GB2592574A - A modular stator arrangement - Google Patents

Abstract

An electrical machine 300 for use in an aircraft, comprising a rotor and a plurality of stator modules 300A, 300B, 300C, wherein each module comprises a stator core 302 having an annular cross section and slots 304 for receiving conductors 306 to form windings of the stator. The windings form phases where each phase comprises a set of conductors wound through the slots and where the windings are connected to a set of power electronics modules 314A, 314B, 314C. The electronics modules are mounted radially around the core. The electronics modules may be located between axial ends 312A, 312B of the respective core. Each set of conductors may comprise end windings at first and second longitudinal ends of the core, wherein the end windings 310A, 310B, extend perpendicularly to the longitudinal axis of the core, the end windings extending beyond an outer edge of the core defining a space between the end windings. The electronics modules of each stator module may then be mounted in that space. The electronics may comprise AC to DC power converters, rectification and may be connected to a common DC output. The machine may also comprise cooling circuit located between the core and electronics. Conductive bridges may be provided between different conductive elements of the windings. The machine may be a three-phase machine suitable for use in an aircraft propulsion system.

Description

A Modular Stator Arrangement The present invention relates to an electrical machine for use in an aircraft comprising a modular stator arrangement. In particular, the invention relates to an electrical machine comprising multiple stators arranged on a common rotor, wherein the power electronics are integrated with each stator module.

Background to the Invention

Electric aircraft propulsion systems typically comprise a fan (propeller), which is connected to an electrical machine. The electrical machine is typically formed of an assembly of magnetic circuit components, comprising a rotor and a stator. As is well known, in an electric machine, such as a generator, rotation of the rotor relative to the stator causes interaction of the magnetic field generated by the rotor with windings provided on the stator, generating an induced electromotive force (ElvIF) and/or electrical current. In a permanent magnet generator, the rotor's magnetic field is produced by permanent magnets, which induces an AC voltage in the stator windings as the stator windings pass through the moving magnetic field of the permanent magnet. The AC voltage is then converted into a DC voltage using suitable electronics that are typically located remote from the generator.

In certain applications, variable voltage DC may be used to provide power in an electric or hybrid-electric aircraft. Active rectifier circuits are highly efficient at turning AC power into controllable voltage DC power, but cooling is required to account for the heat generated as a result of the high power levels dissipated, even in highly efficient conversion circuitry.

Further, there is room for improvement in the reliability and resilience of known electric machines. In particular, for aircraft employing a hybrid fuel/electric drive system, it is important that the electrical machine does not fail completely.

Therefore, there is a need for an improved permanent magnet generator that can preferably provide variable voltage DC power.

Summary of the Invention

A first aspect of the present invention provides an electrical machine for use in an aircraft, comprising a rotor, and a plurality of stator modules, wherein each stator module comprises a stator core having a substantially annular cross section and having a plurality of slots for receiving conductors to form windings of the stator module, a plurality of phases, wherein each phase comprises a set of conductors wound through a plurality of the slots of the stator core, and a set of power electronics modules for treating the electrical output of the windings, wherein each power electronics module of the set is electrically connected to conductors of a phase of the plurality of phases, and wherein the power electronics modules are mounted radially around the stator core.

As such, an electrical machine is provided that comprises a plurality of stator modules arranged around a common rotor, wherein each stator module provides a separate DC channel. As there is no magnetic cross-linking between the individual stator modules, the electrical machine comprises fully segregated DC channels that are both electrically and magnetically decoupled. Moreover, by configuring the electrical machine in this way, the power electronics for converting the output of the windings can be integrated into the stator modules. This provides a compact arrangement, and enables the hot parts of the electrical machine, that is, the stator windings and the power electronics, to be cooled concurrently by the same cooling system.

For clarity, it will be appreciated that the slots of the stator core of each stator module may be longitudinally extending in a direction of a longitudinal axis of the stator module between a first longitudinal end of the stator core and a second longitudinal end of the stator core.

The power electronics modules of each stator module may be located between axial ends of the respective stator core.

Each set of conductors may comprise a set of end windings at first and second longitudinal ends of the stator core, wherein the end windings extend in a plane substantially perpendicular to the longitudinal axis of the stator core, the end windings extending beyond an outer edge of the stator core to thereby define a space between the set of end windings at the first longitudinal end of the stator core and the set of end windings at the second longitudinal end of the stator core. That is to say, the end windings come out further than the stator core to provide a region of overhang at each end of the stator core.

