Electrical Machines With SMC Cores

Abstract

An electrical machine is disclosed. The electrical machine has a first part and a second part, the first part moveable relative to the second part. One of the first part and the second part has a plurality of cores for current-carrying windings. Each core is of soft magnetic composite material (SMC) and is shaped to be no wider at its ends than along its length.

Claims

1. A rotating or linear electrical axial flux machine comprising a first part and a second part, the first part moveable relative to the second part, one of the first part and the second part having a plurality of cores for current-carrying windings, wherein each core is of powder soft magnetic composite material (SMC) and is shaped to be no wider at its ends than along its length.

2. The electrical machine of claim 1, wherein each core is substantially circular in cross-section or each core is substantially a shape of a sector of an annulus in cross-section, whereby one of the first part and the second part has a plurality of magnets.

3. The electrical machine of claim 2, wherein each magnet is substantially round in cross section or wherein each magnet is substantially a sector of an annulus in cross-section.

4. The electrical machine of claim 2, further comprising an element of SMC coupled to each magnet, each element shaped to increase the density of magnetic flux through a core.

5. The electrical machine of claim 2, wherein the plurality of magnets comprises a first set of magnets and a second set of magnets and the plurality of cores is arranged axially between the first set of magnets and the second set of magnets.

6. The electrical machine of claim 2, wherein the part having the plurality of magnets comprise a substrate formed of SMC on which the magnets are mounted.

7. The electrical machine of claim 6, wherein the substrate is made up of a plurality of substrate segments and wherein each substrate segment is arranged to substantially abut radially two other substrate segments.

8. The electrical machine of claim 7, wherein a magnet is mounted at each point where two substrate segments substantially abut.

9. The electrical machine of claim 2, wherein the plurality of cores for current-carrying windings is a first plurality of cores, and there is a second plurality of cores, and wherein the plurality of magnets is a first plurality of magnets and there is a second plurality of magnets, and wherein the first plurality of cores is arranged at a radial distance from the axis substantially equal to the distance from the axis at which the first plurality of magnets is arranged, and the second plurality of cores is arranged at a radial distance from the axis substantially equal to the distance from the axis at which the second plurality of magnets is arranged.

10. The electrical machine of claim 1, wherein a back plate of a stator comprising compressed soft magnetic composite powder material (SMC) is in contact with each core made from compressed soft magnetic composite powder material (SMC).

11. The electrical machine of claim 1, wherein a rotor adjacent to a stator of the electrical machine comprises a plurality of magnets wherein at least a part of the rotor in contact with the magnets is made from compressed soft magnetic composite powder material (SMC).

12. The electrical machine of claim 1, comprising at least a single stator adjacent to one single rotor, one single rotor between two stators or one single stator between two rotors.

13. The electrical machine of claim 1, wherein the electrical machine is part of an axle apparatus for a vehicle.

14. The electrical machine of claim 1, wherein the electrical machine is a part of a flywheel apparatus for an internal combustion engine, whereby the first part is arranged to be mounted a flywheel for connection to a crankshaft of an internal combustion engine.

15. The electrical machine of claim 1, wherein the electrical machine is at least one of a linear electrical machine, a part of a medical device, a part of a blood or heart pump, a compressor, an air conditioning compressor, a fluid pump, a part of a wind turbine, a part of a vertical axis wind turbine, a part of a Darrieus wind turbine, an automotive part, a bicycle part, a motorbike part, a train part, a gear par and a driveline part.

16. A method for manufacturing a core of an electrical axial flux machine wherein each core of the electrical axial flux machine is made of soft magnetic composite material (SMC) and is shaped to be no wider at its ends than along its length, wherein the method comprises (a) compressing SMC powder to form a first part of substantially uniform density, (b) compressing SMC powder to form a second part of substantially uniform density, and (c) compressing the first part and the second part together to form a core of substantially uniform density.

17. A method for manufacturing a core of an electrical axial flux machine wherein each core of the electrical axial flux machine is made of soft magnetic composite material (SMC) and is shaped to be no wider at its ends than along its length, wherein the method comprises gradually building up a part, by placing SMC powder in a die, compressing the powder using a punch, withdrawing the punch, adding more SMC powder, and compressing this, to finally build the core.

18. A method according to claim 16, wherein all cores of the electrical axial flux machine together are integrated in a base plate made of soft magnetic composite material (SMC) which serves for a magnetic reflux whereby the base plate is made by compressed SMC powder.

