Rotor for a permanent magnet electrical machine

Abstract

A rotor 201 for a permanent magnet electrical machine 200 comprising a structural member 210, an array of permanent magnets 221, 222, 223, 224, disposed around a rotational axis 250 of the structural

Description

Technical Field 5 The invention relates to a rotor for a permanent magnet electrical machine. In particular, the invention relates to electrical machines having permanent magnets mounted to the rotor. Background of the Invention 10 15 Electrical machines, such as those employed as generators driven by aircraft engines, typically comprise a rotor and a stator in an assembly of magnetic components. As is well known, 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 (EMF) and / or electrical current. In a permanent magnet electrical machine, such as a permanent magnet generator, the rotor's magnetic field is produced by permanent magnets, which, in the case of a generator, induces an AC voltage in the stator windings as they pass through the moving magnetic field of the permanent magnets. Iron losses such as eddy currents induced in magnetic parts of the machine can cause inefficiencies due to electrical resistance and related heat losses, for example. There exists a need for improvements in rotors for permanent magnet machines. Summary of the Invention 25 The inventors have identified several problems with prior rotor assemblies for permanent magnet electrical machines. A typical rotor assembly may comprise a structural ferromagnetic shaft having a plurality of permanent magnets disposed around it. Rotation of the rotor within a stator can induce eddy currents in the rotor shaft leading to unwanted losses via heat dissipation. For higher frequency electrical machines, the magnitude of 30 these losses can be such that using a laminated rotor may not be practical due to the very small thickness of laminations required to mitigate losses. The inventors have found that most of the material of the ferromagnetic shaft is located within a DC electromagnetic field, meaning that minimal losses due to eddy currents are experienced. In contrast, a thin layer of the ferromagnetic shaft that is closest to the permanent magnets experiences 35 a high frequency electromagnetic field in which eddy current losses can be high. In view of the above, the present disclosure seeks to reduce losses of the electrical machine by minimising losses experienced in a thin surface layer of the rotor shaft. To achieve this, an improved rotor for a permanent magnet electrical machine and a method of assembly therefor is disclosed. The rotor has a structural member, such as a shaft which can rotate about a rotational axis, and permanent magnets positioned around the rotational axis. The rotor has an intermediate layer between the shaft and the magnets. 5 The intermediate layer comprises a soft magnetic composite. Such a material may comprise a powder of a ferromagnetic material, such as steel, with particles of such powder being interspersed with, and optionally coated in, an electrically insulating layer. The particle size being less than a tenth of a millimetre can increase the electrical resistivity and thereby decrease eddy current losses within each particle. 10 According to one aspect of the present invention, there is provided a rotor for a permanent magnet electrical machine, comprising any or all of the following features: a structural member, 15 an array of permanent magnets disposed around a rotational axis of the structural member, and an intermediate layer comprising a soft magnetic composite material, disposed between a surface of the structural member and at least one permanent magnet of the array of permanent magnets; wherein the intermediate layer comprises a gap that is aligned with at least one of the permanent magnets in the array. A 'soft magnetic composite' can be defined as a material comprising a ferromagnetic particulate material. The particles can be interspersed in an electrically insulating material. The average particle size may be less than 100 pm. The material can have a powdered or 25 powder-like particle size and distribution. The intermediate layer can be contrasted with a laminated component that may be formed of alternating layers of ferromagnetic material and insulating material. Unlike grains or adjacent particles which may be found within a single ferromagnetic sheet or layer in a laminated component, the particles of the soft magnetic composite in the intermediate layer may be substantially electrically insulated 30 from one another. In this way, the maximum dimension across which eddy currents may be induced is significantly reduced in the intermediate layer as compared to those in laminated components. In particular, eddy currents in the rotor can be significantly reduced by reducing the maximum geometric extent of any single conductive portion of ferromagnetic material. For a laminated component, such an extent may depend on the 35 surface area of a single layer, whereas in a soft magnetic composite, the maximum uninterrupted extent of conductive ferromagnetic material is limited by the particle size. The structural member may comprise a ferromagnetic material. The structural member may comprise a non-magnetic material and a ferromagnetic layer provided therearound. The structural member may consist of a non-magnetic material. There is provided a path, be it through the structural member and / or the intermediate layer, through which magnetic flux can permeate the rotor to pass between the magnets. The intermediate layer may be disposed around at least a portion of the circumference of the structural member. The