GB2511542A

GB2511542A - Axial flux electrical machines

Info

Publication number: GB2511542A
Application number: GB201304077A
Authority: GB
Inventors: Lloyd Ash
Original assignee: Ashwoods Automotive Ltd
Current assignee: Dana TM4 UK Ltd
Priority date: 2013-03-07
Filing date: 2013-03-07
Publication date: 2014-09-10
Anticipated expiration: 2033-03-07
Also published as: GB201304077D0; GB2511542B

Abstract

A rotor shaft 7 extends through the stator assembly 4 mounting a permanent magnet rotor 8 by a ball-spline 11, the rotor being rotatable with respect to the stator assembly 4 and movable axially with respect to the shaft 7 so as to provide an adjustable air gap. An air gap adjusting plate 12 is mounted for relative rotation to the rotor by bearing 31, axial movement of the plate being transmitted to the rotor via the bearing. Gear 16, sector 14 and cams 20 move the plate 12 which displaces the rotor 8 and its associated ball-spline axially along the shaft 7 whilst the shaft is rotating so as to adjust the air gap between the rotor 8 and stator assembly 4. Alternatively the adjustment means may comprise a electromagnetic element (Fig 11 not shown) in the form of a coil located in the ball-spline (or the shaft) which acts on the shaft (or the ball-spline). Hydraulic, screw-thread or a weighted element variator actuation is also disclosed. A second rotor defining a further air gap may be mounted on the shaft and this air gap may be varied independently or in concert with the first air gap.

Description

AXIAL FLUX ELECTRICAL MACHINES

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. In addition, existing designs do not provide straightforward controllability of power and torque characteristics of the machine during operation.

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, a first rotor attached to the shaft so as to be rotatable with respect to the stator assembly and to be movable axially with respect to the shaft, the first rotor and stator assembly being arranged so as to define a first air gap therebetween, and actuation means operable to move the first rotor axially along the shaft so as to adjust the air gap between the rotor and the stator assembly, wherein the actuation means are operable to adjust the air gap between the rotor and stator assembly concurrently with rotation of the shaft.

In an example, the actuation means comprises a positioning plate, located axially with the shaft, and rotatable with respect to the shaft, the positioning plate being engaged with the rotor for axial movement therewith with respect to the shaft, a cam surface which extends from the stator assembly, from a first position to a second position, the first and second positions being located at different axial positions with respect to the stator assembly, a cam located on the positioning plate for axial movement therewith with respect to the shaft, and arranged to engage with a corresponding cam surface on the stator assembly, drive means operable to rotate the positioning plate with respect to the cam surface such that the cam moves along the cam surface, thereby causing the cam and positioning plate to move axially with respect to the shaft.

In one example, the actuation means comprises a threaded element which attaches the first rotor to the shaft, and an actuator operable to rotate the threaded element so as to move the first rotor axially with respect to the shaft.

In one example, the actuation means comprises an electromagnetic solenoid.

One example further comprises a second rotor attached to the shaft so as to be rotatable with respect to the stator assembly, the second rotor and stator assembly being arranged so as to define a second air gap therebetween, wherein the actuation means is operable to move the second rotors axially along the shaft so as to adjust the first and second air gaps.

In one such example, the actuation means may be operable to move the first rotor independently of the second rotor, such that the first air gap is adjusted independently of the second air gap. Alternatively, the actuation means may be operable to move the first and second rotors in concert, such that the first and second air gaps are adjusted in concert.

The first and second air gaps may be substantially equal in size.

An example axial flux electrical machine may include a stator assembly comprising 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 first rotor and the stator winding assembly, and such that the second cover is located between the second rotor and the stator winding assembly.

The stator assembly may comprise a stator housing that defines a substantially cylindrical aperture therethrough, the shaft extending through the aperture, coaxially therewith, a stator winding assembly located in the aperture, and a bearing located in the aperture for supporting the shaft, the shaft being supported only by the bearing.

