WO2011076579A1

WO2011076579A1 - Stator element for a modulated pole machine

Info

Publication number: WO2011076579A1
Application number: PCT/EP2010/069269
Authority: WO
Inventors: Göran NORD
Original assignee: Höganäs Ab (Publ)
Priority date: 2009-12-21
Filing date: 2010-12-09
Publication date: 2011-06-30
Also published as: TW201141010A

Abstract

Disclosed is a stator for an electrical rotary machine, said stator comprising: a first and a second annular stator core section each including a respective plurality of teeth protruding from the respective stator core section; a coil arranged axially between the first and second stator core sections, wherein the first and second stator core sections and the coil encircle a common geometric axis; wherein the first stator core section comprises a recess forming a wire channel so as to establish a radial passage for a wire leading to or from the coil; wherein the second stator core section comprises an indexing protrusion for guiding assembly of the first and second stator core sections with each other in a predetermined circumferential position of the teeth of the second stator core section relative to the teeth of the first stator core section; and wherein the indexing protrusion is adapted to axially extend into the wire channel to define a circumferential position of the second stator core section relative to the first stator core section.

Description

Stator element for a modulated pole machine

Field of the invention This invention generally relates to electric machines. More particularly, the invention relates to a rotating modulated pole machine, such as a rotating transverse flux machine with a permanent magnet rotor. This type of machine could be utilized either as a motor or a generator depending on the area of application.

Background of the invention

The transverse flux machine (TFM) topology is an example of a modulated pole machine. It is known to have a number of advantages over conventional machines. The basic design of a single-sided radial flux stator is characterized by a single, simple phase coil parallel to the air gap and with a generally U-shaped yoke section surrounding the coil and exposing in two parallel rows of teeth facing the air gap. An example of an efficient rotor is the use of so called buried magnets combined with soft magnetic pole sections or pieces to allow the permanent magnet field to flux-concentrate or be flexible in a direction transverse to the motion as e.g. described in the patent application WO2007/024184 by Jack et al.

WO2007/024184 discloses an electrical, rotary machine, which includes a first stator core section being substantially annular and including a plurality of teeth, a second stator core section being substantially annular and including a plurality of teeth, a coil arranged between the first and second annular stator core sections, and a rotor including a plurality of permanent magnets. The first stator core section, the second stator core section, the coil and the rotor encircle a common geometric axis, and the plurality of teeth of the first stator core section and the second stator core section are arranged to protrude towards the rotor. Additionally the teeth of the second stator core section are circumferentia!ly displaced in relation to the teeth of the first stator core section, and the permanent magnets in the rotor are separated in the circumferential direction from each other by axially extending pole sections made from soft magnetic material.

A multi-phase machine may be obtained by axially stacking a plurality of stator phase sections. The stacking of the individual stator phase sections beneficially makes use of a physical magnetic separation in-between the individual phase-sections to reduce the magnetic coupling in-between the phases that possibly can have an effect of reducing the effective flux in the air gap during operation.

The stator core sections may advantageously be manufactured from metal powder by a suitable compaction process. The first and second stator core sections may be manufactured as two separate elements which are assembled during manufacturing of the stator. WO 2009/116936 discloses an efficient compaction process where both the first and the second stator core sections are compacted as a single integrated component.

The coil normally has two connecting wires, a leading and a trailing wire, providing electrical current to the coil which need to be led from the coil to a connection area of the stator.

It is generally desirable to provide components of a modulated pole machine that can be manufactured at low manufacturing costs and that can efficiently be assembled so as to obtain a modulated pole machine. It is further generally desirable to provide components of a modulated pole machine that fulfill one or more of the following criteria: high stability, high durability, high resistance against stress, low weight, high performance numbers or values of the resulting modulated pole machine, such as torque pr. volume and/or torque pr. current. Summary

Disclosed herein is a stator for an electrical rotary machine, said stator comprising:

• a first and a second annular stator core section each including a respective plurality of teeth protruding from the respective one of the first and second annular stator core section;

• a stator core back providing an axial magnetic flux bridge between the first and second annular stator core sections;

• a coil arranged axially between the first and second stator core sections, wherein the first and second stator core sections and the coil encircle a common geometric axis wherein the first stator core section comprises at least one recess forming a wire channel so as to establish a radial passage for a wire leading to or from the coil; wherein the second stator core section comprises an indexing protrusion for guiding assembly of the first and second stator core sections with each other in a predetermined circumferential position of the teeth of the second stator core section relative to the teeth of the first stator core section; and wherein the indexing protrusion is adapted to axially extend into the wire channel to define a circumferential position of the second stator core section relative to the first stator core section.

