HK40000064B - Stator for an axial flux machine and method for producing the same - Google Patents

Description

Stator for axial flux machine and method for producing the same

Technical Field

The present invention relates to the field of axial flux machines. More particularly, the present invention relates to a stator for an axial flux electric machine and a method of producing the same.

Background

There are many different types of electric machines (both motors and generators) in the prior art, such as brushless direct current (BLDC) motor/generators, synchronous or asynchronous ac motor/generators, switched reluctance motor/generators (SRM), Axial Flux (AF) motor/generators, etc., and each type has its specific advantages and disadvantages, such as in terms of the physical machine itself, in terms of its drive circuitry (e.g., requiring a DSP or simple microcontroller), in terms of the required sensors (e.g., current sensors, position sensors), in terms of characteristics (e.g., torque ripple), etc.

The present invention relates to "axial flux machines" of the type and, more particularly, to so-called unbounded axial flux machines.

Axial flux machines are known per se in the art, for example from WO2010/092400 and US2015/0364956, and therefore, the basic operating principle need not be explained here.

Readers unfamiliar with axial flux machines may review the paper by a. parvainen: "Design of axial flux permanent magnet low-speed and performance company radial-flux and axial-flux machines" (Ph. university of Labrana, La Penlan, 2005). In fig. 1.3 of the paper, reproduced herein as fig. 12, several axial flux topologies are shown: (a) a single-sided motor; (b) an electric machine having a dual stator and a single rotor; (c) a motor having dual rotors and a single stator; (d) a multi-stage motor. The present invention relates only to topologies (c) and (d).

Within these two topologies, two variants are possible, depending on the orientation of the magnets on the two rotor disks: toroidal type and yokeless segmented armature (YASA) type. This is shown in fig. 13. The invention relates only to the second variant: type YASA. This variant has no stator yoke, while the first variant has a stator yoke. Fig. 13 clearly shows the necessity of such a yoke to close the magnetic circuit in the first variant, whereas the magnetic circuit is closed without using a yoke in the second (YASA) variant. The dashed lines represent magnetic field lines. The above-mentioned motor has slots for the windings on the stator.

Fig. 14 shows a schematic configuration of a YASA-type motor with slots. More information can be found in Axis Flux Permanent magnetic Brushless Machines, Kluwer, 2004, ISBN: 1-4020-.

Since the invention is mainly concerned with the Construction of stators, reference is also made to the publications "Mechanical Construction and Analysis of an Axial Flux Segmented Armature Torus Machine" (B.Zhang, Y.Wang, M.doppelbauer and M.Gregor, International conference on electric machines (ICEM), 9.month 2 to 5 days 2014, Berlin, pages 1293 to 1299).

For example, the advantages of axial flux machines over radial flux machines are higher power and torque densities, which are inherent to this type of machine, and in the case of YASA machines, they are combined with higher efficiency, mainly due to the absence of a stator yoke and its associated losses.

However, the YASA topology does present some technical challenges. The compactness increases the importance and technical difficulty of cooling the motor. Furthermore, there are challenges associated with mechanically securing discrete stator teeth in an electric machine with adequate precision and sufficient rigidity.

Document DE 10048492 discloses a stator for an axial field machine. The stator has a number of stator coils contained within a stator housing having a stator ring with a number of inwardly projecting radial spokes between which the coils are located. Each coil has a coil core of ferromagnetic material and an outer stator winding. The coils are arranged between the radial spokes and glued to these spokes. The coils are in thermal contact with adjacent radial spokes and the inner circumferential surface of the stator ring.

Application US2014/009009 relates to an axial gap rotary electric machine. The stator includes a number of stator cores arranged in a circumferential direction. The coils are configured to be wound around the outer periphery of the respective stator cores. The resin molds the stator core wound with the coil. The stator cores each include a protruding portion that partially protrudes from the coil in the direction of the rotation axis. The conductive member is provided so as to be brought into contact with the outer peripheral surface of the protruding portion of the stator core.

JP 2000295801 proposes a split stator core for a radial flux machine. The plurality of core blocks comprise laminated ferromagnetic steel sheets. Each core block has a comb-like portion which is made to protrude and plastically deform on the outer circumferential side of the adjacent core block. A protruding curved protrusion or notch is formed on the surface of each ferromagnetic steel sheet coming into contact with the electromagnetic steel sheet belonging to the adjacent core block.

US2008/098587 discloses a method for manufacturing a laminated stator core of a radial flux machine. The yoke body of the ferromagnetic stator core is laminated.

Disclosure of Invention

It is an object of embodiments of the present invention to provide a yokeless stator for an axial-flux electric machine and an axial-flux electric machine comprising such a yokeless stator, wherein the yokeless stator and the axial-flux electric machine have a structure with good or improved mechanical stability and/or good or improved cooling capacity, while avoiding cooling liquid approaching or approaching the stator teeth or stator coils.

It is a further object of embodiments of the present invention to provide a method of producing such a yokeless stator and a method of producing such an axial-flux electric machine, wherein the method is relatively easy to perform, in particular with regard to accurately positioning the stator teeth in a manner that does not negatively affect mechanical stability and/or cooling capacity or both.

These objects are achieved by a method and apparatus according to embodiments of the present invention.

According to a first aspect, the present invention provides a yokeless stator for an axial-flux electric machine, comprising: a housing including a circumferential portion; a plurality of discrete stator teeth disposed within the circumferential portion, each discrete stator tooth comprising a ferromagnetic material and an electrical winding wound around the ferromagnetic material; the housing further comprises a plurality of elongated portions extending in a radially inward direction from the circumferential portion, the elongated portions having proximal ends mechanically connected to and in thermal contact with the circumferential portion and having distal portions or ends physically located between the electrical windings adjacent the stator teeth; an electrically isolating filler material filling empty spaces within the circumferential portion between the plurality of stator teeth and the plurality of elongated portions. The circumferential portion of the housing is made of a first non-ferromagnetic material and the elongated portion is made of a second non-ferromagnetic material. The housing includes a laminate structure. This laminate structure comprises at least some of said plurality of inwardly directed elongated portions. The non-ferromagnetic structure does not form part of the magnetic circuit due to the high magnetic resistance of the non-ferromagnetic material.

The plurality of discrete stator teeth are preferably arranged at equidistant angular positions inside the circumferential portion.

One advantage is that the elongated portion can perform four functions: (1) defining cavities for receiving discrete stator teeth during assembly, (2) holding the discrete stator teeth in place during production of the stator, (3) holding the discrete stator teeth in place during actual use of the axial-flux electric machine, and (4) cooling the stator teeth during actual use of the electric machine by expelling heat from inside the stator to the circumference.

The elongate portion may be formed integrally with the circumferential portion of the housing, but this is not absolutely required. In some embodiments, they may be formed separately from the circumferential portion and then fixedly attached thereto.