intermediate layer may separate at least one face of a permanent magnet of the array from the structural member. The intermediate layer may comprise a plurality of soft magnetic composite sections. The plurality of soft magnetic composite sections may be disposed about the rotational axis. The number of permanent magnets in the array may be a multiple of the number of soft magnetic composite sections. The permanent magnets may be spaced equally about the rotational axis. The intermediate layer may comprise at least one circumferential gap. The at least one circumferential gap may comprise a space between the soft magnetic composite sections. At least one circumferential gap may be aligned with a centre of at least one of the plurality of permanent magnets. The intermediate layer may be disposed around at least one tenth, at least one eighth, at least one sixth, at least one quarter, at least one half, or at least three quarters of the rotational axis. The intermediate layer may be disposed around an entirety or substantially an entirety of the rotational axis. The intermediate layer may have a thickness between 0.1 and 10 mm, preferably between 0.5 to 5 mm, preferably between 1 and 3 mm. According to an aspect of the present invention, there is provided an electrical machine, for a generator of an aircraft engine, comprising the rotor as described hereinabove. The electrical machine may comprise a stator. The stator may comprise a plurality of stator windings. The electrical machine may be configured to have an electrical frequency which is greater than 1kHz. The electrical frequency may be greater than 5kHz, preferably greater than 10kHz. The electrical machine may further comprise power electronics. The power electronics may be electrically connected to the stator windings. The power electronics may be configured to treat an electrical current delivered to or from the stator windings for operation of the electrical machine. The power electronics may be configured to operate at a frequency of at least eight times the electrical frequency of the electrical machine. The power electronics may be configured to operate at a frequency of at least ten times the electrical frequency 5 of the electrical machine. According to an aspect of the present invention, there is provided an aircraft comprising an aircraft engine, the aircraft engine being connected to an electrical machine as described hereinabove. 10 According to another aspect of the present invention, there is provided a method for assembly of a rotor for an electrical machine comprising the steps of: providing a structural ferromagnetic member, forming an intermediate layer by disposing at least one soft magnetic composite 15 section on a surface of the structural ferromagnetic member, and providing a plurality of permanent magnets disposed around the structural ferromagnetic member such that the soft magnetic composite is located between the structural ferromagnetic member and at least one of the plurality of permanent magnets; wherein the intermediate layer comprises a gap that is aligned with at least one of the 20 permanent magnets. The step of forming the intermediate layer may comprise pressing the at least one soft magnetic composite section around the rotor. This step may be performed while the soft magnetic composite is in an unsintered state. 25 The step of forming the intermediate layer may comprise a step of pressing the at least one soft magnetic composite section onto the structural ferromagnetic member. The step of forming the intermediate layer may comprise a subsequent step of sintering the at least one soft magnetic composite section. 30 The step of forming the intermediate layer may comprise a step of sintering the at least one soft magnetic composite section. The step of forming the intermediate layer may comprise a subsequent step of gluing the at least one soft magnetic composite section onto the structural member. 35 The method may further comprise a step of grinding the soft magnetic composite layer. 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: 5 Figure 1 is a cross-sectional diagram illustrating an example of part of an electrical machine. Figure 2 is a schematic cross-sectional diagram of an electrical machine comprising a rotor according to aspects of the invention. 10 Figure 3 is a schematic diagram of an aircraft comprising a generator and comprising a rotor according to aspects of the invention. Figure 4 is a flow diagram illustrating an embodiment of a method according to an aspect 15 of the present invention. Detailed Description CM £\j Various factors in permanent magnet machines can increase the prevalence of harmonics, ■^“20 which can cause high iron losses as a result of induced eddy currents in the rotor, for example. Such losses typically cause additional heat to be dissipated in the rotor, which £\j can adversely affect the life and performance of the electrical machine. One solution is to actively cool the rotor using a coolant, but this adds additional weight and complexity to the assembly of the electrical machine. Another solution is to form the rotor at least 25 partially from a laminated component, which can be preferable at lower frequencies at which the eddy currents are sufficiently mitigated by the separation of the layers of the lamination. However, for a laminated material to be effective at higher frequencies, the layers of the laminated material would need to very thin, which may introduce complexity to the manufacture and assembly of the rotor, and may decrease its mechanical 30 properties. To illustrate one of the problems with prior devices, Figure 1 shows a section of