The present invention relates to a synchronous type permanent magnet rotating machine that can be used as a motor, a generator, or the like and, in particular, relates to an axial gap type rotating machine in which rotors and a stator face each other in a direction along the rotating shaft.
Permanent magnet rotating machines are classified into a radial gap type and an axial gap type based on their structures. In the radial gap type, a plurality of permanent magnets having radial magnetization directions are arranged in a circumferential direction of a cylindrical rotor and a stator is disposed along the outer peripheral side or the inner peripheral side of the rotor so as to face the permanent magnets. Generally, the stator has a structure in which coils are wound on an iron core having a plurality of teeth in its surface facing the rotor. By the use of the iron core, the magnetic flux from the poles of the rotor can efficiently cross the coils so as to produce a large torque in the case of a motor and a large voltage in the case of a generator.
On the other hand, in the axial gap type, disc-shaped rotors are mounted on a rotating shaft and a stator is disposed to face the rotors in a direction along the rotating shaft and, as in the radial gap type, a plurality of permanent magnets are attached to the rotors and a plurality of coils are attached to the stator so as to face the permanent magnets (JP 2009-33946 A, JP 2010-200518 A). Differently from the radial gap type, the axial gap type is configured such that the stator is sandwiched between the two rotors, and therefore, the area ratio of the permanent magnets occupied in the entire rotating machine can be made large so that, for example, it is possible to improve the characteristics such as the torque density of the rotating machine. In view of this advantage, the axial gap type rotating machine is applied, for example, to a driving motor of a hybrid vehicle or an electric car, for which the high output performance is required, or to a compressor of an air conditioner, a refrigerator, a freezer, a showcase, or the like.
Under the current circumstances, however, the radial gap type is more common. One of reasons for this includes that the axial gap type is complicated in structure and difficult in assembly.
As shown in FIG. 14, a general axial gap type rotating machine 100 comprises, in a housing 109, a rotating shaft 101 rotatably supported by the housing 109 via bearings 110, two rotors 102a and 102b each having permanent magnets 104 and a rotor core 103, and a stator 108 having coils 106 and a stator core 107, and in some cases, has a structure in which the two rotors 102a and 102b are mounted on the rotating shaft 101 with a spacer 111 interposed therebetween and the stator 108 is disposed in a gap between the two rotors 102a and 102b. 
In the case of a motor, by supplying three-phase sinusoidal currents to the coils fixed into the stator core, the rotors are rotated by a reluctance torque in addition to a magnetic torque due to attraction and repulsion forces between the coils and the permanent magnets. In the case of a generator, the induced voltage is generated by forcibly rotating the rotors from the outside so as to allow magnetic flux generated by the permanent magnets to cross the coils.
As shown in FIG. 15, the rotors 102a and 102b used in the general axial gap type rotating machine 100 are each configured such that the permanent magnets 104 are arranged on the rotor core 103 at regular intervals along the circumferential direction and that the adjacent permanent magnets 104 have opposite polarities in a direction along the rotating shaft. The two rotors 102a and 102b having such configuration are disposed so that the permanent magnets 104 of the one rotor 102a and the permanent magnets 104 of the other rotor 102b face each other in such a way as to have opposite polarities, thereby generating magnetic forces 112. The mutual distance (interval) between the permanent magnets 104 arranged on each rotor core 103 should be narrow and the magnets having opposite polarities should be adjacent to each other. In order to simplify the assembly process, post-magnetization is attempted in which unmagnetized magnets are first bonded to the rotor core 103 and then are magnetized.