The main advantage of the elongated portion is that it acts as a mechanical holding means during operation of the electrical machine (by providing structural strength to resist forces exerted on the stator teeth, in particular tangential forces), while significantly reducing the thermal resistance of the medium through which heat can flow from the location where heat is generated (stator teeth and windings) to the location where heat is dissipated (circumference) during normal operation of the electrical machine. In other words, the elongated portion provides increased cooling capacity inside the stator structure. This advantage should not be underestimated.

A motor having such an elongated portion provides an improved power density, which means that a motor having such an elongated portion can have a higher power than the same motor without such an elongated portion.

The main advantage of such a motor is that it does not require internal water cooling, especially since water cooling adds complexity due to the need for additional equipment such as pumps. Nevertheless, the motor is compatible with water cooling strategies. The housing can be cooled from the outside in any conventional manner, for example by means of forced air or water cooling or the like. In some embodiments, it is possible to provide a water passage in a circumferential portion of the housing.

It is advantageous to fill the empty space with the filler material without leaving air bubbles because the resin has better thermal conductivity than air (e.g., about 0.3W/m.k for resin and about 0.026W/m.k for air).

Preferably, the elongated portion extends radially inwards up to the innermost radial position of the stator teeth or even further, as this allows for more heat transfer and thus improved cooling capacity.

In one embodiment, the stator further comprises a central element arranged inside said housing; the plurality of discrete stator teeth are arranged in a space between the circumferential portion and the central element; the elongated portion extending in a radially inward direction from the circumferential portion toward a central element; an electrically isolating filler material fills empty spaces between the circumferential portion and the central element and the plurality of stator teeth and the plurality of elongated portions.

In one embodiment, the central element is an element adapted to hold a bearing.

In one embodiment, the central element is or comprises a bearing.

In one embodiment, the first non-ferromagnetic material and the second non-ferromagnetic material are the same.

In one embodiment, the stator housing comprises a plurality of laminates stacked on top of each other, the laminates having a shape comprising at least a portion of a circumferential portion.

In one embodiment, the elongated portions forming part of the laminated structure are electrically isolated from each other. This avoids circulating currents known as "eddy currents". In another embodiment, where the comb member is connected to the various laminates of the elongated portion via the back side of that comb member, electrical isolation is apparently only partially created. Even in this case, eddy currents are largely avoided. In other words, the various laminates of the elongate portion may be completely electrically isolated from each other, or may be connected to each other at their radially innermost or outermost points, but not at both points.

One advantage is to provide a housing made of an electrically isolating laminate sheet instead of a monolithic piece made of an electrically conductive material, since the laminate sheet allows to further reduce losses due to eddy currents. A thin oxide layer or coating may be sufficient as an isolation. The thickness of the laminate may be in the range of 1.0mm to 6.0mm, for example in the range of 1.0mm to 4.0mm, for example in the range of 1.5mm to 2.5 mm.

In one embodiment, at least some of the laminate sheets comprise two or more laminate segments each having a portion of a circumferential portion spanning an arc of less than 360 °.

In one embodiment, at least some of the laminates further include a plurality of radially inwardly directed elongated portions integrally formed with the circumferential portion.

One advantage is that such laminates can be easily produced using classical production techniques such as cutting, rolling, stamping, laser cutting, plasma cutting, and the like. Another advantage is that the elongated portion is formed integrally with the circumferential portion, thus providing, by definition, excellent mechanical and thermal contact and automatically resulting in a correct positioning of the stator teeth. One advantage is that in this case no brazing or soldering or other fastening technique is required to connect e.g. the comb element to the circumferential part.

In one embodiment, all of the laminates are the same; or the laminate sheet comprises a first type of laminate sheet comprising a plurality of radially inwardly directed elongated portions integrally formed with a circumferential portion, and a second type of laminate sheet comprising only the circumferential portion and no radially inwardly directed elongated portions.

Where all of the laminates have elongate portions, they are preferably coated or anodized or otherwise provided with a barrier layer to prevent electrical contact between elongate portions of adjacent laminates.

In case both types of laminates are used, the coating or oxidation or other forms of isolation of the laminates can be omitted without increasing the losses due to eddy currents, since eddy currents are smaller at the circumference of the housing and prevented in the elongated portion due to the axial distance.

In one embodiment, at least some of the laminates include a plurality of notches or grooves for mounting one or more of the plurality of elongated portions. The elongated portion is optionally included in a plurality of comb-like elements.

In one embodiment, the inner wall of the circumferential portion of the housing comprises a plurality of notches or grooves.

In one embodiment, the stator housing comprises a one-piece annular body as the circumferential portion.

One advantage of this embodiment is that the annular body can be produced as a single piece, which may provide production advantages. Another advantage of this embodiment is that a monolithic body made of a single piece may improve the mechanical stability even further. It is believed that losses due to eddy currents flowing in this annular structure are relatively small and, in addition, any heat generated therein may be transferred directly to the external environment.

One advantage is that such materials provide relatively low magnetic permeability and relatively high thermal conductivity. Preferably, aluminum or copper or an aluminum alloy or a copper alloy is used. Another advantage of aluminum (or aluminum alloys) is its low mass density (only about 2700 kg/m)³In contrast, copper: about 8900kg/m³Or steel: 7800kg/m³). The first non-ferromagnetic material may be the same as the second non-ferromagnetic material, or may be different. One advantage of using the same non-ferromagnetic material is that it can be more easily connected via welding or soldering or brazing. Another advantage is that the structures will experience the same thermal expansion (same coefficient of thermal expansion), which reduces thermal stresses.

In one embodiment, the filler material is or includes a resin and a fibrous material.

Preferably, a material having a relatively high thermal conductivity (e.g., at least 0.2W/m.K) and a relatively low electrical conductivity (e.g., less than 1X 1) is selected0¹⁶Ohm. cm) and relatively low permeability resins.

One advantage is to use a resin with a relatively low viscosity (e.g. below 500mpa.s at a temperature of 50 to 250 ℃) in that it flows easily and fills all gaps so that substantially no air bubbles remain.

Preferably, a resin is chosen which has sufficient mechanical strength and which does not weaken even at operating temperatures (temperature class IEC60085) of up to 200 ℃ or even up to 225 ℃ or even up to 250 ℃. In a particular embodiment, the resin is a thermosetting resin such as an epoxy resin, a BMI resin, a benzoxazine resin, a phenolic resin, or a thermoplastic resin such as PA, PPS, PPSU, PAI, PEEK.

It is advantageous if the resin further comprises fibers having a length of 3mm to 15mm selected from the group of non-conductive materials such as glass fibers or aramid fibers or powders selected from the group consisting of inorganic fillers such as alumina, silica, wollastonite, boron nitride or aluminum nitride or organic additives such as CSR (core shell rubber) impact modifiers or other polymeric tougheners to further improve the mechanical and/or thermal properties.

According to a second aspect, the present invention provides an axial flux electric machine comprising: a yokeless stator according to the first aspect and at least one rotor rotatably mounted to the stator.

