WO2012042209A2

WO2012042209A2 - An electrical machine and a rotor for an electrical machine

Info

Publication number: WO2012042209A2
Application number: PCT/GB2011/001410
Authority: WO
Inventors: Frank H. Moeller
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-09-28
Filing date: 2011-09-28
Publication date: 2012-04-05
Anticipated expiration: 2013-03-28
Also published as: GB201307704D0; WO2012042209A3; GB2503971A; GB201016324D0

Abstract

An external rotor for an electrical machine. The rotor comprises a plurality of permanent magnets, a plurality of backing segments and support structure, the permanent magnets arranged circumferentially to form poles of the rotor, and the plurality of backing segments arranged radially outside the permanent magnets to provide a flux path between the poles and to provide an abutment to at least radially locate the permanent magnets. Each backing segment is retained by the support structure.

Description

AN ELECTRICAL MACHINE AND A ROTOR FOR AN ELECTRICAL

MACHINE

FIELD

This invention relates to an electrical machine, such as a motor, a generator or a motor- generator; and also relates to a rotor for such an electrical machine. In particular, embodiments of this invention relate to an external rotor for such an electrical machine. BACKGROUND

Electrical machines in which the flux passes between the rotor and stator in a radial direction are often termed "radial-flux" electrical machines. Such machines can be configured either with the rotor arranged radially inside the stator, or with the rotor arranged radially outside the stator. The use of permanent magnets, particularly permanent magnets of rare-earth materials, on external rotors can be used to produce a machine with, when used as a motor, high a torque-to-weight ratio.

There are certain challenges associated with mounting magnets on external rotors. In one mounting arrangement, the magnets are mounted to the inside of a backing ring that counteracts the centrifugal force acting on the magnets during rotation, thereby locating the magnets radially. Consideration must however be given to the construction of the backing ring. For example, where the backing ring is simply formed from electrical steel, eddy currents will exist in the ring during use and will result in energy being lost as heat - so- called eddy current iron losses (PR losses or "iron losses"). These losses are

particularly significant at higher frequencies. An alternative is to laminate the backing ring from a plurality of generally identical, thin, insulated, rings that are axially stacked to form the complete backing ring. This reduces the eddy currents and resulting losses, but it is difficult and expensive:

stamping the individual thin rings from sheet material results in much waste material (the centre of the ring is wasted). In addition, providing bolts through the laminated stack of rings can introduce short-circuits and so give rise to eddy currents and associated losses.

A further alternative is to form the backing ring rather like a "slinky" spring from a long, thin, strip of insulated material that is spirally- wound to give an axially-laminated stack. Again, fixing the laminated stack together is a problem and usually requires bolts through the stack: if the holes are pre-formed before winding, it is difficult to get them to line-up; if the holes are formed after winding, burrs created by drilling encourage short-circuits and losses.

Finally, the formation of the backing ring from sintered iron would reduce the eddy currents and associated losses, but it is difficult to form the sintered iron into thin lings; whereas thick rings are too heavy and so have too much inertia for a rotating component.

Additionally, a suitable sinter material has very low strength and could not be used without additional support to withstand the centrifugal forces occurring in a high speed machine.

An object of at least certain embodiments is to address one or more of these problems.

SUMMARY

In general terms, embodiments provide a segmented external rotor for an electrical machine in which magnets are mounted on the segments and the segments held against radial forces acting outwards. One segment per pole may be provided, but with magnets of each pole spread across neighbouring segments.

According to an aspect of this invention, there is provided an external rotor for an electrical machine, the rotor comprising a plurality of permanent magnets, a plurality of backing segments and support structure, the permanent magnets arranged

circumferentially to form poles of the rotor, and the plurality of backing segments arranged radially outside the permanent magnets to provide a flux path between the poles and to provide an abutment to at least radially locate the permanent magnets, each backing segment being retained by the support structure. In at least certain embodiments, by providing a segmented backing for the magnets, less material is wasted than is the case with, for example, a complete annular backing ring made up of laminated stamped rings. The problems associated with a slinky-type arrangement and sintered material can also be avoided.