an example of a permanent magnet electrical machine 100. The electrical machine 100 has a rotor 101, rotatable around a rotational axis (not shown) within a stator 140 disposed 35 concentrically around the rotor 101. The rotor 101 comprises a structural member 110 and a plurality of permanent magnets 120 disposed therearound. The stator 140 comprises a plurality of slots 141 having a plurality of stator windings 142 therein. The stator windings 142 may be connected to power electronics (not shown). As is well-known, rotation of the rotor 101 relative to the stator 140 induces an 5 electromotive force in the stator windings 142, thereby acting as a generator. Conversely, provision of an electrical current in the stator windings 142 generates a force which can act to turn the rotor 101, thereby acting as a motor. In either case, there are several sources in the electrical machine 100 that can contribute to the generation of harmonics, such as the switching of the power electronics, uneven or non-linear loads, and slot effects. 10 Such harmonics can induce losses such as eddy currents in the rotor 101. It has been found that the most significant eddy current losses are experienced in a layer 110a of the structural member 110 that is closest to the magnets 120. In the illustrated example, the layer 110a is the radially outermost layer of the structural member 110, disposed between the magnets 120 and the remaining part of the structural member 110. 15 Figure 2 shows a rotor 201 according to a first aspect of the present disclosure. The rotor 201 comprises a structural member 210 which may comprise a ferromagnetic material. The rotor 201 further comprises an array of permanent magnets disposed around a rotational axis 250 of the structural member 210. As shown in Figure 2, there may be an array of four permanent magnets 221, 222, 223, 224 equally spaced around the rotational axis 250. The rotor 201 further comprises an intermediate layer 230 comprising a soft magnetic composite material. The intermediate layer 230 is disposed between a surface of the structural member 210 and at least one of the permanent magnets 221-224. 25 The rotor 201 in Figure 2 is shown in the context of an electrical machine 200, more specifically a permanent magnet electrical machine, such as one suitable for use as a permanent magnet generator. Therefore, the electrical machine 200 may further comprise a stator 202, relative to which the rotor 201 may rotate about the rotational axis 250. In the illustrated arrangement, the stator 202 comprises a stator core 240, arranged around 30 the rotational axis 250 and comprising a plurality of slots 241, defined by a plurality of stator teeth 243, having stator windings 242 arranged therein. The structural member 210 of the rotor 201 may be a cylindrical shaft, preferably a hollow shaft, arranged to rotate around the rotational axis 250. The structural member 210 may 35 have a substantially annular cross section. The structural member 210 may comprise a ferromagnetic material, an example of which includes steel. The structural member 210 may comprise or consist of electrical steel, such as silicon steel. The ferromagnetic material may be continuous in the axial, radial and / or hoop directions of the structural member 210. The structural member 210 may comprise a laminated component, comprising alternating layers of steel and an electrically insulating material. Laminations may be positioned in a plane perpendicular to the rotational axis 250. However, forming the structural component from a solid mass of material such as a ferromagnetic material can 5 provide advantages in structural strength. In some examples, the structural member 210 can comprise or be comprised of a non-magnetic material, such as a carbon fibre composite. Alternatively or additionally, the structural member 210 may comprise a layer of ferromagnetic material. The layer of ferromagnetic material may be disposed between the structural member and the intermediate layer. 10 The magnets of the array of permanent magnets 221-224 may comprise any suitable magnetically hard material such as ferrite, alnico and / or rare-earth materials such as neodymium. The magnets 221-224 may be angularly displaced from each other around the rotational axis, for example by 90 degrees. A first magnet 221 may be diametrically 15 opposite a third magnet 223, while a second magnet 222 may be diametrically opposite a fourth magnet 224. The magnets 221-224 may be placed so as to have alternate north and south polarities around the angular extent of the rotor 201. In Figure 2, the magnets may be arranged such that the first magnet 221 and the third magnet 223 are north poles, while the second magnet 222 and the fourth magnet 224 are south poles. It will be 20 appreciated that this spatial arrangement may be different for an array of a different number of permanent magnets. One or more magnets of the array may comprise a plurality of sections. In the illustrated example, the first magnet 221 comprises a first section 221a and a second section 221b. 25 The first section 221a and the second section 221b are optionally separated by a gap 221c. The first section 221a and the second section 221b provide a single pole. It will be appreciated that similar equivalent arrangements may be employed for each of the remaining permanent magnets 222-224. Furthermore, alternative arrangements to that shown in Figure 2 may be envisaged in which at least one magnet comprises more than 30 two sections. While an arrangement with four permanent magnets is illustrated and described herein, it will be appreciated that this disclosure may extend to a rotor having a different number of permanent magnets to thereby provide a rotor with a different number of poles. 