According to a third aspect, the present invention provides a method of producing a yokeless stator for an axial-flux electric machine. The method comprises the following steps: a) providing a housing comprising a circumferential portion made of a first non-ferromagnetic material and a plurality of elongated portions made of a second non-ferromagnetic material and extending from the circumferential portion in a radially inward direction, the elongated portions having proximal ends mechanically connected to and in thermal contact with the circumferential portion, the elongated portions being adapted to define a plurality of cavities for receiving a plurality of discrete stator teeth; at least some of the elongated portions are included in a laminated structure of the stator housing; b) arranging the plurality of discrete stator teeth in the plurality of cavities, each discrete stator tooth comprising a ferromagnetic material or core and an electrical winding wound around the ferromagnetic material or core; c) increasing the temperature of the arrangement to a temperature in the range of 50 ℃ to 250 ℃ and, while maintaining this temperature, filling the empty spaces within the circumferential portion with an electrically isolating filler material; d) the filler material is allowed to harden and/or cure.

In one embodiment, the method further comprises a step x) of arranging a central element or bearing inside the housing between steps a) and b) or between steps b) and c).

In one embodiment, the method further comprises a step e) of actively or passively cooling the stator after step d).

In one embodiment, step a) comprises one of the following alternatives: i) stacking a plurality of laminates on top of each other, at least some of the laminates having a shape including a circumferential portion and a plurality of elongated portions extending from the circumferential portion in a radially inward direction; ii) stacking a plurality of laminates on top of each other, a first group of the laminates having a first shape comprising a circumferential portion and a plurality of elongated portions extending from the circumferential portion in a radially inward direction, a second group of the laminates having a shape comprising only an outer circumferential portion without elongated portions extending from the circumferential portion in a radially inward direction; iii) stacking a plurality of laminates on top of each other, at least some of the laminates having a shape comprising a circumferential portion and a plurality of recesses for mounting one or more of the plurality of elongated portions comprised in the comb-shaped element; iv) providing an integral annular body comprising a plurality of notches or grooves for mounting one or more of the plurality of elongate portions comprised in the comb element.

In the case of alternative iii) or iv), the method may further comprise: the comb element is mounted to the circumferential portion of the stator housing by means of welding, soldering, brazing, press-fitting or gluing.

According to a fourth aspect, the present invention also provides a method of producing an axial-flux electric machine, comprising the steps of: producing a yokeless stator according to the third aspect; one or more rotors are rotatably mounted to the yokeless stator.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Drawings

Fig. 1 illustrates an exploded view of an exemplary axial-flux electric machine that includes a yokeless stator (illustrated in the middle) and two rotors, according to one embodiment of the present invention.

Fig. 2 shows the exemplary stator of fig. 1 in more detail as an embodiment of the invention.

Fig. 3 is an axial flux electric machine known in the art having an embedded water cooled stator. Fig. 3 is a reproduction of fig. 8 of the publication by Bo Zhang mentioned in the background section.

Fig. 4 shows an example of a so-called "discrete stator tooth" with "concentrated winding" known per se in the art that may be used in embodiments of the present invention.

Fig. 5 shows an example of a laminate that may be used in embodiments of the present invention. The laminate sheet has a circumferential portion and a plurality of elongated portions extending radially inwardly.

Fig. 6 is an example showing how the stator of fig. 2 may be constructed as a lamination stack. Fig. 6(a) shows a single laminate of fig. 5. Fig. 6(b) shows a partial stack comprising a plurality of laminates stacked on top of each other. Fig. 6(c) shows a complete lamination stack forming the housing of the stator shown in fig. 2.

Fig. 7 is a cross-sectional view of the stator of fig. 2 in a plane perpendicular to the rotor axis. An example of the relative position and size of one lamination sheet and a plurality of discrete stator teeth is shown.

Fig. 8(a) and 8(b) show examples of two different layers of preforms that may be used together in an embodiment of a stator according to the present invention. The laminate of fig. 8(a) is an integral laminate having a circumferential portion and a plurality of elongated portions extending radially inward. The laminate of fig. 8(b) has a circumferential portion but no elongated portion.

Fig. 8(c) shows an example of the type of laminates that may be used in an embodiment of a stator according to the present invention in combination with a comb element as shown, for example, in fig. 9, and optionally in combination with the laminate of fig. 8 (b).

Fig. 10 shows an example of another embodiment of a stator housing for forming a yokeless stator according to the present invention. The stator housing includes a single monolithic body having an annular shape and having a plurality of grooves on an inner surface thereof adapted to receive a plurality of comb-like elements each including a plurality of elongated portions.

Fig. 9 shows the one-piece annular body and comb member in exploded view, separated, prior to assembly. Fig. 10 shows the stator casing after assembly, including a one-piece toroid forming a circumferential portion of the casing, and including a plurality of elongated portions extending radially inward from the toroid.

Fig. 11 illustrates an example of a method of manufacturing a yokeless stator for an axial flux machine according to one embodiment of this disclosure. Optional steps show further steps for manufacturing an axial-flux electric machine including such a yokeless stator.

Fig. 12 is a copy of fig. 1.3 from a paper by a.

Fig. 13(a) shows a torus electrode with the north pole on the first rotor facing the north pole on the second rotor. Fig. 13(b) shows a YASA motor known in the art with the north pole on the first rotor facing the south pole on the second rotor.

Fig. 14 shows a schematic configuration of a YASA-type motor with slots as described by j.

The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Any reference signs in the claims shall not be construed as limiting the scope. The same reference numbers in different drawings identify the same or similar elements.

Detailed Description

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and relative dimensions are not consistent with a practical implementation of the present invention.

Furthermore, the terms "first," "second," and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Furthermore, the terms "above," "below," and the like in the description and in the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term 'comprising', used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It should therefore be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising the components a and B" should not be limited to devices consisting of only the components a and B. This means that with respect to the present invention, the only relevant components of the device are a and B.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments, as will be apparent to one of ordinary skill in the art from this disclosure.

Similarly, it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Moreover, although some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are intended to be within the scope of the invention and form different embodiments, as will be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

In this document, the terms "laminate" and "sheet" are used as synonyms.

In this document, the term "lamination stack" is used to indicate a plurality of laminated sheets (or sheets) stacked on top of each other. The stacked laminates may be the same or may be different.

In this document, the terms "monolithic" or "integral" are used as synonyms and mean that a certain piece of material is formed as a single continuous part (as can be produced, for example, by moulding, extrusion, rolling, cutting, etc.).

In this document, the term "discrete stator teeth" is used to indicate stator teeth (typically made of ferromagnetic material) around which a wire (e.g., copper wire) (referred to herein as "winding" or "concentrated winding") is wound. The word "discrete" is sometimes omitted in this document.

The present invention relates to electrical machines, in particular electric motors and generators, comprising a fixed part (called stator) and a movable part (called rotor), and more particularly to electrical machines of the kind having a stator without a yoke. While removing the yoke adds advantages in terms of, for example, reducing weight or improving motor efficiency, it creates serious challenges/problems associated with proper positioning of the stator teeth, mechanical stability (also referred to as "structural integrity"), and cooling capability.