The backing segments may form a substantially continuous ring. Each segment may abut each neighbouring segment. The abutment interface may be substantially radial. The segments may extend between the poles. Each segment may extend between a pair of poles. Each segment may extend between a respective part of the poles between which it extends. For each pole, two segments may extend therefrom, each to a respective one of the circumferentially neighbouring poles. Each segment may abut each of its neighbouring segments substantially at a mid-point of the respective pole. By providing such an arrangement, very little flux flows between neighbouring segments, and so the axial thickness of the segments can be reduced at their

circumferential ends to accommodate locating structure that abuts the segments, without any significant loss of performance. Each pole may comprise a plurality of the permanent magnets. For each pole, at least one of the permanent magnets thereof may be backed by one of the segments that extends from that pole, and at least one of the magnets thereof may be backed by the other of the segments that extends from that pole. Half of the magnets of each pole may be backed by the one of the segments; the other half of the magnets may be backed by the other of the segments.

Each segment may be of lesser radial thickness at circumferential ends thereof than therebetween. In other words, each segment may be of lesser radial thickness adjacent the interfaces with the neighbouring segments. Each segment may be of lesser radial thickness adjacent the mid-point of each pole than adjacent the mid-point between neighbouring poles. Each segment may be shaped so as to taper from a greater radial thickness away from its circumferential ends to a lesser radial thickness adjacent its circumferential ends. Each segment may have a radially outer surface that is shaped such that an external force acting thereon and normal thereto has both a radially inwards and tangential component thereto.

The support structure may be arranged to retain the segments against forces acting on the segments substantially in a radially outward direction. The support structure may be arranged to abut the segments substantially radially to provide such radial retention. The support structure may be arranged to retain the segments against forces acting substantially in a tangential direction. The support structure may be arranged to abut the segments substantially in a radial direction. The support structure may be arranged to abut the segments in a direction oblique to a radius so as to be arranged to react against the segments with a force having both radial and tangential components, thereby providing both radial and tangential retention.

The support structure may comprise annular structure radially outside the segments that encloses the segments. The annular structure may comprise composite structure wound around the segments. The composite structure may comprise circumferentially orientated fibres or filaments.

The support structure may comprise a plurality of retention members that project to provide the radial and/or tangential retention of the segments. The retention members may abut the segments in the regions thereof of lesser radial thickness. The retention members may project substantially axially. The segments may be arranged such that the retention members can be accommodated in the regions thereof of lesser thickness without projecting radially beyond the radially thickest part of the segments. The arrangement may be such that the retention members, when so accommodated, project to the same radial distance as do the radially outermost parts of the segments.

The retention members may be a plurality of axially-extending members that project between and are retained by first and second axially-spaced annuluses. At least one of the annuluses may be mounted for rotation. The remainder of the rotor may be supported on the at least one annulus for rotation therewith.

According to another aspect of this invention, there is provided an electrical machine comprising a rotor as defined hereinabove. The electrical machine may be a radial electrical machine. It may have eight pole-pairs. There may be four permanent magnets making up each pole. Each pole may be of opposite polarity to each neighbouring pole. The electrical machine may comprise a wound internal stator. The electrical machine may be arranged to operate as a motor and/or a generator.

According to a further aspect of this invention, there is provided a vehicle, for example a bicycle, comprising an electrical machine as defined hereinabove, the electrical machine housed in a wheel hub and/or a crank housing of the vehicle.

The vehicle may be, for example, an aircraft, boat, car, bicycle or other vehicle. In other embodiments, the electrical machine may find use in other applications, for example, a turbocharger or windturbine.

In an embodiment, the rotor may comprise a plurality of axially-stacked layers, each layer comprising a plurality of circumferentially-arranged layer elements, wherein each layer element is a layer of each of a plurality of the segments, the segments in each layer element being joined together. The segments in each layer element may be integrally formed. The layer elements in each layer may not be integrally formed with each other.

The layer elements may by arranged to make up a substantially complete ring. Each layer element may abut each neighbouring layer element. The abutment interface may be substantially radial.

Each layer element may be substantially the same shape as each other layer element. Each layer element may have substantially the same grain orientation as each other layer element. The grain orientation may be substantially circumferential. The grain orientation may be tangential to a point on the layer element. The layer elements may be coated to increase electrical resistance between layers.

