Cooled electric machine

1. A yokeless electric machine comprising a rotor having permanent magnets and a stator having coils wound on stator bars for interaction with the magnets across an air gap defined between them, wherein the rotor has two stages arranged one at either end of the stator bars, with air gaps between the ends of the bars and the rotor stages, wherein the bars and coils thereon are enclosed by a stator housing comprising two annular plates and two cylindrical walls defining a chamber incorporating cooling medium in contact with the coils to cool the coils, wherein the coils are coated and are disposed within the chamber, the annular plates extend through the air gaps, said stator housing including a port for supply and a port for drainage of said cooling medium, wherein the cooling medium flows back and forth between the outer and inner radius of the stator housing on plural occasions by reason of blocks disposed between the coils and stator housing, whereby the cooling medium is forced through gaps in between the coils by the blocks.

2. The yokeless electric machine as claimed in claim 1 that is an axial flux machine, said bars being disposed circumferentially spaced around a rotational axis of the rotor and, preferably, parallel thereto, the rotor comprising two stages each having permanent magnets interacting with each end of the bars.

3. The yokeless electric machine as claimed in claim 1 , in which the material of the stator housing is non-magnetic.

4. The yokeless electric machine as claimed in claim 3 , in which the material of the stator housing is electrically non-conducting.

5. The yokeless electric machine as claimed in claim 1 , in which the material of the stator housing, at least in the air gap, is thermally insulating.

6. The yokeless electric machine as claimed in claim 1 , wherein the annular plates including recesses to locate the bars within the chamber.

7. The yokeless electric machine as claimed in claim 6 , in which said annular plates are integral with said cylindrical walls, which cylindrical walls are each split around their circumference and connected together along inner and outer circumferential seams at said splits.

8. The yokeless electric machine as claimed in claim 7 , in which said split is central defining two clamshells.

9. The yokeless electric machine as claimed in claim 8 , in which said clamshells are substantially identical.

10. The yokeless electric machine as claimed in claim 6 , in which said annular plates are separate from said cylindrical walls, and wherein said cylindrical walls are aluminium and said annular plates are plastics material.

11. The yokeless electric machine as claimed claim 6 , in which said annular plates are thinned at the ends of the bar to minimize the gap between the bars and the magnets on the rotor.

12. The yokeless electric machine as claimed claim 6 , in which said cylindrical walls are an inner and outer wall, said outer wall having means to mount the machine and said inner walls comprising means to mount bearings for the rotor.

13. The yokeless electric machine as claimed in claim 12 , in which the rotor stages each comprise an annular dish, whose outer rims mount said permanent magnets and whose inner rims are connected together enclosing said bearings.

14. The yokeless electric machine as claimed in claim 13 , in which the rotor stages complete a magnetic circuit.

15. The yokeless electric machine as claimed in claim 1 , in which the stator housing insulates the magnets from heat generated in said coils.

16. The yokeless electric machine as claimed in claim 1 , in which there are between two and eight transitions of the cooling medium flow between the coils.

17. The yokeless electric machine as claimed in claim 1 in which the chamber and coils are lined with a lacquer or resin coating that insulates electrically the chamber and its contents from direct contact with the cooling medium.

18. A method of constructing the yokeless electric machine as claimed in claim 17 comprising the steps of: assembling the machine; filling the chamber with liquid resin or lacquer to wet all the internal surfaces of the chamber including its contents; removing the resin to leave a coating of the resin on the internal surfaces of the chamber; and curing the resin to form an electrically insulating layer on said internal surfaces.

19. The method as claimed in claim 18 , comprising the step of, prior to removing the resin, evacuating the chamber of air to assist penetration of the resin into small spaces in the chamber.