The or each rotor may comprise a rotor disk having first and second substantially planar sides, a plurality of magnets engaged with the first side of the disk, and a plurality of flux conduit portions engaged with the second side of the disk, the magnets and conduit portions being arianged to overlap one another in circumferential diiection such that one conduit portion overlaps a pair of adjacent magnets.

In such an example, the conduit portions may be of glain oiiented laminated steel, or ot powder iron, or of a combination of grain oriented laminated steel and powder iron.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a first perspective view of an axial flux electrical machine assembly; Figure 2 is a perspective view of an electrical machine embodying one aspect of the present invention; Figure 3 is a side perspective view of the machine of Figure 2; Figure 4 is a side view of the machine of Figures 2 and 3; Figure 5 is a first end view of the machine of Figures 2 to 4; Figure 6 is a cross-sectional side view of the machine of Figures 2 to 5; Figure 7 is a cross-sectional side view of a ball-spline assembly of the machine of Figures 2 to 6; Figures 8 and 9 aie end and side views respectively of a lotor of the machine of Figures 2 to 7; Figure 10 is an end view of a stator assembly of the machine of Figures 2 to 7; and Figure 11 is a schematic view of another air gap adjustment mechanism embodying the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electrical machine embodying the present invention will be described in 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.

Figure 1 shows a front view of an electrical machine assembly. The assembly 1 includes an electrical machine 2 embodying an aspect of the present invention, and an upper housing 3.

The electrical machine 2 includes a stator assembly 4, a rotor assembly 5 and a first cover 6.

A shaft (not shown) extends centrally from the electrical machine 2, and, as will be described in more detail below, is mounted for rotation with a rotor of the rotor assembly 5, with respect to the stator assembly 4. In a first mode of operation, electrical power is supplied to the machine, so that the machine operates as a motor, causing the shaft to rotate. In a second mode of operation, the machine acts as a generator of electricity when the shaft is rotated by an external force.

The upper housing 3 provides a location for electrical connections and control equipment and circuitry for the electrical machine 2.

Figures 2 to 6 show the electrical machine 2 of Figure 1 in more detail. The electrical machine 2 comprises a stator assembly 4 onto which is mounted axially a rotor assembly 5.

A shaft 7 extends axially through the stator assembly 4 and the rotor assembly 5. The stator assembly 4 comprises a motor housing 26 in which stator components are located. The motor housing 26 includes electrical terminal apertures 10 through which electrical connections are made to the stator of the machine.

The rotor assembly 5 comprises a rotor 8 on which is mounted a plurality of magnets 9. The construction of the rotor 8 will be described in more detail below with reference to Figure 8 and 9. The rotor 8 is mounted on the shaft 7 for rotation with the shaft 7 by way of a ball spline assembly 11 which will be described in more detail with reference to Figure 7. The rotor 7 is a substantially circular flat disk having a substantially planar surface which is adjacent a corresponding surface of the stator assembly 4. The plurality of magnets 9 are arranged to be adjacent to the stator assembly 4. An air gap is defined between the magnets 9 of the rotor 8 and the stator assembly 4.

Electrical machines embodying an aspect of the present invention include actuation means for adjusting the size of the air gap between the rotor 8 and the stator assembly 4. Rotor-stator air gap control is a useful technique for controlling the torque and speed characteristics of the motor. For example, at start-up (that is from zero revolutions per minute -rpm), a small air gap is desirable as this increases the level of torque generated by the motor. As the rotational speed of the motor rises, for example as the vehicle in which the motor is mounted increases its speed, the amount of torque required decreases. The actuation means then operate to increase the size of the air gap between the rotor 8 and the stator assembly 4, so as to reduce the amount of torque generated by the motor. This also serves to maintain a desired power output level, since the power output of the motor is equal to the product of the generated torque and the rotational velocity of the rotor. As the rotational velocity of the rotor changes, the air gap can be adjusted by the actuation means, so that desired torque and speed characteristics can be maintained.

The actuation means may be provided by any suitable device, such as a mechanical arrangement, an electromechanical arrangement, or a hydraulic arrangement. Any suitable combination of techniques may also be used to provide the actuation means.