35 The intermediate layer 230 comprises a soft magnetic composite material. While being newly adopted in this disclosure, a soft magnetic composite material is commonly referred to using the acronym 'SMC'. A SMC material comprises soft magnetic particles. The particles may comprise a ferromagnetic material, such as iron. The SMC may originate from a powder. The intermediate layer 230 may be formed using a powder metallurgical process. The average particle size in such powders may range from 5 to 200 pm. The average particle size may advantageously be less than 200 pm, less than 150 pm, less than 100 pm, less than 75 pm, or less than 50 pm. A smaller particle size is preferable in 5 reducing the extent of any in-particle eddy currents that may be induced. 10 15 25 The particles of the powder may be at least partially coated in an electrically insulating material. This can be achieved by the particles being mixed or interspersed in an electrically insulating material. In this way, electrically and magnetically conductive particles in the SMC may be electrically insulated from at least some adjacent electrically and magnetically conductive particles by virtue of a layer or partial layer of electrical insulation. A soft magnetic composite material can be contrasted with a laminated component that has alternating macroscopic layers of, for example, steel and an electrically insulating resin. In a soft magnetic composite material, the maximum dimension along which electrical current can flow may be limited by the particle size. Unlike a ferromagnetic layer of a laminated component, the particles in a soft magnetic composite may be non-contiguous. Furthermore, the magnetic and electrical properties of a soft magnetic composite may be isotropic, meaning that such physical properties are similar when measured in different directions; this is in contrast to a laminated component which is anisotropic such that its physical properties depend on the direction in which they are measured. In a laminate a single layer of electrically and magnetically conductive material may extend over significant distances in a single plane, cylinder or linear direction. Electrically and magnetically conductive particles in a SMC will generally have a maximum overall dimension of less than a centimetre, or less than a millimetre, and may have a maximum overall dimension smaller than that, such as the sizes described above. Overall, in the portion of the rotor 201 in which the soft magnetic composite is employed, conductivity is reduced and the magnetic properties are retained, even if at reduced levels. Therefore, the intermediate layer 230 reduces the effect of eddy current losses while still being permeable by magnetic flux. 30 35 The intermediate layer 230 is disposed between a surface of the structural member 210 and at least one of the permanent magnets 221-224. More specifically, the intermediate layer 230 may be disposed between a surface of the structural member 210 and a surface of at least one of the permanent magnets 221-224. The intermediate layer 230 may be provided radially outward of the structural member 210. The intermediate later 230 may be provided radially inward of at least one of the permanent magnets 221-224. The intermediate layer 230 may be positioned between a radially outer surface of the structural member 210 and a radially inner surface of at least one of the permanent magnets 221- 224. At least a portion of the intermediate layer 230 may be sandwiched between the structural member 210 and at least one of the permanent magnets 221-224. In this way, the intermediate layer 230 may separate at least one face of a permanent magnet from the structural member 210. 5 In some examples, the intermediate layer 230 may be a continuous annulus of soft magnetic composite material extending around the entire circumference of the structural member 210, as shown in Figure 2. Therefore, the intermediate layer 230 may be disposed around the entirety of a surface of the structural member 210 in such an example. Non- 10 continuous part-annular layers can also be provided. The intermediate layer 230 may comprise a plurality of sections, or segments, which are separated from one another by at least one circumferential gap or interruption (not shown). In this way, the intermediate layer 230 may comprise a plurality of discrete sections disposed around the rotational axis 250. The intermediate layer 230, or parts of it, may be disposed around a portion of the 15 angular extent of the structural member 210. The intermediate layer may be disposed around a portion of an outer surface of the structural member 210. 25 30 35 The intermediate layer 230, which may be disposed around only a portion of the structural member 210, may do so as a plurality of discrete sections disposed around the structural member 210. The portion may be one tenth, preferably at least one eighth, preferably at least one sixth, preferably at least one quarter, preferably at least one half, preferably at least three quarters, preferably at least seven eighths of the angular extent of the structural member 210. For an intermediate layer 230 comprising a plurality of sections, at least one gap between an adjacent pair of sections may be aligned with the centre of at least one of the permanent magnets 221-224. More specifically, at least one of such gaps may be provided at a similar angular position as that of the geometric centre of one of the permanent magnets 221-224. At least one of these optional gaps may be an air gap. By way of illustration, such