The power electronics modules of each stator module may be mounted in the space between the set of end windings at the first longitudinal end of the stator core and the set of end windings at the second longitudinal end of the stator core. That is to say, the power electronics modules may be mounted such that they sit outside of the stator core and between these overhanging portions of the end windings The power electronics modules of each stator module may be configured to convert AC power to DC power. Preferably, the power electronics modules of each stator module are connected to a common DC output. Consequently, as the phase windings and power electronics modules within each module are outputting to the same DC channel, problems with magnetic cross-linking within the stator module can be minimised. Each stator module thus provides a single DC channel that is neither electrically nor magnetically coupled to the DC channels of the other stator modules. As such, if a DC channel in one of the stator modules fails, it will not significantly disturb the operation of the remaining DC channels.

For example, the power electronics modules of each stator module may be comprise rectifier switches. In many cases, active rectifier switches will be used. In another example, the power electronic modules may comprise diodes.

The electrical machine may further comprise a coolant circuit, wherein the coolant circuit is configured to deliver cooling fluid to the windings and the power electronics modules of each stator module. By locating the power electronics modules in close relation to the stator core, the cooling circuit used to cool the windings of the stator module can also be used to cool the power electronics modules. As no separate or additional cooling system is required for the power electronics, this contributes to reducing the overall weight of the system.

The coolant circuit may be configured to simultaneously or sequentially deliver cooling fluid to the windings and the power electronics modules of each stator module.

In some cases, the coolant circuit comprises a carrier located between the stator core and the power electronics modules, the carrier having one or more channels for receiving cooling fluid. For example, the carrier may be a heat sink ring mounted around the outside of the stator core.

The plurality of stator modules may be arranged in series along the rotor.

In some arrangements, the set of conductors of each phase are wound in a concentrated winding configuration. Alternatively, the set of conductors of each phase may be wound in a distributed winding configuration.

Each set of conductors of a respective phase may comprise 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 extending between respective first ends of the first and second conductor elements in a direction substantially perpendicular to the first and second conductor elements.

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, 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, and a step portion extending longitudinally relative to the stator core between the first and second bridge portions by which the first and second bridge portions are electrically connected.

By configuring the windings in this way, the conductive bridges can be arranged to sit flat against the end face of the stator core of each stator module, thus enabling the stator modules to be stacked closely together to thereby minimise the overall length of the rotor required.

In such arrangements, the conductive bridges of the plurality of windings may extend beyond an outer edge of the respective stator core. This provides the region of overhang between which the power electronics modules are mounted. Furthermore, this overhang enables easy access to the side of the power electronics modules laying against the stator core, which also makes that region a suitable place for connecting the windings to the power electronics.

In some arrangements, the plurality of phases comprises three phases.

A further aspect of the present invention provides an aircraft propulsion system comprising an electrical machine as described above.

Brief Description of the Drawings

Further features and advantages of the present invention will become apparent from the following description of embodiments thereof, presented by way of example only, and by reference to the drawings, wherein: Figure 1 is a block diagram illustrating a schematic view of an electrical machine according to the present invention; Figure 2 illustrates one stator module according to the present invention; Figure 3 illustrates a plurality of stator modules according to the present invention; Figure 4 shows part of a stator module according to the present invention for illustrating a winding arrangement; Figure 5 shows a partial cross-section of the stator module shown in Figure 3; Figure 6 shows detail of an end of the stator module shown in Figures 4 and 5; Figure 7 illustrates an enlarged perspective view of the end of the stator module shown in Figures 5-6, in which a sub-set of windings are shown; Figure 8 shows a cross-section through a portion of the stator module of Figures 5-7; Figures 9A and 9B illustrate a schematic cross-sectional view of the stator module of Figures 4-8, in which a single winding is shown at different stages of manufacturing; and Figure 10 a schematic of an aircraft propulsion system comprising an electrical machine according to the present invention.