19. A method according to claim 16, wherein the parts of the cores are compressed together to achieve a core length of at least 1.5 times of its width.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0096] Specific embodiments of the invention are described below by way of example only and with reference to the accompanying drawings, in which:

[0097] FIG. 1a is an axial schematic view of a first embodiment of the electrical machine;

[0098] FIG. 1b is a cross-sectional schematic view of the electrical machine of the first embodiment;

[0099] FIG. 2 is an axial scrap schematic view of the electrical machine of the first embodiment;

[0100] FIG. 3 is a schematic depiction of the general arrangement of cores, coils and magnets and magnetic flux lines in the described embodiments;

[0101] FIG. 4a is an axial schematic view of a second embodiment of the electrical machine;

[0102] FIG. 4b is a cross-sectional schematic view of the electrical machine of the second embodiment;

[0103] FIG. 5 an axial scrap schematic view of the electrical machine of the second embodiment;

[0104] FIG. 6 is a side scrap schematic view of a third embodiment of the electrical machine;

[0105] FIG. 7 is a cross-sectional schematic view of a fourth embodiment of the electrical machine;

[0106] FIG. 8 is a cross-sectional schematic view of a fifth embodiment of the electrical machine;

[0107] FIG. 9 is a schematic depiction of the arrangement of magnets and SMC segments in a rotor yoke forming part of a sixth embodiment of the electrical machine;

[0108] FIG. 10 is a schematic depiction of the arrangement of segments in a rotor yoke forming part of a seventh embodiment of the electrical machine;

[0109] FIG. 11 is a schematic depiction of the cross-sectional shape of a core, a magnet and a coil around a core, all forming part of an eighth embodiment of the electrical machine;

[0110] FIG. 12 is a part cross-sectional schematic view of a ninth embodiment of the electrical machine of the electrical machine, with SMC elements, or “flux concentrators” mounted on the magnets;

[0111] FIGS. 13a, 13b and 13c show steps in a process of making a core for an electrical machine; and

[0112] FIGS. 14, 15 and 16 each shows a specific stator-rotor arrangement of the electrical machine.

[0113] One or more features of one or more embodiments above or below might be used with one or more features of another embodiment or embodiments to show different opportunities of the invention. Therefore, any feature of an embodiment is not restricted only to this specific embodiment but is to be understood in a broader sense only as an example.

SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS

[0114] [Outer Stator Rotating Electrical Machine]

[0115] FIGS. 1a and 1b show an axial view and a cross-sectional view, respectively, of a first embodiment of the electrical machine. In this embodiment, the electrical machine is a transverse flux (or axial flux) electrical machine 10. The electrical machine 10 has a first part in the form of a rotor 11 and a second part in the form of a stator 17. In this embodiment, the stator 17 has twelve SMC cores 1 fixed to it. Each core 1 is surrounded by current-carrying windings in the form of a coil 2. The cores 1 have no pole shoes.

[0116] The rotor 11 is encased within the stator 17. This can be seen most clearly in FIG. 1b. The arrangement of the rotor 11 and the stator 17 to form the electrical machine 10 will be described in more detail below.

[0117] With reference to FIG. 1b, the stator 17 consists of two housing side plates 14 and a coil support 6. The coil support 6 holds the coils 2 and their respective cores 1 in place. The housing side plates 14 protect the rotor 11 and hold the coil support 6 in place relative to a shaft 12 of the rotor 11.

[0118] The coil support 6 is in the shape of a section of a tube. That is, it is in the shape of a ring whose inner and outer surfaces are generally flat in their axial directions. The coil support 6 is made from aluminium. The coils 2, containing the cores 1, are fixed to the radially inner surface of the coil support 6. The coils 2 and cores 1, and their attachment to the coil support 6, will be described in more detail below.

[0119] As mentioned above, the housing side plates 14 protect the rotor 11 and provide a mount for the coil support 6. The housing side plates 14 are mirror images of one another, in all but one respect. Namely, one of the side plates 14 defines a hole through which, when the electrical machine 10 is assembled, the shaft 12 of the rotor 11 extends, while the other side plate 14 does not. The housing side plates 14 are plates formed of aluminium. The housing side plates 14 are in the shape of a square with its four corners cut off. There is a central, circular, hole cut in one of the housing side plates 14 to allow a shaft 12 of the rotor 11 (described in more detail below) to pass through the housing side plate 14. One side of each housing side plate 14 is flat. This is the outer side when the electrical machine 10 is assembled. The other side of each housing side plate 14 (the inner side when the electrical machine 10 is assembled) defines protrusions 19, 18. There is an inner protrusion 19 in the shape of a ring, which is adjacent to the central hole in the housing side plate 14.