The electric motor converts electrical energy into mechanical energy (motion) and the generator converts mechanical energy (motion) into electrical energy, but in practice such conversion does not occur at 100.00% efficiency and losses occur due to "electrical losses" caused for example by the electrical resistance of the copper windings, "magnetic losses" in for example the stator teeth and "mechanical losses" from for example friction in the bearings. These losses generate heat that needs to be transferred.

A first particular problem facing the inventors is adequate cooling, which is particularly important for electrical machines having a relatively high power density. The inventors are particularly interested in finding a solution that does not use cooling fluid close to the stator teeth or windings, as that would complicate the machine.

A second particular problem faced by the inventors is to accurately locate the stator teeth in the stator housing. In fact, the absence of a yoke in such machines allows the stator teeth to be substantially in an indeterminate physical position, while the precise position of these teeth with respect to each other and with respect to the rotor is advantageous for obtaining a high efficiency and good operation of the machine. The axial position affects the axial air gap between the stator and the rotor, which has an impact on performance.

A third particular problem faced by the inventors is to keep the stator teeth in a rigid fixed position during operation of the electrical machine, despite the large mechanical forces exerted thereon, in particular due to the axial forces caused by the magnets and the tangential forces caused by the torque being transmitted, and despite the stator being subjected to high temperatures.

The present invention addresses at least one, preferably both and most preferably all of these issues.

The inventors contemplate the idea of providing an axial-flux electric machine 1 with a yokeless stator 4, the yokeless stator 4 comprising: a housing 41 comprising circumferential portions 82, 87; a plurality of discrete stator teeth 5 arranged inside the circumferential portion 82, 87, each discrete stator tooth comprising a ferromagnetic material or a magnetic core and an electrical winding wound around the ferromagnetic material or the magnetic core. The housing 41 further comprises a plurality of elongated portions 45, 72 extending in a radially inward direction from said circumferential portions 82, 87. The elongated portions 45, 72 have proximal ends mechanically connected (e.g., mechanically fixed) to and in good thermal contact with the circumferential portions 82, 87 (e.g., by being integrally formed with the circumferential portions, or by being welded or soldered or brazed or press-fit thereto), and have distal or distal ends physically located between the electrical windings adjacent the stator teeth 5. The stator further comprises an electrically isolating filling material 6 filling the empty spaces within said circumferential portions 82, 87, in particular between said plurality of stator teeth 5 and said plurality of elongated portions 45, 72.

The plurality of discrete stator teeth are preferably arranged at equidistant angular positions. The plurality of discrete stator teeth are preferably arranged such that each stator tooth is located between first and second coaxial imaginary cylindrical surfaces, and such that the tooth is located between two imaginary planes perpendicular to the axis of the cylindrical surfaces, the two imaginary planes being tangent to opposite sides of the stator tooth.

The circumferential portion may be a substantially cylindrical portion or an annular portion having, for example, a circular cross-section or a polygonal cross-section.

Said elongated portions 45, 72 may be integral parts of the circumferential portion of the casing 41 or may be mechanically firmly connected to the circumferential portion of the casing 41 so as to be able to withstand axial and tangential forces of a predefined magnitude (the value of which depends on the rated power of the motor). The elongate portions 45, 72 are also in "good thermal contact" with the circumferential portion of the housing 41, as understood by the skilled person to provide a relatively low thermal resistance, typically of the order obtained when two metals are welded or brazed or soldered or press-fit together.

As will be further explained, the elongated portions 45, 72 have four functions: (1) defining cavities for receiving the discrete stator teeth 5 during assembly, (2) holding the discrete stator teeth 5 in predefined angular positions during production of the stator 4, (3) holding the discrete stator teeth 5 in place during actual use of the axial-flux machine, and (4) cooling the stator teeth 5 during actual use of the electrical machine by transferring heat from the interior of the stator 4 to the circumferential portions 82, 87 of the housing 41.

Turning now to the drawings.

Fig. 1 illustrates an exploded view of an axial-flux electric machine 1 according to one embodiment of the present invention, including a stator 4 (illustrated in the middle) and two rotors 2a, 2b arranged on opposite sides of stator 4, although the present invention is not limited to this particular embodiment. The rotors 2a, 2b can be mounted to the bearing or bearing assembly 3 in a known manner, for example directly or indirectly via a shaft (not shown). In the particular example shown in fig. 1, the rotor has the form of a disc and has a plurality of permanent magnets 21 fixedly mounted to the disc, but other rotor types may also be used. For example, instead of a permanent magnet, the disc may contain an electromagnet, or may contain no magnet at all.

However, the main focus of the invention is on the structure of the yokeless stator 4, which will be explained in more detail further on. As mentioned above, the stator 4 of the present invention also comprises an electrically isolating filler material 6 which preferably fills all empty spaces between the housing 41 and the bearing 3 which are not occupied by the discrete stator teeth 5 or the elongated portions 45, but for illustrative purposes the filler material 6 is not shown in the drawings.

In fact, it is not necessary that the bearing 3 is located inside the stator, since the bearing (for example) may also be located outside the stator. Three variations are contemplated in this regard: (1) the bearing is positioned in the stator; (2) a stator having a central element adapted to hold or mount a bearing; (3) a stator without a discrete central element. Only the first variant will be described in more detail herein, but the invention is not limited thereto.

Fig. 2 shows the stator 4 of fig. 1 in more detail. The stator 4 includes a housing 41. The casing 41 of fig. 2 is formed as a laminated stack forming a tubular body. The tubular body includes a circumferential portion having a generally cylindrical shape and having a plurality of elongate elements extending radially inward from the circumferential portion. The housing 41 further comprises a plurality of discrete stator teeth 5 arranged inside said housing 41.

The terms "tubular body" and "cylindrical" and "annular" should not be understood to mean or imply a dimension in the axial direction that is greater than the cross-sectional dimension. It is only intended to mean that the structure is open in the axial dimension. The ratio of the axial dimension H to the cross-sectional dimension D of the circumferential portion of the housing of the stator according to the invention may be greater than 1.0 or, in general, less than 1.0, for example less than 0.5.

The discrete stator teeth 5 are arranged at an angular distance from each other, preferably equidistantly. The housing 41 forms a housing around the plurality of discrete stator teeth 5 and further comprises means for holding the plurality of discrete stator teeth 5 in place during production of the stator 4 and also during normal operation of the electrical machine. These members are elongate portions 45 extending radially inwardly from the circumferential portion. The elongated portion 45 may be integrally formed as part of a laminate 43 (or sheet) that is stacked to form the tubular housing 41 (as will be described in more detail in fig. 5-8), or may be provided in the form of fingers 72 as part of a comb member 7 that is formed separately from and fixedly connected to a circumferential portion 87 of the housing 41 (as will be described in more detail in fig. 9 and 10), such as by welding, soldering, brazing, press fitting, or any other suitable technique that provides good mechanical and thermal contact between the circumferential and elongated portions.