The layer elements may be arranged such that each layer and/or layer element is angularly displaced relative to an axially adjacent layer and/or layer element by a whole number of segments. The whole number of segments may be less than the number of segments of the or each layer element. The layers elements may be arranged such that each layer and/or layer element is angularly displaced relative to an axially juxtaposed layer and/or layer element by a whole number of segments. A first group of the layers and/or layer elements may each be angularly aligned with one another; a second group of the layers and/or layer elements may each be angularly aligned with one another; the first group and the second group being angularly displaced relative to each other by a whole number of segments. The angularly displaced layers and or layer elements may be displaced by one or more segments in the same circumferential direction. Some angularly displaced layers and/or layer elements may be displaced by one or more segments in one circumferential direction and others by one or more segments in the opposite circumferential direction. For example, a first layer and/or layer element may be displaced by one or more segments in a first circumferential direction relative to a layer axially adjacent to it and a successive layer and/or layer element may be displaced by one or more segments in the opposite circumferential direction to the displacement of the first layer and/or layer element. This pattern of relative displacement may then be repeated for successive layers and/or layer elements.

The radial interfaces between layer elements may be angularly distributed in different axially-stacked layers. The layer elements may be attached to one or more

circumferentially adjacent layer elements in the same layer at their radial interfaces, for example by welding. The layer elements may abut radially without being attached.

In an embodiment, the rotor may comprise a plurality of circumferentially arranged backing segments, the segments being spaced apart from each other.

The support structure may comprise an annular structure abutting the segments substantially axially. The segments may be joined substantially axially to the support structure. This joint may, for example, be formed by casting the segments into the support structure. The support structure may be located on a rim of a vehicle wheel. The support structure may be the rim of a vehicle wheel. The support structure may be arranged to provide axial retention, radial retention, tangential retention or any combination of these.

According to a further aspect of this invention, there is provided a method of manufacturing a rotor of an electrical machine as defined above, the method comprising the steps of:

(a) assembling a plurality of the layer elements to form one of the layers; and (b) assembling another plurality of the layer elements to form a successive one of the layers axially adjacent the previous layer and such that the layer elements of the successive layer are angularly displaced from the layer elements of the previous layer by a whole number of segments. Optional features of the other aspects of the invention are also optional features of this aspect.

BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments will be described below by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows an axial view of components of an electrical machine, including a rotor; Figure 2 shows a sectional view of the electrical machine, the section being taken through a radial plane;

Figure 3 shows in more detail certain of the components shown in Figure 1. Figure 4 shows a rotor with the topology of that of Figures 1 to 3, but constructed in a different way;

Figure 5 is a detailed view of a component of the rotor of Figure 4; and Figure 6 is a view of a rotor of another embodiment in an in-wheel application. SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS Figure 1 shows certain parts of a rotor 10 of an electrical machine. The electrical machine is an radial-flux, permanent magnet, electrical machine having an external rotor 10. The stator is not shown, but it is envisaged that the stator be a stator with salient poles around which are wound coils for receiving an excitation current in the event that the electrical machine is used as a motor, and from which a current can be drawn in the event that the electrical machine is used as a generator. The particular arrangement of the stator is not relevant to the innovations disclosed in this document and so will be described no further.

The rotor 10 is generally made up of permanent magnets 20 which are mounted on backing segments 30, which in turn are supported by support structure 40. Each of these will now be described in greater detail.

The magnets 20 are a plurality of rare-earth magnets, which in this embodiment, are Neodymium-Iron-Boron magnets, although it is envisaged that any form of magnet may be used. The magnets are circumferentially distributed to form poles 22 of the rotor. The distribution is such that, in the present embodiment, four magnets 20 are grouped together into a side-by-side circumferential series, with faces of like polarity facing the same direction, to form a pole. Each pole is circumferentially spaced from each neighbouring pole and is arranged with opposite polarity. In the present embodiment, there are 16 poles.