Hence, by combining the indexing feature with the wire channel, the stator core sections may be kept relatively simple with few three-dimensional features, thus providing a robust manufacturing process with low risk of tool break down, e.g. when the stator core sections are made from magnetic powder by a powder compaction process.

By providing a wire channel for feeding a connecting wire of the coil to a connection area of the stator, the size of the coil may be optimized relative to the size of the annular cavity provided between the stator core sections. It will be appreciated that the coil may include a single winding or multiple windings. The indexing protrusion may have an axial dimension such that the indexing protrusion only extends a part of a depth of the wire channel into the wire channel so as to leave a passage for a wire between the indexing protrusion and a bottom of the wire channel. The indexing protrusion may have a lateral dimension matching a cross-sectional width of the wire channel so as to allow a snug fit of the indexing protrusion in the wire channel and providing a precise circumferential alignment o the first and second stator core sections.

In some embodiments the stator core section is a soft magnetic structure. It is an advantage that the improved utilization of soft magnetic structure causes improved performance per volume. In one embodiment the stator core sections are made of magnetic powder such as soft magnetic powder. By making the stator core sections from magnetic powder the manufacturing of the stator device may be simplified and magnetic flux concentration, utilizing the advantage of effective three dimensional flux paths, may be more efficient.

The wiring channel may be formed by incorporating a corresponding protrusion into the punch that is used for compaction or shaping of the stator core section from magnetic powder. When the wire channel is provided on a first side of the first stator core section and when the first stator core section further comprises a protrusion, e.g. in the form of a ridge, on a second side of the first stator core section, opposite the first side, the protrusion being axially aligned with the wire channel, the first stator core section may be produced with a simple tool and with a substantially uniform compaction ratio, because variations in the axial thickness of the first stator core section are reduced, or even eliminated. It is a further advantage of the stator described herein, that its manufacture only requires simple, robust, and relatively inexpensive tooling.

In some embodiments, the first and the second stator core sections each comprise a recess forming a wire channel so as to establish a radial passage for a wire leading to or from the coil; and an indexing protrusion for guiding assembly of the first and second stator core sections with each other in a predetermined circumferential position of the teeth of the second stator core section relative to the teeth of the first stator core section; and wherein the indexing protrusion is adapted to axially extend into the wire channel of the corresponding other one of the first and second stator core sections to define a circumferential position of the second stator core section relative to the first stator core section. Furthermore, the indexing protrusion of each stator core section may be circumferentially displaced from the wire channel of said stator core section. Hence, the stator comprises two circumferentially and axially separated radial wire channels matching the winding geometry of the coil, and simplifying the mounting of the coil. In some embodiments, the coil comprises two connection wires, a leading and a trailing wire, arranged on respective axial sides of the coil, thus further facilitating an efficient assembly of the stator.

In some embodiments, one or both of the stator core sections may comprise more than one recess each forming a respective wire channel. For example, the stator may comprise a plurality of coils, e.g. multiple parallel windings to bring down the wire size that can improve the smallest critical mechanical bending radius or hinder skin-depth causing high Ohmic resistance at increased frequencies.

The first and second stator core sections may be provided as two identical components, thus providing an efficient manufacturing by use of one single design for the two stator core halves so as to be able to manufacture both stator core sections by a single tooling set.

The stator core back may be located radially on a first side of the coil while the teeth are located radially on a second side of the coil opposite the first side. Hence, when the wire channel provides a radial passage through the stator core back, the wire channel provides a passage from the winding area to the connection area of the stator, The stator core back providing the flux bridge may be a stator yoke member arranged concentrically with the first and second stator core sections. In some embodiments, each annular stator core section has an inner side proximal to the coil and an outer side distal from the coil; and the first stator core section comprises an annular flange axially protruding from the inner side of the first stator core section, the annular flange forming at least a part of the stator core back. The flange may be substantially continuous in the circumferential direction and concentric with the stator core section.

