Axial flux machine shoe optimisation

1. An axial flux machine comprising:

a rotor bearing a set of permanent magnets and mounted for rotation about an axis of the axial flux machine; and

a stator having a cavity formed between walls of a stator housing and containing therein a plurality of electro-magnetic coil assemblies disposed circumferentially at intervals about the axis, wherein said rotor and said stator are spaced apart along said axis, each of the plurality of electro-magnetic coil assemblies comprising:

an axially extending stator tooth;

one or more coils each wound around the stator tooth; and

one or more end shoes at an end of said stator tooth, said one or more end shoes each comprising an inner radial edge, an outer radial edge, and at least one circumferential edge adjacent to a neighbouring one of the plurality of electro-magnetic coil assembles;

wherein the at least one circumferential edge comprises a radially innermost portion and a radially outermost portion, and

wherein the radially innermost portion and the radially outermost portion are separated by a step projecting perpendicularly from the at least one circumferential edge, wherein the step has a thickness less than a thickness of the radially innermost portion of the at least one circumferential edge.

2. The axial flux machine of claim 1 , wherein the radially innermost portion of the at least one circumferential edge has a thickness approximately equal to a thickness of the inner radial edge.

3. The axial flux machine of claim 2 , wherein said radially outermost portion of the at least one circumferential edge of said shoe projects beyond said radially innermost portion of the at least one circumferential edge in a radial direction.

4. The axial flux machine of claim 1 , wherein said radially innermost portion of the at least one circumferential edge of said shoe is located at a surface axially closest to said rotor.

5. The axial flux machine of claim 1 , wherein said one or more end shoes comprise a first end shoe at a first axial end of said stator tooth and a second end shoe at a second axial end of said stator tooth,

wherein said first axial end and said second axial end are opposing ends of said stator tooth.

6. The axial flux machine of claim 1 , wherein said one or more end shoes are formed from a soft magnetic composite material.

7. The axial flux machine of claim 1 , wherein said one or more end shoes each comprise two opposing circumferential edges adjacent to different neighbouring electro-magnetic coil assemblies,

wherein at least a part of each of the two circumferential edges has a thickness less than a thickness of the inner radial edge.

8. The axial flux machine of claim 1 , wherein the axial flux machine has a YASA (Yokeless and Segmented Armature) topology.

9. A method of optimising a shoe shape of the one or more stator shoes of claim 1

the method comprising:

determining an axial force ripple at a plurality of radial distances from the axis for a plurality of shoe shapes; and

combining the plurality of shoe shapes at one or more of the plurality of radial distances to determine an optimised shoe shape, wherein the optimised shoe shape minimises an axial force ripple.

10. An axial flux machine comprising:

a rotor bearing a set of permanent magnets and mounted for rotation about an axis of the axial flux machine; and

a stator having a cavity formed between walls of a stator housing and containing therein a plurality of electro-magnetic coil assemblies disposed circumferentially at intervals about the axis, wherein said rotor and said stator are spaced apart along said axis, each of the plurality of electro-magnetic coil assemblies comprising:

an axially extending stator tooth;

one or more coils each wound around the stator tooth; and

one or more end shoes at an end of said stator tooth, said one or more end shoes each comprising an inner radial edge, an outer radial edge, and at least one circumferential edge adjacent to a neighboring one of the plurality of electro-magnetic coil assembles;

wherein the at least one circumferential edge comprises a radially innermost portion and a radially outermost portion;

wherein at least part of the at least one circumferential edge has a thickness less than a thickness of the radially innermost portion thereby reducing a leakage flux between neighboring electro-magnetic coil assemblies.

11. The axial flux machine of claim 10 , wherein said part of the at least one circumferential edge comprises the radially outermost portion of the at least one circumferential edge.

12. The axial flux machine of claim 11 , wherein the radially innermost portion of at least one circumferential edge has a thickness approximately equal to the thickness of the inner radial edge.

13. The axial flux machine of claim 12 , wherein said part of the at least one circumferential edge of said shoe projects beyond said radially innermost portion of at least one circumferential edge in a radial direction.

14. The axial flux machine of claim 13 , wherein said part of the at least one circumferential edge and said radially innermost portion of at least one circumferential edge are separated by a step on the at least one circumferential edge.

15. The axial flux machine of claim 10 , wherein said part of the at least one circumferential edge of said shoe is located at a surface of the one or more end shoes axially closest to said rotor.

16. The axial flux machine of claim 10 , wherein said one or more end shoes comprise a first end shoe at a first axial end of said stator tooth and a second end shoe at a second axial end of said stator tooth, and

wherein said first axial end and said second axial end are opposing ends of said stator tooth.

17. The axial flux machine of claim 10 , wherein said one or more end shoes are formed from a soft magnetic composite material.

18. The axial flux machine of claim 10 , wherein said one or more end shoes each comprise two opposing circumferential edges adjacent to different neighbouring electro-magnetic coil assemblies, and

wherein at least a part of each of the two circumferential edges has a thickness less than a thickness of the inner radial edge.

19. The axial flux machine of claim 10 , wherein the axial flux machine has a YASA (Yokeless and Segmented Armature) topology.