Each backing segment 30 is generally the same as each other backing segment 30. The shape of each backing segment 30 is such that it approximates to a segment of a ring that is coaxial with the axis of rotation of the electrical machine. More specifically, the inner surface of each segment 30 is intended to match as closely as practical an arcuate surface coaxial with the axis of rotation, but the outside surface is shaped such that each circumferential end thereof is closer to the radially inner surface than is the portion of the outer surface that is between those two ends. In other words, the radially outer surface of each segment tapers towards the radially inner surface towards each circumferential end. As a consequence, each segment 30 is of greater axial thickness at portions thereof between the two ends, than it is at its two ends. This arrangement is more easily seen in Figure 3. With reference to Figure 1, the segments 30 are arranged end-to-end and coaxial with the axis of rotation to form a backing ring for the magnets 20. As already mentioned, the magnets 20 are mounted on the backing segments 30. The mounting is such that the adjacent magnets 20 that go to make up any one particular pole are mounted on two different, but neighbouring, segments 30: some magnets 20 of the pole are mounted on one segment 30; the other magnets 20 of the pole are mounted on the other, neighbouring, segment 30. In the present embodiment, in which each pole is made up of four magnets 20, two magnets 20 are each mounted on a respective one of the neighbouring segments 30. Each pole is spread across two segments 30 in this way such that each segment 30 supports, at one circumferential end thereof, magnets 20 of a first pole and, at the other circumferential end thereof and spaced therefrom, magnets 20 of a neighbouring pole. Thus, each segment 30 has mounted thereon magnets 20 from each of two neighbouring poles.

With reference to Figure 3, the circumferential ends of each segment 30 that are thinner (in an axial direction) than parts of the segment 30 therebetween, terminate in an end surface 32 that extends in a radial plane. Accordingly, when the segments 30 are arranged circumferentially end-to-end, the end surfaces 32 abut one another with face- to-face contact. The support structure 40 will now be described. With reference to Figure 1, the support structure 40 includes a rotor carrier 42, a clamp ring 44, a series of bolts 46 and a component known as a composite "bandage" 48. The general arrangement of these components is that the rotor carrier 42 is mounted for rotation, the backing segments 30 and the magnets 20 are axially clamped between the rotor carrier 42 and the clamp ring 44, with the bolts 46 extending between the clamp ring 44 and the rotor carrier 42 in order to exert the necessary axial clamping force on the segments 30 and the magnets 20. This will, however, be described in more detail. With reference to Figure 2, the rotor carrier 42 is a substantially annular disc, but with an axially projecting lip 43 projecting to one side of the disc. The lip includes a plurality of holes formed therein, each having a screw thread formed in the internal wall thereof; and also has a wall 143 projecting from the radially inner edge of the lip 43 in the same axial direction in which the lip 43 projects.

With continued reference to Figure 2, the clamp ring 44 is a flat annular disc with the same inside and outside diameter as the lip 43 of the rotor carrier 42. The clamp ring 44 also includes an axial projecting wall 45, and also includes a plurality of holes formed through the ring 44 and arranged to correspond to the holes formed in the rotor carrier 42.

The arrangement is such that the segment 30 with the magnets 20 thereon (in this embodiment, it is envisaged that the magnets 20 be glued to the segments 30) fit between the lip 43 of the rotor carrier 42 and the clamp ring 44, with the segments 30 sandwiched between the lip 43 of the rotor carrier 42 and the clamp ring 44, and with the magnets 20 sandwiched between the wall 143 of the rotor carrier 42 and the wall 45 of the clamp ring 44. The bolts 46 pass through the holes in the clamp ring 44 into the threaded holes in the rotor carrier 42 and are tightened to clamp the segments 30 and the magnets 20 between the rotor carrier 42 and the clamp ring 44. This provides axial location of the segments 30 and magnets 20 relative to the rotor carrier 42.