In some embodiments the second stator core section comprises an annular flange axially protruding from the inner side of the second stator core section and the annular flange of the first stator core section abuts the annular flange of the second stator core section so as to form the stator core back.

Hence, in such embodiments, the stator core back is provided as an integrated part of one or both stator core sections. By arranging such a stator core section with integrated stator core back, the manufacturing process of the parts of the stator assembly and the assembling process of the stator assembly may be facilitated and more cost-effective.

The stator core sections may thus provide respective annular walls providing radial flux paths and defining side walls of a cavity for accommodating the coil. The teeth protrude radially from a first circumferential edge of the annular walls. The protrusion defining at least a part of the stator core back is arranged near or at a second circumferential edge of the annular walls, opposite the first edge.

The present invention relates to different aspects including the stator described above and in the following, and corresponding methods, devices, and/or product means, each yielding one or more of the benefits and advantages described in connection with the first mentioned aspect, and each having one or more embodiments corresponding to the embodiments described in connection with the first mentioned aspect and/or disclosed in the appended claims.

In particular, disclosed herein is a stator core section for a stator as described above and in the following.

Furthermore, disclosed herein does an electric rotary machine comprise a rotor and a stator described above and in the following. In some embodiments the rotor comprises a plurality of permanent magnets. In some embodiments the permanent magnets in the rotor are separated from each other by pole sections made from soft magnetic material. In particular, the permanent magnets in the rotor may be separated in the circumferential direction from each other by axially extending pole sections. The pole sections may be made of a magnetic powder. The permanent magnets may be magnetised in the circumferential direction and with alternating orientation.

In some embodiments, also referred to as outer-rotor configuration, the rotor is arranged around the first and second stator core sections and wherein the teeth extend radially outward. In alternative embodiments, also referred to as inner-rotor configuration, the stator is arranged around the rotor and the teeth extend radially inward. In some embodiments the electrical rotary machine is a modulated pole machine. In conventional machines, the coils explicitly form the multi-poie structure of the magnetic field, and the magnetic core function is just to carry this multi-pole field to link the magnet and/or other coils. In a modulated pole machine, it is the magnetic circuit which forms the multi-pole magnetic field from a much lower, usually two, pole field produced by the coil. In a modulated pole machine, the magnets usually form the matching multi-pole field explicitly but it is possible to have the magnetic circuit forming multi-pole fields from a single magnet. During operation, embodiments of a modulated pole machine have a three-dimensional (3D) flux path, including a flux path component in the transverse direction relative to the direction of movement. The stator and/or the rotor may comprise such a transverse flux path component, i.e. a flux path component in the axial direction.

A benefit of having the modulation is that every pole sees all of the magneto motive force (MMF) of the coil, so that as the pole number rises, the magnetic field strength (MMF/metre) rises with it without any change in the coil. This may be compared with a conventional machine in which, as the pole number rises, so does the number of coils and hence the smaller each coil is. The pole pitch however also falls with pole number, so that as the pole number rises, the magnetic field strength is more or less constant in a conventional machine as the MMF/coil reduction balances with the reduction in pole pitch.

In some embodiments the modulated pole machine comprises a claw pole arrangement or extension. For modulated pole machines, taking as fixed a geometry which forms torque from a circumferential/axial surface i.e. a radial field machine, the field may be carried radially across the air gap with the magnetic circuit, circumferentially by one pole pitch, which can be done in the stator or the rotor or partially in both, and axially in both directions to enclose the coil. If the axial circuit is closed in the stator around the coil, the claw pole arrangement is produced.

