An Axial Flux Machine

Abstract

An axial flux machine comprises a stator, a rotor 10 and a rotor housing 40. The stator comprises a stator housing 100 enclosing stator bars disposed circumferentially at intervals around an axis 30 o

Description

FIELD OF THE INVENTION The present invention relates to an axial flux machine, and in particular to improvements in containment in the instance of structural integrity failure of a rotor in axial flux machines. BACKGROUND OF THE INVENTION Preventing debris from causing damage to surroundings when a high-speed rotor disintegrates is crucial for safety in various industrial and engineering applications, and well-established engineering methods and principles are followed to minimise this risk. Nevertheless, even with over specification on strength and multiples on safety margins, aerospace certification requires a statistically low probability of failure for propulsion systems, and furthermore mandate that such a rotor failure leads to a low probability for the release of “high energy debris” (HED). HED is any object that because of its kinetic energy could cause damage to surrounding parts of the aircraft. A typical source for HED is the mechanical failure of high-speed rotational components such as propellers, turbine discs etc. The rotor in an electric motor is a high-speed rotational component and therefore a potential source of HED. Whereas approaches are taught for containing the smaller components within a motor rotor assembly, such as magnets, little attention is given to the potential for fracture of rotor substrates i.e., rotor discs found in single and double rotor, single stator axial flux topologies, because this substantive component, on and around which subcomponents are built, is usually deemed of such low probability of failure. Nevertheless, failure of rotor discs or similar substantive components forming the basis of axial flux rotors is of consequence in aerospace applications and containment strategies are required. Conventional approaches in aerospace for achieving an acceptably low probability of HED include using detailed computer modelling and validated mechanical stress models set to high multiples of safety margins at the design and test stage and following strict process controls and using traceable specified materials in manufacture. Nevertheless, failure remains a possibility and to this end there is usually provided a secondary means of mechanical retention that can prevent HED release in the case of failure of the primary means of retention and beyond this there is finally provided some form of containment structure around the source of HED. Increased mass, complexity, development time and final cost are some of the inherent challenges associated with every methodology aimed at achieving a functional solution. Typically, multiple approaches are employed concurrently to effectively mitigate the risk of HED release. US11008887B2 teaches an annular hollow casing surrounding a turbofan, the casing having an inner abradable wall, i.e., next to the fan blade tips, beyond which there is an energy absorbing layer comprising, a solid or honeycomb structure made from energy absorbing materials beyond which there is a ‘nesting’ cavity also comprising energy absorbing material(s) into which a detached blade can be safely guided and brought to rest. IIS’887. Jet aircraft gas turbines have multibladed fans, having a central fan ring I disk I wheel to which blades are attached. The fan disk or wheel is generally a cylindrical mass having a similar shape to that of an axial flux rotor with both turbine and rotor disks subject to substantial tensile stress during their operation. US4521160A teaches a method of secondary containment for the most severe loss of rotor integrity in which the rotor wheel I disk fractures. The fan wheel of US’160 extends axially beyond the width of fan blades attached to the ring. On these axially extended fan wheel regions, that is, either side of protruding fan blades are placed containment rings made of aluminium matrix high tensile strength wound filaments. US’160 takes effort to stress that the containment rings are not wound on to the fan wheel to retard bursting of the wheel but the aluminium fibre composite rings are placed and retained on the fan wheel ends to contain wheel fragments in case a fan ring should burst. In providing secondary containment, US’160 adds mass and complexity. EP3116102A1 teaches a double stator single rotor machine in which the rotor rim is composed of high-strength insulating material wound around the outer peripheral edge of the rotor. EP102 teaches this approach provides a x2 safety factor against a burst test, but does not provide a containment solution in case of rotor burst. In the light of a paucity of solutions available for containing fracture pieces of substantive axial flux rotor substrates, it is apparent there is a need for a high reliability containment solution. SUMMARY OF THE INVENTION The present invention is defined by the independent claims appended hereto. Further advantageous embodiments are also defined by the dependent claims, also appended hereto. We describe an axial flux machine comprising: a stator comprising a stator housing enclosing a plurality of stator bars disposed circumferentially at intervals around an axis of the machine, each of the stator bars having a set of coils wound therearound forming a respective stator coil stack for generating a magnetic field; a rotor comprising a set of permanent magnets mounted on a front face of the rotor, the rotor being mounted for rotation about the axis of the machine, the rotor being spaced apart from the stator along the axis of the machine to define a gap between the stator and rotor and in which magnetic flux in the machine is generally in an axial direction, and the rotor having a rear face opposing the front face; and a rotor housing enclosing the rotor, the rotor housing having an inner axial surface spaced apart from and facing the rear face of the rotor, and an inner circumferential surface spaced apart from and facing a circumferential outer edge of the rotor, wherein each of the rotor and rotor housing comprise one or more protrusions arranged and configured with respect to each other to engage with each other during mechanical failure of the rotor. Advantageously, providing these one or more protrusions provides for an axial flux machine that is capable of containing or at least retaining the High Energy Debris (HED) when the rotor fails when rotating. When we discuss failure of the rotor, this could be, for example, a full or partial falling apart or disintegration of the rotor disc whilst rotating, or a displacement of the rotor in a radial or axial direction due to, for example, bearing failure or failure of other parts of the machine associated with the mounting of the rotor relative to the stator. The one or more rotor projections may comprise one or more circular projections projecting axially from the rear face of the rotor, the rotor projections being centred around the axis of the machine and located radially away from the axis of the machine. The rotor housing may comprise one or more circular projections corresponding with each of the respective one or more rotor projections, the rotor housing projections projecting axially towards the rear face of the rotor from the inner axial surface of the rotor housing and being centred around the axis of the machine and located radially away from the axis of the machine. Each respective corresponding rotor housing projection and rotor projection may be arranged to overlap axially with each other. The amount of axial overlap of the rotor projections and the rotor housing projections is preferably greater than the gap between the rotor and stator. When there are two or more rotor projections and corresponding two or more rotor housing projections, the two or more rotor projections may be concentric with respect to each other, and the rotor housing projections may be concentric with respect to each other. The one or more rotor projections may comprise a respective lip projecting radially away from the axis of the machine, and the one or more rotor housing projections may comprise a lip projecting radially towards the axis of the machine, the respective one or more lips of rotor projection overlap axially with respect to the lips of the one or more of the rotor housing projections and are separated radially with respect to the lips of the one or more rotor housing projections. The one or more rotor projections may project perpendicularly from the rear face of the rotor and the