In the specific example shown in fig. 2, the stator 4 includes a housing 41 having a tubular shape. The tubular shape has a circumferential portion and a plurality of elongated portions extending radially inward from the circumferential portion. The circumferential portion may have a circular cross-section, but a polygonal cross-section would also be possible.

In the example shown, the stator 4 comprises 15 discrete stator teeth 5 arranged circumferentially between a circumferential portion of the lamination stack and the bearing 3, but the invention is not limited thereto and the number of stator teeth 5 may also be less than 15 or greater than 15. The number of discrete stator teeth 5 may be even or may be odd.

The bearing 3 may be any suitable type of bearing known in the art.

In the example shown in fig. 2, the ratio of the axial dimension H to the radial dimension D (outer diameter) of the stator is about 30%, but the invention is not limited thereto and other ratios may also be used.

Fig. 3 illustrates an axial-flux electric machine known in the art. A yokeless stator 200 with embedded water cooling is shown. While water cooling may be advantageous for some applications, it may be undesirable in other applications. The structure of fig. 2 is further described in the publication by Bo Zhang mentioned in the background section, in particular in fig. 8 thereof, and will therefore not be described in more detail herein. Suffice it to say that such a construction does not have an elongate portion extending between the windings of the stator teeth.

Fig. 4 shows a discrete stator tooth 5 that may be used in embodiments of the present invention. Discrete stator teeth are known per se in the art and, therefore, are only briefly discussed herein.

The discrete stator teeth 5 have a core of ferromagnetic material preferably surrounded by a first spacer layer, at least one electrically conductive winding or coil (typically a copper winding), preferably a second spacer layer. The core is preferably made as a laminated stack comprising a plurality of pieces of ferromagnetic material (e.g. comprising Fe or Ni or a FeNi alloy). In the example of fig. 4, for illustrative purposes, stator teeth with only eight laminations are shown, but in practice the number of stator teeth laminations can be much higher. The use of a laminated core reduces losses caused by so-called "eddy currents". The shape of the laminate is preferably selected to optimize the performance of the motor in a manner known in the art. Despite these measures, there are still electrical losses in the copper winding (due to electrical resistance) and magnetic losses in the ferromagnetic material, resulting in heat (when the machine is running) that needs to be transported away from the source (inside the stator 4) to the outside of the stator, to the environment.

According to an important aspect of the invention, this heat transfer may advantageously be achieved or improved by means of a plurality of elongated portions 45, 72, which are preferably located in the vicinity of the location where heat is generated. The elongated portion may, for example, have a distal end portion or end located between or outside the discrete stator teeth 5 and have a proximal end that is integrally formed with the circumferential portion of the housing 41 or fixedly connected to the circumferential portion of the housing 41 (e.g., by means of welding, brazing, soldering or press-fitting) in a manner that provides good mechanical and thermal contact with the circumferential portion of the tubular housing 41. The elongated portions 45, 72 are preferably made of a non-ferromagnetic material having a relatively high thermal conductivity, such as copper or aluminum or a copper alloy or aluminum alloy. It is preferably shaped and oriented in a manner that reduces losses due to "eddy currents". Even though the individual elongated portions 45, 72 may not provide as great a mechanical strength or a large heat flow as they do as a group. The filler material 6 further improves the mechanical rigidity of the structure.

The inventors have found two particularly interesting embodiments of such a stator 4, which can be produced conveniently. The first embodiment is based on a lamination stack 82, wherein each lamination sheet 43 comprises a circumferential portion 44 and a plurality of elongated portions 45, the stack of circumferential portions 44 forming a circumferential body, as will be described in fig. 5 to 8. The second embodiment is based on a single sheet body 87 having an annular shape forming a circumferential portion of the housing 41, having a plurality of recesses or grooves 49 or the like to which a plurality of comb members 7 are mounted, each comb member 7 including a plurality of elongated portions 72. These embodiments will be described in more detail below.

With respect to the orientation of the laminate, the following points should be noted. Consider a coordinate system where r is the radial direction, fi is the circumferential direction, and z is the axial direction. From a thermal perspective, the laminate may be positioned along the 2D plane using the equations fi ═ constant or z ═ constant. In both cases, a suitable heat transfer is achieved. From an electromagnetic point of view, the laminate may lie in a plane r-constant or a plane z-constant. In both cases, a large amount of eddy currents are avoided. Therefore, to achieve both effects simultaneously, a solution in which z is constant is preferred.

First preferred embodiment:

fig. 5 shows an example of a laminate 43 or sheet that may be used in an embodiment according to the invention, and fig. 6 shows how such laminates 43 may be stacked on top of each other to form a laminate stack 82, said laminate stack 82 forming a stator housing 41, said stator housing 41 having a circumferential portion and an elongate element 45 extending radially inwardly therefrom. The laminate 43 has a shape including a circumferential portion 44 and a plurality of elongated portions 45 extending radially inward from the circumferential portion 44.

Preferably, the laminate 43 is made of a non-ferromagnetic metal or metal alloy (such as copper or aluminum or a copper alloy or aluminum alloy) because the non-ferromagnetic metal or alloy is a material having a relatively low magnetic permeability and a relatively high thermal conductivity. This provides low magnetic losses and high heat transfer.

Such a laminate 43 may be produced, for example, by cutting or stamping or rolling a metal sheet, such as by laser cutting or plasma cutting, although other suitable techniques may also be used. The metal sheet typically has a thickness of about 1.5mm or 2.0mm or 2.5mm, but the invention is not limited thereto and metal sheets having a thickness of less than 1.5mm or more than 2.5mm, for example any thickness in the range of 1.0mm to 6.0mm, may also be used.

The elongated portion 45 shown in the example of fig. 5 has a rectangular shape with a predefined length "Lx" (see fig. 7) and a predefined constant width "W", but the invention is not limited thereto and other shapes, such as triangular or trapezoidal or even saw tooth shaped, or any other suitable shape may also be used. To avoid mechanical stress concentrations, and to closely follow the contour of the windings 52 on the discrete stator teeth 5 in order to keep the thermal resistance between the windings 52 and the elongated portions 45 as low as possible, the proximal ends of the elongated portions (i.e. the ends close to the circumferential portion 44) preferably do not have sharp 90 ° edges, but are preferably rounded or tapered or conical, as shown for example in fig. 5. Between its proximal end and its distal end, the elongated portion 45 preferably has a shape that is substantially complementary to the shape of the winding 52. A small air gap may be provided between the windings 52 of the stator teeth 5 and the elongated portion 45. This gap is preferably less than 3.0mm, for example less than 2.0mm, for example about 1.0mm, depending on the size of the stator teeth 5 with concentrated windings 52. Selecting a smaller gap reduces the thermal resistance, thereby improving heat transfer, but makes assembly of the stator 4 somewhat more difficult.