[0120] The inner protrusion 19 of each bearing side plate 14 accommodates a bearing 13 which, when the electrical machine 10 is assembled, is adjacent to the shaft 12. When the electrical machine 10 is assembled, the bearings 13 hold the side plates 14 against the shaft 12 of the rotor 11, while allowing the side plates 14 to remain stationary with respect to the shaft 12 while the shaft 12 is rotating.

[0121] The outer protrusion 18 is also in the shape of a ring. The outer protrusion is radially outward of the inner protrusion 19. The outer protrusion helps to support the coil support 6. Specifically, the coil support 6 is positioned axially inwardly of the two housing side plates 14 and radially outwardly of their outer protrusions 18. It is thus held axially in place by the housing side plates 14 and radially in place by the inner protrusions 19.

[0122] The arrangement of the coils 2 on the coil support 6 of the stator will now be described in more detail, with reference to FIG. 2. FIG. 2 shows schematically part of the coil support 6. Mounted on the radially inner surface of the coil support 6 are clips 21. There are twelve clips 21 (although only four are shown in FIG. 2), one for each stator core 1 and coil 2. Each clip 21 has arms 23 which are connected to the coil support 6 and which curve around the coil 2 to hold the coil 2 in place. In this way, each of the twelve coils 2 is supported by a clip 21 connected to the coil support 6, which is in turn connected to the housing side plates 14. The coils 2 have their axis parallel to the axis of the stator 17. That is, the axis of each coil 2 is perpendicular to the housing side plate 14 and parallel to the coil support 6.

[0123] In other embodiments, adhesive (e.g. resin) is used in place of the clips 21 to hold the coils 2 to the coil support 6. For example, in one alternative embodiment, the radially inner surface of the coil support 6 defines twelve indentations which are semi-circular in cross-section. A coil 2 is glued into each of these indentations in the coil support 6.

[0124] Each coil 2 is formed from electrically conducting wire. In this embodiment the wire is copper and is round in cross section. In other embodiments, other materials can be used for the wire provided they are electrically conducting. In other embodiments, the wire is square or rectangular in cross section. The coils 2 themselves are circular in cross section. The coils are wound by machine.

[0125] Once assembled, operation of the electrical machine 10 as a motor depends on the generation of a sufficiently high magnetic field by the passing of current through the coils 2. Conversely, operation of the electrical machine 10 as a generator depends on the generation by permanent magnets 4 (described below) of a sufficiently high current in the coils 2. To permit the maximum flux density through the coils 2, a core 1 of high permeability is provided in the centre of each coil 2. Once a coil 2 has been wound, a core 1 is inserted into the middle of the coil 2. The coils 2 define a central space which is circular in cross section and has approximately the same diameter as a core 1, such that the core 1 can easily be slotted into the coil 2.

[0126] In conventional electrical machines, cores within coils would have pole shoes to spread the flux in the air gap between magnets and the coils and so to reduce the flux density in the air gap. However, the cores 1 of the present electrical machine 10 are formed of SMC and so, as discussed above, pole shoes are unnecessary. Each core 1 is formed of soft magnetic compound. SMC is made up of iron particles which are covered in an electrically insulating coating, such that each particle is electrically insulated from the other particles. The particles are then formed into a shape and heat treated or cured such that the particles hold their shape. Each core 1 is manufactured in this way. Each core 1 is cylindrical in shape. That is, in side section it is rectangular, and in cross section it is circular. Each core 1 is therefore simple in shape.

[0127] With reference once more to FIG. 1b, the rotor 11 will now be described in more detail. The rotor 11 is generally yo-yo-shaped in cross-section. That is, in overall shape, the rotor 11 has two generally disc-shaped parts arranged parallel to one another and axially connected at their centres. The rotor 11 is made up of twin backing elements for its magnets 4, twin support rings 16 for these backing elements, and two discs of matrix material 5 which hold the magnets in place on the backing elements.

[0128] In this embodiment, the backing elements are in the form of yoke rings 3 formed of SMC. Each of the two yoke rings 3 is in the shape of a disc defining a circular hole (not visible in cross-section) at its centre. In other words, each of the two yoke rings 3 is annular in shape. Each yoke ring 3 is radially wide enough to accommodate the magnets 4. That is, the yoke ring 3 is wider than each of the magnets 4.

