1. Field of the Invention
The invention relates to a rotor and a motor including the rotor.
2. Discussion of Background
There are conventional motors that include a so-called embedded magnet-type rotor in which permanent magnets are embedded and fixed in a rotor core. In a motor including the embedded magnet type rotor, reluctance torque is produced in addition to magnet torque produced by the permanent magnets. Therefore, this motor has an advantage of being able to produce higher torque than a motor that includes a so-called surface magnet type rotor in which permanent magnets are fixed to a surface of a rotor core.
There is known an embedded magnet type rotor that uses, for example, V-shaped permanent magnets that protrude toward a rotating shaft-side (e.g., Japanese Patent Application Publication No. 2011-214086 (JP 2011-214086 A)). In the rotor described in JP 2011-214086 A, the surface area of the permanent magnets can be made large, as compared to a rotor in which flat plate-shaped permanent magnets are disposed to be orthogonal to radial directions (e.g., US2011/0148246 A1). Therefore, in the rotor described in JP 2011-214086 A, it is possible to secure a sufficient number of magnetic fluxes even if permanent magnets whose maximum energy product is small are used.
If permanent magnets of a rotor are subjected to a strong external magnetic field whose direction is opposite to the magnetization direction of the permanent magnets of the rotor, for example, in the case where a coil of a stator is supplied with overcurrent, there is a possibility that a part of the permanent magnets may be magnetized in the opposite direction and demagnetization (irreversible demagnetization) may occur. In the rotor including V-shaped permanent magnets as in JP 2011-214086 A, two circumferential end portions of each permanent magnet are located more radially outward than its circumferential center portion. Therefore, there is a problem that the two circumferential end portions (radially outer portions) of each permanent magnet are likely to be demagnetized.
This problem is not limited to the rotors including the V-shaped permanent magnets, and may similarly occur in any rotor as long as the permanent magnets have magnetic pole-facing portions that radially extend. For example, the problem may occur in a rotor including U-shaped permanent magnets, or a rotor including permanent magnets each of which is formed of a pair of permanent magnet pieces arranged in a V shape (e.g., Japanese Patent Application Publication No. 2012-23804 (JP 2012-23804 A)).