge portion 231. The second base plate 320 may have radially inner and outer spacer elements 321 and 322 by which the second base plate 320 is spaced from the first base plate 310 in the longitudinal direction of the stator 100. The second base plate 320 may be fixed to the first base plate 310 by the spacer elements 321 and 322. The step portion 240 is fabricated on top of the first bridge portion 231 using the additive manufacturing process. The step portion 240 may be fabricated before or after the second base plate 320 has been positioned over the first base plate 320. The second bridge portion 232 is then fabricated directly on the second base plate 320 using an additive manufacturing process to form one or more layers of the electrically conductive material until the required height has been achieved for the second bridge portion 232. This stage is shown in Figure 7.

As will be understood by the skilled person, the first base plate 310 could be omitted and replaced with an electrically insulative coating on the first end face of the stator core 100. The coating is preferably also heat resistant. For example, the first base plate 310 could be replaced with a ceramic coating on the first end face of the core. As will be appreciated, some, the majority, or substantially all of the windings may be manufactured using the process described above. One or more further base plates may be positioned on top of and spaced from the second base plate. For example, the stator may comprise an outer base plate positioned over the second bridge portion. One or more further bridge portions may be fabricated on the one or more further base plates. One or more phase connections for the windings can be formed by fabricating one or more layers of the phase connections on the one or more further base plates using the additive manufacturing process. This can avoid the need for bending and brazing the conductors at the start and end points of each phase. In view of the flexible manner in which the conductive bridge is fabricated, it is possible to vary the cross-section of the conductive bridge as desired. This can allow the design of the windings to be optimised for the desired electrical performance characteristics, such as improved electrical performance and/or reduced overall resistance of the overall winding.

Figure 8 shows a view of a second end 120 of the stator assembly 10 of Figure 1. The second end 120 of the stator assembly 10 is shown for illustrative purposes, and a skilled person will appreciate that second ends of the conductor elements of each winding 200 can be connected to one another in a conventional manner, as is known for multi-phase electrical machines of the type discussed herein, to create the necessary electrical paths and connections to inputs and/or outputs of the electrical machine. As with the first end of the core, a single winding will exit the second end of the stator core at a first point, will extend circumferentially around the end face of the stator core outside of the core 100, and will re-enter a second slot in the stator core having travelled a certain angular distance around the end of the stator. The connections of the conductor elements at the second end 120 of the assembly may be the same as discussed in respect of the connections at the first end of the stator 110. In the example shown in Figure 8, the conductor elements are connected at the second end of the stator core 100 by a plurality of second conductive bridges 250. Each of the second conductive bridges is configured to extend between respective second ends of the first conductor element of a first winding and the second conductor element of a second winding in a direction substantially perpendicular to the conductor elements. Preferably, each of the second conductive bridges is substantially the same in structure and manufacture as the first conductive bridges. As will be appreciated, the methods of manufacture and the stators of the present invention can provide a more efficiently manufactured, or more efficiently configured, stator resulting in weight, material and efficiency saving, as can be realised by any or all of the features described above and herein and illustrated in the figures.