Rotor, dynamo-electric machine having the rotor and rotor manufacturing method

1. A rotor comprising:

a rotatable shaft that is rotatable about a rotational axis of the rotatable shaft;

a rotor core that includes a boss, which is fixed to the rotatable shaft, and a plurality of projections, which radially outwardly project from the boss and are spaced from each other in a circumferential direction, wherein each of the plurality of projections is formed as a soft magnetic pole and has a radially outer surface, which includes a circumferential center portion that is located radially outward of at least one of two circumferential end portions of the radially outer surface of the projection;

a plurality of permanent magnets, each of which is formed as a magnet pole and is circumferentially placed between corresponding adjacent two of the plurality of projections, wherein each of the plurality of permanent magnets has a radially outer surface that is an arcuate surface, which extends arcuately in the circumferential direction; and

a cover that is configured into a tubular form and is fitted to the radially outer surface of each of the plurality of projections and the radially outer surface of each of the plurality of permanent magnets, wherein:

the circumferential center portion of the radially outer surface of each of the plurality of projections contacts a radially inner surface of the cover; and

the at least one of the two circumferential end portions of the radially outer surface of each of the plurality of projections is radially inwardly spaced from the radially inner surface of the cover.

2. The rotor according to claim 1 , wherein a ratio of a surface area of a contacting portion of the radially outer surface of each of the plurality of projections, which contacts the cover, relative to a total surface area of the radially outer surface of the projection is in a range of 50% to 80%.

3. The rotor according to claim 1 , wherein the radially outer surface of each of the plurality of projections is an arcuate surface that extends arcuately in the circumferential direction.

4. The rotor according to claim 3 , wherein:

a radius of curvature of the radially outer surface of each of the plurality of projections is smaller than a radius of curvature of the radially outer surface of each of the plurality of permanent magnets; and

a center of curvature of the radially outer surface of each of the plurality of projections is further spaced from the rotational axis of the rotatable shaft in comparison to a center of curvature of the radially outer surface of each of the plurality of permanent magnets.

5. The rotor according to claim 1 , wherein a radial wall thickness of each of the plurality of permanent magnets is progressively reduced from a circumferential center portion of the radially outer surface of the permanent magnet toward each of two circumferential ends of the radially outer surface of the permanent magnet.

6. The rotor according to claim 1 , wherein a circumferential extent of each of the plurality of permanent magnets is equal to or larger than a circumferential extent of each of the plurality of projections.

7. The rotor according to claim 1 , wherein the radially outer surfaces of all of the plurality of permanent magnets have an identical polarity.

8. The rotor according to claim 1 , wherein:

the rotor core includes a plurality of steel plates, which are stacked one after another in a direction of the rotational axis of the rotatable shaft; and

a radial wall thickness of the cover is equal to or smaller than a plate thickness of each of the plurality of steel plates, which is measure in a direction perpendicular to a plane of the steel plate.

9. The rotor according to claim 1 , wherein a circumferential extent of a contacting area between the radially outer surface of each of the plurality of projections and the radially inner surface of the cover is smaller than a circumferential extent of a contacting area between the radially outer surface of each of the plurality of permanent magnets and the radially inner surface of the cover.

10. A dynamo-electric machine comprising:

the rotor of claim 1 ; and

a stator that is placed radially outward of the rotor and is configured to generate a magnetic field that is rotatable about the rotational axis of the rotatable shaft.

11. A rotor manufacturing method, comprising:

forming a rotor core that includes a boss, which is configured to be fixed to a rotatable shaft, and a plurality of projections, which radially outwardly project from the boss and are spaced from each other in a circumferential direction, wherein each of the plurality of projections is formed as a soft magnetic pole and has a radially outer surface, which includes a circumferential center portion that is located radially outward of at least one of two circumferential end portions of the radially outer surface of the projection;

installing a plurality of permanent magnets to the rotor core such that each of the plurality of permanent magnets is formed as a magnet pole and is circumferentially placed between corresponding adjacent two of the plurality of projections, wherein each of the plurality of permanent magnets has a radially outer surface that is an arcuate surface, which extends arcuately in the circumferential direction; and

fitting a cover, which is configured into a tubular form, to the radially outer surface of each of the plurality of projections and the radially outer surface of each of the plurality of permanent magnets such that the circumferential center portion of the radially outer surface of each of the plurality of projections contacts a radially inner surface of the cover, and the at least one of the two circumferential end portions of the radially outer surface of each of the plurality of projections is radially inwardly spaced from the radially inner surface of the cover.

12. The rotor manufacturing method according to claim 11 , wherein the forming of the rotor core includes forming each of the plurality of projections such that a ratio of a surface area of a contacting portion of the radially outer surface of each of the plurality of projections, which contacts the cover, relative to a total surface area of the radially outer surface of the projection is in a range of 50% to 80% after the fitting of the cover.

13. The rotor manufacturing method according to claim 11 , wherein the forming of the rotor core includes forming the radially outer surface of each of the plurality of projections as an arcuate surface that extends arcuately in the circumferential direction.

14. The rotor manufacturing method according to claim 13 , wherein the forming of the radially outer surface of each of the plurality of projections includes forming the radially outer surface of each of the plurality of projections such that a radius of curvature of the radially outer surface of each of the plurality of projections is smaller than a radius of curvature of the radially outer surface of each of the plurality of permanent magnets, and a radial distance between a center of curvature of the radially outer surface of each of the plurality of projections and the rotational axis of the rotatable shaft is larger than a radial distance between a center of curvature of the radially outer surface of each of the plurality of permanent magnets and the rotational axis of the rotatable shaft.