one or more rotor housing projections project perpendicularly from the inner axial surface of the rotor housing. Alternatively, the one or more rotor projections may project non-perpendicularly from the rear face of the rotor and the one or more rotor housing projections project non-perpendicularly from the inner axial surface of the rotor housing. In any of the above, the radially outermost of the one or more rotor protrusions and a corresponding one of the one or more rotor housing protrusions may be located adjacent the circumferential edge of the rotor. The one or more rotor projections may comprise one or more projections on the circumferential outer edge of the rotor projecting radially outward of the circumferential outer edge of the rotor, and the one or more rotor housing projections may comprise one or more projections mounted to a coiled spring that is coupled to the inner circumferential surface of the rotor housing and projecting radially inward towards the circumferential outer edge of the rotor. Alternatively, the one or more rotor projections may comprise one or more projections on the circumferential outer edge of the rotor projecting radially outward of the circumferential outer edge of the rotor, and the one or more rotor housing projections may comprise one or more projections located on the inner circumferential surface of the rotor housing and projecting radially inward towards the circumferential outer edge of the rotor. In either of the above, the one or more rotor projections may comprise one or more hooked projections, and the one or more rotor housing projections comprise one or more hooked projections, and wherein the hooked projections of the rotor projections may be arranged to face the hooked projections of the rotor housing such that they engage and catch each other during mechanical failure of the rotor. In any of the above, the radial distance between the one or more rotor protrusions and the one or more rotor housing protrusions may be sufficient to allow the rotor to rotate around the axis of the machine when there is no mechanical failure of the rotor. In any of the above, the axial flux machine may comprise a second rotor comprising a set of permanent magnets on a front face and mounted for rotation about the axis of the machine, the second rotor being spaced apart from the stator along the axis of the machine to define a gap between the stator and second rotor, the second rotor being disposed on a side of the stator opposed to the rotor and the second rotor having a rear face opposing the front face. The axial flux machine may also comprise a second rotor housing enclosing the second rotor, the second rotor housing having an inner axial surface spaced apart from and facing the rear face of the second rotor, and an inner circumferential surface spaced apart from and facing a circumferential outer edge of the second rotor, wherein each of the second rotor and second rotor housing comprise one or more protrusions arranged and configured with respect to each other to engage with each other during mechanical failure of the second rotor. The one or more second rotor projections may comprise one or more circular projections projecting axially from the rear face of the second rotor, the second rotor projections being centred around the axis of the machine and located radially away from the axis of the machine, and wherein the second rotor housing may comprise one or more circular projections corresponding with each of the respective one or more second rotor projections, the second rotor housing projections projecting axially towards the rear face of the second rotor from the inner axial surface of the second rotor housing and being centred around the axis of the machine and located radially away from the axis of the machine. Each respective corresponding second rotor housing projection and second rotor projection may be arranged to overlap axially with each other. The amount of axial overlap of the second rotor projections and the second rotor housing projections is preferably greater than the gap between the second rotor and stator. When there are two or more second rotor projections and corresponding two or more second rotor housing projections, the two or more second rotor projections may be concentric with respect to each other, and the second rotor housing projections may be concentric with respect to each other. The one or more second rotor projections may comprise a respective lip projecting radially away from the axis of the machine, and the one or more second rotor housing projections comprise a lip projecting radially towards the axis of the machine, the respective one or more lips of second rotor projection overlap axially with respect to the lips of the one or more of the second rotor housing projections and are separated radially with respect to the lips of the one or more second rotor housing projections. The one or more second rotor projections may project perpendicularly from the rear face of the second rotor and the one or more second rotor housing projections project perpendicularly from the inner axial surface of the second rotor housing. The one or more second rotor projections may project non-perpendicularly from the rear face of the second rotor and the one or more second rotor housing projections may project non-perpendicularly from the inner axial surface of the second rotor housing. The radially outermost of the one or more second rotor protrusions and a corresponding one of the one or more second rotor housing protrusions may be located adjacent the circumferential edge of the second rotor. The one or more second rotor projections may comprise one or more projections on the circumferential outer edge of the second rotor projecting radially outward of the circumferential outer edge of the second rotor, and the one or more second rotor housing projections may comprise one or more projections mounted to a coiled spring that is coupled to the inner circumferential surface of the second rotor housing and projecting radially inward towards the circumferential outer edge of the second rotor. The one or more second rotor projections may comprise one or more projections on the circumferential outer edge of the second rotor projecting radially outward of the circumferential outer edge of the second rotor, and the one or more second rotor housing projections may comprise one or more projections located on the inner circumferential surface of the second rotor housing and projecting radially inward towards the circumferential outer edge of the second rotor. The one or more second rotor projections may comprise one or more hooked projections, and the one or more second rotor housing projections may comprise one or more hooked projections, and wherein the hooked projections of the second rotor projections are arranged to face the hooked projections of the second rotor housing such that they engage and catch each other during mechanical failure of the second rotor. The radial distance between the one or more second rotor protrusions and the one or more second rotor housing protrusions may be sufficient to allow the second rotor to rotate around the axis of the machine when there is no mechanical failure of the second rotor. In any of the above, the machine may be a motor or generator. LIST OF FIGURES The present invention will be described by way of example only and with reference to the accompanying figures, in which: Figure 1 shows a simplified view of a rotor for the worst-case scenario of a “disc burst” whereupon a third of the disc is released; Figure 2 shows a partial cross-section of an axial flux machine incorporating the features of the present invention; Figure 3 illustrates potentially unprotected areas of the rotor; Figure 4 shows an alternative projection arrangement; Figure 5 shows a further alternative projection arrangement; Figure 6 shows an alternative arrangement of projections in which the projections project non-perpendicularly from their respective surfaces; and Figure 7 shows a further alternative projection arrangement. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In brief, the present invention provides an axial flux machine comprising a stator, at least one rotor and a rotor housing enclosing the rotor. Each of the rotor and rotor housing are provided with one or more protrusions that are arranged and configured with respect to each other to engage with each other during mechanical failure of the rotor. Advantageously, providing these one or more protrusions provides for an axial flux machine that is capable of containing or at least retaining the High Energy Debris (HED) when the rotor fails when rotating. When we discuss failure of the rotor, this could be, for example, a full or partial falling apart or disintegration of the rotor disc whilst rotating, or a displacement of the rotor in a radial or axial direction due to, for example, bearing failure or failure of other parts of the machine associated with the mounting of the rotor relative to the stator. Whilst axial flux machines are known, we will briefly describe the structure of such a machine. The axial flux machine comprises a stator and at least one rotor, although some implementations use two rotors. We will focus our discussion on the two rotor variants. The stator is a collection of separate stator bars spaced circumferentially about a rotation axis of the rotors. Each bar has its own axis which is preferably, but not essentially, disposed parallel to the rotation axis. Each end of each stator bar is provided with a shoe which serves a physical purpose of confining a coil stack, which stack is preferably of square / rectangular section insulated wire so that a high fill factor can be achieved. The coils are connected to an electrical circuit that, in the case of a motor, energizes the coils so that the poles of the resultant magnetic fields generated by the current flowing in the coils is opposite in adjacent stator coils. The two rotors carry permanent magnets that face one another with the stator coil between. Two air gaps are disposed between respective shoe and magnet pairs. There are an even number of coils and magnets spaced around the axis of rotation and, preferably, there are a different number of coils and magnets so that the coils do not all come into registration with the corresponding magnet pair at the same time and at the same rotational position of the rotor with respect to the stator. This serves to reduce cogging. In a motor the coils are energized so that their polarity alternates serving to cause coils at different times to align with different magnet pairs, resulting in torque being applied between the rotor and the stator. The rotors are generally connected together (for example by a shaft and rotate together about the axis relative to the stator. The magnetic circuit is provided by two adjacent stator bars and two magnet pairs and a back iron for each rotor links the flux between the back of each magnet facing away from the respective coils. The stator coils are enclosed within a housing that extends through the air gap and which defines a chamber supplied with a cooling medium. Conventional approaches in aerospace for achieving an acceptably low probability of HED release include providing some form of containment structure around the source of HED Solutions can include significant development cost / complexity / time / risk, unit cost increase and unit mass increase. Selection of the optimal approach is an important aspect of the “certification strategy” for development of a new aerospace propulsion system. Therefore, in turn, minimizing the penalties associated with each of the potential approaches is an important driver in aerospace business development and innovation. Based on the well-established certification approach for gas turbines (which contain rotor discs somewhat similar to those in axial flux machines), a rotor containment structure is preferably designed (and proven) to contain a worst-case scenario “disc burst” that results in 1 / 3 of the rotor / disc sector being released. Figure 1 shows a simplified view of a rotor for the worst-case scenario of a “disc burst” that results in a 1 / 3 of the rotor / disc sector being released. This relatively large mass can require a very heavy containment structure to guarantee retention under all conditions. Figure 1 shows a rotor disc 10 divided into 1 / 3 segments 20, and the axis of rotation 30 of the rotor disc 10. The present invention attempts to address the issues associated with providing a containment structure around the source of the HED (that is, the rotor). The present invention aims to achieve containment without the typically-associated penalties of significantly increased propulsion system size and mass. It does this by: 1. Utilizing an existing housing structure of the motor to transmit the forces generated by the retention mechanism. 2. Engaging the retention mechanism as soon as possible after the disc fragment has been released i.e. before the fragment is able to displace significantly from its normal location. This approach is known as “containment at source” and is known to significantly simplify the analysis. 3. Tightly constraining the possible arrangement / positioning of the interacting parts (i.e. the static housing and the HED) at the point at which the retention mechanism engages, making design &analysis significantly simpler. 4. Maximizing the area / volume of material engaged on both sides of the mechanism (i.e. the static part and the HED), reducing stresses. 5. Adopting straightforward manufacturing processes compatible with the those typically used in the production of the motor casing and rotor. Figure 2 shows a partial cross-section of an axial flux machine incorporating the features of the present invention. Only an upper portion of the rotor and stator housing are shown for the sake of simplicity. Whilst we will describe an axial flux machine comprising a single rotor and single stator, the techniques described below, may also be used with double rotor single stator variants of the axial flux machine. Figure 2 shows the axial flux machine comprising a rotor 10, stator housing 100 (enclosing the stator assembly comprising the plurality of stator bars, stator coil stacks and the like as described above) and rotor housing 40. The rotor 10 and stator housing 100 are co-located in the machine to be centred around the axis 30 of the machine. An (axial) air gap 70 is provided between the rotor 10 and stator housing 100 in which magnetic flux in the machine is generally in an axial direction. The rotor 10 is arranged to rotate about the axis 30 of the machine. As discussed above, only the upper half of the rotor 10, stator housing 100 and rotor housing 40 are shown for the sake of clarity; it would be clear that the mirror image of these features would be present below the axis line 30. Whilst not shown (for the sake of simplicity), the rotor 10 comprises a set of permanent magnets mounted on a front face of the rotor 10, and the rotor has a rear face opposing the front face. A rotor housing 40 encloses the rotor 10 and has an inner axial surface spaced apart from and facing the rear face of the rotor 10, and an inner circumferential surface spaced apart from and facing a circumferential outer edge of the rotor 10. In its broadest sense, the present invention provides each of the rotor 10 and rotor housing 100 with one or more protrusions arranged and configured with respect to each other to engage with each other during mechanical failure of the rotor. The protrusions are configured and arranged such that they do not engage with each other when the rotor 10 is working within normal parameters within the machine. As such the rotor 10 is able to rotate unhindered despite the presence of the protrusions. However, should the rotor 10 experience a failure, for example fully or partially fall apart or disintegrate whilst rotating, or significantly displace in a radial direction due to, for example, bearing failure, the protrusions are configured and arranged to engage with one another. When engaged with each other, the protrusions are configured to slow down and stop the rotor 10 from rotating further, and also to capture or prevent release / ejection of the HED portions of the rotor 10 that have become separated from the main body of the rotor 10. The protrusions are able to be located on various parts of the rotor 10 and correspondingly on the rotor housing 40. We will discuss different options below. In a first implementation, as shown in Figure 2, the rotor 10 is provided with one or more projections 60 that are circular projections projecting axially from the rear face of the rotor 10. The one or more the rotor projections 60 are preferably centred around the axis 30 of the machine and located radially away from the axis of the machine. Furthermore, the rotor housing 40 comprises one or more circular projections 50 corresponding with each of the respective one or more rotor projections 60. The rotor housing projections 50 project axially towards the rear face of the rotor 10 from the inner axial surface of the rotor housing 40, The rotor housing projections 50 are preferably centred around the