[0129] There are fourteen magnets 4 mounted on each yoke ring 3. The magnets are mounted on the face of each yoke ring 3 which is adjacent to the cores 1 and coils 2 when the electrical machine 10 is assembled. The magnets 4 are equally radially spaced around each yoke ring 3.

[0130] Each yoke ring 3 is in turn mounted on a support ring 16. The two support rings 16 are formed of composite material. Each support ring 16 acts to stiffen the yoke ring 3 which is mounted to it, to prevent it from flexing under magnetic forces. This is advantageous since in being formed of SMC, the yoke rings 3 are somewhat brittle. The support rings 16 therefore help to prevent damage to the yoke rings 3. Each support ring 16, like the yoke rings 3, is annular in shape. The radial distance from the central hole in each support ring 16 to the outer edge of each support ring is greater than the radial distance from the central hole in each yoke ring 3 to the radially outer edge of each yoke ring 3. The outer edge of the yoke ring 3 axially abuts the outer edge of its respective support ring 16.

[0131] Each yoke ring 3—support ring 16 pair is held in place on the rotor shaft 12 by a disc of matrix material 5. One disc of matrix material 5 is glued to the axially-inner face of one of the yoke rings 3 and also to the part of the axially-inner face of the support ring 16 on which the yoke ring 3 is mounted that is exposed. The other disc of matrix material 5 is likewise glued to the other yoke ring 3 and support ring 16. The magnets 4 are exposed through respective circular holes in the discs of matrix material 5.

[0132] The two discs of matrix material 5 each have a central hole. These central holes are of equal dimensions. They are smaller in diameter than the central holes in the yoke rings 3 and support rings 16. A shaft 12 extends axially through the holes in each of these components. The shaft 12 has a radial protrusion to which the discs of matrix material 5 are fixed by countersunk screws 15. One disc of matrix material 5 is fixed on one side of the radial protrusion on the shaft 12. The other disc of matrix material 5 is fixed on the axially opposite side of the radial protrusion on the shaft 12. Thus, although the yoke rings 3 do not touch the shaft 12, they are held in place on the shaft 12 by the discs of matrix material. This holds the magnets 4 at a radial distance from the shaft 12 such that they radially align with the cores 1 and coils 2 of the stator 17.

[0133] The two discs of matrix material 5 each have an axial depression towards their centre. The axial depressions slope radially inwards from the part of each disc of matrix material 15 that is fixed to the support ring 16. Thus, when the discs of matrix material 5 are fixed to the radial protrusion on the shaft 12, a central part of each disc of matrix material 5 is axially closer to the other disc of matrix material 15 than a radially outer part of each disc of matrix material 5. In other words, the radially outer parts of the discs of matrix material 5 are further apart from one another than the radially-inner parts of each disc of matrix material 5. The greater axial distance between the radially outer edges of the discs of matrix material 15 allows the cores 1 and coils 2 mounted on the coil support 6 of the stator 17 to be accommodated between the two yoke rings 3 of the rotor 11.

[0134] Thus, when the electrical machine 10 is assembled, the magnets 4 mounted on each yoke ring 3 face each other, with the cores 1 and coils 2 axially between them. In having two yoke rings 3, each supporting an equal number of magnets 4, the axial magnetic forces can be balanced. To keep the axial magnetic forces balanced, the two discs of matrix material 5 are mirror images of each other to ensure that the air gap between the magnets 4 and coils 2 is even.

[0135] [Operation of Outer Stator Rotating Electrical Machine]

[0136] FIG. 3 shows a schematic radial view of the arrangement of magnets 4, coils 2 and cores 1 in the electrical machine described above with reference to FIGS. 1 and 2.

[0137] The electrical machine 10 can be operated either as a motor or as a generator. In operation as a motor, the coils 2 are connected to a supply of alternating current (not shown). FIG. 3 shows the flux lines 7 of the magnetic field generated by passing current through the coils 2. The alternating direction of the current through the coils 2 urges the pairs of magnets 4 towards successive coils 2. Since, as described above, the magnets are connected by the matrix material 5 to the shaft 12, the rotation of the magnets 4 causes a torque to be applied to the shaft 12. Conversely, when the electrical machine 10 is operated as a generator, rotation of the shaft causes the magnets 4 to rotate about the coils 2, inducing an alternating current in the coils 2.