Detailed Description of Preferred Embodiments

The present disclosure relates to an electrical machine for use in an aircraft in which a modular stator is arranged around a single rotor. The disclosure addresses drawbacks which the inventors have identified in known electric machines. An eVTOL aircraft typically employs a larger number of rotors than the typical single main rotor plus tail rotor design used by conventional helicopters. For safety of flight, it is preferable that these should be as independent as possible. Conventional permanent magnet generators used in such aircraft use a 3 phase winding design with multiple stars to allow for multiple DC channels per generator, that is, a number of segregated sets of windings on a single stator. However, whilst the DC channels are not connected electrically, there is still magnetic cross-linking within the stator. In stators that are fitted with two or more stars, each star is electrically separate, in that they do not share a common neutral. However, they are still magnetically coupled in that they share the same stator core and rotor magnets. Therefore, any sudden change in current on one star will be communicated magnetically through the stator to the other star(s). It is common for two wires from different stars to be sharing the same slot, so in the event of a fault, for instance a short circuit, two stars could be lost rather than just one. Sudden loss of one channel therefore risks disturbance or loss to other channels. This approach is complicated to scale -a new winding and potentially iron design would be required to scale the size up and down, even if the same rotor size was retained. For example, starting with a 100kW stator that is 100mm long and produces 100V, to obtain a 200kW stator, this could be achieved with the same design by simply stretching it to 200mm long. However, this would increase the voltage to 200V in the process, which would exceed the maximum allowable voltage for minimising the weight of the electrical system as a whole.

In the disclosed electric machine, each stator module is integrated with power electronics for converting the generated AC voltage into DC voltage. By providing multiple stators around a common rotor, fully segregated DC channels are provided that are neither electronically nor magnetically connected. In doing so, variable DC power is provided, with each DC channel operating independently of the others, such that loss of one channel does not adversely affect any of the other channels. Furthermore, each stator module can be integrated with the power electronics in such a way as to enable shared cooling of both parts. As with most conventional stators, the power electronics for conventional permanent magnet generators have a relatively high thermal power density, up to about 1 MW/m2, and thus cooling of both is required. As such, by integrating the two components together so that the cooling system may be routed through both simultaneously, the overall weight of the whole system may also be significantly reduced. Furthermore, this arrangement provides improved scalability, since doubling the power output can be easily achieved by adding another stator module, with the voltage remaining the same in the process.

Figure 1 illustrates an example of an electrical machine 100 according to the present invention, comprising two stator modules 102A, 1025 arranged around a common rotor 104. Whilst not shown in detail here, it will be appreciated that the rotor 104 will typically comprises a rotating component, preferably in the form of a ferromagnetic back iron, and an array of permanent magnets distributed around the circumference of the rotating component.

Each stator module 102A, 102B comprises a magnetic stator core 106A, 1065 with a plurality of longitudinal slots (not shown) extending through the stator core 106A, 1065 in the direction of the longitudinal axis of the machine 100 (denoted by arrow labelled "X"). A plurality of conductors, typically formed from copper wire, are wound around the slots of the stator cores 106A, 106B to define phases of the stator modules 102A, 102B, as will be described in more detail below. In Figure 1, only the end windings 108, 1085 are shown.

For a conventional variable frequency generator, the power is used directly by all the loads on an aircraft, and so the generator must be of sufficient specification to provide the required quality of power for all equipment under all required circumstances. These power quality requirements therefore force the use of quite a thick back iron for the stator core, enabling the outer diameter of the conductors to match that of the stator core quite closely. For any generator where the power is run through power electronics, the power electronics take the raw generator output and convert it. This is true of both a "smart" permanent magnet generator, where all of the power electronics for the whole generator are housed within the same casing to combine them with the electrical machine, as well as those with the power electronics mounted remotely from the generator. This removes a need for such high power quality to be output from the generator, because the power electronics can adapt the way they convert the power to maintain a constant output, and means that the same power quality requirements are not placed on the stator. Consequently, a very thin back iron layer can be used in the stator.

As can be seen in Figure 1, this can result in the end windings 108A, 108B coming out to a greater diameter than the stator core 106A, 1066, creating a void between the end windings, outside of the stator core 106A, 1066, that must be filled in order to fit the stator module 102A, 1026 into its housing (not shown) securely. This void can thus be filled with at least one thermally conductive housing 110A, 110B containing the power electronics for the respective stator module 102A, 102B, the power electronics comprising at least the rectifier switches for converting AC voltage to DC voltage, optionally along with any other control electronics that may be required. In some cases, the rectifier switches are active rectifier switches.

This enables simple cooling of the rectifier switches since the hot parts are closely co-located with the stator module 102A, 102B. For example, a carrier 112A, 112B such as a heat sink ring may be placed around the stator core 106A, 1066 of each stator module 102A, 1026 through which cooling fluid may be passed, the deliver cooling fluid thus being delivered to both the windings of the stator modules 102A, 1026 and the power electronics modules 110A, 110B mounted around the outside of the stator core 106A, 106B. Alternatively, the rotor may comprise a fluid delivery system comprising jets that are configured to spray cooling fluid outwards in order to simultaneously cool the stator modules 102A, 1026 and the power electronics modules 110A, 110B. It will of course be appreciated that any cooling system that is suitable for delivering cooling fluid to the stator modules 102A, 102B may be used.