In the example shown in Figures 2 to 6, the actuation means is provided by a rotor positioning plate 12 mounted coaxially with the shaft 7 and ball spline assembly 11. The positioning plate 12 is supported by a plurality (in this example 3) of cam followers 18, on a corresponding plurality of cam surfaces 19 provided by cam portions 20 which extend from the stator assembly 4. Each cam portion 20 extends axially from the stator assembly 4 and provides a cam surface 19 which extends from a high point 21 to a low point 22. The high point 21 is at furthest distance from the stator assembly, and the low point 22 is closer to the stator assembly 4 in an axial direction. The positioning plate 12 is located so as to be able to move axially along the shaft 7, thereby moving the rotor 8 and ball spline assembly 11 along the shaft 7. The rotor 8 and ball spline assembly 11 are arranged to rotate with the shaft 7 relative to both the stator assembly 4 and the positioning plate 12. The positioning plate 12 is rotatable within a predetermined angular range with respect to the stator assembly 4.

The positioning plate 12 incorporates a sector gear 14 which extends from an outer edge region of the positioning plate 12. The sector gear 14 engages with a drive gear 16. The drive gear 16 is mounted for rotation with respect to the stator assembly 4, and is driven by a drive gear motor and gearing assembly 17. The drive gear motor assembly 17 is preferably provided by a stepper motor, and serves to turn the drive gear 16 in known increments of angular rotation. As the drive gear 16 rotates, since it is engaged with the sector gear 14, the positioning plate 12 is rotated with respect to the stator assembly 4 and cam portions 20.

As the positioning plate 12 is rotated with respect to the cam portions 20, the cam followers 18 follow respective cam surfaces 19. This in turn moves the positioning plate 12 axially with respect to the shaft 7 since the cam followers' positions are changed relative to the stator assembly 4. As the positioning plate 12 moves axially with respect to the shaft 7, the air gap between the rotor 8 and the stator assembly 4 is changed.

The air gap is at its maximum when the cam followers 18 are at the cam high points 21 of the cam surfaces 19. As the drive gear 16 is driven by the drive gear motor assembly 17, the positioning plate 12 is rotated such that the cam followers 18 follow the cam surfaces 19 towards the cam low points 22. The arrangement of stepper motor 17, drive gear 16, sector gear 14, cam surfaces 19, and cam followers 18 enable the axial position of the positioning plate 12, and hence the rotor 8, to be adjusted with very fine tolerance relative to the stator assembly 4.

Figure 6 illustrates the electrical machine in more detail in cross-sectional side view. The stator assembly 4 includes a stator housing 26 and shaft bearings 25 on which the shaft 7 is supported. The shaft 7 extends through the stator assembly 4 and out of a rear portion of the housing 26. A cover 29 is provided to seal the stator assembly 4. The stator includes stator pole portions 27 of, for example, iron or steel around which stator windings 28 are located. Details of the stator assembly are shown in Figures 9 and 10, to be described below.

The ball spline assembly 11 engages with the shaft 7 by way of inter-engaging splines 24.

The splines 24 extend along the outer surface of the shaft 7, in an axial direction, such that the ball spline assembly can move axially along the shaft 7 by way of the splines 24. The rotor 8 is attached for movement with the ball spline assembly 11 and so is able to rotate with the shaft 7. The positioning plate 12 does not rotate with the shaft 7, but carries bearings upon which the shaft 7, the rotor 8 and the ball spline assembly 11 are able to rotate. The ball spline assembly 11 is arranged to move axially with the bearings of the positioning plate 12 such that as the positioning plate 12 moves axially along the shaft?, the ball spline assembly 11 moves along the shaft 7.

Figure? illustrates the ball spline assembly 11, positioning plate 12 and rotor Bin more detail in cross sectional side view. Figure 7 is a somewhat simplified view of the arrangement, for the sake of clarity.

As described above, the shaft 7 extends axially through the electrical machine, and provides external splines 24 for engagement with corresponding splines on a ball spline nut 30.