axis of the machine 30 and located radially away from the axis of the machine. Whilst Figure 2 shows multiple projections 50, 60 on both the rear face of the rotor 10 and the rotor housing 40, in its simplest form, the invention also works with a single projection 60 on the rotor 10 and a single projection 50 on the rotor housing 40. The rotor projections 60 and rotor housing projections 50 are arranged to overlap axially with each other, but be separated by a radial gap. This permits the rotor to rotate freely without colliding with the projections under normal operation of the rotor. Preferably the amount of axial overlap of the rotor projections 60 and the rotor housing projections 50 is greater than the air gap 70 between the rotor and stator. The air gap 70 is typically in the region of 1mm for permanent magnet axial flux machines. As such, during a mechanical failure of the rotor 10, the released fragment may shift axially all the way to impinge upon the stator housing 100 and the projections 50, 60 may still catch on one another to retain the fragment in the radial direction. In the event of failure of the rotor’s primary retention mechanism, the resultant radial (outward) motion of the released rotor segment / sector fragment will cause the rings to clash and hence the released fragment to be retained. The nominal radial clearance between the static rings (rotor housing projections 50) and rotating rings (rotor projections 60) should be as small as possible so that the released fragment moves very little before the projections clash. This is to minimize or eliminate the possibility of the released fragment rotating significantly, which could complicate analysis and / or testing. When there are two or more rotor projections 60 and corresponding two or more rotor housing projections 50, as shown in Figure 2 for example, the two or more rotor projections 60 are concentric with respect to each other, and the rotor housing projections 50 are concentric with respect to each other. Multiple projections are shown to illustrate the principle that more than a single projection on each of the rotor 10 and rotor housing 40 may be utilized. Multiple projections 50, 60 could serve to help very small fragments of rotor to be captured by at least one ring. Furthermore, multiple projections 50, 60 could increase the load-carrying area / volume of material, thereby reducing the stress levels and mass impact of the mechanism. Also, multiple projections 50, 60 may provide redundancy in case of excessive deflection / failure of one projection. Figure 3 illustrates potentially unprotected areas of the rotor 10. Preferably, the radially outermost projections 50, 60 should be positioned as close as possible to the circumferential edge of the rotor 10 to minimize the size of “unprotected” region 12 of the rotor that, if released, would always escape the mechanism (since it would not impinge upon any projections). If the stator integrity cannot guarantee containing the released fragment in the axial direction, then alternative projections may be used, as described below. Figure 4 shows such an alternative projection arrangement. The projections 50, 60 remain similar to those shown in figures 2 and 3 and the operating principle remains the same. However, as shown in the close up in Figure 4, the rotor projections 60 are provided with a “hook” or lip 62 that projects radially away from the axis of the machine 30. Similarly, each of the rotor housing projections 50 is provided with a correspondingly shaped “hook” or lip 52 that projects radially towards the axis of the machine from the rotor housing projection 50. Preferably there is clearance in the radial direction between the rotor projection lips 62 and the rotor housing projection lips 52 to enable the casing to be assembled axially onto the motor without the hooks clashing. Figure 5 shows a further alternative projection arrangement. The arrangement of projections 54, 64 shown in Figure 5 is similar to the projections 50, 60 shown in Figure 4. However, the projections 54, 64 in Figure 5 are provided with additional material to strengthen the respective projection 54, 64. In some of the above arrangements, it is preferable that the one or more rotor projections 60 project substantially perpendicularly from the rear face of the rotor 10 and the one or more rotor housing projections 50 project substantially perpendicularly from the inner axial surface of the rotor housing 40. However, alternative arrangements may be possible in which the one or more rotor projections 60 project non-perpendicularly from the rear face of the rotor 10 and the one or more rotor housing projections 50 project non-perpendicularly from the inner axial surface of the rotor housing 40. Figure 6 shows an alternative arrangement of projections in which the projections project non-perpendicularly from their respective surfaces. As can be seen, the rotor housing projection 56 in this arrangement extends non-perpendicularly from the inner surface of the rotor housing 40. A correspondingly angled projection 66 projects from the rotor 10 in order to engage and catch with the rotor housing projection 56 in the event of failure of the rotor 10. A potential advantage offered by this arrangement is the tendency for the released rotor fragment to be pulled / drawn axially towards the rotor housing via its radial motion. This keeps the fragment from contacting the surface of the stator housing 100 and thereby causing mechanical and / or thermal damage (via friction). Similar to the arrangements shown above, there is an axial overlap that is greater than the air gap 70 between the rotor 10 and stator housing 100. Whilst Figure 6 shows also an axial overlap, it is preferable that there is little or no axial overlap to enable assembly of the rotor 10 and rotor housing 40. As shown in Figure 3, portions of the rotor 10 may remain unprotected even with the use of the projections 50, 60. Figure 7 shows an arrangement of projections that may be used in conjunction with the above-mentioned arrangements or may be used in isolation, that is without the arrangements shown above. In this arrangement, the one or more rotor 10 projections comprise one or more projections 220 on the circumferential outer edge of the rotor 10 projecting radially outward of the circumferential outer edge of the rotor. The one or more rotor housing projections comprise one or more projections 210 mounted to a coiled spring 200 that is coupled to the inner circumferential surface of the rotor housing 40 and projecting radially inward towards the circumferential outer edge of the rotor 10. Similar to other arrangements, the rotor projections 220 and rotor housing projections 210 comprise hooks, or they are arranged at a corresponding angle with respect to each other such that they catch when the rotor is in a failure condition. Under normal operating conditions, that is when the rotor is not in a failed condition, the rotor is able to rotate freely without the projections 210, 220 catching each other. In the event that the rotor 10 does fail, the rotor (or rotor fragment) moves radially outwards and catches the spring hooks which now rotate with the rotor rotation direction and thereby tighten the spring. The spring both absorbs the rotation energy and clamps the rotor parts. Alternatively, the one or more rotor projections comprise one or more projections on the circumferential outer edge of the rotor projecting radially outward of the circumferential outer edge of the rotor, and the one or more rotor housing projections comprise one or more projections located on the inner circumferential surface of the rotor housing and projecting radially inward towards the circumferential outer edge of the rotor. This is similar to the arrangement shown in Figure 7, however without the spring arrangement. Instead the rotor housing projections are mounted directly to the inner circumferential face of the rotor housing 40. As mentioned above, the principles described above may also be used on variants of the axial flux machine that are double-rotor single-stator arrangements. In the case of double-rotor single stator variants, for example, projections on the second rotor and second rotor housing may be implemented using the same arrangements as described above. As such, the second rotor would be protected from failure by the use of the projections on the second rotor and second rotor housing. Throughout the description, we have been referencing an axial flux machine. In practice, this may be used as a motor or generator. No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims

1. An axial flux machine comprising:a stator comprising a stator housing enclosing a plurality of stator bars disposed circumferentially at intervals around an axis of the machine, each of the stator bars having a set of coils wound therearound forming a respective stator coil stack for generating a magnetic field;a rotor comprising a set of permanent magnets mounted on a front face of the rotor, the rotor being mounted for rotation about the axis of the machine, the rotor being spaced apart from the stator along the axis of the machine to define a gap between the stator and rotor and in which magnetic flux in the machine is generally in an axial direction, and the rotor having a rear face opposing the front face; anda rotor housing enclosing the rotor, the rotor housing having an inner axial surface spaced apart from and facing the rear face of the rotor, and an inner circumferential surface spaced apart from and facing a circumferential outer edge of the rotor, wherein each of the rotor and rotor housing comprise one or more protrusions arranged and configured with respect to each other to engage with each other during mechanical failure of the rotor.

2. An axial flux machine according to claim 1, wherein the one or more rotor projections comprise one or more circular projections projecting axially from the rear face of the rotor, the rotor projections being centred around the axis of the machine and located radially away from the axis of the machine, and wherein the rotor housing comprises one or more circular projections corresponding with each of the respective one or more rotor projections, the rotor housing projections projecting axially towards the rear face of the rotor from the inner axial surface of the rotor housing and being centred around the axis of the machine and located radially away from the axis of the machine.