Furthermore, by filling the void with power electronics 110A, 1106, the amount of wasted space is reduced. This minimises the packaging weight, and makes modularising the machine 100 straightforward, since the filtering stage can be located outside the stator modules 102A, 1026, filling up the same axial length as the stator module 102A, 1026. In this respect, filtering is often preferred, to deal with the noise in the DC power signal generated by the power electronics, which may be noise caused by residual AC that the rectifier switches have not quite dealt with, or noise at the switching frequency of the rectifier switches and the like. Such noise is often at levels above that which is acceptable by an aircraft power system, and so filters such as a capacitor and/or inductor can be used to clean up the signal to obtain DC power with less noise. The casing of the power electronics modules 110A, 110B may be configured to link together to effectively form a housing around the outside of the electrical machine 100, with the filtering components then being located outside of said housing.

Figure 2 illustrates a single stator module 200 in more detail. As before, the stator module 200 comprises a stator core 202 comprising a plurality of longitudinal slots 204 distributed evenly around the circumference of the stator module 200 and that extend through the stator core 202 in the direction of the longitudinal axis. The stator module 200 further comprises a plurality of windings for each of the three phases (denoted A, B and C), which may be wound in a concentrated or distributed winding configuration, although other suitable winding arrangements known to the skilled person can be used in the alternative. For each phase, a plurality of conductors 206 may be wound around diametrically opposing groups of adjacent slots 204, depending on the number of poles. For example, a four pole machine with three phase output, as in the case of the presently illustrated example, the conductors 206 are wound around two diametrically opposing groups of adjacent slots 204. For a six pole machine, the conductors 206 can be wound around three diametrically opposing groups of adjacent slots 204 that are 120° apart, while other similarly configured arrangements are envisaged for other pole numbers.

The end windings 210A, 21B may extend in the radial direction beyond the stator core 202 such that there is an empty region between the end windings 210A of a first end 212A of the stator module 200 and the end windings 2106 of a second end 2126 of the stator module 200. This space can be used to integrate the power electronics, specifically, those associated with the AC to DC conversion, into the stator module 200. For each phase, a power electronics module 214A, 2146 and 214C is provided on the outer circumference of the stator, and may be located in a space formed between parts of the end windings 210A, 210B. Each power electronics module 214A, 21413 and 214C may comprise a thermally conductive carrier containing the power electronics, in particular, the active rectification switches, for converting the AC voltage generated by the respective phase windings into a DC voltage. Each set of phase windings and their respective power electronics modules 214A, 2146 and 214C will preferably correspond to a single DC channel. Each stator module 200 is preferably a single star stator, in this case with three phases, wherein the phase wires may be connected together so that they can share a common neutral. As such, all of the phase windings and power electronics modules 214A, 214B and 214C may be electrically coupled. Rectifying only a single phase to a DC channel is possible, but may be excessively noisy for certain applications. For reduced noise levels, rectifying all phases of a star (in the illustrated case, 3 phases) to a common DC channel provides a reasonably smooth DC output with little noise, which can then be run through a relatively light weight filter to obtain a good quality DC signal.

Each power electronics module 214A, 2146 and 214C is connected to the respective phase windings via the end conductors 208A, 208B and 208C of each set of phase windings and a corresponding busbar 216A, 2166 and 216C. As such, the first power electronics module 214A is connected to the end conductors 208A corresponding to the phase A windings, the second power electronics module 2145 is connected to the end conductors 208B corresponding to the phase B windings, and the third power electronics module 214C is connected to the end conductors 208C corresponding to the phase C windings. A neutral busbar 218 is then also provided, which connects the neutrals (not shown) of all three phases together.

As such, the power electronics having a high thermal power density are located in close proximity to the stator, which itself has high thermal power density. This provides a compact arrangement that enables simultaneous cooling of both components.

Figure 3 illustrates an example of a modular stator system 300 according to the present invention comprising a plurality of stator modules 300A, 3005 and 300C. As described previously with reference to Figure 2, each stator module 300a, 3005 and 300C comprises a stator core 302 comprising a plurality of longitudinal slots 304 distributed evenly around the circumference of the stator module 300A, 300B and 300C, and that extend through the stator core 302 in the direction of the longitudinal axis. Each stator module 300A, 300B and 300C further comprises a plurality of conductors 306 wound around groups of adjacent slots to provide plurality of windings for each of the phases, as illustrated and described in relation to Figure 2.

