Permanent magnet rotor

1. A permanent magnet rotor for an electrical machine having a stator and a rotor and an air gap arranged between an outer circumference of the rotor and the stator, the rotor comprising:

a rotor body configured for rotating around a central rotational axis, and further comprising

principal permanent magnets that are substantially rectangular in an axial cross-section and are embedded in the rotor body and are arranged inclined with respect to a radial plane extending through a center of the principal magnets and such as to have a circumferential magnetic orientation, wherein

the principal magnets have an inner end and an outer end, the outer end of the principal magnet being arranged in the proximity of the air gap of the outer circumference of the rotor, and outer bridges of magnetically conductive material being formed between the outer ends of the principal magnets and the outer circumference of the rotor, wherein

auxiliary magnets are arranged near the outer ends of the principal magnets to magnetically saturate the outer bridges, and wherein

the principal magnets are made of materials having superior magnetic characteristics than the auxiliary magnets.

2. The permanent magnet rotor according to claim 1 , wherein outer cooling channels are provided near the outer ends of the principal magnets.

3. The permanent magnet rotor according to claim 2 , wherein the outer cooling channels are provided between the outer ends of the principal magnets and the outer bridges.

4. The permanent magnet rotor according to claim 1 , wherein inner bridges of magnetically conductive material are formed between the inner ends of the principal magnets and portions of non-magnetic material, and wherein

the auxiliary magnets are arranged near the inner ends of the principal magnets to magnetically saturate the inner bridges.

5. The permanent magnet rotor according to claim 4 , wherein inner cooling channels are provided near the inner ends of the principal magnets.

6. The permanent magnet rotor according to claim 5 , wherein the inner cooling channels are provided between the inner ends of the principal magnets and the portions of non-magnetic material.

7. The permanent magnet rotor according to claim 1 , wherein the auxiliary magnets are smaller than the principal magnets.

8. The permanent magnet rotor according to claim 1 , wherein the auxiliary magnets are made of a cheaper different material than the principal magnets.

9. The permanent magnet rotor according to claim 1 , wherein the auxiliary magnets have a circumferential magnetic orientation.

10. The permanent magnet rotor according to claim 1 , wherein the auxiliary magnets have a radial magnetic orientation.

11. The permanent magnet rotor according to claim 8 , wherein the auxiliary magnets are made of ferrite.

12. The permanent magnet rotor according to claim 1 , wherein the rotor body comprises a rim and a plurality of permanent magnet modules arranged on an outer or inner circumference of the rim,

each of the permanent magnet modules extending generally along an axial direction and being of substantially constant axial-cross section, the permanent magnet modules comprising

a base adapted to be fixed to the rotor rim, and

one or more axial rows of the permanent magnets and rows of the auxiliary magnets, and one or more pole pieces.

13. The permanent magnet rotor according to claim 12 , wherein the rim is made of a substantially non-magnetic material.

14. The permanent magnet rotor according to claim 12 , further comprising a plurality of anchors for fixing the permanent magnet modules to the rotor rim, wherein

the permanent magnet modules comprise a base having a bottom surface adapted to be fit to an inner or outer circumference of the rotor rim, and two axially extending side surfaces, and

the two side surfaces of the base of the permanent magnet modules each comprise an axially extending groove, and wherein

the anchors have a shape that fits in axially extending grooves of neighbouring permanent magnet modules.

15. The permanent magnet rotor according to claim 14 , wherein the anchors are made of a substantially non-magnetic material.

16. The permanent magnet generator according to claim 12 , wherein each of the permanent magnet modules comprises two axial rows of principle and auxiliary magnets.

17. The permanent magnet generator according to claim 12 , wherein the base and pole pieces are formed by a stack of metal sheets.

18. A direct drive wind turbine comprising a generator having the rotor according to claim 1 .

19. A permanent magnet rotor for an electrical machine having a stator and a rotor and an air gap arranged between the outer circumference of the rotor and the stator, the rotor comprising:

a rotor body configured for rotating around a central rotational axis and further comprising

principal permanent magnets that are substantially rectangular in an axial cross-section and are embedded in the rotor body and are arranged inclined with respect to a radial plane extending through a center of the principal magnets and such as to have a circumferential magnetic orientation, wherein

the principal magnets have an inner end and an outer end, the outer end of the principal magnet being arranged in the proximity of the air gap of the outer circumference of the rotor, outer bridges of magnetically conductive material being formed between the outer ends of the principal magnets and the outer circumference of the rotor, and inner bridges of magnetically conductive material are formed between the inner ends of the principal magnets and portions of non-magnetic material, and wherein

auxiliary magnets are arranged near the inner ends of the principal magnets to magnetically saturate the inner bridges and

auxiliary magnets are arranged near the outer ends of the principal magnets to magnetically saturate the outer bridges.

20. The permanent magnet rotor of claim 19 , wherein the auxiliary magnets are made from a cheaper material than the principal magnets.