With reference to Figure 1 , it should be noted that the arrangement is also such that the bolts 46 are positioned adjacent the interface of each segment 30 with its neighbouring segment 30, such that each bolt 46 abuts the radially outer surface of each of the two and neighbouring interfacing segments, adjacent where those two segments abut one another. Figure 3 shows in more detail the abutment between one bolt 46 and two neighbouring segments 30. As can be seen, the outer surface of each segment 30 is shaped to follow the outer diameter of the bolts 46 for the length of the outer surface that is in abutment with the bolt 46. Thus, contact area between the bolts and the segments 30 is maximised. The abutment between the bolts 46 and the backing segments 30 provides circumferential and radial (at least in an outwards direction) location of the backing segments, and hence of the magnets mounted thereon, relative to the rotor carrier 42. The final component of the support structure 40 is the composite bandage 48 that is wound around the outside of the segments 30 and the bolts 46. In this embodiment, the composite bandage 48 is a length of elongate, initially flexible, composite material comprising flexible fibres of high tensile strength in a resin matrix that can be wrapped around the segments 30 and the bolts 46 and then cured to provide a strong ring that can react against radially outwards forces acting on the bolts 46, and the segments 30, thereby assisting in providing radial location of the components of the rotor 10 at high rotation speeds.

In operation, the poles of the rotor 10 set up a magnetic field having flux loops circulating through neighbouring poles of the rotor 10. The positioning of some magnets 20 of each pair of neighbouring poles on the same backing segment 30 causes that backing segment 30 to provide a flux path for the flux linking those two poles. (Although not shown, it will be understood that this flux also crosses an air gap with the rotor, passes through the pole of the rotor to a neighbouring pole thereof, before crossing back across the air gap into the neighbouring pole of the rotor 10).

For each pole, some of the flux is carried by one backing segment 30 in one circumferential direction, and the remainder of the flux is carried by a neighbouring segment 30 in the opposite circumferential direction. Not much flux therefore passes between neighbouring segments and so the contact area between neighbouring segments constituted by the end surfaces 32 can be made small, thereby providing space for the bolts 46 without increasing the outer radius of the rotor and in a way that allows the bolts to abut the segments 30 with both an axial and a circumferential component, without restricting flux flow in the rotor.

In the present embodiment, it is envisaged that the segments 30 can be formed of electrical steel. In alternative embodiments they may be laminated in order to reduce eddy currents. In other embodiments, the segments may, perhaps in addition to being laminated, may be arranged such that the grain orientation of the segments is approximately circumferential so as to be conducive to flux flow therein. It is also envisaged that the backing segments may be formed by sintering. In this and other embodiments, it is envisaged that the other components of the rotor may be formed from a ferrous material or a non-ferrous material.

In embodiments where rotation speeds are not high, it is envisaged that the composite bandage may be omitted.

Various construction techniques may be used to arrive at the rotor 10 described above with reference to Figures 1 to 3. For example, the rotor 10 may made up of a plurality of axially-stacked layers of electrical steel. Each layer would be identical to each other layer in being a cross-section through a plane perpendicular to the axis of the rotor 10. Each layer would stamped, or in another example laser-cut, from a sheet of electrical steel as a complete, integral, ring of material. The layers would then be welded together, or in other examples held together by other means such as by gluing or by external clamping structure.

One drawback of such an approach is the large amount of waste electrical steel produced by stamping complete rings from a sheet of that material. Another drawback is the disadvantageous grain orientation in certain parts of the ring: in some parts the grain orientation will be substantially tangential and so be aligned with the direction of flux, which is beneficial; in other parts the grain orientation will be substantially radial and so not aligned with the direction of flux, which is not beneficial.

Accordingly, in order to address these drawbacks, in the present embodiment each segment 30 is made up of a plurality of axially-stacked segment layers. Each segment layer is a cross section though a segment 30 and is identical to each other segment layer. Again, the segment layers are stamped, or in an alternative example laser-cut, from a sheet of electrical steel. As shapes with the cross-section of a single segment 30 are being cut in this example, rather than a complete ring having the shape of a cross- section through the complete rotor 10, the segment layers can be cut from sheet material in a way that avoids much of the waste material produced when complete rings are cut. In addition, each of the segment layers can be cut from the sheet such that the grain orientation lies substantially circumferentially in each segment layer, thereby substantially aligning with the direction of flux in the segment layer. Segment layers are assembled into segments 30 by axially stacking the segment layers and then welding them together.