[0138] [Inner Stator Electrical Machine]

[0139] FIGS. 4a and 4b show an axial view and a cross-sectional view, respectively, of a second embodiment of the electrical machine. In this second embodiment, the machine is an inner stator rotating electrical machine 20. The rotor forms part of the hub of an electric bicycle (not shown). Like the electrical machine 10 of the first embodiment, the electrical machine 20 of the second embodiment has a first part in the form of a rotor 11 and a stationary part in the form of a stator 17. Like the electrical machine 10 of the first embodiment, this inner stator electrical machine 20 is a transverse flux (or axial flux) electrical machine. Unlike the electrical machine 10 of the first embodiment, in this second embodiment, the stator 17 is encased within the rotor 11. In this embodiment, the electrical machine 20 has 76 permanent magnets. It has 38 magnets mounted on each of two yokes 3 of the rotor 11. The electrical machine 20 has 36 coils 2 mounted on its stator 17. Each of the 36 coils 2 has a respective core 1 formed of SMC within it.

[0140] With reference to FIG. 4b, the stator 17 consists of two circular plates, each having a hole at its centre. The central spindle 42 extends axially through the central hole in each of the plates of the stator 17. In much the same way as the discs of matrix material 5 of the first embodiment 10 are fixed to a radial protrusion on the shaft 12 of the first embodiment, the stator 17 plates of the second embodiment 20 are each fixed to a radial protrusion on a central spindle 42 by tie bolts 45. The radially outer edges of the two plates of the stator 17 are secured to one another by further tie bolts 45.

[0141] Fixed in a ring around the radially outer edges of the plates of the stator 17 is a coil support 46. The coil support 46 is annular in shape. That is, the coil support 46 is shaped like a section of a hollow tube. On the radially-outer surface of the coil support 46, there is a ring of composite reinforcement 16. To this ring of composite reinforcement 16 are mounted 36 coils 2. These coils 2 are generally the same as the coils of the first embodiment. Within each coil 2 is a core 1 of SMC. Again, these cores 1 are as described above in relation to the first embodiment. The mounting of the coils 2 to the composite reinforcement 16 is achieved in this second embodiment in the same way as the mounting of the coils 2 to the coil support 6 of the first embodiment. That is, each coil is clipped to the layer of composite reinforcement 16 around the coil support 46. FIG. 5 shows schematically an axial view of the arrangement of 4 of the cores about the coil support 46. In overview then, the stator 17 of this embodiment provides a mount for coils arranged radially outwardly of the stator 17.

[0142] One of the two plates of the stator 17 has bores through it adjacent the connection to the central spindle 42. These bores provide cable access 47 so that the coils 2 mounted on the stator 17 can be connected to an electric circuit (not shown).

[0143] With continued reference to FIG. 4b, also mounted on the central spindle 42 is a rotor 11. The rotor 11 consists of two hub side plates 44, one on one axial side of the coils 2 supported by the stator 17, and the other hub side plate 44 on the other axial side of the coils 2. The hub side plates 44 are connected to one another around their radially outer edges by an annulus of composite reinforcement 16. This composite reinforcement 16 holds the hub side plates 44 at an axial distance from one another sufficient to accommodate the coils 2 between them. A yoke ring 3 formed of SMC is mounted on the inner face of each of the hub side plates 44. The yoke ring 3 is of the same shape as the yoke ring 3 described above in relation to the first embodiment.

[0144] To each of the two yoke rings 3, there are mounted 38 magnets. The arrangement of the magnets 4 and yoke ring 3 is a described above with reference to FIGS. 1 and 2. In this embodiment, however, there are more magnets 4 than in the first embodiment. The magnets 4 are mounted on the yokes 3 such that they face one another in pairs across the stator 17.

[0145] One of the hub side plates 44 is mounted directly to the central spindle 42 by a bearing 13. The other hub side plate 44—the one that is axially adjacent the plate of the stator 17 that has the bores for cable access 47—is mounted to a radially-inner, axially-outer part of the stator 17. Again, it is mounted on bearings 13 to enable it to rotate with the other plate of the rotor 11 about the central axis.

[0146] Operation of this electrical machine 20 is as described above in relation to the first embodiment 10 of the electrical machine, except that since the rotor 11 in this second embodiment is external to the stator 17, it is the outer part of this electrical machine 20 which rotates.

[0147] [Linear Actuator]

[0148] The principles described above in relation to the first two embodiments can also be applied to a linear actuator 30. A schematic side view of such a linear actuator is shown in FIG. 6. In operation, the magnets 4, visible in this figure outlined in dots, are mounted on a first, moving part, while the coils 2 are mounted on a second, stationary part. In other embodiments, the coils 2 can be mounted on a first, moving part, while the magnets 4 are mounted on a second, stationary part.

