Electric machine rotor sleeve

1. An electric machine comprising:

a stator;

a rotor radially inward of the stator and comprising a plurality of surface permanent magnets;

a rotor sleeve surrounding the rotor, the rotor sleeve comprising:

an inner layer having a radial thickness between 0.1 mm and 1 mm; and

an outer layer composed of a plurality of layers of fibre reinforced matrix material including:

a first layer of carbon fibre reinforced polymer comprising carbon fibres oriented substantially 90° to a rotational axis,

a second layer of fibres having a lower modulus of elasticity than that of the carbon fibres and being oriented between 50° and 75° in either one of a clockwise direction and an anti-clockwise direction relative to the rotational axis, and

a third layer of the fibres having the lower modulus of elasticity and being oriented between 50° and 75° in the other one of the clockwise direction and the anti-clockwise direction relative to the rotational axis,

the first layer, the second layer, and the third layer being stacked in this order,

the outer layer having a radial thickness between 3 mm and 15 mm, wherein:

a ratio of a radius of the rotor sleeve to a radial thickness of the rotor sleeve including the inner and outer layers is between 20 and 40; and

the electric machine has a maximum rotor rotational speed greater than 15000 RPM.

2. The rotor sleeve according to claim 1 , wherein the plurality of layers comprises a plurality of layers of carbon fibre reinforced polymer, each layer of the carbon fibre reinforced polymer comprising fibres oriented substantially 90° to the rotational axis.

3. The rotor sleeve according to claim 1 , wherein the rotor sleeve comprises at least another layer of carbon fibre reinforced polymer, and the second and third layers of fibres having the lower modulus of elasticity are provided between the first layer and the another layer of carbon fibre reinforced polymer.

4. The rotor sleeve according to claim 1 , wherein the lower modulus of elasticity fibres in the second layer are oriented between 50° and 75° in the clockwise direction relative to the rotational axis, and the lower modulus of elasticity fibres in the third layer are oriented between 50° and 75° in the anti-clockwise direction relative to the rotational axis.

5. The rotor sleeve according to claim 2 , wherein the carbon fibres of each carbon fibre reinforced polymer layer are oriented between 89° and 90° to the rotational axis.

6. The rotor sleeve according to claim 2 , wherein one or more layer of carbon fibre reinforced polymer defines a radial thickness of no more than 2 mm.

7. The rotor sleeve according to claim 3 , the sleeve comprises between 5 and 20% low modulus of elasticity fibre reinforced composite.

8. The rotor sleeve according to claim 1 , wherein the lower modulus of elasticity fibres in the second and third layers comprise any of glass fibre, aramid, ultra-high molecular weight polyethylene (such as Dyneema™) and poly (p-phenylene-2,6-benzobisoxazole) (PBO).

9. The rotor sleeve according to claim 1 , wherein the rotor sleeve comprises the inner layer of fibre reinforced matrix material provided radially inwardly of the outer layer of fibre reinforced matrix material, the inner layer comprising a first fibre material having a modulus of elasticity lower than the fibre material of the outer layer.

10. The rotor sleeve according to claim 9 , wherein the inner and/or outer layer of fibre reinforced matrix materials comprise pre-stressed fibres, and the fibres of the outer layer are pre-stressed to a stress of greater than 1000 MPa.

11. The rotor sleeve according to claim 9 , wherein the inner layer of fibre reinforced matrix materials comprise any of glass fibre, aramid, PBO, E-glass, S-glass, nylon, or Dyneema, or a mixture of one of more of these materials.

12. The rotor sleeve according to claim 9 , wherein the outer layer comprises carbon fibre or ceramic fibre.

13. The rotor sleeve according to claim 9 , wherein the outer layer comprises ply angles of approximately 90° relative to the rotational axis of an electric machine rotor when installed.

14. The rotor sleeve according to claim 5 , wherein the carbon fibres of each carbon fibre reinforced polymer layer are oriented between 89.2° and 89.9° to the rotational axis.

15. The rotor sleeve according to claim 6 , wherein the radial thickness of the one or more layer of carbon fibre reinforced polymer is between 1 and 2 mm.

16. The rotor sleeve according to claim 9 , wherein the inner and/or outer layer of fibre reinforced matrix materials comprise pre-stressed fibres, and the fibres of the outer layer are pre-stressed to a stress of between 500 and 1500 MPA.

17. The rotor sleeve according to claim 1 , wherein the radial thickness of the inner layer is approximately 0.5 mm.