Accordingly, the ball spline nut 30 is able to rotate with the rotor 7 with respect to the stator assembly 4. The rotor 8, carrying the plurality of magnets 9, is attached to the ball spline nut for rotation therewith. The rotor 7 is supported on bearings 31 which are located within, and fixed to, the positioning plate 12. The rotor 8 is, therefore, able to rotate with respect to the positioning plate 12.

Accordingly, as the positioning plate 12 moves axially with respect to the shaft 7, the ball spline nut 31 and rotor 8 also move axially with respect to the shaft 7. The air gap between the magnets 9 on the rotor 8 and the stator assembly 4 is adjusted by movement of the positioning plate 12.

In use, the drive gear 16 engages with the sector gear 14 in order to rotate the positioning plate 12 with respect to the motor housing 26. Since the cam portions 20 extend from the motor housing 26, any rotational movement of the positioning plate 12 with respect to the housing 26 results in movement of the cam followers 18 with respect to the respective cam surfaces 19. As the cam followers 18 follow the cam surfaces 19! the axial position of the positioning plate, and hence of the rotor 8, is changed with respect to the stator housing 26.

Figure 8 and 9 show end and side views respectively of the rotor 8. The rotor 8 is provided by a substantially circular planar body having first and second planar faces. On the first planar face, the plurality of magnets 9 is mounted. The magnets 9 are arranged in a spaced apart series around the first face. An inner portion of the rotor 8 includes features for attachment to the ball spline nut 31 of the ball spline assembly 11. The second planar face may carry a plurality of flux conduit portions, each of which is arranged to span between adjacent magnets of the plurality of magnets carried by the first side of the rotor 8.

Figure 10 illustrates a cross-sectional end view of parts of the stator assembly 4. The stator assembly 4 includes a plurality of pole portions 27, arranged in a circular series around an aperture 33 through which the shaft 7 extends. A set of electrical windings 34 are arranged around the pole portions 27, and include electrical terminals 35.

It will be readily appreciated that, although the example shown includes a single rotor 8 located to one side of a single stator assembly 4, the principles of the invention may be applied to electrical machines with any number and arrangement of rotors and stator assemblies.

In the example described above, the actuation means is provided by the drive gear 16 and positioning plate 12 combination.

In an alternative example, illustrated in Figure 11, the actuation means is provided by an electromagnetic element 37 which acts to move the ball spline assembly along the shaft 7.

A controller (not shown for clarity) energises the electromagnetic element 37 in order to produce a magnetically induced force between the element 37 and the shaft 7. In the example shown in Figure 11, the electromagnetic element 37 is provided by a coil located in the ball spline nut 30. In an alternative arrangement, the electromagnetic element may be located separately from the ball spline nut 30, rotor 8 and shaft 7. In a further alternative, the shaft 7 may include the electromagnetic element 37. Any suitable arrangement of electromagnetic elements may be provided. The electromagnetic element provides a solenoid arrangement, in which the plunger" or moveable element drives the position of the rotor with respect to the shaft.

In a further alternative arrangement, the ball spline assembly 11 may be moved along the shaft 7 by a directly driven screw-thread arrangement. In such an example, a drive motor acts on a threaded adjuster to move the ball spline nut along the shaft.

In a further alternative arrangement a hydraulic actuator may be provided to drive the position of the ball spline nut and rotor on the shaft. In a further alternative arrangement, the actuation means is provided by an assembly based around a mechanical speed control device known as a variator. A variator is mechanical system that includes weighted elements that rotate with the shaft, and which are able to move radially with respect to the shaft. As the rotational speed of the shaft increases, the weighted elements are caused to move outwardly radially with respect to the shaft. As the weighted elements move, a linkage is caused to move such that the ball spline nut is moved along the shaft to increase the air gap between the rotor and the stator assembly. As the rotational speed of the shaft decreases, the weighted elements move inwardly toward the shaft, thereby moving the rotor into closer proximity of the stator assembly.