When segments and/or layers are welded, it is envisaged that the segment layers or layers are welded together by laser and so as to give a thin weld bead. The segment layers or layers are welded along radially outer edges, towards the circumferential ends of the segments to keep the welds substantially away from the flux path. This is to minimise and short-circuiting of the segment layers or layers. Segments 30 assembled in this way are then welded to one another to form the rotor 10.

A further example for arriving at the rotor topology of Figures 1 to 3 will now be described with reference to Figure 4. In this example, the topology of the rotor 1 10 is substantially the same as that described with reference to Figure 1 to 3, but the rotor 110 differs in the way it is constructed..

As can be seen from Figure 4, the rotor 1 10 is made up of magnets 120 mounted on segments 130 in much the same way as in the first embodiment. The shape of the rotor 110 and the segments 130 is, in this embodiment, the same as before. However, this further example differs in that the way in which this rotor 110 and segments 130 are formed.

Specifically, the rotor 110 is made up of a plurality of axially-stacked layers in the form of laminations of electrical steel. With reference to Figure 5, each layer is made up of several layer elements 132. In the present example, each layer element is a portion of a layer that is a contiguous, integrally-formed slice of each of two juxtaposed segments 130. Each layer element is laser-cut from a sheet of electrical steel. The electrical steel is coated with a ceramic layer to provide electrical insulation. In this embodiment, it is envisaged that the layer elements 132 be cut such that the grain orientation in each layer element 132 is the same and is aligned substantially circumferentially with respect to the layer element 132. The ceramic layer increases electrical resistance between axially juxtaposed layer elements 132 and therefore reduces eddy currents between layers and associated losses. To form each complete layer of the rotor 110, the layer elements 132 are placed end-to- end to form a ring. In this embodiment, the layer elements 132 are not specifically joined together along radially-extending interfaces of each layer element 132 in a particular layer. As mentioned, complete layers are axially stacked to form the rotor 1 10. However, layers are not stacked such that the layer elements 132 in each layer are angularly aligned with the layer elements 132 in each other layer, with the interfaces between juxtaposed layer elements 132 in one layer on top of those in the or each neighbouring layer. Instead, the layers and the layer elements 132 from which they are made up, are rotationally staggered, that is angularly displaced, with respect to each other. As will be explained below, this benefits structural integrity of the rotor 1 10.

During the axial stacking of the layers, interfacing faces of the layers are coated in glue to bond the layers together.

In the present example, starting from one axial end of the rotor 110 and moving towards the other end, each successive layer is angularly displaced in the same angular direction from the previous layer by a single segment 130. Thus, each layer is rotationally "indexed" from each previous layer. This arrangement results in interfaces between layer elements 132 in a layer being angularly displaced from interfaces between layer elements 132 in the or each neighbouring layer. Thus, interfaces in adjacent layers do not coincide. As these interfaces will be points of weakness and may also give rise to electromagnetic imbalance, distributing these interfaces angularly throughout the axial stack avoids a concentration of these points of weakness or imbalance and so minimises the potential effects of them. In other embodiments, different approaches to angularly displacing the interfaces between adjacent layer elements 132 in one layer with respect the interfaces between layer elements 132 in adjacent layers are envisaged. For example, in one alternative embodiment, a group of adjacent layers are rotationally aligned with each other such that the interfaces between layer elements in each layer of that group are angularly aligned with the interfaces in each other layer of the group. A second group of adjacent layers is also arranged in the same way (and, optionally, further groups), with the second group being angularly displaced with respect to the first by one segment. In the two examples just described, the layer elements 132 are made up of contiguous portions, or slices, of two segments. The permutations of angular staggering are therefore limited: layers can be staggered by just a single segment. In other

embodiments, it is envisaged that the layer elements be made up of portions of more than two segments and so more permutations for angular staggering are possible and envisaged: layers may be staggered by one, two or more segments, with different staggering arrangements being used between different layers. In short, any arrangement is envisaged that angularly distributes the interfaces between layer elements 132 when the rotor 1 10 is considered as a whole.