a gap in the intermediate layer 230 may be aligned with the gap 221c in the first magnet 221. Eddy current losses are higher in the region of the spaces between adjacent pairs of permanent magnets 221-224. For example, the rotor components in the region of the space 225 between the first magnet 221 and the second magnet 222 may experience the highest eddy current losses. Therefore, it may be desirable to preferentially provide soft magnetic composite material in these regions to mitigate such high eddy current losses. In this way, gaps in the intermediate layer 230 may be positioned where there is a reduced necessity to mitigate eddy currents. Therefore, the number of poles of the electrical machine may be a multiple of the number of soft magnetic composite sections. By way of illustration, for a rotor in which the structural member 210 is a complete annulus around the rotational axis 250, an intermediate layer 230 being disposed around half of the angular extent of the structural member 210 would be disposed around a total of 180 degrees around the rotational axis 250. Such a portion of the angular extent of the 5 structural member 210 around which the intermediate layer 230 extends need not be continuous, but may instead be formed of separate sections or segments, which each may extend around smaller angular extents of the structural member 210. For the case in which the intermediate layer 230 extends around the rotational axis 250 by 180 degrees, the separate segments may comprise two segments each extending 90 degrees around the 10 rotational axis, or three segments each extending around 60 degrees of the rotational axis. Therefore, any desired portion of the angular extent of the intermediate layer 230 around the structural member 210 may be realised by employing one or more separate segments, which may or may not be equally spaced and / or equally sized. In view of the above, it will be understood that any reference to the intermediate layer 230 may be 15 interpreted as referring to a continuous layer of soft magnetic composite material or to a plurality of discrete sections of soft magnetic composite material. The intermediate layer 230 may directly abut the structural member 210, or it may be fixed to the structural member 210 via a retaining layer (not shown) which may comprise an adhesive. Similarly, at least one of the permanent magnets 221-224 may directly abut the intermediate layer 230, or it may be fixed to the intermediate layer 230 via a retaining layer (not shown) which may comprise an adhesive. There may be a rotor sleeve (not shown) disposed around the rotor 201, preferably around the plurality of permanent magnets 221-224, to secure the components of the rotor 201. 25 The intermediate layer 230 may have a thickness of between 0.1 and 10 mm, preferably between 0.5 to 5 mm, more preferably between 1 and 3 mm. Such a thickness may be a mean thickness averaged around the intermediate layer 230. Such a thickness may be measured in the radial direction relative to the rotational axis 250. The intermediate layer 30 230 may comprise a plurality of sublayers. The optional sublayers may be stacked on top of one another in a radial direction. The cumulative thickness of the optional sublayers may fall within one of the aforementioned ranges. The rotor 201 is shown in Figure 2 to be arranged concentrically within the stator 202. In 35 the illustrated arrangement, the array of permanent magnets 221-224 is located at a radial position between the radial positions of the structural member 210 and the stator 202. Also in the illustrated example, the intermediate layer 230 is located at a radial position between the radial positions of the structural member 210 and the stator 202. In the example shown, the rotatable part of the electrical machine 200, that is to say the rotor 201, is arranged within the stationary part of the electrical machine 200, that is to say the stator 202. Although the illustrated arrangement may be preferred, it will be understood that the device shown in Figure 2 could be modified such that the rotational part of the 5 electrical machine were located at a position radially outwards from the stationary part. Instead of the stator 202 being disposed around the rotor 201, the stator 202 may instead be disposed within the rotor 201. In this alternative arrangement, the array of permanent magnets and the intermediate layer may still be positioned between the structural member and the stator. Furthermore, the intermediate layer may still be positioned between at 10 least one permanent magnet and the structural member. The stator 202 may comprise a stator core 240, preferably comprising a ferromagnetic material. The stator core 240 may comprise a laminated component. The stator core 240 may comprise a plurality of radially extending teeth 243. The teeth 243 may define a 15 plurality of longitudinal slots 241. The slots 241 may be evenly distributed around the stator core 240 and may extend through the stator core 240 in an axial direction. The stator 202 may comprise a double winding arrangement, having a first stator winding 242a CM and a second stator winding 242b. The first stator winding 242a may be positioned radially outwards from the second stator winding 242b. CM20 Rotation of the rotor 201 relative to the stator 202 can induce an electromotive force in the stator windings 242 as a result of the varying magnetic field that they experience from ^M the rotating array of permanent magnets 221-224. In this way, the electrical machine 200 may be employed as an electrical generator. As shown schematically in Figure 3, the stator 25 windings 242 may be