Rotor for axial gap-type permanent magnetic rotating machine

1. An axial gap-type permanent magnetic rotating machine comprising a rotor comprising a rotating shaft having an axis of rotation, a rotor yoke of disc shape radially extending from the shaft, and a plurality of permanent magnet segments arranged on a surface of the rotor yoke and circumferentially about the axis of rotation such that each permanent magnet segment have a magnetization direction parallel to the axis of rotation, and a stator having a plurality of coils arranged circumferentially about the axis of rotation, the rotor and the stator being disposed to define an axial gap therebetween, wherein

each of said permanent magnet segments is an assembly of two or more divided permanent magnet pieces divided along parallel to a direction of the axis of rotation,

each of the divided permanent magnet pieces has a magnetization direction parallel to the axis of rotation, and a coercive force at a surface region and

a central region of the divided permanent magnet piece, and

in each of the divided permanent magnet pieces, the coercive force in the surface region of the magnet piece is higher than that in the central region of the magnet piece.

2. The axial gap-type permanent magnetic rotating machine of claim 1 wherein the magnet pieces are of a sintered Nd base rare earth magnet.

3. The axial gap-type permanent magnetic rotating machine of claim 2 wherein each piece of sintered Nd base rare earth magnet has a coercive force profile from the surface region toward the central region, which is created by letting Dy or Tb diffuse from the surface toward the interior of the magnet piece.

4. The axial gap-type permanent magnetic rotating machine of claim 3 wherein the step of letting Dy or Tb diffuse from the surface toward the interior of the sintered Nd base rare earth magnet piece includes applying a Dy or Tb oxide powder, a Dy or Tb fluoride powder or a Dy or Tb-containing alloy powder to surfaces of the magnet piece, then holding the magnet piece at a high temperature sufficient to diffuse Dy or Tb.

5. The axial gap-type permanent magnetic rotating machine of claim 2 wherein each piece of sintered Nd base rare earth magnet has a coercive force profile from the surface region toward the central region, which is created by letting Dy or Tb diffuse from the surface toward the interior of the magnet piece mainly via grain boundaries.

6. The axial gap-type permanent magnetic rotating machine of claim 1 wherein

said each of the divided permanent magnet pieces has heat resistance at a surface region and a central region of the divided permanent magnet piece, and

in each of the divided permanent magnet pieces, the heat resistance in the surface region of the magnet piece is higher than that in the central region of the magnet piece.

7. The axial gap-type permanent magnetic rotating machine of claim 1 wherein each of said permanent magnet segments is an assembly of 4 to 50 pieces.

8. The axial gap-type permanent magnetic rotating machine of claim 1 wherein the divided permanent magnet pieces are stacked.

9. The axial gap-type permanent magnetic rotating machine of claim 1 wherein each of said permanent magnet segments is an assembly of 2 to 50 pieces, and the divided permanent magnet pieces are bonded with an adhesive to form the assembly.

10. The axial gap-type permanent magnetic rotating machine of claim 1 wherein

the coils are circumferentially spaced apart as viewed in an axial direction, and

the circumference where the coils are arranged is concentric with the circumference where the permanent magnet segments are arranged on the rotor yoke.

11. The axial gap-type permanent magnetic rotating machine of claim 1 wherein the coils are disposed on the stator separately from the rotating shaft.

12. The axial gap-type permanent magnetic rotating machine of claim 1 wherein the coercive force in whole of the surface region of the magnet piece is higher than that in the central region of the magnet piece.