[0149] [Powered Wheel]

[0150] FIG. 7 shows, in cross-section, two wheels 78 and an axle 72 of a train (not shown), with an electrical machine 40, 50 mounted to each wheel 78. When the electrical machines 40, 50 are operated as motors, this arrangement propels the train by turning the axle 72 and the wheels 78 mounted to the axle 72. Other axles of the train are also provided with similar arrangements.

[0151] The two electrical machines 40, 50 are mirror-images of each other. Only the electrical machine 40 shown on the left of FIG. 7 will therefore be described in detail here. The stator 17 of the electrical machine 40 is mounted to the axle 72 of the wheel 78 via bearings 13 and a support structure 77. Permanent magnets 84 mounted to the wheel 78 form a rotor 11. Cores 1 and coils 2 are arranged on the stator 17 as described above with reference to FIGS. 1b and 2. The permanent magnets 84 are arranged relative to the cores 1 and coils 2 as also described above with reference to those figures. Instead of being mounted on SMC yoke rings 3 to discs 5 of matrix material, one set of magnets 84 is mounted on an SMC lining ring 73 to the axially inner face of the wheel 78, and the other set of magnets 84 is mounted on another SMC lining ring 73 to a brake disc 71, axially half-way between the two wheels 78. The SMC lining rings 73 will be described in more detail below, with reference to FIG. 9. In overall shape, however, each SMC lining ring 73 is generally line each SMC yoke ring 3, although larger, in order to support the magnets 84 on the wheels 78 and brake disc 71.

[0152] The wheels 78 and brake disc 71 are made of steel. Steel is a good backing material for the magnets 84. However, because of the large homogeneous mass of the wheels 78 and brake disc 71, without the SMC lining rings 73, they would generate large iron losses, especially at high operating speeds. Lining the wheels 78 and brake disc 71 with SMC lining rings 73 reduces iron losses. This makes it practical to mount the electrical machine to the axle 72 and wheels 78 without gear transmission between them. In turn, this reduces the cost and weight of arrangements for driving the axle 71 of a train.

[0153] [SMC Lining Ring]

[0154] FIG. 9 shows a substrate of SMC in the form of an SMC lining ring 73 that can be used with the arrangement described above with reference to FIG. 7. The SMC lining ring 73 is in the shape of a disc defining a circular hole at its centre. In other words, it is annular in shape. The radial distance between the edge of the hole and the radially-outer edge of the SMC lining ring 73 is great enough to accommodate magnets 84. That is, the SMC lining ring 73 is wider than each of the magnets 84. In shape, it is therefore similar to the SMC yoke ring 3 described above with reference to FIG. 1b. For applications such as that described above with reference to FIG. 7, that is, applications in which the SMC lining ring 73 is required to be of large dimensions, the SMC lining ring 73 is made up of substrate segments. There are seven segments in this embodiment. In embodiments where a larger lining ring 73 is required, more segments are used in order to keep each segment of a size which can easily be formed of SMC. The segments are the shape of sectors of an annulus. In other words, the lining ring 73 is divided along radial lines. In this embodiment there are seven radial dividing lines 99.

[0155] SMC is brittle and so forming and curing large parts in SMC is difficult. Assembling the SMC lining ring 73 from segments of SMC means that smaller parts are required to be made than if it were formed in one piece and manufacture of the SMC lining ring 73 is therefore made easier.

[0156] The dashed circles in FIG. 9 indicate the position 98 of magnets 84 on the SMC lining ring 73 when it is used in an electrical machine. The magnets 84 are evenly spaced around the SMC lining ring 73. In this embodiment, fourteen magnets 84 are positioned radially around the SMC lining ring 73. One magnet 84 is positioned over each radial dividing line 99. The remaining seven magnets 84 are positioned with one magnet 84 between each magnet 84 that is positioned over a radial dividing line 99.

[0157] [Engine Flywheel]

[0158] FIG. 8 shows in cross section an electrical machine 80 with a first part in the form of a modified flywheel 81 of an internal combustion engine (ICE) 89. The electrical machine 80 is arranged to function as a starter-generator. Like a standard flywheel, the modified flywheel 81 of this embodiment is mounted to the crankshaft 82 of the IC. Unlike a standard flywheel, however, the modified flywheel 81 of this embodiment resembles the rotor 11 of the embodiment described above in relation to FIG. 1b. Specifically, the modified flywheel 81 is formed of two discs, co-axially connected, with magnets 84 mounted on their axially-inner faces. The magnets 84 are fixed to an SMC lining ring 73, as described above with reference to FIGS. 7 and 9. Coils 2 are mounted on the inner surface of an outer casing 86 as described above in relation to the coils 2 and coil support 6 of the first embodiment.