The electrical rotary machine may be a multi-phase machine where the stator is a multi-phase stator, where the phases are arranged side-by-side in the axial direction, and where each phase comprises two stator core sections each having a respective set of teeth, a flux bridge connecting the stator core sections, and a coil, and where the teeth are arranged to protrude towards the rotor. Brief description of the drawings

The above and/or additional objects, features and advantages of the present invention, will be further elucidated by the following illustrative and non- limiting detailed description of embodiments of the present invention, with reference to the appended drawings, wherein:

Fig. 1 shows a schematic, exploded, perspective view of an example of a transverse flux machine. Fig. 2 shows a cross-sectional view of the assembled machine of fig. 1. Fig 3 shows a 3-phase stator containing 3 sets of stator component pairs, each holding one circumferential winding. In particular, fig. 3a shows a schematic, perspective of the 3-phase stator, and fig. 3b shows a schematic, exploded, perspective of the 3-phase stator.

Fig. 4 shows a single phase stator arrangement comprising two stator core halves and one winding.

Fig. 5 shows a stator core section embodied as a single component compacted in one piece.

Fig. 6 shows a detailed view of a radial wire channel and indexing feature of a stator core section. Fig. 7 shows detailed views of wire channels formed when assembling two stator core sections.

Detailed description

In the following description, reference is made to the accompanying figures, which show by way of illustration how the invention may be practiced.

This invention relates to the field of an electric modulated pole machine 00 of which one example is shown in figs. 1 and 2. The electric modulated po!e machine stator 10 comprises a central coil 20, e.g. a single winding, that will magnetically feed multiple teeth 102 formed by the soft magnetic core structure. The stator core is then formed around the coil 20, while for other common electrical machine structures the coil is formed around individual teeth core sections. Examples of the modulated pole machine topology are sometimes recognised as e.g. Claw-pole-, Crow-feet-, Lundell- or TFM- machines. More particularly the shown electric modulated pole machine 100 comprises two stator core sections 14, each inc!uding a plurality of teeth 102 and being substantially annular, a coil 20 arranged between the first and second circular stator core sections, a flux bridge 18 providing an axial flux path between the stator core sections 14,16, and a rotor 30 including a plurality of permanent magnets 22. Further, the stator core sections 14, the coil 20 and the rotor 30 are encircling a common geometric axis 103, and the plurality of teeth of the two stator core sections 14 are arranged to protrude towards the rotor 30 for forming a closed circuit flux path. The machine in figure 1 is of the radial air gap flux outer rotor type as the stator teeth protrude in a radial direction towards the rotor in this case with the stator surrounding the rotor. However, the stator could equally well be placed interiorly with respect to the rotor which type is also illustrated in some of the following figures. The stator described herein may be used in connection with different types of electric modulated pole machines, e.g. machines of both the axial and the radial type and for both interiorly and exteriorly placed stators relative to the rotor. Similarly, the stator described herein may be applied to single phase machines as well as to multi phase machines. In Fig. 1 , the stator core sections 14 are shown as laminated, i.e. made from stacked sheets of soft magnetic material separated by a thin electrical insulation. The general technique of laminating stator cores is well known to persons skilled in the art. However, it will be appreciated that the stator core sections may be made as compacted components from soft magnetic metal powder. In particular, the stator core sections may be provided with a wire channel and an indexing protrusion as described herein.

The active rotor structure 30 is built up from an even number of segments 22, 24 whereas half the numbers of segments - also called pole sections 24 - are made of soft magnetic material and the other segments are made of permanent magnet material 22. The state of art method is to produce these segments as individual components. Often the number of segments can be rather large, typically around 10 - 50 individual pieces. The permanent magnets 22 are arranged so that the magnetization directions of the permanent magnets are substantially circumferential, i.e. the north and the south pole, respectively, is facing in a substantially circumferential direction. Further, every second permanent magnet 22, counted circumferential!y is arranged having its magnetization direction in the opposite direction in relation to the other permanent magnets. The magnetic functionality of the soft magnetic pole pieces 24 in the desired machine structure is fully three dimensional and it is required that the soft magnetic pole piece 24 is able to efficiently carry magnetic flux with high magnetic permeability in all three space directions. A traditional design using laminated steel sheets will not show the required high permeability in the direction perpendicular to the plane of the steei sheets and it is here beneficial to use a soft magnetic structure and material that shows a higher magnetic flux isotropy than a state of the art laminated steel sheet structure.

