The present invention relates to axial flux electrical machines. BACKGROUND OF THE INVENTION

Electrical machines, including motors and generators, are important in a wide range of applications, including vehicle propulsion systems, power generation systems including wind, and water power generation systems, and in industrial applications. One particular application is in hybrid vehicle power systems in which an electric motor is used in combination with an internal combustion engine. Axial flux electrical machines are particularly suited to vehicle applications, due to their relatively high torque density.

However, existing designs of axial flux electrical machines can be difficult and expensive to assemble with a desired high level of quality.

It is therefore desirable to provide a design of axial flux electrical machine which overcomes the drawbacks of the previously-considered designs.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided an axial flux electrical machine comprising a stator assembly, a shaft that extends through the stator assembly for rotation with respect to the stator assembly, and a rotor attached to the shaft so as to be rotatable with respect to the stator assembly, wherein the stator assembly comprises a stator housing which has first and second ends and which defines a substantially cylindrical aperture that extends from the first end to the second end, a stator winding assembly located in the aperture, and first and second covers which engage with the first and second ends of the stator housing respectively, so as to close the aperture, the first and second covers defining respective apertures through which the shaft extends, such that the first cover is located between the rotor and the stator winding assembly, wherein the first and second covers include respective bearing housings in which are located respective bearing for supporting the shaft, and wherein the shaft is supported by only such bearings.

According to another aspect of the present invention, there is provided a method of manufacturing an axial flux electrical machine comprising the steps of providing a first cover having a first surface; locating a plurality of stator pole portions on the first surface of the first cover; locating a plurality of windings around respective pole portions; locating a bearing housing substantially centrally on the first cover; locating the first cover on a first end of a stator housing such that the stator pole portions components, the windings, and the bearing housing are located within a substantially cylindrical aperture defined by the stator housing; engaging a magnetic flux conductor element on the first stator pole portions; locating a second cover on a second end of the stator housing, so as to close the aperture, the second cover engaging with the second stator pole portion components and the bearing housing; engaging a rotor with a shaft; locating the shaft through the stator housing such that it is supported by at least one bearing located in the bearing housing, and such that the rotor is adjacent the first cover; and providing at least one external cover to enclose the rotor.

According to another aspect of the present invention, there is provided a stator assembly for an axial flux electrical machine the stator assembly comprising a stator housing that defines a substantially cylindrical volume therein, an integrated stator cover affixed to a first end of the housing, thereby to close that first end of the volume, the integrated stator cover including a plurality of stator pole components which extend from the cover into the volume, and a bearing for supporting a shaft, a stator winding assembly located in the volume, and arranged such that the stator pole components extend at least partially into respective winding elements of the winding assembly, and a second cover affixed to the housing, so as to close a second end of the volume, the second cover including a bearing for supporting such a shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an end view of an axial flux electrical machine embodying the present invention;

Figure 2 is a side view of the machine of Figure 1 ;

Figure 3 is a side cross-sectional side view of the machine of Figure 1 ;

Figure 4 is a side cross-sectional side view of a stator assembly of the machine of Figure 1 ;

Figure 5 is an end view of the stator assembly of Figure 4;

Figure 6 is a side view of part of a stator component;

Figure 7 is a perspective view of the part of Figure 6;

Figure 8 is a side view of a rotor assembly of the machine of Figure 1 ;

Figure 9 is a cross-sectional side view of the rotor assembly of Figure 8; Figure 10 illustrates a stator component;

Figure 11 is a flowchart illustrating steps in an example method of manufacturing the stator component of Figure 10;

Figure 12 illustrates an example winding component;

Figure 13 is a flowchart illustrating steps in an example method of manufacturing the winding component of Figure 12;

Figure 14 illustrates a single sided stator assembly;

Figure 15 is a flowchart illustrating steps in an example method of manufacturing the stator assembly of Figure 14;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electrical machine embodying the present invention will be described in more detail below. The embodiment to be described is an electric motor, but such a design may also be used as a generator. The principles of construction to be described apply equally to both types of electrical machine.

Figures 1 and 2 show an electric motor 1 embodying one aspect of the present invention, which motor 1 comprises a casing 10 provided with external covers 1 1a and 1 1 b. An output shaft 13 extends through the first external cover 11 b. An electrical terminal housing 12 is provided by the casing 10 and electrical connections 14 are provided in the housing 12. The casing 10 is provided with coolant connections 15, and a number of cooling fins 16. It will be readily appreciated that the coolant connection 15 and cooling fins 16 are not essential to the construction of the motor 1.