3. An axial flux machine according to claim 2, wherein each respective corresponding rotor housing projection and rotor projection are arranged to overlap axially with each other.

4. An axial flux machine according to claim 3, wherein the amount of axial overlap of the rotor projections and the rotor housing projections is greater than the gap between the rotor and stator.

5. An axial flux machine according to any one of claims 2 to 4, wherein when there are two or more rotor projections and corresponding two or more rotor housing projections, the two or more rotor projections are concentric with respect to each other, and the rotor housing projections are concentric with respect to each other.

6. An axial flux machine according to any one of claims 2 to 5, wherein the one or more rotor projections comprise a respective lip projecting radially away from the axis of the machine, and the one or more rotor housing projections comprise a lip projecting radially towards the axis of the machine, the respective one or more lips of rotor projection overlap axially with respect to the lips of the one or more of the rotor housing projections and are separated radially with respect to the lips of the one or more rotor housing projections.

7. An axial flux machine according to any one of claims 2 to 6, wherein the one or more rotor projections project perpendicularly from the rear face of the rotor and the one or more rotor housing projections project perpendicularly from the inner axial surface of the rotor housing.

8. An axial flux machine according to any one of claims 2 to 6, wherein the one or more rotor projections project non-perpendicularly from the rear face of the rotor and the one or more rotor housing projections project non-perpendicularly from the inner axial surface of the rotor housing.

9. An axial flux machine according to any preceding claim, wherein the radially outermost of the one or more rotor protrusions and a corresponding one of the one or more rotor housing protrusions are located adjacent the circumferential edge of the rotor.

10. An axial flux machine according to any preceding claim, whereinthe one or more rotor projections comprise one or more projections on the circumferential outer edge of the rotor projecting radially outward of the circumferential outer edge of the rotor, andthe one or more rotor housing projections comprise one or more projections mounted to a coiled spring that is coupled to the inner circumferential surface of the rotor housing and projecting radially inward towards the circumferential outer edge of the rotor.

11. An axial flux machine according to any one of claims 1 to 9, whereinthe one or more rotor projections comprise one or more projections on the circumferential outer edge of the rotor projecting radially outward of the circumferential outer edge of the rotor, andthe one or more rotor housing projections comprise one or more projections located on the inner circumferential surface of the rotor housing and projecting radially inward towards the circumferential outer edge of the rotor.

12. An axial flux machine according to claim 10 or 11, wherein the one or more rotor projections comprise one or more hooked projections, and the one or more rotor housing projections comprise one or more hooked projections, and wherein the hooked projections of the rotor projections are arranged to face the hooked projections of the rotor housing such that they engage and catch each other during mechanical failure of the rotor.

13. An axial flux machine according to any preceding claim, wherein the radial distance between the one or more rotor protrusions and the one or more rotor housing protrusions is sufficient to allow the rotor to rotate around the axis of the machine when there is no mechanical failure of the rotor.

14. An axial flux machine according to any preceding claim, comprising a second rotor comprising a set of permanent magnets on a front face and mounted for rotation about the axis of the machine, the second rotor being spaced apart from the stator along the axis of the machine to define a gap between the stator and second rotor, the second rotor being disposed on a side of the stator opposed to the rotor and the second rotor having a rear face opposing the front face.

15. An axial flux machine according to claim 14, comprising a second rotor housing enclosing the second rotor, the second rotor housing having an inner axial surface spaced apart from and facing the rear face of the second rotor, and an innercircumferential surface spaced apart from and facing a circumferential outer edge of the second rotor,wherein each of the second rotor and second rotor housing comprise one or more protrusions arranged and configured with respect to each other to engage with each other during mechanical failure of the second rotor.

16. An axial flux machine according to claim 15, wherein the one or more second rotor projections comprise one or more circular projections projecting axially from the rear face of the second rotor, the second rotor projections being centred around the axis of the machine and located radially away from the axis of the machine, and wherein the second rotor housing comprises one or more circular projections corresponding with each of the respective one or more second rotor projections, the second rotor housing projections projecting axially towards the rear face of the second rotor from the inner axial surface of the second rotor housing and being centred around the axis of the machine and located radially away from the axis of the machine.

17. An axial flux machine according to claim 16, wherein each respective corresponding second rotor housing projection and second rotor projection are arranged to overlap axially with each other.

18. An axial flux machine according to claim 17, wherein the amount of axial overlap of the second rotor projections and the second rotor housing projections is greater than the gap between the second rotor and stator.

19. An axial flux machine according to any one of claims 16 to 18, wherein when there are two or more second rotor projections and corresponding two or more second rotor housing projections, the two or more second rotor projections are concentric with respect to each other, and the second rotor housing projections are concentric with respect to each other.

20. An axial flux machine according to any one of claims 16 to 19, wherein the one or more second rotor projections comprise a respective lip projecting radially away from the axis of the machine, and the one or more second rotor housing projections comprise a lip projecting radially towards the axis of the machine, the respective one or more lips of second rotor projection overlap axially with respect to the lips of the one or more ofthe second rotor housing projections and are separated radially with respect to the lips of the one or more second rotor housing projections.