[0159] In operation, the electrical machine 80 can function as a starter motor. To start the ICE 89, current is passed through the coils 2. This causes the flywheel 81 to turn, in the same manner as passing a current through the coils 2 of the outer stator electrical machine 10 described with reference to FIG. 1b causes its rotor 11 to turn. Turning the flywheel 86 causes the crankshaft 82 to turn, starting the ICE.

[0160] The electrical machine 80 can also operate as a generator. Spinning of the flywheel 86 by pistons of the ICE 89 induces a current in the coils 2 which can be used, for example, to charge a vehicle battery.

[0161] Incorporating the electrical machine 80 with the engine flywheel 81 provides a more compact arrangement for a starter-generator. It also eliminates the need for a gear transmission to allow the crankshaft 82 to be turned by an electrical machine operating as a motor, or to have the rotor of the electrical machine turned by the crankshaft when the electrical machine is operated as a generator. Such an electrical machine 80 can be used to convert electric vehicles to series hybrid operation.

[0162] [Segment Magnets]

[0163] FIG. 10 shows an alternative arrangement of magnets to be used in an electrical machine for applications where cogging is to be minimised. In this arrangement, there are four magnets 109 mounted on an SMC yoke ring 103. The SMC yoke ring 103 is generally the same as the SMC yoke ring 3 described above with reference to FIG. 1b. By contrast to the magnets 4 of the first embodiment, however, the magnets 109 of this embodiment are not round but are shaped like sectors of an annulus so that when assembled and mounted on the SMC yoke ring 103, the magnets 109 make up a ring.

[0164] This arrangement has particular advantages when a low number of magnetic poles are required in an electrical machine. When a low number of magnetic poles are required, round magnets give good running efficiency due to the low volume of magnetic material, but can lead to bad cogging when the machine is operated. In electrical machines to be used where full torque is required from starting the machine, such as in a bicycle drive (where cogging will be felt as “lumpiness” when accelerating), it is desirable to minimise cogging. Magnets 109 of the shape described above reduce cogging, because spaces between the magnets are minimised.

[0165] FIG. 11 shows yet another shape of magnets to be used in an electrical machine for applications where cogging is to be minimised. The magnet 114 shown is shaped such that when many similarly-shaped magnets are arranged side-by-side, they form a ring. Specifically, the magnet 114 has a cross-sectional shape having an outline described by a two circles joined by two non-intersecting tangents to the circles. In other words, the magnet 114, in cross-section, is shaped like a trapezoid, with a semicircle having a diameter equal to the length of one of the parallel edges of the trapezoid joined to that edge of the trapezoid along the straight edge of the semicircle, and a second semicircle having a diameter equal to the length of the other parallel edge of the trapezoid joined to that edge of the trapezoid along the straight edge of the semicircle. The diverging edges of the trapezoid form straight edges of the magnet. Placing magnets 114 adjacent one another with their straight edges abutting forms a ring of magnets. As with the magnets 109 described above in relation to FIG. 10, therefore, when assembled and mounted on the SMC yoke ring, these magnets 114 make up a ring.

[0166] FIG. 11 also shows a shape of cores 111 to be used with the magnets described above in relation to this figure. The cores 111 have the same cross-sectional shape as the magnets 114, but have a smaller cross-sectional area. This helps to minimise stray flux in the electrical machine and enhance its performance.

[0167] [Flux Concentrators]

[0168] FIG. 12 shows part of a ninth embodiment 120 of the electrical machine in which SMC elements in the form of flux concentrators 121 made from SMC are used to attenuate the flux levels of the magnets 4. In this embodiment, the magnets 4 are Ferrite-based. These are generally cheaper than Neodymium-based magnets, but can be demagnetised at lower flux densities through the magnets 4. The flux concentrators 121, by spreading magnetic flux through the magnets 4, allow the SMC cores 1 to be used at similar effective flux levels to more expensive magnets, such as Neodymium-based magnets.

[0169] Each flux concentrator 121 is shaped like a conical frustum (that is, a truncated cone) mounted at its base to a section of a cylinder. In cross-section, therefore, each flux concentrator 121 is wider at one end and tapers towards its other end. At the wider end, each flux concentrator 121 has a first face 122. Each flux concentrator 121 narrows towards a second face 123 of smaller cross-sectional area than the first face 122.