Figure 3 illustrates a cross-sectional side view of the motor 1 of Figures 1 and 2, and shows a stator assembly 20 through which the shaft 13 extends, and a rotor 22 which is mounted on the shaft 13. As will be described in more detail below, the shaft 13 and rotor 22 are mounted to be rotatable with respect to the stator assembly 20, and casing 10.

The motor of the Figure 1 , 2 and 3 incorporates a single stator assembly and a single rotor, located to one side of the stator assembly. The stator assembly incorporates bearings for supporting the shaft 13. These internal bearings provide the only support for the shaft 13 and rotor 22 inside the motor 1.

The motor 1 shown in Figures 1 , 2 and 3 also includes a shaft encoder and processor 24 within the overall casing of the motor 1. The processor 24 receives shaft position

information from the encoder for determining shaft speed and position. In addition, the motor 1 includes a number of sensors (not shown for clarity) which operate to produce measurement signals in dependence upon operating characteristics of the motor 1. For example, the sensors may include temperature sensors for determining the temperatures of the stator windings, stator pole portions, rotor components, rotor bearings, and other components of the motor 1.

The processor 24 operates to receive such operating measurement signals and processes those signals to produce operating data which is stored in a memory device within the motor 1. The stored data can be used and processed by the processor to determine possible future wear or failure modes, such as local high temperatures indicative of a decaying electrical connection, and can be output, either fully, or as an alarm condition. Such data processing within the motor can serve to optimise the life of the motor by providing advance notification for servicing and maintenance requirements. The processor is preferably connected to an industry-standard shielded connector, such as a CAN (controller area network) bus connector, for enabling output of processed data and alarm condition data. The processor may be connected with a second output connector which can be accessed directly by a service technician to provide diagnostic information and servicing inputs. The processor may also be connected to a wireless transmitter/receiver which is operable to transmit and receive data for the processor. In such a manner, the motor 1 can be remotely accessed and can indicate status and alarm conditions remotely.

As will be described in more detail below, a motor embodying the present invention is manufactured as a series of modules, which enable the manufacturing process to be kept straightforward, and hence less expensive than previous designs. A motor embodying the present invention also has the advantage that the manufacturing process works with the high forces provided by the magnets and stator, rather than working against them. Such a design enables the manufacturing process to be kept straightforward.

The stator assembly 20 is shown in more detail in the cross-sectional view of Figure 4. The stator assembly 20 comprises a stator housing 201 (which also provides the motor casing 10, and the electrical connector housing 12 for the motor 1 of Figures 1 , 2 and 3). The stator housing 201 defines a cylindrical aperture 202 in which stator components are located. The cylindrical aperture 202 is defined between the casing 201 and first and second stator covers 203 and 213. The first and second stator covers 203, 213 are secured to the stator casing 201 via stator cover fixings 204 and 214 respectively. The fixings 204, 214 may be provided by any suitable means, such as bolts, screws or a bonding material. Located centrally in the cylindrical aperture 202, and attached to the first and second stator covers 203, 213, is a rotor bearing assembly 205. The rotor bearing assembly 205 is attached to the first and second stator covers 203 and 213. The rotor bearing assembly 205 defines a cylindrical passage 208 therethrough to which the shaft 13 extends when the motor is assembled. Each of the stator covers 203 and 213 defines a circular aperture through which the shaft 13 can pass. The rotor assembly housing 205 defines bearing regions 207 and 217 into which support bearings for the rotor are located.

The stator assembly 20 also comprises a plurality of stator pole portions 210 and a conductive element 212 around which stator windings 211 are located. The conductive element 212 is an annular member located coaxially with the cylindrical passage 208, and is in contact with the plurality of stator pole portions 210. Electrical connections 215 are provided with the housing 201.

In line with conventional axial motor construction, the stator is provided with a plurality of windings, which when in use carry electric current to provide a rotating magnetic flux. This magnetic flux is reacted by the magnets on the rotors which causes the rotors and hence the drive shaft, to turn. In order to maximise the effect of the magnetic flux produced in the stator, the windings are located around central cores (or "stator pole portions"). These stator pole portions can be of an iron-based material. In an embodiment of the present invention are provided by grain oriented laminated steel sheets, or powder iron, or a combination of the two. Such constructions serves to reduce magnetic losses and, therefore, improve efficiency of the machine.