Figure 2 shows the same radial electric modulated pole machine as in fig 1 but in a cross-sectional view of the assembled machine showing more clearly how the stator teeth 102 extend towards the rotor and how the stator teeth of the two stator core sections 14 are rotationally displaced in relation to each other.

This design of the rotor and the stator 10 has the advantage of enabling flux concentration from the permanent magnets 22 so that the surface of the rotor 12 facing a tooth of the stator 10 may present the total magnetic flux from both of the neighboring permanent magnets 22 to the surface of the facing tooth. The flux concentration may be seen as a function of the area of the permanent magnets 22 facing each pole section 24 divided with the area facing a tooth. In particuiar, due to the circumferential displacement of the teeth, a tooth facing a pole section results in an active air gap that only extends partly across the axial extent of the pole section. Nevertheless, the magnetic flux from the entire axial extent of the permanent magnets is axia!ly and radially directed in the pole section towards the active air gap.

These flux concentration properties of each pole section 24 make it possible to use weak low cost permanent magnets as permanent magnets 22 in the rotor and makes it possible to achieve very high air gap flux densities. The flux concentration may be facilitated by the pole section being made from magnetic powder enabling effective three dimensional flux paths. Further, the design also makes it possible to make more efficient use of the magnets than in corresponding types of machines. Yet another advantage of the design is that the magnets see substantially the same reluctance independent of the rotor position alleviating problems with pulsating flux.

An example of a stator disclosed herein will now e described in more detail with reference to figs. 3-7.

Fig. 3 shows an example of a 3-phase stator containing 3 sets of stator component pairs, each holding one circumferential winding. The stator of fig. 3 comprises three stator phase sections 10a-c, each similar to the stator of fig. 1 , but for an outer rotor configuration, i.e. with teeth 102 protruding radially outward.

As in the example of fig. 1 , each electric modulated pole machine stator 10a- c of fig. 3 comprises a central coil 20a-c, e.g. a single winding, that magnetically feeds multiple teeth 102 formed by the soft magnetic core structure. More particularly, each stator phase 10a-c of the shown electric modulated pole machine 100 comprises two stator core sections 14, each including a plurality of teeth 102 and being substantially annular, a coil 20 arranged between the first and second circular stator core sections. Further, the stator core sections 14 and the coil 20 of each stator phase encircle a common axis defined by a central shaft 303, and the plurality of teeth 102 of the stator core sections 14 are arranged to protrude radially outward. In the example of fig. 3 a rotor (not explicitly shown) may be arranged coaxially with the stator and encircling the stator so as to form an air gap between the teeth 102 of the stator and the rotor. The rotor may be provided as alternating permanent magnets and pole pieces as described in connection with fig. 1 but with a radius large enough for the rotor to encircle to stator. it will be appreciated that the stator described herein may also be used in a single phase machine or in a multi-phase machine having a different number of phases than three.

The stator core sections are mounted on a cylindrical sleeve 315 that axially projects through the central openings of the stator core sections. The stator core sections are arranged in pairs, with a coil sandwiched between the stator core sections of a pair forming a stator phase. The stator arrangement is axially fixated by annular cover plates (not explicitly shown) which are connected to the central cylindrical sleeve 315, e.g. by means of screws. The stator phase segments 10a-c may further be axially separated from each other by annular separator plates (not shown). Fig. 4 shows a single phase stator comprising two stator core sections 14 and one coil 20. The single phase stator 10 of fig, 4 may be used as a stator of a single-phase machine and/or as a stator phase of a multi-phase machine, e.g. one of the stator phases 10a-c of the machine of fig. 3. The stator comprises two identical stator sections 14, each made of compacted magnetic powder, where each stator section comprises a number of teeth 102. Each stator core section is made of magnetic powder metal and separately compacted to shape in a press tool. When the stator core sections have identical shapes, they may be pressed in the same tool. The two stator core sections are then joined in a second operation, and together form the stator core with radially extending stator core teeth, where the teeth of one stator core section are axially and circumferentially displaced relative to the teeth of the other stator core section.