As will be appreciated by the skilled reader, in a fully assembled electrical machine, a rotor will be present within a void 320 provided in the stator modules 300A, 3005 and 300C, and will be arranged to rotate relative to the stator modules 300A, 3005 and 300C about a rotation axis X. The axis X extends in a longitudinal direction relative to the axis of rotation of the modular stator assembly 300. Magnetic flux created in the rotor interacts with the windings 306 of each stator module 300A, 3005 and 300C to generate a voltage and/or current in the stator windings in a conventional manner.

The end windings 310A, 3105 extend in the radial direction beyond the stator core 302 such that there is an empty region between the end windings 310A in the direction of a first end 312A of the modular stator system 300 and the end windings 310B in the direction of a second end 3125 of the modular stator system 300. As described above, this space is used to integrate the power electronics, specifically, those associated with the AC to DC conversion, into the stator modules 300A, 3005 and 300C. For each phase of each stator module 300A, 3005 and 300C, a power electronics module 314A, 3145 and 314C may be provided in the space between the end windings 310A, 3105. Each power electronics module 314A, 3145 and 314C may contain the power electronics, in particular, the active rectification switches, for converting the AC voltage generated by the respective phase windings into a DC voltage. Each power electronics module 314A, 314B and 314C may be connected to the respective phase windings via the end conductors 308A, 308B and 3018C of each set of phase windings and their corresponding busbars 316A, 316B and 314C. As such, the first power electronics module 314A of each stator module 300A, 300B and 300C can be connected to the end conductors 308A corresponding to the phase A windings of said stator module 300A, 300B, 300C, the second power electronics module 314B of each stator module 300A, 3005 and 300C can be connected to the end conductors 3085 corresponding to the phase B windings of said stator module 300A, 300B and 300C, and the third power electronics module 214C of each stator module 300A, 300B and 300C can be connected to the end conductors 308C corresponding to the phase C windings of said stator module 300A, 300B and 300C. As before, each stator module 300A, 300B and 300C has a neutral busbar 318 for connecting the neutrals of all three phases together.

As before, each stator module 300A, 3005 and 300C provides a single DC channel, wherein the power electronics modules 314A, 314B and 314C and the respective phase windings of each stator module 300A, 300B and 300C are connected to a common DC channel. As such, the power electronics modules 314A, 314B and 314C of each stator module 300A, 3005 and 300C work together to provide a single DC output per stator module 300A, 3005 and 300C.

In this arrangement, there is no magnetic cross-linking within or between the stator modules 300A, 300B and 300C. Consequently, if a DC channel in the first stator module 300A was suddenly lost, its loss will not disturb the operation of the DC channels corresponding to the other two stator modules 300B and 300C.

As such, the modular stator system 300 shown in Figure 3 provides three fully segregated DC channels, which are both electrically and magnetically decoupled. By providing a modular stator system 300 where each module provides a single DC channel, the required magnetic decou piing is achieved with sufficient electrical isolation to ensure that the failure of one DC channel will not cascade to the other DC channels of the system.

It will also be appreciated that each stator module may comprise any suitable number of phases, and indeed machines requiring a large amount of power may benefit from having higher multiples of phases being rectified to a common DC channel. Furthermore, the filtering requirements are reduced at a higher number of phases, since the DC ripple is at a higher frequency, can be filtered out of the output more easily, with less complex filtering electronics. Likewise, any number of stator modules may be used depending on the amount of power required. For example, an electrical machine may comprise six stator modules with each stator module comprising six phases to provide six fully segregated DC channels that are both electrically and magnetically decoupled.

The modular stator system 300 can then be arranged around a common rotor (not shown) to provide an electrical machine that is fault redundant, compact and easy to manufacture. In this respect, the number of stator modules 300A, 300B and 300C can be selected depending on the power requirements of each channel and the multiple redundancy requirements, for example, 2 stator modules may combine to provide 200kW overall, 3 stator modules may provide 300kW etc. Based on the number of stator modules 300A, 300B and 300C to be used, the rotor length can be adjusted accordingly with no other changes to the design required. In this respect, the axial length of the electrical machine can be a critical parameter because of the very high speeds involved, and thus it can be necessary to keep the rotor length as short as possible to avoid any issues with bending and vibration of the rotating shaft and associated rotor module. As such, it is also preferable to have a short design for each stator module. For example, to ensure that the final electric machine has an acceptable length and weight, the conductors may need to be designed as hairpins that lie flat against the ends of the stator module, as will now be described in detail. Arranging the conductors such that they have a different cross-section and shape in the end windings compared to through the stator core can be particularly advantageous because of the very high electrical frequency such machines operate at.