Embodiments of the present invention enable the rotor-stator air gap to be adjusted whilst the electrical machine is in use. Accordingly, the power and torque characteristics of the machine can be adjusted to provide the most appropriate operating characteristics during operation of the machine.

In order to achieve this appropriate control, the electrical machine embodying an aspect of the present invention includes suitable sensors, including a rotational speed sensor, and a controller for providing speed and air gap control. The controller uses data derived from the machine sensors to provide suitable control inputs to the rotor position actuation means.

The controller is housed in the upper housing 3 shown in Figure 1.

The controller operates the air gap actuator in order to optimise the desired operating characteristics of the electrical machine whilst the machine is in use. For example, when in the first operating mode (motor), the controller may control the air gap to give the most efficient use of energy by the motor. This is particularly relevant at low rotational speeds and under relatively low loading. When operating as a generator, in the second mode of operation, the air gap can be adjusted to provide optimal energy recovery efficiency.

Claims

CLAIMS: 1. 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; a rotor aftached to the shaft so as to be rotatable with respect to the stator assembly and to be movable axially with respect to the shaft, the rotor and stator assembly being arranged so as to define a first air gap therebetween; and actuation means operable to move the rotor axially along the shaft so as to adjust the air gap between the rotor and the stator assembly, wherein the actuation means are operable to adjust the air gap between the rotor and stator assembly during rotation of the shaft.
2. An axial flux electrical machine as claimed in claim 1, wherein the actuation means comprises: a positioning plate, located axially with the shaft, and rotatable with respect to the shaft, the positioning plate being engaged with the rotor for axial movement therewith with respect to the shaft; a cam surface which extends from the stator assembly, from a first position to a second position, the first and second positions being located at different axial positions with respect to the stator assembly; a cam located on the positioning plate tor axial movement therewith with respect to the shaft, and arranged to engage with a corresponding cam surface on the stator assembly; drive means operable to rotate the positioning plate with respect to the cam surface such that the cam moves along the cam surface, thereby causing the cam and positioning plate to move axially with respect to the shaft.
3. An axial flux electrical machine as claimed in claim 1, wherein the actuation means comprises a threaded element which attaches the first rotor to the shaft, and an actuator operable to rotate the threaded element so as to move the first iotor axially with respect to the shaft.
4. An axial flux electrical machine as claimed in claim 1, wherein the actuation means comprises an electromagnetic solenoid.
5. An axial flux electrical machine as claimed in any one of the preceding claims, wherein the rotor is attached to the shaft by way of a ball spline assembly, such that the lotor is iotatable with the shaft, and movable along the shaft axially.
6. An axial flux electrical machine as claimed in claim 1, further comprising a second rotor attached to the shaft so as to be rotatable with respect to the stator assembly, the second rotor and stator assembly being arranged so as to define a second air gap therebetween, wherein the actuation means is operable to move the second lotois axially along the shaft so as to adjust the fiist and second aft gaps.
7. An axial flux electrical machine as claimed in claim 6, wherein the actuation means is operable to move the fiist rotor independently of the second rotor, such that the first air gap is adjusted independently of the second air gap.
8. An axial flux electrical machine as claimed in claim 6, wherein the actuation means is operable to move the fiist and second rotois in concert, such that the fiist and second air gaps are adjusted in concert.
9. An axial flux electrical machine as claimed in any one of claims 6 to 8, wherein the first and second air gaps are substantially equal in size.
10. An axial flux electrical machine as claimed in any one of the preceding claims, wherein the statol 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 first rotor and the stator winding assembly, and such that the second covei is located between the second rotor and the statol winding assembly.
11. An axial flux electrical machine as claimed in any one of the preceding claims, wherein the stator assembly comprises a stator housing that defines a substantially cylindrical aperture therethrough, the shaft extending through the aperture, coaxially therewith, a stator winding assembly located in the aperture, and a bearing located in the aperture for supporting the shaft, the shaft being supported only by the bearing.