Fig. 5 shows a stator core section embodied as a single stator core section component 14 compacted in one piece, e.g. one of the stator core sections of the stator core sections of the stator of fig. 4. Each stator core section 14 is formed as an annular disc having a central, substantially circular opening defined by a radially inner edge 551 . The teeth 102 protrude radially outward from a radially outer edge of the annular disc. The annular part between the inner edge 551 and the teeth 102 provides a radial flux path and a side wall of a circumferential cavity accommodating the coil 20. Each stator core section comprises a circumferential flange 434 at or near the inner edge 551. In the assembled stator the circumferential flange 434 is arranged on the inner side 535 of the stator core section, i.e. the side facing the coil 20 and the other stator core section. In the embodiment shown in figs 3-7, the stator core sections 14 are formed as identical components. In particular both stator core sections comprise a flange 434 protruding towards the respective other stator core section. In the assembled stator, the flanges 434 abut each other and form an axial flux bridge allowing the provision of an axial magnetic flux path between the stator core sections. In the assembled stator for an outer rotor machine the coil thus encircles the stator core back formed by flanges 434.

Each coil 20 has two connecting wires 321 for providing electrical current to the coils. The connecting wires may connect to the coil at different circumferential and/or radial positions. In the example shown in figs. 3-7, each coil is formed as a single winding where one connecting wire, designated 321 a in fig. 7, is connected to the coil at the radially inward edge of the winding, while the other connecting wire, designated 321 b in fig. 7, is connected to the coil at the radially outward edge of the winding. The connecting wires 321 are connected next to each other in a circumferential direction, thus allowing a maximum number of complete turns of the coil. The connecting wires are radially led towards the part of the stator distal from the rotor, i.e. towards the radially inner edge of the stator in the example of the outer rotor design shown in figs. 3-7. The connecting wires 321 are then led in an axial direction along the radially inner edge 551 of the stator core sections. In the 3-phase stator of fig. 3, the connecting wires of all stator phases are led to an axial end of the stator between the inner edge of the stators and the sleeve 315. To this end, the sleeve 315 is provided with a recess 325 on its outer surface for accommodating the connecting wires 321. The connecting wires 321 may thus conveniently be connected to a control and/or power supply circuit (not shown) of the electric machine. The connection to of the wires 321 may be performed in a connection area on one axial side of the stator. At least one of the connecting wires 321 is led in a radial direction along the coil 20. In the example of the outer rotor stator design of figs. 3-7, one connecting wire is led from the radially outer edge of the coil 20 radially inward, as illustrated by wire 321b in fig. 7. The other connecting wire, designated 321a in fig. 7, is led from the radially inner edge radially inward, thus only utilising a part of the wire channel of the other stator core section. Both connecting wires 321 a-b are radially led through the stator core back formed by the flanges 434. As can be seen in fig. 6, the edges 654 of the recess in the flange 434 that face the coil may be provided with a curvature having a radius large enough to avoid any sharp edges at which the wires may get damaged.

To this end, each stator core section is provided with an elongated recess forming a wire channel 431 that extends radially along the inner side 535 of each stator core section, i.e. on the side of the stator core section from which the flange 434 extends. Fig. 6 shows a detailed view of the radial wire channel 431 and a corresponding indexing feature 432 of the stator core section 14.

The recess 431 extends from the inner edge 551 of the stator core section to the outer edge of the annular disc from which the teeth 102 extend radially outward. It will be appreciated that if the coil is radially thinner than the annular disc (i.e. only extends partly from the flange to the outer edge of the annular disc), the wire channel may only need to extend along a part of the radial thickness of the stator core section.

The wire channel 431 in figs. 3-7 is shown as a straight, radially directed wire channel. Even though, this provides for the simplest design, it will be appreciated that other layouts of the channel may be used, as long as it provides a radial passageway to the inner edge of the stator core section. Similarly, it will further be appreciated that the wire channel may not need to extend all the way to the inner edge of the stator core section, but may alternatively terminate in a cut-out in the inner edge, or even a hole at the inner end of the wire channel that axial!y extends through the stator core section. However, when the wire channel extends all the way to the inner edge, a particularly simple assembly and wiring layout is achieved, in particularly for multi-phase machines. The wire channel 431 may have any suitable cross section, e.g. a rectangular cross section, a u-shaped cross section, or a trapezoidal cross-section, as in the example of figs. 3-7. The wire channel 431 may be positioned circumferentially between two teeth, as in the example of figs. 3-7, thus reducing any undesired influence on the magnetic flux.