Figure 4 shows an example of stator 400 for illustrating a possible configuration of the stator windings in aspects of the present invention. The stator 400 includes a stator core 402 and a plurality of windings 404. As can be seen and as is typical for known electrical machines, the core 402 has a substantially cylindrical outer surface 403 and a substantially cylindrical inner surface 405. Whilst the stator core 402 is shown here to extend radially outwards to the same diameter as the end windings 404A and 404B, it will be understood that in embodiments of the present invention the stator core 402 will be thinner to provide a space for the power electronics, as described above with reference to Figures 1-3 above.

As described previously, the stator 400 comprises a plurality of slots 406 which extend longitudinally along the length of the stator core 402. In the illustrated example, the slots 406 extend substantially parallel with the rotation axis X. The slots 406 are arranged in a substantially circular array and are arranged with a longitudinal opening toward the inner surface 405 of the core 402. This longitudinal opening preferably extends along substantially all of the length of the core 402. 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 406 and through the stator core 402, 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 406 preferably occupy substantially the same circumferential position relative to the core 403, i.e. they are at the same angular position around the circumference of the core 402 in the same slot 406.

At the first longitudinal end 408 of the stator core 402, 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 disclosure relates principally to the arrangement of the windings at the first end 408 of the stator 400, 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 402 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.

As discussed previously, the windings 404 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 410 at either end of the electrical path by which the winding group may be connected to power electronics, such as contained in the thermally conductive carriers shown in Figures 1-3.

As can be appreciated from the overall assembly shown in Figure 4, the longitudinal extent of the overhang portion outside of the core 402, where the conductor elements of each winding 404 are connected to one another by the conductive bridge is relatively small, when compared to conventional arrangements of stator windings, where bent conductor elements and/or brazed connections have been used.

For the present invention, by configuring the conductive bridges such that they sit flat against the end face of the stator core of each stator module, the stator modules can be stacked closely together to thereby minimise the overall length of the rotor required.

Figure 5 shows a partial cross-section view of the first end 408 of the stator 400 shown in Figure 4. As can be seen, the stator 400 includes first and second base plates 510 and 520 arranged on the first end face of the stator core 402 on which the windings 404 are supported. The first base plate 510 is fixed on the end face of the stator core 402 substantially parallel to the first base plate 510 and the second base plate 520 is spaced apart from the first base plate 510 in the longitudinal direction. The first and second base plates 510 and 520 are preferably planar. The first and second base plates 510 and 520 are preferably annular. The first and second base plates 510 and 520 preferably include a plurality of apertures which are aligned with the slots 406 in the stator core 402 and through which the conductor elements of each winding extend. Alternatively, the first and second base plates 510 and 520 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 510, 520. Preferably, the base plates 510 and 520 are formed from a heat resistant material. Preferably, the base plates 510 and 520 are formed from an electrically and thermally insulating material. Preferably, the base plates 510 and 520 are formed from a ceramic material.

