In a general interior permanent magnet motor, as many magnet insertion holes as the number of poles are formed in advance in a circumferential direction at approximately regular intervals through a rotor core formed by laminating and fixing a plurality of magnetic steel sheets punched into a predetermined shape, and one permanent magnet, which has an approximately rectangular cross-sectional shape when viewed in an axial direction, is inserted into each magnet insertion hole. There is a thin portion between both ends of the magnet insertion hole and an outer circumferential surface of the rotor core. This is because a portion between both the ends of the magnet insertion hole and the outer circumferential surface of the rotor core serves as a path for a so-called leakage magnetic flux, which is a magnetic flux flowing out of a surface of the permanent magnet and into another surface of the permanent magnet without passing through a stator core and contributing to an output, and thus this portion is often designed to have a smallest thickness required for securing strength.
However, in the rotor core having such a thin portion, a portion on a radially outer side and a portion on a radially inner side of the magnet insertion hole are connected to each other via the thin portion. Thus, when an upper limit on the number of revolutions of a motor is raised, the strength of the rotor core is required to be improved because the magnitude of a centrifugal force acting on a rotor is proportional to the square of the number of revolutions.
To handle such a situation, for example, in Patent Literature 1, there is disclosed a structure in which the magnet insertion hole of one magnetic pole is divided into a plurality of holes in a circumferential direction (thus, a permanent magnet of one magnetic pole is also divided into a plurality of permanent magnets in a circumferential direction), and a bridge configured to connect a portion on a radially outer side and a portion on a radially inner side is formed in that divided position, to thereby intend to achieve improvement of the strength of the rotor core against the centrifugal force. Further, in Patent Literature 1, there is also disclosed forming an arc portion at a corner portion formed at a root of the bridge in consideration of the fact that stress is concentrated on the corner portion formed at the root of the bridge.
Further, in Patent Literature 2, there is disclosed forming the bridge itself into a curved shape similarly in consideration of the fact that stress is concentrated on the corner portion formed at the root of the bridge.