The effect of this angular displacement is that the radial interfaces between parts of layers are angularly distributed throughout the axial stack of layers. This provides the rotor 110 with better mechanical stability when compared to discrete segments made up of a stack of segment layers, each the shape of a single segment in cross-section.

Furthermore, this approach allows for better grain orientation than the arrangement in which complete, integral, rings are used to make up the rotor.

With reference to Figure 6, a further embodiment is disclosed. In this further embodiment, another external rotor 210 of an electrical machine is provided. The rotor 210 of this embodiment is similar to that 10 of the first embodiment described above with reference to Figures 1 to 3. Specifically, the rotor 210 of this embodiment is made up of a plurality of circumferentially arranged rotor segments 230 that are, taken individually, substantially the same as the segments 30 of the first embodiment in terms of their shape and magnet 220 polarity and location. The rotor segments 230 of the present embodiment are made up of axially-stacked segment layers, each having the shape of a cross-section through a single segment 230. These segment layers are each cut from a sheet of electrical steel made and assembled into segments 230 as described above.

An example application for the rotor 210 of the present embodiment is an in- wheel motor for a vehicle such as an electric passenger car (not shown). The rim of a wheel is shown at 240 in Figure 6. In this embodiment, this rim 240 is a light alloy cast rim. The rotor segments 230 are cast into the rim 240, i.e. they are positioned during casting of the rim 240 so as to be held in position by surrounding structure of the rim 240 once casting is complete.

In this embodiment, the segments 230 are positioned so as to be circumferentially spaced from each circumferentially adjacent segment 230. This is the striking difference between the present embodiment and those described above. The reason for this is that, in wheel-hub mounted applications, it can be advantageous to increase the diameter of the motor, as this greatly increases torque produced by the motor. If the motor of Figures 1 to 3 were simply scaled up for a wheel-hub application, there would be a large increase in the amount of electrical steel used, which in turn would greatly increase cost and weight. It will be understood that both of these are undesirable. In particular, increasing weight in a wheel hub is undesirable as it is an unsprung and rotating mass and so would lead to undesirable handling effects. The present embodiment avoids these drawbacks by introducing spacing between the segments 240 and by using surrounding structure of the wheel rim 240 to locate the segments 230.

Abutment structure 246 may also be cast into the rim 240 to serve the same function as the bolts 46 of the first embodiment described above with reference to Figures 1 to 3 in providing location for the segments 230. For example, the abutment structure 246 may also take the form of bolts and may interact with a framework to maintain relative positioning of the segments 240 before and during casting of the rim 240.

It will be understood that, because of the flux path of the rotor 210 (which is substantially the same as that of the rotor 10 of the first embodiment), the segments 230 of this embodiment can be spaced in the way described without a loss of performance of the electrical machine. Indeed, spacing the segments 230 of the present embodiment actually improves electrical performance in some instances by reducing flux leakage and eddy currents between adjacent segments 230 and so reducing associated losses. This is made possible by the flux path of the present electrical machine.

Claims

1. An external rotor for an electrical machine, the rotor comprising a plurality of permanent magnets, a plurality of backing segments and support structure, the permanent magnets arranged circumferentially to form poles of the rotor, and the plurality of backing segments arranged radially outside the permanent magnets to provide a flux path between the poles and to provide an abutment to at least radially locate the permanent magnets, each backing segment being retained by the support structure. 2. A rotor according to claim 1, wherein, for each pole, two segments extend therefrom, each to a respective one of the circumferentially neighbouring poles.

3. A rotor according to any preceding claim, wherein each segment abuts each of its neighbouring segments substantially at a mid-point of the respective pole.

4. A rotor according to any preceding claim, wherein, for each pole, at least one of the permanent magnets thereof is backed by one of the segments that extends from that pole, and at least one of the magnets thereof are backed by the other of the segments that extends from that pole.

5. A rotor according to any preceding claim, wherein half of the magnets of each pole are backed by the one of the segments.

6. A rotor according to claim 5, wherein the other half of the magnets are backed by the other of the segments.

7. A rotor according to any preceding claim, wherein each segment is of lesser radial thickness at circumferential ends thereof than therebetween.

