Machine cooling systems

1. A method of manufacturing an axial flux permanent magnet machine, the machine having a stator comprising a stator housing enclosing a set of coils wound on respective stator bars and disposed circumferentially at intervals about an axis of the machine, and a rotor bearing a set of permanent magnets and mounted for rotation about said axis, and wherein said rotor and stator are spaced apart along said axis to define a gap between the rotor and stator in which magnetic flux in the machine is generally in an axial direction, the method comprising:

providing ports for flowing coolant through said stator housing around said coils such that said coolant flows between said coils;

defining a gap for coolant flow between adjacent said coils, wherein each of said coils includes a single layer of windings over said stator bar, said layer of windings comprising a ribbon-shaped wire having a greater width across a surface of the ribbon-shaped wire than thickness through the ribbon-shaped wire; wherein said windings are stacked horizontally along said stator bar such that inner edges of said coil abut said stator bar and each of the adjacent surfaces of said windings abut one another, wherein said width of said single layer of windings defines a distance across said coil perpendicular to a direction in which said windings are stacked, and wherein said gap for coolant flow is defined by outer edges of adjacent coils;

designing said machine by controlling said gap between said adjacent coils to within a tolerance limit using a tolerance of said width of said wire; and

manufacturing said axial flux permanent magnet machine to the design.

2. A method as claimed in claim 1 wherein said controlling said gap between said coils comprises controlling a percentage variation from an average gap between said coils of said stator to within said tolerance limit by using said single layer of windings of said ribbon-shaped wire to limit variation in spacing between outer edges of said coils of said stator.

3. A method as claimed in claim 2 wherein a minimum value of said gap between said coils is less than 2 mm and wherein said tolerance limit is 20% of said gap.

4. A method as claimed in claim 1 further comprising providing said coils of said stator with a shoe at each end, and stopping said windings short of one or both said shoes to define a coolant channel between an end of said coil of said stator and a respective said shoe.

5. A method as claimed in claim 1 further comprising preloading each of said coils in compression to urge turns of each of said coils of said stator towards one another.

6. An axial flux permanent magnetic machine, the machine having a stator comprising:

a stator housing enclosing a set of coils wound on respective stator bars and disposed circumferentially at intervals about an axis of the machine, and a rotor bearing a set of permanent magnets and mounted for rotation about said axis, and wherein said rotor and stator are spaced apart along said axis to define a gap therebetween in which magnetic flux in the machine is generally in an axial direction;

wherein:

each of said coils includes a single layer of windings over said stator bar, said single layer of windings comprising a ribbon-shaped wire having a greater width across a surface of the ribbon-shaped wire than thickness through the ribbon,

said windings are stacked horizontally along said stator bar such that inner edges of said coil abut said stator bar and adjacent surfaces of said windings abut one another,

said width of said single layer of windings defines a distance across a said coil perpendicular to a direction in which said windings are stacked,

outer edges of adjacent coils define a gap for coolant flow between said adjacent stator coils for cooling said machine; and

said gap between said adjacent coils is controlled to within a tolerance limit that is using a tolerance of said width of said wire.

7. An axial flux permanent magnetic machine as claimed in 6 wherein each of said coils is preloaded in compression to urge turns of each of said coils towards one another.

8. An axial flux permanent magnetic machine as claimed in claim 6 wherein a respective one of said stator bars has a shoe at each end, and wherein a gap between said coil and said shoe defines, at one or both ends of said coil, a coolant channel.

9. An axial flux permanent magnetic machine as claimed in claim 8 further comprising a spacer between an end of said coil and the respective shoe to define said channel.

10. An axial flux permanent magnetic machine as claimed in claim 6 further comprising barriers or blocks between a subset of said coils of said stator and radially inner and outer walls of said housing to force flow of a coolant through said gaps between said coils of said stator.

11. An axial flux permanent magnetic machine as claimed in claim 10 wherein a respective one of said stator bars has a shoe at each end, wherein said shoes at each end of said stator bars contact a respective facing pocket of said stator housing; wherein said stator housing comprises coolant inlet and outlet ports, and wherein around said stator said barriers are provided alternately connecting a said coil of said stator to said inner wall and to said outer wall after intervals of a plurality of coils of said stator to define a first plurality of coolant flow channels between adjacent coils of said stator in a first radial direction and a second plurality of coolant flow channels between adjacent coils of said stator in a second, opposite radial direction; and wherein said defining of said distance across a said coil of said stator controls widths of said coolant flow channels in the same radial direction between adjacent said coils of said stator to balance coolant flows in said coolant flow channels in the same radial direction.