connected to power electronics 260. The power electronics 260 may be configured to treat an electrical current derived from the stator windings 242, which may have an electrical frequency of the fundamental frequency of the generator. The power electronics may sample the electrical current at a switching frequency which is a multiple of the fundamental frequency of the generator. The multiple may be greater than 30 5, more preferably greater than 8, more preferably greater than 10. Therefore, the switching frequency of the power electronics 260 may be 10 times greater than the fundamental frequency of the electrical machine 200. By way of illustration only, for a four-pole electrical machine such as that shown in Figure 2, rotating at a speed of 40,000 rotations per minute (rpm), the generator can induce an electrical current having a 35 fundamental frequency of approximately 1.3 kilohertz (kHz). Therefore, the power electronics may switch the electrical current at a frequency of 13 kHz. The electrical machine 200 may also be operated as an electrical motor by providing an electrical current to the stator windings 242. The electrical current supplied to the electrical machine may be treated by the power electronics 260 to provide a fundamental frequency that can drive the motor at a desired rotational speed. 5 The electrical machine 200 may be configured to have a fundamental frequency greater than 1 kHz. The fundamental frequency may be greater than 5 kHz, preferably greater than 10 kHz, more preferably greater than 20 kHz, more preferably greater than 30 kHz, more preferably greater than 40 kHz. An electrical machine capable of generating a higher 10 fundamental frequency may be lighter which is preferable for a range of applications, particularly for electrical machines adopted in lighter aircraft. This disclosure provides the possibility for an electrical machine to operate at high frequencies without requiring the use of complex coolant systems to handle the heat 15 generated by eddy currents. While prior solutions to this problem have involved the adoption of a laminated component, the very thin laminations needed to sufficiently mitigate the eddy current losses are expensive to manufacture and difficult to assemble. Furthermore, rotors made from laminated components may suffer from reduced mechanical strength, which is especially important for high rotational speeds. This disclosure provides an improved electrical machine by the identification of the problem that eddy currents are most significant in a layer of the structural member that is closest to the permanent magnets, and further by the provision of a solution of employing an intermediate layer comprising a soft magnetic composite. 25 Figure 3 is a schematic illustration of an aircraft 280. The aircraft 280 comprises an aircraft engine 270. The aircraft engine 270 may transfer rotational drive to the electrical machine 200 via a drive shaft 271. As described above, the electrical machine 200 comprises a rotor 201 and a stator 202. The stator 202, in particular the stator windings 242, may be connected to power electronics 260. 30 Figure 4 illustrates a method of assembly for a rotor 201 for an electrical machine 200. The method may comprise one or more steps of incorporating any aspect of the rotor described above. The method comprises a step S4.1 of providing a structural ferromagnetic member for a rotor. The method further comprises forming an intermediate 35 layer by a step S4.4 of disposing at least one soft magnetic composite section around the rotor. This step S4.4 may be performed by disposing a single annular section around the rotor. The step of forming the intermediate layer may comprise a step S4.2 of providing a soft magnetic composite powder. The soft magnetic composite powder may comprise PG G I- Ug particles having electrically insulating layers. The step of forming the intermediate layer further comprises a step S4.3 of forming a green (i.e. unsintered) section of soft magnetic composite, the section optionally being an annulus. This step S4.3 may be performed by pressing the soft magnetic composite powder using a hydraulic press to form the green 5 soft magnetic composite section. The method may comprise a step of mounting, for example by clamping, at least one soft magnetic composite section around the rotor, for example by disposing at least one soft magnetic composite section around the structural member. 10 The step of forming the intermediate layer may comprise a sintering step which may be performed after the step S4.4 of disposing the at least one soft magnetic composite section around the rotor. The sintering step may be performed by firing the soft magnetic composite section in place. In other words, the soft magnetic composite may be fired while it is disposed around the rotor. Alternatively, at least one soft magnetic composite section 15 may be sintered before it is disposed around the rotor. In this alternative example, the at least one soft magnetic composite section may be sintered and may then be subsequently glued to the rotor to form a soft magnetic composite layer. The method may comprise a step of grinding the soft magnetic composite layer. 20 The method further comprises a step S4.5 of disposing a plurality of permanent magnets around the rotor. This step may comprise arranging at least one magnet on the intermediate layer. At least one magnet may be glued onto the soft magnetic composite layer. The method may further comprise a step of wrapping a rotor sleeve, such as one made from carbon fibre, around the array of permanent magnets. 25 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