8. A rotor according to any preceding claim, wherein each segment is of lesser radial thickness adjacent the mid-point of each pole than adjacent the mid-point between neighbouring poles. 9. A rotor according to any preceding claim, wherein each segment is shaped so as to taper from a greater radial thickness away from its circumferential ends to a lesser radial thickness adjacent its circumferential ends.

10. A rotor according to any preceding claim, wherein, each segment has a radially outer surface that is shaped such that an external force acting thereon and normal thereto has both a radially inwards and tangential component thereto.

11. A rotor according to any preceding claim, wherein the support structure is arranged to abut the segments substantially in a radial direction.

12. A rotor according to any preceding claim, wherein the support structure is arranged to abut the segments in a direction oblique to a radius so as to be arranged to react against the segments with a force having both radial and tangential components, thereby providing both radial and tangential retention.

13. A rotor according to any preceding claim, wherein the support structure comprises annular structure radially outside the segments that encloses the segments. The annular structure may comprise composite structure wound around the segments. 14. A rotor according to any preceding claim, wherein the support structure comprises a plurality of retention members that project to provide the radial and/or tangential retention of the segments.

15. A rotor according to claim 14, wherein the retention members abut the segments in the regions thereof of lesser radial thickness.

16. A rotor according to claim 14 or claim 15, wherein the segments are arranged such that the retention members can be accommodated in the regions thereof of lesser thickness without projecting radially beyond the radially thickest part of the segments.

17. A rotor according to claim 16, wherein the arrangement is such that the retention members, when so accommodated, project to the same radial distance as do the radially outermost parts of the segments.

18. A rotor according to any one of claim 14 to claim 17, wherein the retention members are a plurality of axially-extending members that project between and are retained by first and second axially-spaced annuluses. 19. A rotor according to any preceding claim and comprising a plurality of axially- stacked layers, each layer comprising a plurality of circumferentially-arranged layer elements, wherein each layer element is a layer of each of a plurality of the segments and is integrally formed. 20. A rotor according to claim 19, wherein each layer element is substantially the same shape as each other layer element.

21. A rotor according to claim 19 or claim 20, wherein each layer element has a substantially circumferential grain orientation.

22. A rotor according to any of claim 19 to 21, wherein the layer elements are arranged such that each layer and/or layer element is angularly displaced relative to an axially adjacent layer and/or layer element by a whole number of segments. 23. A rotor according to any of claim 19 to 21, wherein a first group of the layers and/or layer elements are all angularly aligned with each another; a second group of the layers and or layer elements are all angularly aligned with each another; the first group and the second group being angularly displaced relative to each other by a whole number of segments.

24. A rotor according to any one of claim 19 to 23, wherein some angularly displaced layers and/or layer elements are displaced by one or more segments in one

circumferential direction and others by one or more segments in the opposite circumferential direction.

25. A rotor according to claim 24, wherein a first layer and/or layer element is displaced by one or more segments in a first circumferential direction relative to a layer axially adjacent to it and a successive layer and/or layer element is displaced by one or more segments in the opposite circumferential direction to the displacement of the first layer and/or layer element.

26. A rotor according to any of claim 19 to claim 25, wherein some or all of the layer elements in a particular layer are joined at their radial interfaces, for example by welding.

27. A rotor according to any of claim 19 to claim 26, wherein some or all of the layer elements in a particular layer abut radially without being joined. 28. An electrical machine comprising a rotor according to any preceding claim.

29. A vehicle comprising an electrical machine according to claim 28, the electrical machine housed in a wheel hub, flywheel and/or drive hub of the vehicle. 30. A rotor according to any of claim 1 to claim 18, wherein the backing segments are circumferentially spaced apart.

32. A rotor according to claim 30, wherein the support structure comprises a wheel rim for a vehicle.

33. A rotor according to claim 32, wherein the segments are cast into the wheel rim.

34. A method of manufacturing a rotor according to any of claim 19 to claim 27, the method comprising the steps of:

(a) assembling a plurality of the layer elements to form one of the layers; and

(b) assembling another plurality of the layer elements to form a successive one of the layers axially adjacent the previous layer and such that the layer elements of the successive layer are angularly displaced from the layer elements of the previous layer by a whole number of segments.