The stator core section 14 is further provided with a radial ridge 433 on its outer side opposite the inner side 535 on which the flange 434 and the wire channel 431 are provided. The ridge 433 is circumferentially aligned with the wire channel 431 , i.e. provided directly opposite the wire channel. Consequently, the axial thickness of the stator section is substantially constant along the circumference, despite the provision of the wire channel, thus reducing any variations of the compaction ratio of the compacted component.

When both stator core sections are provided with a wire channel there is provided a wire channel on each side of the coil 20 in the assembled stator. Thus when the connecting wires 321 are led radially along the coil on respective axial sides of the coil, each connecting wire may be laid out inside one of the wire channels.

Each stator core section 14 is further provided with an indexing protrusion 432, protruding axially from the inner side 535 of the stator core section 14 towards the other stator core section. The indexing protrusion 432 is provided as a part of the flange 434, i.e. protrudes axially further than the remainder of the flange. The axial dimension and the shape of the indexing protrusion 432 are selected such that the indexing protrusion fits into the wire channel 431 of the respective other stator core section in the assembled stator, but such that the indexing protrusion only extends part of the depths of the wire channel of the other stator core section into said other wire channel, thus still leaving a passageway for a connecting wire 321 between the indexing protrusion and the bottom of the wire channel. Generally, the depth of the wire channel and the height of the indexing protrusion are adapted to the thickness of the connecting wires.

The circumferential position of the indexing protrusion 432 relative to the wire channel 431 is chosen such that the indexing protrusion fits into the wire channel of the other stator core section when the stator core sections are assembled with each other with the desired circumferential positioning of the stator teeth 102 relative to the other stator core section. The indexing protrusion 432 may be positioned ciose to the wire channel, e.g. directly adjacent to the wire channel as shown in figs 3-7, thus facilitating easy assembly of a coil comprising an integer number of complete turns. However, it will be appreciated that the indexing protrusion and the wire channel can be placed in any circumferential position, as long as the circumferential position difference between the stator core sections relative to the magnetic pole pitch defined by the pitch distance between the teeth is ½ of said pitch distance. This may be useful when it is desirable to allow for partial winding turns, e.g. in low-voltage applications were the wire thickness is large and the number of turns low.

Similarly the indexing protrusion may be positioned radially displaced from the flange, i.e. the indexing protrusion may protrude from the annular disc. However, as the relative depth of the wire channel is larger in the area of the protrusion, a higher indexing protrusion may be provided while still allowing a passageway for the connecting wire. This allows for a more accurate and secure fit of the indexing protrusion into the wire channel.

By providing the indexing protrusion so as to engage the wire channel, a circumferential alignment and efficient assembly of the stator core sections is facilitated with a small number of complicated features in the stator core sections.

Generally, the stator structures described herein may be made of a magnetic powder. The magnetic powder may e.g. be a soft magnetic iron powder or powder containing Co or Ni or alloys containing parts of the same. The soft magnetic powder could be a substantially pure water atomised iron powder or a sponge iron powder having irregular shaped particles which have been coated with an electrical insulation. In this context the term "substantially pure" means that the powder should be substantially free from inclusions and that the amount of the impurities O, C and N should be kept at a minimum. The average particle sizes are generally below 300 μηη and above 10 μιη.

However, any soft magnetic metal powder or metal alloy powder may be used as long as the soft magnetic properties are sufficient and that the powder is suitable for die compaction.

The electrical insulation of the powder particles may be made of an inorganic material. Especially suitable are the type of insulation disclosed in US 6348265 (which is hereby incorporated by reference), which concerns particles of a base powder consisting of essentially pure iron having an insulating oxygen- and phosphorus-containing barrier. Powders having insulated particles are available as Somaloy®500, Somaloy®550 or Somaloy®700 available from Hoganas AB, Sweden.

Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilised and structural and functional modifications may be made without departing from the scope of the present invention. For example, while the invention has mainly be described with reference to an outer rotor machine, it will be appreciated that the invention may equally be applied to an inner rotor machine. In such an embodiment, the references to radial directions (inner and outer and inward and outward) are generally interchanged.