Figures 6 and 7 illustrate a stator pole component 2101 of a stator pole portion 210. The stator pole component 2101 has a body portion 2102 and an enlarged end portion 2103 which extends from the body portion 2102. The body portion 2102 and enlarged end portion 2103 define a winding receiving region 2104 in which the stator windings 211 are located. In position in the stator assembly, the end faces of the body portions 2102 of the stator pole portions 210 contact a face of the conductive element 212. This arrangement serves to provide a magnetic return path between the stator pole portions 210.

In one embodiment, the stator windings 21 1 are formed of relatively thick, flat copper bar which is bent into a suitable shape. Alternatively, the windings can be provided by the more conventional multi-wound copper wire type. In any event, in order to allow speedy and efficient manufacturing of the stator part, the windings are pre-wound before being located onto the first part of the stator portions, before attachment of the second half of the stator portion. The required number of stator portions are produced, and can be produced in advance and stored ready for manufacture into a stator assembly, and then into a motor. In one example manufacturing process, the second stator cover 213 is placed in a jig with its inner surface accessible to a worker and/or assembly machine. The rotor bearing assembly 205 is located on the cover 213 and secured to it using the appropriate fixings or bonding. The plurality of stator pole portions 210 is placed in position on the cover 213 around the bearing assembly 205, and are bonded in place on the cover 213. The prefabricated windings 21 1 are then placed over the stator pole portions 210, and the appropriate electrical connections are made to the windings 211. The cover 213 is then secured to the stator housing, so that one end of the cylindrical aperture is closed, and so that the stator components and bearing housing are located within the stator housing. The conductive element 212 is located on the plurality of stator pole portions 210. The remaining cover 203 is then located on the casing and secured to the casing 210 using fixings (or suitable bonding material). The cover 203 holds the conductive element 212 in place in the stator assembly 20 in contact with the end faces of the stator pole portions 210. After the cover 213 is placed on the assembly, the assembly may be filled with a suitable potting material, such as gel, resin or phase-change material.

The first and second covers 203, 213 are of a heat insulating material in order to provide a thermal barrier to reduce heat transfer to and from the stator assembly. Reducing heat transfer serves to help maintain the components within an appropriate temperature range.

An optional feature of a motor embodying the present invention is the provision of a fluid conduit located in the stator housing. The conduit may be provided by a suitable flexible tube, for example of glass fibre. The conduit defines a fluid flow path around the internal space of the stator housing 201 through which cooling fluid, such as water, can be pumped. The fluid conduit is in fluid communication with the coolant connections 15, for transfer of cooling fluid in and out of the motor housing 10. The stator assembly is then ready for the introduction of the rotor assembly, which includes the rotor 22 and the shaft 13.

The rotor assembly is illustrated in Figures 8 and 9, and comprises the shaft 13 onto which the rotor 22 is mounted. The rotor 22 is fixed to the shaft 13 and is rotatable with the shaft 13. In Figure 8, the rotor assembly is shown mounted on the rotor bearing assembly housing 205, which is shown in the stator assembly described previously. Figure 9 shows a cross-sectional view of the rotor assembly of Figure 8.

The shaft 13 extends through the rotor shaft bearing housing 205 and is supported by bearings 226 and 236. The shaft is solely supported by these internal bearings 226 and 236 in the motor. The rotor 22 comprises a steel, or otherwise magnetically conductible, rotor disk 221 and a plurality of magnets 222 mounted on a surface of the rotor disk 221. The rotor disk 221 provides a magnetic flux path between the magnets, thereby increasing the efficiency of the rotor. In another example, the rotor disk is of a composite material, such as carbon fibre composite, and is provided with magnetic flux conduit portions, to provide the magnetic flux path between the magnets 222 of the rotor 22.

In order to assemble the motor, bearings 226 and 236 are inserted into the bearing locating regions 207 and 217 of the rotor bearing housing 205 in the stator assembly 20. The rotor 22 is attached to the shaft 13, and then the shaft 13 is inserted into the stator assembly through the cylindrical aperture 208. The drive shaft 13 is supported in the stator assembly by the bearings 226 and 236.

Such a motor design enables the shaft 13 to be located in the bearing in an accurate manner. In addition, provision of the internal bearing enables the shaft insertion to be completed without the need for complex manipulation tools, since the magnetic forces exerted between the rotor magnets and the stator assembly serve to pull the rotor and shaft into position. Any off centre forces are resisted by the shaft in the bearing.

In order to complete the motor, the first and second external covers 11 a and 1 1 b are attached to the casing 10.