Still referring to fig. 5, the circumferential portion 44 of the laminate 43 may have the shape of a closed circle or a closed polygon, and may optionally have one or more openings or gaps 83. This optional gap 83 may be present in some of the laminates 43 of the laminate stack 41, but preferably not in all of the laminates. When present, the openings or gaps 83 may, for example, be used to facilitate electrical connection with the stator windings 52, but of course, the invention is not limited to this connection and any other suitable means for connecting the windings may also be used.

The circumferential portion 44 may further comprise a plurality of through holes 47 which may be used for aligning the lamination sheets 43 during production (e.g. in the XY plane) and/or for clamping the stack 41 when filling with the filling material 6 (e.g. in the Z direction) and/or for exerting an axial clamping force on the lamination sheets 43 during actual use of the stator 4. For example, it is possible to insert threaded steel rods in these holes 47 and clamp them together by means of nuts (not shown) or in any other suitable manner known in the art.

Although fig. 5 shows a laminate made of a single piece, it is also possible to provide two or more "laminate segments" which may be arranged to form substantially the same object as shown in fig. 5 and each comprise a circumferential portion (e.g. two portions spanning an arc of about 180 °, or three portions spanning an arc of about 120 °, or four portions spanning an arc of about 90 °) and have at least one elongate portion 45 directed radially inwardly. Each of these parts may have at least two through holes 47, but this is not absolutely necessary. As will be appreciated from fig. 6, the laminate segments may be stacked on top of each other and held together by suitable means. The advantage of providing the laminate 43 as a "single piece" is that it provides increased mechanical strength and requires less handling during production. The advantage of providing the laminate 43 as "two or more complemental pieces" is that it allows the laminate to be produced with less waste of material. This is particularly important for axial flux machines with relatively large diameters.

The circumferential portion 44 of the laminate 43 may include additional holes (not shown) for forming channels in the stack, or for receiving tubes or conduits (oriented in an axial direction) that may be used to allow cooling using a fluid, such as a water jacket. Further, the circumferential portion 44 may have a radially outward extension (not shown) for acting as a heat sink to the environment.

Fig. 6(a) shows a single laminate 43 of fig. 5 in perspective view. Fig. 6(b) shows how a plurality of identical laminates 43 may be stacked on top of each other to form a partial stack 81. Fig. 6(c) shows an example of a laminated stack 82 composed of a plurality of laminated sheets 43 stacked on top of each other for forming the housing 41 of the stator 4.

In the example of fig. 6, all the laminae 43 are of the same type and have openings 83, all situated above one another, but the invention is not limited thereto and the openings 83 of the different laminae may be provided at different angular positions (not shown), which can be easily obtained during assembly by simply rotating the laminae before or at the same time as stacking the laminae.

As can be seen, the stacked elongated portions 45 form an "interior wall" or "interior grid" or "interior mesh," depending on whether the elongated portions 45 are spaced apart, collectively referred to herein as an "interior wall. These internal walls or grids or meshes 84 help to easily and accurately position the stator teeth 5 during production of the stator 4. These "interior walls" 84 cause only minimal losses (by preventing or reducing large eddy currents) as they are made from the laminate stack. Since the elongated portion 45 is formed integrally with the circumferential portion 44, it also provides excellent mechanical and thermal connection with the circumferential portion 44, allowing the housing 41 to withstand relatively large axial and tangential forces and allowing efficient heat transfer from the interior of the housing 41 where heat is generated to the exterior of the circumferential portion 44 and the housing 41 where the heat is removed.

In practice, not all of the laminates 43 have openings 83, but some of them may have openings 83. Preferably at least two laminates 43 at the bottom of the stack 82 and preferably at least two laminates 43 at the top of the stack are closed, i.e. without openings 83. In the example of fig. 1, five laminates at the bottom (left side of fig. 1) and five laminates at the top (right side of fig. 1) are closed.

Fig. 7 shows an example of the relative positions of the discrete stator teeth 5 (see fig. 4) and the laminate 43 of fig. 5 and 6 when producing the stator 4. As mentioned above, the stator teeth 5 may simply be inserted in the cavities 88 (see fig. 6c) formed between the two "inner walls" 84a, 84b and between the inner surface 42 of the circumferential portion of the housing and the outer surface of the bearing 3 (not shown in fig. 6 c). Alternatively, the bearing or bearing assembly 3 may be added after inserting the discrete stator teeth 5 in said cavity 88.

Generally, the greater the surface area of the "inner wall" 84 that is located near or in close proximity to and thus in good thermal contact with the concentrated winding 52 on the stator teeth, the more efficient the heat transfer (i.e., the more heat that is transferred for a given temperature difference) and, therefore, the lower the temperature inside the stator 4.

Not only the length Lx of the elongated portion 45, but also its shape and size (e.g., width W and thickness thereof in fig. 5) have an effect on heat transfer. Empirically, for a given thickness of the elongated portion 45, the greater the width W, the greater the heat transfer capability.

Another important aspect is the distance between the elongated portion 45 and the electrical coil 52, wherein heat has to pass through the filling material 6. This distance is preferably as small as possible. Preferably, the shape of the elongated portion 45 is chosen to complement the outer shape of the concentrated winding 52 of the stator teeth 5.

As mentioned above, all remaining empty spaces inside the stator 4 between the inner surface 42 (see fig. 6c) of the circumferential portion of the housing 41 and the outer surface of the bearing (or bearing assembly) 3, which are not occupied for example by the elongated portions 45 and the stator teeth 5, will be filled with an electrically isolating but preferably highly thermally conductive filling material 6, such as epoxy resin. It is advantageous not to leave air bubbles inside the filling material 6, since such air bubbles increase the thermal resistance and thus reduce the effectiveness of the cooling.

In the embodiment of fig. 6(c), all the laminations 43 are identical and the number of elongated portions "Nep" is equal to the number of stator teeth "Nst", and hence Nep-Nst, which allows one elongated portion 45 to be provided between each pair of adjacent stator teeth 5 for each layer of the stack. In this case, the laminates 43, or at least the elongated portions 45 thereof, of the two laminates 43 stacked on top of each other should be electrically isolated from each other, for example by means of an insulating coating and/or an insulating epoxy and/or an insulating glue between them, in order to reduce "eddy currents". It should be noted in this respect that lamination of the circumferential portion of the casing 41 itself is not absolutely necessary, since the "eddy currents" are relatively small at the circumference of the casing, but are very important between the stator teeth.

Reference is now made to fig. 8. In another embodiment of the stator according to the present invention, the stator 4 may include at least two different types of laminated sheets, for example, a first type 43a shown in fig. 8(a) and a second type of laminated sheet 43b shown in fig. 8 (b). The first type of laminate has an elongated portion 45, while the second type of laminate 43b does not have an elongated portion 45. The first type 43a may for example occupy even layers of the laminate stack and the second type may for example occupy odd layers of the stack, or vice versa. This is another way of avoiding electrical contact between the elongate portions 45 of adjacent laminates in order to reduce eddy currents, i.e. by providing an axial spacing between them.