Embodiments of the invention disclosed herein may be used for a direct wheel drive motor for an electric-bicycle or other electrically driven vehicle, in particular a light-weight vehicle. Such applications may impose demands on high torque, relatively low speed and low cost. These demands may be fulfilled by a motor with a relatively high pole number in a compact geometry using a small volume of permanent magnets and wire coils to fit and to meet cost demands by the enhanced rotor assembly routine.

In device claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage. It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Claims

Claims:

1. A stator for an electrical rotary machine, said stator comprising:

2. A stator according to claim 1 ; wherein the wire channel provides a radial passage through the stator core back.

3. A stator according to claim 1 or 2; and wherein the indexing protrusion only extends a part of a depth of the wire channel into the wire channel so as to leave a passage for a wire between indexing protrusion and a bottom wall of the wire channel.

4. A stator according to any one of the preceding claims wherein the teeth of each annular stator core section protrude in a radial direction from a circumferential edge of the annular stator core section.

5. A stator according to any one of the preceding claims, wherein the stator core back is an annular yoke member arranged to encircle said common geometric axis.

6. A stator according to any one of the preceding claims, wherein the stator core back is made of soft magnetic powder.

7. A stator according to any one of the preceding claims, wherein each annular stator core section has an inner side proximal to the coil and an outer side distal from the coil; and wherein the first stator core section comprises an annular flange axially protruding from the inner side of the first stator core section, the annular flange forming at least a part of the stator core back.

8. A stator according to claim 7, wherein the flange is substantially continuous in the circumferential direction.

9. A stator according to claim 7 or 8, wherein the second stator core section comprises an annular flange axially protruding from the inner side of the second stator core section and wherein the annular flange of the first stator core section abuts the annular flange of the second stator core section so as to form the stator core back,

10. A stator according to any one of the preceding claims, wherein the teeth of the second stator core section are circumferentia!ly displaced in relation to the teeth of the first stator core section.

11 . A stator according to any one of the preceding claims, wherein each stator core section is a compacted metallic powder component.

12. A stator according to any one of the preceding claims, wherein said stator core sections are made of soft magnetic powder.

13. A stator according to any one of the preceding claims, wherein the teeth of the first stator core section and of the second stator core section protrude radially, and wherein the teeth of first stator core section and the teeth of second stator core section are axially displaced in relation to each other.

14. A stator according to any one of the preceding claims, wherein the teeth extend radially outward.

15. A stator according to any one of claims 1 -12, wherein the teeth of the first stator core section and the second stator core section protrude axialiy and wherein the first stator core section is arranged encircling the second stator core section.

16. A stator according to any one of the preceding claims wherein the first and the second stator core sections each comprise

• at least one recess forming a wire channel so as to establish a radial passage for a wire leading to or from the coil;

• an indexing protrusion for guiding assembly of the first and second stator core sections with each other in a predetermined circumferential position of the teeth of the second stator core section relative to the teeth of the first stator core section; and wherein the indexing protrusion is adapted to axially extend into the wire channel of the corresponding other one of the first and second stator core sections to define a circumferential position of the second stator cores section relative to the first stator core section.

17. An electrical rotary machine comprising a rotor and a stator as defined in any one of claims 1 through 16, wherein the plurality of teeth of the first stator core section and the second stator core section are arranged to protrude towards the rotor; wherein the rotor comprises a plurality of permanent magnets separated in the circumferential direction from each other by axially extending pole sections made from soft magnetic material.

18. An annular stator core section for a stator as defined in any one of claims 1 through 16, the stator core section comprising:

• a plurality of teeth protruding from the annular stator core section;

• at least one recess forming a wire channel so as to establish a radial passage for a wire leading to or from a coil arranged axially adjacent to the annular stator core section;

• an indexing protrusion for guiding assembly of the annular stator core section with another, like, annular stator core section in a predetermined circumferential position of the teeth of the annular stator core section relative to the teeth of the another stator core section; and wherein the indexing protrusion is adapted to axially extend into the wire channel of the another stator core section to define a circumferential position of the another stator core section relative to the annular stator core section.