[0170] The first face 122 of each flux concentrator 121 is glued to the face of a magnet 4 that is axially closest to the cores 1. The first face 122 of each flux concentrator 121 is the same shape as that face of the magnet and has the same surface area. Thus all magnetic flux that passes through the magnet 4 passes through the flux concentrator 121. The second face 123 of the flux concentrator 121 is the same shape as a cross-section of any of the cores 1. The surface area of the second face 123 is the same as the cross-sectional area of any one of the cores 1. Thus, when a core 1 is radially aligned with one of the flux concentrators 121, magnetic flux is channelled by the flux concentrator 121 through the core 1 and magnet 4 on which the flux concentrator 121 is mounted. The matrix material 5 helps to keep the flux concentrators 121 in place on the magnets 4.

[0171] The flux concentrators 121 work in a similar way to pole shoes; in use, they spread the flux axially adjacent the magnets 4 and concentrate it through the cores 1. The flux concentrators 121 thus provide similar advantages without the need for pole shoes. The flux concentrators 121 are easier to manufacture and less fragile than SMC cores with pole shoes, since they are of simpler shape.

[0172] [Method of Manufacturing Cores]

[0173] FIGS. 13a, 13b and 13c show steps in a process of making a core 135 for an electrical machine. In a first step, shown in FIG. 13a, SMC powder 136 is placed in a tubular die 131 and a punch 133 having a circular cross-section with a diameter slightly less than the inner diameter of the die 131 is inserted into the die 131. Force is applied to the punch 133. This compresses the powder 136 so that it is of roughly uniform density throughout. In this way, a first cylindrical part is formed of SMC. In a second step, a second cylindrical part is formed in the die 131 in the same way. In this embodiment, a third cylindrical part is also formed in the same way. Next, all three parts are inserted into a second die 132. The second die 132 is shaped like the first die 131, but is long enough to accommodate the three parts end to end. A punch 133 having a circular cross-section with a diameter slightly less than the inner diameter of the die 132 is inserted into the die 132. Force is applied to the punch 133 so that the three parts are compressed together within the die. Because the three parts are within the die, there is no burr where they join. The method can thus be used to produce a core having a length of at least 1.5 times its diameter, with a uniform density and no burrs.

[0174] [Multiple Arrays of Poles and Magnets]

[0175] In another alternative embodiment, the electrical machine is as described above with reference to FIG. 1, but has two arrays of magnets and two arrays of cores and coils. As with the embodiment described with reference to FIG. 1, an array of magnets is arranged in a ring around the axis of rotation of the rotor, and an array of cores is also arranged in a ring around the axis. In this alternative embodiment, however, there is a second array of magnets also arranged in a ring around the axis of rotation of the rotor, and a second array of cores also arranged in a ring around the axis. The second array of magnets is radially inside the first array of magnets. Similarly, the second array of cores is radially inside the first array of cores. The radial distance from the axis to the second array of magnets is equal to the radial distance from the axis to the second array of cores.

[0176] In operation the second arrays of cores and magnets interact in the manner described above in relation to the cores and magnets of the electrical machine described with reference to FIG. 1. The second arrays increase the number of active parts within the electrical machine. Since the second arrays are positioned radially inside the first arrays, the size of the electrical machine in this embodiment is not much greater than the size of the electrical machine described with reference to FIG. 1. In this embodiment, there are therefore more active parts within an electrical machine that is not much bigger than the electrical machine described above. Thus the performance of the electrical machine is enhanced without greatly increasing its size.

[0177] [Schematic View of Potential Electrical Machines]

[0178] FIGS. 14, 15 and 16 each show a specific stator-rotor arrangement of the electrical machine. Each stator S might be a single piece like in FIG. 15 or a multipart piece like shown in FIG. 14 and FIG. 16. There, each core C made from powder SMC might have a step at least at one of its ends by which fixation is possible, preferably with a press fit. The stator is preferably made from powder SMC, at least the stator's back plate BP. The core might be mounted in the back plate using different technologies, even use of glue is possible. The rotor R and its back plate BP is made also preferably from powder SMC. The Rotor might also comprise an appropriate, nonmagnetric material P, like Pertinax.

[0179] The magnets M are either mounted by press fit, see e.g. FIG. 15, or might be fixed with an appropriate nonmagnetic material P, like Pertinax on the back plate whereby the back plate preferably is made from powder SMC, as shown e.g. in FIG. 14. An alternative rotor R is shown in FIG. 15 adjacent to the stator-rotor-stator arrangement.