21. An axial flux machine according to any one of claims 16 to 20, wherein the one or more second rotor projections project perpendicularly from the rear face of the second rotor and the one or more second rotor housing projections project perpendicularly from the inner axial surface of the second rotor housing.

22. An axial flux machine according to any one of claims 16 to 20, wherein the one or more second rotor projections project non-perpendicularly from the rear face of the second rotor and the one or more second rotor housing projections project non-perpendicularly from the inner axial surface of the second rotor housing.

23. An axial flux machine according to any one of claims 14 to 22, wherein the radially outermost of the one or more second rotor protrusions and a corresponding one of the one or more second rotor housing protrusions are located adjacent the circumferential edge of the second rotor.

24. An axial flux machine according to any one of claims 15 to 23, whereinthe one or more second rotor projections comprise one or more projections on the circumferential outer edge of the second rotor projecting radially outward of the circumferential outer edge of the second rotor, andthe one or more second rotor housing projections comprise one or more projections mounted to a coiled spring that is coupled to the inner circumferential surface of the second rotor housing and projecting radially inward towards the circumferential outer edge of the second rotor.

25. An axial flux machine according to any one of claims 15 to 23, whereinthe one or more second rotor projections comprise one or more projections on the circumferential outer edge of the second rotor projecting radially outward of the circumferential outer edge of the second rotor, andthe one or more second rotor housing projections comprise one or more projections located on the inner circumferential surface of the second rotor housing and projecting radially inward towards the circumferential outer edge of the second rotor.

26. An axial flux machine according to claim 24 or 25, wherein the one or more second rotor projections comprise one or more hooked projections, and the one or more second rotor housing projections comprise one or more hooked projections, and wherein the hooked projections of the second rotor projections are arranged to face the hooked5 projections of the second rotor housing such that they engage and catch each other during mechanical failure of the second rotor.

27. An axial flux machine according to any one of claims 14 to 26, wherein the radial distance between the one or more second rotor protrusions and the one or more second 10 rotor housing protrusions is sufficient to allow the second rotor to rotate around the axis of the machine when there is no mechanical failure of the second rotor.

28. An axial flux machine according to any preceding claim, wherein the machine is a motor or generator.15Amendments to the Claims have been filed as follows:23CLAIMS:

1. An axial flux machine comprising:a stator comprising a stator housing enclosing a plurality of stator bars disposed 5 circumferentially at intervals around an axis of the machine, each of the stator bars having a set of coils wound therearound forming a respective stator coil stack for generating a magnetic field;a rotor comprising a set of permanent magnets mounted on a front face of the rotor, the rotor being mounted for rotation about the axis of the machine, the rotor being10 spaced apart from the stator along the axis of the machine to define a gap between the stator and rotor and in which magnetic flux in the machine is generally in an axial direction, and the rotor having a rear face opposing the front face; anda rotor housing enclosing the rotor, the rotor housing having an inner axial surface spaced apart from and facing the rear face of the rotor, and an inner circumferential15 surface spaced apart from and facing a circumferential outer edge of the rotor, wherein each of the rotor and rotor housing comprise one or more protrusions arranged and configured with respect to each other to engage with each other during mechanical failure of the rotor, wherein the one or more rotor projections comprise one or more circular projections projecting axially from the rear face of the rotor, the rotor projections20 being centred around the axis of the machine and located radially away from the axis of the machine, andwherein the rotor housing comprises one or more circular projections corresponding with each of the respective one or more rotor projections, the rotor housing projections projecting axially towards the rear face of the rotor from the inner axial surface of the25 rotor housing and being centred around the axis of the machine and located radially away from the axis of the machine.

2. An axial flux machine according to claim 1, wherein each respective corresponding rotor housing projection and rotor projection are arranged to overlap30 axially with each other.

3. An axial flux machine according to claim 2, wherein the amount of axial overlap of the rotor projections and the rotor housing projections is greater than the gap between the rotor and stator.

4. An axial flux machine according to any preceding claim, wherein when there are two or more rotor projections and corresponding two or more rotor housing projections, the two or more rotor projections are concentric with respect to each other, and the rotor housing projections are concentric with respect to each other.

55. An axial flux machine according to any preceding claim, wherein the one or more rotor projections comprise a respective lip projecting radially away from the axis of the machine, and the one or more rotor housing projections comprise a lip projecting radially towards the axis of the machine, the respective one or more lips of rotor projection 10 overlap axially with respect to the lips of the one or more of the rotor housing projections and are separated radially with respect to the lips of the one or more rotor housing projections.

156. An axial flux machine according to any preceding claim, wherein the one or more rotor projections project perpendicularly from the rear face of the rotor and the one or more rotor housing projections project perpendicularly from the inner axial surface of the rotor housing.

7. An axial flux machine according to any one of claims 1 to 5, wherein the one or more rotor projections project non-perpendicularly from the rear face of the rotor and the one or more rotor housing projections project non-perpendicularly from the inner axial surface of the rotor housing.

8. An axial flux machine according to any preceding claim, wherein the radially 25 outermost of the one or more rotor protrusions and a corresponding one of the one or more rotor housing protrusions are located adjacent the circumferential edge of the rotor.

9. An axial flux machine according to any preceding claim, whereinthe one or more rotor projections further comprise one or more projections on the 30 circumferential outer edge of the rotor projecting radially outward of the circumferential outer edge of the rotor, andthe one or more rotor housing projections comprise one or more projections mounted to a coiled spring that is coupled to the inner circumferential surface of the rotor housing and projecting radially inward towards the circumferential outer edge of the rotor.