A further example manufacturing method will now be described with reference to Figures 10 to 15. The principle of this further example method is to enable a modular process, in order that desirably high quality of manufacture can be achieved. Figure 10 illustrates a stator cover 213 having stator pole components 2101 integrated therein, and Figure 11 illustrates steps in a method of manufacturing such a stator cover 213. The stator cover 213 is manufactured so as to secure the stator pole components 2101 into the cover. The cover may be of a composite material, or may be of a plastics material. Steps in example manufacturing processes for both material types are illustrated in Figure 1 1.

In order to manufacture the integrated stator cover 213 of Figure 10, a predetermined number of stator pole components 2101 are arranged in a predetermined pattern in a jig or moulding tool (Figure 11 , step 400). Where the cover is of a composite material, the relevant fibre material, for example carbon fibre, is layed up around the positioned stator pole components 2101 (step 402), to provide the cover 213. The fibre lay-up is undertaken in such a manner as to provide the final cover with sufficient stiffness and strength. The fibre lay-up also provides the cover 213 with a bearing receiving feature 217, and defines a shaft aperture 208. The bearing receiving feature 217 is adapted to receive a bearing for carrying the rotor shaft of the machine.

The composite material is then completed by injection of a suitable resin, vacuum, heating and curing steps in accordance with well-known and understood practice (step 404). As an alternative, the cover may be prefabricated and the stator pole positions attached thereto.

Where the cover is of a plastics material, a chosen plastics material is injection moulded over the stator pole components 210 (step 402a), thereby forming the cover 213 and providing insulation for the stator components. The completed cover is removed from the mould tool (step 404a). In such a manner, the cover 213 can be formed in a single injection moulding step, without the need for additional processing to provide suitable insulation for the stator components.

The stator cover with integrated pole components is then ready for use in the manufacture of the stator assembly, as will be described below.

In one particular example, the stator windings are provided by a single assembly, for example as illustrated in Figure 12. In one example, the assembly is manufactured from a continuous piece of winding material, such as copper bar, and provides a continuous winding assembly having a series of interlinked windings 2018, one for each stator pole. The windings 2018 are interlinked by connecting portions 2019. In another example, the winding assembly is manufactured from a series of separate windings that are connected together to provide separate phase connections for the stator. The result in both examples is a complete pre-assembled winding assembly.

Providing a pre-manufactured single piece winding assembly greatly reduces the complexity of manufacturing a motor embodying an aspect of the present invention.

Figure 13 illustrates steps in an example method of manufacturing the winding component of Figure 12. The winding material is prepared (step 410), and then a single continuous winding component is wound (step 412), in one example using a computer controlled winding machine. The winding component is then ready (step 414) for use in the

manufacture of a stator assembly, as described below.

A single sided stator assembly 20 is illustrated in Figure 14, and Figure 15 illustrates steps in an example method of manufacturing the same. A previously-manufactured integrated stator cover 213 is located on a jig (step 430), and the stator housing 10 is bonded or otherwise attached thereto (step 432), such that the stator pole components 2101 of the cover 213 extend axially into an internal volume of the housing 10. In this manner, the cover, and hence pole components can be located accurately within the housing 10.

A pre-manufactured winding component 21 1 is then inserted into the housing 10 (step 434), such that each pole components 2101 extends at least partially into a winding element 211 1A of the winding component 21 1. The winding component 21 1 is dimensioned such that it extends only partially into the internal volume of the housing, and so that the pole components of the integrated cover 213 extend substantially fully into respective winding elements 211 1.

The magnetic flux conductor 212, in this example in the form of and annular ring, is then located (step 236) in the internal volume of the housing 10, and the open end of the housing is closed with a cover 242 (step 438). The cover 242 may be secured with bolts, or by bonding with an appropriate bonding material. The conductor 240 may be attached, by bonding or other means, to the cover 242, such that the cover 242 and conductor 240 are located on the housing in the same single step.

A bearing is then bonded into the bearing receiving feature (step 439) using a bonding agent. The bearing bonding agent may be thermally activated, such that the bearing can be removed from the cover by application of heat to the bonding agent. Preferably, the choice of bonding agent is made so that the bonding agent is released at a lower temperature than the composite material resin.

Electrical connections 17 are then made to the windings (step 440). The single sided stator assembly is then available for manufacture into an electrical machine, as will be described below. This single sided stator assembly 210 can then be considered as a pre- manufactured module, and can be manufactured independently of other components of the machines.

In order to complete the electrical machine, the rotor assembly is located in the stator assembly, such that the shaft is supported by the internal bearings. The external covers are then attached to the casing in order to complete the machine.