12. An axial flux electrical machine as claimed in any one of the preceding claims, wherein the or each rotor comprises a rotor disk having first and second substantially planar sides, a plurality of magnets engaged with the first side of the disk, and a plurality of flux conduit portions engaged with the second side of the disk, the magnets and conduit portions being arranged to overlap one another in circumferential direction such that one conduit portion overlaps a pair of adjacent magnets.
13. An axial flux electrical machine as claimed in claim 12, wherein the conduit portions are of grain oriented laminated steel, or of powder iron, or of a combination of grain oriented laminated steel and powder iron.
14. An axial flux electrical machine substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.Amendments to the claims have been filed as follows: CLAIMS: 1. 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; a rotor attached to the shaft so as to be rotatable with respect to the stator assembly and to be movable axially with respect to the shaft, the rotor and stator assembly being arranged so as to define a first air gap therebetween; and actuation means operable to move the rotor axially along the shaft so as to adjust the air gap between the rotor and the stator assembly, wherein the actuation means are operable to adjust the air gap between the rotor and stator assembly during rotation of the shaft, wherein the actuation means comprises: a positioning plate, located axially with the shaft, and rotatable with respect to the shaft, the positioning plate being engaged with the rotor for axial movement (0 therewith with respect to the shaft; a cam surface which extends from the stator assembly, from a first position to a second position, the first and second positions being located at different axial positions with respect to the stator assembly; a cam located on the positioning plate for axial movement therewith with respect to the shaft, and arranged to engage with a corresponding cam surface on the stator assembly; and drive means operable to rotate the positioning plate with respect to the cam surface such that the cam moves along the cam surface, thereby causing the cam and positioning plate to move axially with respect to the shaft..2. An axial flux electrical machine as claimed in any one of the preceding claims, wherein the rotor is attached to the shaft by way of a ball spline assembly, such that the rotor is rotatable with the shaft, and movable along the shaft axially.3. An axial flux electrical machine as claimed in claim 1, further comprising a second rotor attached to the shaft so as to be rotatable with respect to the stator assembly, the second rotor and stator assembly being arranged so as to define a second air gap therebetween, wherein the actuation means is operable to move the second rotors axially along the shaft so as to adjust the first and second air gaps.4. An axial flux electrical machine as claimed in claim 3, wherein the actuation means is operable to move the first rotor independently of the second rotor, such that the first air gap is adjusted independently of the second air gap.5. An axial flux electrical machine as claimed in claim 3, wherein the actuation means is operable to move the first and second rotors in concert, such that the first and second air gaps are adjusted in concert.6. An axial flux electrical machine as claimed in any one of claims 3 to 5, wherein the first and second air gaps are substantially equal in size.cf 7. An axial flux electrical machine as claimed in any one of claims 3 to 6, wherein the T" is stator assembly comprises a stator housing which has first and second ends and 04 which defines a substantially cylindrical aperture that extends from the first end to the 0 second end, a stator winding assembly located in the aperture, and first and second (0 covers which engage with the first and second ends of the stator housing (\J 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 first rotor and the stator winding assembly, and such that the second cover is located between the second rotor and the stator winding assembly.8. An axial flux electrical machine as claimed in any one of the preceding claims, wherein the stator assembly comprises a stator housing that defines a substantially cylindrical aperture therethrough, the shaft extending through the aperture, coaxially therewith, a stator winding assembly located in the aperture, and a bearing located in the aperture for supporting the shaft, the shaft being supported only by the bearing.9. An axial flux electrical machine as claimed in any one of the preceding claims, wherein the or each rotor comprises a rotor disk having first and second substantially planar sides, a plurality of magnets engaged with the first side of the disk, and a plurality of flux conduit portions engaged with the second side of the disk, the magnets and conduit portions being arranged to overlap one another in circumferential direction such that one conduit portion overlaps a pair of adjacent magnets.10. An axial flux electrical machine as claimed in claim 9, wherein the conduit portions are of grain oriented laminated steel, or of powder iron, or of a combination of grain oriented laminated steel and powder iron.11. An axial flux electrical machine substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings. (4 (0