Fig. 8(c) shows an example of a type of lamination 43c comprising a plurality of recesses 46, wherein corresponding recesses of different layers of laminations 43c stacked on top of each other are adapted to receive comb-like elements 7, such as, for example, those shown in fig. 9. The comb element 7 may be glued or welded or brazed or soldered or press-fitted in the recess or groove 46, for example. In a particular example, the recess 46 may have a conical or dovetail shape, and the comb element 7 may be slid into the stack in the axial direction and then fixedly mounted thereto. The tubular housing 41 may consist essentially of a laminate sheet 43c of the type shown in fig. 8(c) or a combination of laminate sheets 43c (see fig. 8c) alternating with laminate sheets 43b (see fig. 8 b). Similar to as described above, the laminates 43c may include openings 83, but preferably at least a predefined number of laminates (e.g., at least two laminates at the bottom of the stack and at least two laminates at the top of the stack) do not include such gaps 83, but rather have closed circumferential portions 44.

In another embodiment (not shown), the number "Nep" of the elongated portions 45 of at least some of the laminations 43 is only equal to half the number "Nst" of stator teeth 5, so Nep equals Nst/2. This configuration may be of interest if the number of stator teeth is an integer multiple of 2. In this case, the laminates 43 are preferably arranged so that the elongated portions 45 of the even and odd laminates are not directly on top of each other. This is another way of providing axial spacing between the elongate portions 45 to avoid direct contact.

Of course, it would also be possible in principle to provide a lower lamination sheet 43 in which the number of elongate portions "Nep" is only 1/3 for the number of stator teeth "Nst", and therefore Nep ═ Nst/3, which allows an even greater axial distance to be created between the elongate portions 45 of the different layers. Such a configuration may be of interest and may be suitable for lower power density machines if the number of stator teeth is an integer multiple of 3, but such stators have lower mechanical stability and provide less efficient heat transfer than the above example, since the total number of elongate portions 45 is reduced by a factor of three.

Second preferred embodiment:

fig. 9 and 10 show a further embodiment of a stator 4 according to the invention in an exploded view (fig. 9) and in an assembled form (fig. 10). The main difference between the embodiment of fig. 9 and 10 on the one hand and the embodiment of fig. 5 to 8 on the other hand is that in the embodiment of fig. 9 and 10, the stator 4 comprises a monolithic body 87 having an annular shape forming a circumferential portion of the casing 41. The circumferential portion has a plurality of grooves 49 or slits or the like on its inner surface for mounting a plurality of comb elements 7, said comb elements 7 being formed separately but mounted to the circumferential portion, for example by welding, pressing, soldering or brazing.

In this embodiment, the circumferential portion of the housing 41 is not laminated, but as mentioned above, this is not important because eddy currents are smaller at the circumferential position. Instead, the comb element 7 has a shape comprising a first portion 71 adapted to be received in the groove or slit 49 and a plurality of fingers 72 extending orthogonally to the first portion 71. These fingers 72 have the same function as the elongate portion 45 described above. The comb shape ensures that the fingers 72 do not contact each other at their distal ends, thus preventing the formation of a conductive loop and reducing losses due to eddy currents.

Also in this embodiment, the comb element has four functions: (1) defining cavities 89 for receiving the discrete stator teeth 5 during assembly, (2) holding the discrete stator teeth 5 in place during production of the stator 4, (3) holding the discrete stator teeth 5 in place during actual use of the axial flux machine, and (4) cooling the stator teeth 5 by transferring heat from the interior to the circumference of the stator 4 during actual use of the electric machine.

The comb element 7 is fixedly mounted to the circumferential portion 87 in any known manner. For example, the comb element 7 may have at least two cylindrical protrusions to be inserted into corresponding openings (not shown) formed in the circumferential portion, which protrusions are then deformed like rivets. In another embodiment, the comb element 7 is press-fitted. The comb element 7 and the groove 49 may have a dovetail shape, in which case the comb element 7 may be inserted in the groove 49 by axial insertion. In another embodiment, the comb element 7 is a planar element, in which case the comb element 7 may be radially inserted in the groove 49. Preferably, the comb element 7 has a shape complementary to the shape of the groove 49 for allowing good mechanical and thermal contact. Welding or brazing or soldering further improves such mechanical and thermal contact between the circumferential portion 87 and the comb element 7 and allows for efficient heat transfer. The comb element 7 is preferably made of a non-ferromagnetic material or alloy, such as aluminum or copper or an aluminum alloy or a copper alloy.

The one-piece annular body 87 shown in fig. 9 has a relatively large opening 83, but as mentioned above, this is not necessary and this opening 83 may be omitted for the same reasons as described above.

Although not absolutely required, monolithic annular body 87 may further include a plurality of channels 86 for allowing cooling of annular body 87 using a cooling fluid (e.g., water). The inlets and outlets of these channels may be provided on the top and bottom of the body (see left side of fig. 10) or on the outer surface 85 of the body (as shown in the middle of fig. 10).

Fig. 11 shows an example of a method of manufacturing a stator 4 for an axial flux machine 1 according to the invention. The method 1100 comprises:

-providing 1101 a housing 41, 48 comprising a circumferential portion and a plurality of elongated portions 45, 72 extending from said circumferential portion in a radially inward direction; (see the example of figures 6(c) and 10),

providing 1103 a plurality of discrete stator teeth 5 and arranging them in the plurality of cavities 88, 89, each discrete stator tooth 5 comprising a ferromagnetic material 51 or magnetic core and an electrically conductive winding 52 wound around the ferromagnetic material or magnetic core;

-increasing 1104 the temperature of the arrangement to a temperature in the range of 50 ℃ to 250 ℃ or, for example, 100 ℃ to 250 ℃ for thermally expanding the structure, and while maintaining this temperature, filling any remaining empty space within the circumferential portion 82, 87, in particular the empty space between the plurality of discrete stator teeth 5 and the elongated portion 45, 72, with an electrically isolating filling material, such as epoxy;

-allowing 1105 the filling material 6 to harden and/or cure.

The method may further comprise a step 1102 of arranging 1102 a central element or bearing inside the housing between steps 1101 and 1103 or between steps 1103 and 1104.

The method may further comprise a step 1106 of actively or passively cooling the stator 4, thereby contracting the stator (4) and pre-tensioning the housing 41, 48, following step 1105. This pre-tension is caused by the difference between the thermal expansion coefficient of the circumferential part of the housing 41, 48 (e.g. aluminium or aluminium alloy) and the thermal expansion coefficient of the magnetic core (e.g. comprising Fe or Ni), the winding (e.g. copper) and the resin.

Fig. 11 also shows a further step for producing axial-flux electric machine 1 (an example of which is shown in an exploded view in fig. 1), namely step 1107 of mounting at least one (e.g., two) rotors 2a, 2b to stator 4.

Other embodiments are also possible, for example, an embodiment (not shown) comprising a plurality of laminates 43 stacked on top of each other similar to the laminate shown in fig. 8(b) but having a plurality of recesses on the inside of the circumferential portion 44, which recesses form grooves in the axial direction on the inner surface of the circumferential portion, in which grooves comb members 7 similar to the comb members shown in fig. 9 are mounted, for example by welding, soldering, brazing or press-fitting, resulting in an assembly similar to that shown in fig. 10 but comprising circumferential portions in a laminated stack rather than on a monolithic annular body 87.