1. A rotor for a permanent magnet electrical machine, comprising:a structural member,5 an array of permanent magnets disposed around a rotational axis of thestructural member, andan intermediate layer comprising a soft magnetic composite material, disposed between a surface of the structural member and at least one permanent magnet of the array of permanent magnets;10 wherein the intermediate layer comprises a gap that is aligned with at leastone of the permanent magnets in the array.

2. A rotor according to claim 1, wherein the intermediate layer comprises at least one section that is at least part-annular.

153. A rotor according to claim 2, wherein the at least one section is retained to the structural member by adhesion, by sintering and / or by a rotor sleeve.

4. The rotor according to any preceding claim, wherein the intermediate layer is 20 disposed around at least a portion of the circumference of the structural member.

5. The rotor according to any preceding claim, wherein the intermediate layer separates at least one face of a permanent magnet of the array from the structural member.

256. The rotor according to any preceding claim, wherein the intermediate layer comprises a plurality of soft magnetic composite sections disposed about the rotational axis.30 7. The rotor according to any preceding claim, wherein the intermediate layer isdisposed around at least one tenth, preferably at least one eighth, preferably at least one sixth, preferably at least one quarter, preferably at least one half, preferably at least three quarters of the rotational axis.35 8. The rotor according to any preceding claim, wherein the intermediate layer isdisposed around an entirety of the rotational axis.

9. The rotor according to any preceding claim, wherein the intermediate layer has a thickness between 0.1 and 10 mm, preferably between 0.5 to 5 mm, preferably between 1 and 3 mm.

10. An electrical machine, for a generator of an aircraft engine, comprising the rotor according to any of claims 1 to 9.

11. The electrical machine according to claim 10, configured to have an electrical frequency which is greater than 1kHz, preferably greater than 5kHz, preferably greater than 10kHz.

12. The electrical machine according to claim 10 or claim 11, further comprising power electronics, electrically connected to stator windings comprised in a stator and configured to treat an electrical current delivered to or from the stator windings for operation of the electrical machine.

13. The electrical machine according to any of claims 10 to 12, wherein the power electronics are configured to operate at a frequency of at least eight times the electrical frequency of the electrical machine.

14. An aircraft comprising an aircraft engine, the aircraft engine being connected to an electrical machine according to any of claims 10 to 13.

15. A method for assembly of a rotor for an electrical machine comprising the steps of: providing a structural ferromagnetic member, forming an intermediate layer by disposing at least one soft magnetic composite section on a surface of the structural ferromagnetic member, andproviding a plurality of permanent magnets disposed around the structural ferromagnetic member such that the soft magnetic composite is located between the structural ferromagnetic member and at least one of the plurality of permanent magnets;wherein the intermediate layer comprises a gap that is aligned with at least one of the permanent magnets.

16. The method according to claim 15, wherein the step of forming the intermediate layer comprises pressing the at least one soft magnetic composite section around the rotor in an unsintered state.

17. The method according to claim 15 or claim 16, wherein the step of forming the intermediate layer comprises pressing the at least one soft magnetic composite section onto the structural ferromagnetic member and then subsequently sintering the at least one soft magnetic composite section.

518. The method according to claim 15, wherein the step of forming the intermediate layer comprises sintering the at least one soft magnetic composite section and then subsequently gluing the at least one soft magnetic composite section onto the structural ferromagnetic member.10PG G I- Ug

Images