10. An axial flux machine according to any one of claims 1 to 8, whereinthe one or more rotor projections comprise one or more projections on the circumferential outer edge of the rotor projecting radially outward of the circumferential outer edge of the rotor, and5 the one or more rotor housing projections comprise one or more projectionslocated on the inner circumferential surface of the rotor housing and projecting radially inward towards the circumferential outer edge of the rotor.

11. An axial flux machine according to claim 9 or 10, wherein the one or more rotor 10 projections comprise one or more hooked projections, and the one or more rotor housing projections comprise one or more hooked projections, and wherein the hooked projections of the rotor projections are arranged to face the hooked projections of the rotor housing such that they engage and catch each other during mechanical failure of the rotor.15in12. An axial flux machine according to any preceding claim, wherein the radial distance between the one or more rotor protrusions and the one or more rotor housing protrusions is sufficient to allow the rotor to rotate around the axis of the machine when there is no mechanical failure of the rotor.1 2013. An axial flux machine according to any preceding claim, comprising a second rotor comprising a set of permanent magnets on a front face and mounted for rotation about the axis of the machine, the second rotor being spaced apart from the stator along the axis of the machine to define a gap between the stator and second rotor, the second 25 rotor being disposed on a side of the stator opposed to the rotor and the second rotor having a rear face opposing the front face.

14. An axial flux machine according to claim 13, comprising a second rotor housing enclosing the second rotor, the second rotor housing having an inner axial surface 30 spaced apart from and facing the rear face of the second rotor, and an inner circumferential surface spaced apart from and facing a circumferential outer edge of the second rotor, wherein each of the second rotor and second rotor housing comprise one or more protrusions arranged and configured with respect to each other to engage with each 35 other during mechanical failure of the second rotor.

15. An axial flux machine according to claim 14, wherein the one or more second rotor projections comprise one or more circular projections projecting axially from the rear face of the second rotor, the second rotor projections being centred around the axis 5 of the machine and located radially away from the axis of the machine, andwherein the second rotor housing comprises one or more circular projections corresponding with each of the respective one or more second rotor projections, the second rotor housing projections projecting axially towards the rear face of the second rotor from the inner axial surface of the second rotor housing and being centred around 10 the axis of the machine and located radially away from the axis of the machine.1516. An axial flux machine according to claim 15, wherein each respective corresponding second rotor housing projection and second rotor projection are arranged to overlap axially with each other.

17. An axial flux machine according to claim 16, wherein the amount of axial overlap of the second rotor projections and the second rotor housing projections is greater than the gap between the second rotor and stator.20 18. An axial flux machine according to any one of claims 15 to 17, wherein whenthere are two or more second rotor projections and corresponding two or more second rotor housing projections, the two or more second rotor projections are concentric with respect to each other, and the second rotor housing projections are concentric with respect to each other.2519. An axial flux machine according to any one of claims 15 to 18, wherein the one or more second rotor projections comprise a respective lip projecting radially away from the axis of the machine, and the one or more second rotor housing projections comprise a lip projecting radially towards the axis of the machine, the respective one or more lips 30 of second rotor projection overlap axially with respect to the lips of the one or more of the second rotor housing projections and are separated radially with respect to the lips of the one or more second rotor housing projections.

20. An axial flux machine according to any one of claims 15 to 19, wherein the one35 or more second rotor projections project perpendicularly from the rear face of the secondrotor and the one or more second rotor housing projections project perpendicularly from the inner axial surface of the second rotor housing.

21. An axial flux machine according to any one of claims 15 to 19, wherein the one 5 or more second rotor projections project non-perpendicularly from the rear face of the second rotor and the one or more second rotor housing projections project non-perpendicularly from the inner axial surface of the second rotor housing.

22. An axial flux machine according to any one of claims 13 to 21, wherein the radially 10 outermost of the one or more second rotor protrusions and a corresponding one of the one or more second rotor housing protrusions are located adjacent the circumferential edge of the second rotor.1523. An axial flux machine according to any one of claims 14 to 22, whereinthe one or more second rotor projections comprise one or more projections on the circumferential outer edge of the second rotor projecting radially outward of the circumferential outer edge of the second rotor, andthe one or more second rotor housing projections comprise one or more projections mounted to a coiled spring that is coupled to the inner circumferential surface of the second rotor housing and projecting radially inward towards the circumferential outer edge of the second rotor.

24. An axial flux machine according to any one of claims 14 to 22, whereinthe one or more second rotor projections comprise one or more projections on25 the circumferential outer edge of the second rotor projecting radially outward of the circumferential outer edge of the second rotor, andthe one or more second rotor housing projections comprise one or more projections located on the inner circumferential surface of the second rotor housing and projecting radially inward towards the circumferential outer edge of the second rotor.3025. An axial flux machine according to claim 23 or 24, wherein the one or more second rotor projections comprise one or more hooked projections, and the one or more second rotor housing projections comprise one or more hooked projections, and wherein the hooked projections of the second rotor projections are arranged to face the hookedprojections of the second rotor housing such that they engage and catch each other during mechanical failure of the second rotor.

26. An axial flux machine according to any one of claims 13 to 25, wherein the radial 5 distance between the one or more second rotor protrusions and the one or more second rotor housing protrusions is sufficient to allow the second rotor to rotate around the axis of the machine when there is no mechanical failure of the second rotor.

27. An axial flux machine according to any preceding claim, wherein the machine is 10 a motor or generator.21 07 25

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