For the sake of completeness, it is mentioned that once the stator 4 is produced and filled with the electrically isolating filler material 6, it can of course be cooled on its outer surface 85 in any known manner, for example by passive air cooling (e.g. by exposure to the open air), or by active air cooling (e.g. by blowing air through the stator), or by active water cooling. Note that cooling the stator 4 on the outer surface is fundamentally different from the solution provided in fig. 3, in which liquid is used internally to cool the motor in fig. 3.

Reference numbers:

1: axial flux machine, 2: a rotor, 21: permanent magnet, 3: bearing, 31: outer surface of bearing, 32: inner surface of bearing, 4: stator, 41: stator housing comprising a circumferential portion and an elongated portion, 42: inner surface, 43: laminate, 44: circumferential portion of laminate, 45: radially inwardly directed elongated portion, 46: recess (or cavity or groove, etc.) in the laminate, 47: hole, 49: notches (or cavities or grooves, etc.) in the monolithic annular body, 5: discrete stator teeth, 51: ferromagnetic material or ferromagnetic core, 52: winding, 53: electrical isolation, 6: filler, 7: comb member, 71: first portion, 72: finger, 81: partial laminate stack, 82: forming a laminated stack of a tubular body comprising a circumferential portion and a plurality of elongate portions extending therefrom, 83: openings in the (optional) circumferential portion, 84: stack of elongated portions forming "walls", 85: outer surface of housing, 86: passage, 87: integer having an annular shape, 88: cavity, 89: a cavity.

Claims (17)

1. A yokeless stator for an axial-flux electric machine, comprising:

-a housing comprising a circumferential portion;

-a plurality of discrete stator teeth arranged within the circumferential portion, each discrete stator tooth comprising a ferromagnetic material and an electrical winding wound around the ferromagnetic material;

-the housing further comprises a plurality of elongated portions extending in a radially inward direction from the circumferential portion,

the elongated portion having a proximal end mechanically connected to and in thermal contact with the circumferential portion and having a distal portion or end physically located between the electrical windings adjacent stator teeth;

-an electrically isolating filler material filling empty spaces within the circumferential portion between the plurality of stator teeth and the plurality of elongated portions,

wherein the circumferential portion of the housing is made of a first non-ferromagnetic material and the elongated portion is made of a second non-ferromagnetic material, and

wherein the housing comprises a laminate structure comprising at least some of the plurality of elongate portions directed inwardly.

2. The stator of claim 1, further comprising a central element disposed inside the housing;

-the plurality of discrete stator teeth are arranged in a space between the circumferential portion and the central element;

-the elongated portion extends in a radially inward direction from the circumferential portion towards the central element;

-electrically isolating filler material filling empty spaces between the circumferential portion and the central element and the plurality of stator teeth and the plurality of elongated portions.

3. The stator of claim 1, wherein the first non-ferromagnetic material and the second non-ferromagnetic material are the same.

4. The stator of claim 1, wherein the lamination structure comprises a plurality of laminations stacked on one another, the laminations having a shape that includes at least a portion of the circumferential portion.

5. The stator of claim 4, wherein the at least some of the laminations comprise two or more lamination segments, each having the portion of the circumferential portion spanning an arc of less than 360 °.

6. The stator as claimed in claim 4 wherein said at least some of said laminations further comprise said portions of said plurality of elongated portions directed radially inwardly integrally formed with said circumferential portion.

7. The stator as claimed in claim 4 wherein all laminates are identical or wherein the laminates comprise a first type of laminate comprising a plurality of radially inwardly directed elongate portions integrally formed with the circumferential portion and a second type of laminate comprising only a circumferential portion and no radially inwardly directed elongate portions.

8. The stator of claim 1, wherein the housing comprises a single-piece annular body as the circumferential portion.

9. The stator of claim 1, wherein an inner wall of the circumferential portion of the housing includes a plurality of notches or grooves for mounting the plurality of elongated portions.

10. The stator of claim 1, wherein the laminated structure comprises a plurality of elongated plates stacked on top of each other forming at least some of the plurality of elongated portions directed inwardly.

11. A stator according to claim 9, wherein the laminated structure comprises a plurality of comb-like elements forming at least some of the plurality of elongate portions directed inwardly.

12. A stator according to claim 1, wherein the filler material is a resin or comprises a resin and a fibrous material.

13. An axial-flux electric machine comprising the yokeless stator of claim 1 and at least one rotor rotatably mounted to the stator.

14. A method of producing a yokeless stator for an axial-flux electric machine, comprising the steps of:

a) providing a stator housing comprising a circumferential portion made of a first non-ferromagnetic material and a plurality of elongated portions made of a second non-ferromagnetic material and extending in a radially inward direction from the circumferential portion,

the elongated portion has a proximal end mechanically coupled to and in thermal contact with the circumferential portion,

the elongated portion adapted to define a plurality of cavities for receiving a plurality of discrete stator teeth;

at least some of the elongated portions are included in a laminated structure of the stator housing,

b) arranging a plurality of discrete stator teeth in the plurality of cavities, each discrete stator tooth comprising a ferromagnetic material or core and an electrical winding wound around the ferromagnetic material or core;

c) increasing the temperature of the arrangement to a temperature in the range of 50 ℃ to 250 ℃ and, while maintaining this temperature, filling the empty spaces within the circumferential portion with an electrically isolating filler material;

d) allowing the filler material to harden and/or cure.

15. The method of producing a yokes-less stator for an axial-flux electric machine of claim 14, wherein the step of providing the housing includes one of the following alternatives:

i) stacking a plurality of laminates on top of each other, at least some of the laminates having a shape including a circumferential portion and a plurality of elongated portions extending from the circumferential portion in a radially inward direction;

ii) stacking a plurality of laminates on top of each other, a first group of the laminates having a first shape comprising a circumferential portion and a plurality of elongated portions extending from the circumferential portion in a radially inward direction, a second group of the laminates having a shape comprising only an outer circumferential portion without elongated portions extending from the circumferential portion in a radially inward direction;

iii) stacking a plurality of laminates on top of each other, at least some of the laminates having a shape comprising a circumferential portion and a plurality of recesses for mounting one or more of the plurality of elongated portions comprised in the comb-shaped element;

iv) providing an integral annular body comprising a plurality of notches or grooves for mounting one or more of the plurality of elongate portions comprised in the comb element.

16. The method of claim 15, in the case of alternatives iii) or iv), further comprising the step of mounting the comb elements to the circumferential portion of the stator housing by means of welding, soldering, brazing, press-fitting or gluing.

17. A method of producing an axial-flux electric machine, comprising the steps of:

-producing a yokeless stator according to the method of claim 14;

-rotatably mounting one or more rotors to the yokeless stator.