Figure 6 illustrates in more detail an example of a path of a selected winding 404 of the plurality of windings provided in the stator 400. In Figure 6, the first and second base plates are omitted for clarity. As an illustrative example, the winding 404 has a first conductor element 610 extending longitudinally through the stator core 402 in the radially outer position in a first slot 406A and a second conductor element 620 extending longitudinally through the stator core 402 in the radially inner position in a second slot 406B. The first and second slots 406A and 406B are separated from one another by a radial angle around the circumference of the stator. A conductive bridge 630 is formed to connect the first conductor element 610 to the second conductor element 620. As can be appreciated from the figures, the conductive bridge 630 extends in one or more planes which are preferably substantially perpendicular to the direction of extension of the conductor elements 610 and 620 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 630 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 7 illustrates an enlarged perspective view of the first end of the stator showing the winding 404 of Figure 6 in more detail. The base plates and several of the adjacent windings have been omitted from Figure 7 for clarity. As can be seen, the conductive bridge 630 comprises a first bridge portion 631 extending from the first end of the first conductor element 610, and a second bridge portion 632 extending from the first end of the second conductor element 620. The conductive bridge 630 also has a longitudinally extending portion 640 in the form of a step portion, which connects the first bridge portion 631 to the second bridge portion 632. The first bridge portion 631 extends in a first radially outward direction relative to, and in a first circumferential direction about, the core 402. The second bridge portion 632 extends in a radially inward direction, and also in the first circumferential direction around the core 402. Preferably, the second bridge portion 632 extends further in a radial direction than the first bridge portion 631 such that the first conductor element 610 can be located in a radially outer location it its respective slot 406A, while the second conductor element 620 can be located in a radially inner position in its respective slot 406B. The first bridge portion 631 of the conductive bridge 630 passes under a first array of conductive bridges of windings of the assembly, such that the first bridge portion 631 is disposed between the stator core 402 and the second array of conductive bridges, while the second portion 632 of the conductive bridge 630 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 632 and the core 402. As can be appreciated, one or more of the windings 404 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 640 is preferably provided at an outer-most radial position of the conductive bridge 630. In this way, the longitudinal change in path of the conductive bridge 630 occurs at or adjacent to its radially outermost point. It will be appreciated that this is necessary in order for the conductive bridge 630 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 620 in slot 406B. One or both of the first and second bridge portions 631 and 632 may comprise a first curved sub-part 633, 634 which has a tighter radius of curvature than a second subpart 635, 636. The first sub-part may be located adjacent the first conductor element 610 and/or between a second sub-part and the first conductor element 610. The second subpart may be located away from the first conductor element 610. In this manner, the conductive bridge can follow a first path away from the first end of the first conductor element 610 that has a first relatively tight curve 633, which connects to a straighter and less curved portion 635 extending to a radially outward extent of the conductive bridge at a longitudinally extending portion 640. The longitudinally extending portion 640 of the conductive bridge 630 can be connected to a curved sub-part 636, which has a lesser radius of curvature than a further curved sub-part 634 located adjacent to the second conductor element 620 and which preferably connects to the first end of the second conductor element 620. 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 402, as illustrated in the assembly of the earlier figures.

For the purposes of this example, the stator core 402 is shown to extend to the same radial extent as the first and second bridge portions 631, 632. However, it will be appreciated that in the present invention, the stator core 402 is thinner such that the bridge portions 631, 632 extend beyond the outer edge of the stator core 402, thereby defining a space between the bridge portions 631, 632 at each longitudinal end of the stator module in which the power electronics modules are arranged to sit.

Figure 8 shows a cross-section through a portion of the stator 400 showing the arrangement of the slots 406. The required size of the slots 406 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 402. To date, the small slot-size in the core 402 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 402 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 8, it is possible to form one or more of the radially outer conductor elements 610 in the outer position in the slots 406 of the core 402 with a first aspect ratio and to form one or more of the radially inner conductor elements 620 in the inner position in the slots 406 of the core 402 with a second aspect ratio which is different to the first aspect ratio. As illustrated, one or more of the conductor elements 610 located in an outer radial position in the slot 406 may have a relatively wider circumferential dimension, and/or may have a shorter radial dimension, when compared to the conductor elements 620 in the inner radial positions. Conversely, one or more of the radially inner conductor elements 620 located at the radially inner position in the slot 406 may have a greater radial dimension relative to the outer conductor element 610 and/or may have a smaller circumferential dimension than the outer conductor element 610. 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 610. 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 610 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 400.

Figures 9A and 9B 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 610 of each winding is provided in the radially outer position of a first slot while the second conductor element 620 is provided in the radially inner position of a second slot. A first base plate 510 is positioned over the first end 408 of the stator core 402 such that the first and second conductor elements extend through apertures in the first base plate 410. The first portion 631 of the conductive bridge 630 is then fabricated directly on the first base plate 510 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 9A. 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 510 and fused together into the required shape using a laser, thereby forming one or more layers of the first bridge portion 631. This process is repeated to form further layers of the first bridge portion 631, with each layer adhering to the previous layer. Once the required height for the first bridge portion 631 has been achieved, a second base plate 520 is positioned over the first base plate 510 and the first bridge portion 631. The second base plate 520 may have radially inner and outer spacer elements 521 and 522 by which the second base plate 520 is spaced from the first base plate 510 in the longitudinal direction of the stator 402. The second base plate 520 may be fixed to the first base plate 510 by the spacer elements 521 and 522. The step portion 640 is fabricated on top of the first bridge portion 631 using the additive manufacturing process. The step portion 640 may be fabricated before or after the second base plate 520 has been positioned over the first base plate 520. The second bridge portion 632 is then fabricated directly on the second base plate 520 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 632. This stage is shown in Figure 9B.

In the present invention, the outer edge of the stator core 402 will lie in closer relation to the first and second conductor elements 610, 620 such that the first and second bridge portions 631, 632 comprise a region of overhang defining a space in which the power electronics modules are configured to sit.

Figure 10 illustrates an example of a full-electric or hybrid-electric aircraft propulsion system 1000 comprising an electrical machine 1002 as described herein. The electrical machine 1002 comprises a rotor 1004 and a plurality of stator modules 1006A, 1006B and 1006C according to the arrangements described above. In this respect, the stator modules 1006A, 1006B and 1006C are arranged such that the power electronics for converting AC voltage to DC voltage are integrated therein, specifically, in a space between the end windings. Whilst three stator modules 1006A, 1006B and 1006C are shown in Figure 10, it will be appreciated that any suitable number of stator modules may be used depending on the power required by the aircraft.

The electrical machine 1002 is connected to an aircraft propeller 1008 by means of a rotating shaft 1010, wherein the electric machine 1002 drives the shaft 1010 to thereby drive the propeller 1008.

In the context of a full electric or hybrid-electric aircraft, the electrical machine described herein may be used in a propulsive system, wherein the electric motors driving the propellers of the aircraft by converting the electrical power, supplied by electrical generators driven by a turboshaft or given by the battery, to a mechanical power (torque).

In particular, the electrical machine described herein comprising a modular stator system is advantageously used for high speed/low torque machines such as direct-drive generators attached to a prime mover such as a gas turbine.

Various modifications, whether by way of addition, deletion and/or substitution, may be made to all of the above described embodiments to provide further embodiments, any and/or all of which are intended to be encompassed by the appended claims.

Claims (16)

1. CLAIMS1. An electrical machine for use in an aircraft, comprising: a rotor; and a plurality of stator modules, wherein each stator module comprises: a stator core having a substantially annular cross section and having a plurality of slots for receiving conductors to form windings of the stator module; a plurality of phases, wherein each phase comprises a set of conductors wound through a plurality of the slots of the stator core; and a set of power electronics modules for treating the electrical output of the windings, wherein each power electronics module of the set is electrically connected to conductors of a phase of the plurality of phases, and wherein the power electronics modules are mounted radially around the stator core.
2. 2. An electrical machine according to claim 1, wherein the power electronics modules of each stator module are located between axial ends of the respective stator core.
3. 3. An electrical machine according to claims 1 or 2, wherein each set of conductors comprises a set of end windings at first and second longitudinal ends of the stator core, wherein the end windings extend in a plane substantially perpendicular to the longitudinal axis of the stator core, the end windings extending beyond an outer edge of the stator core to thereby define a space between the set of end windings at the first longitudinal end of the stator core and the set of end windings at the second longitudinal end of the stator core.
4. 4. An electrical machine according to claim 3, wherein the power electronics modules of each stator module are mounted in the space between the set of end windings at the first longitudinal end of the stator core and the set of end windings at the second longitudinal end of the stator core.
5. 5. An electrical machine according to any preceding claim, wherein the power electronics modules of each stator module are configured to convert AC power to DC power.
6. 6. An electrical machine according to claim 5, wherein the power electronics modules of each stator module are connected to a common DC output.
7. 7. An electrical machine according to any preceding, wherein the power electronics modules of each stator module comprise rectifier switches.
8. 8. An electrical machine according to any preceding claim, further comprising a coolant circuit, wherein the coolant circuit is configured to deliver cooling fluid to the windings and the power electronics modules of each stator module.
9. 9. An electrical machine according to claim 8, wherein the coolant circuit is configured to simultaneously or sequentially deliver cooling fluid to the windings and the power electronics modules of each stator module.
10. 10. An electrical machine according to claims 8 or 9, wherein the coolant circuit comprises a carrier located between the stator core and the power electronics modules, the carrier having one or more channels for receiving cooling fluid.
11. 11. An electrical machine according to any preceding claim, wherein the plurality of stator modules are arranged in series along the rotor.
12. 12. An electrical machine according to any preceding claim, wherein each set of conductors of a respective phase comprises 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 extending between respective first ends of the first and second conductor elements in a direction substantially perpendicular to the first and second conductor elements.
13. 13. An electrical machine according to claim 12, wherein the conductive bridge comprises 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, 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, and a step portion extending longitudinally relative to the stator core between the first and second bridge portions by which the first and second bridge portions are electrically connected.
14. 14. An electrical machine according to claims 12 or 13, wherein the conductive bridges of the plurality of windings extend beyond an outer edge of the respective stator core.
15. 15. An electrical machine according to any preceding claim, wherein the plurality of phases comprises three phases.
16. 16. An aircraft propulsion system comprising an electrical machine according to any of the preceding claims.