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An electric motor including a stator with a plurality of coils wound around a ring-shaped iron core, and a rotor rotatably disposed inside the stator is provided. The rotor includes a rotor shaft and a yoke. The yoke is provided near its periphery with a plurality of plate-like permanent magnets arr

An electric motor including a stator with a plurality of coils wound around a ring-shaped iron core, and a rotor rotatably disposed inside the stator is provided. The rotor includes a rotor shaft and a yoke. The yoke is provided near its periphery with a plurality of plate-like permanent magnets arranged at equal pitches. Heat generated from the rotor is released through a plurality of air holes formed in the yoke between the rotor shaft and the permanent magnets.

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What is claimed is: 1. An electric motor comprising a stator with a plurality of coils wound around a ring-shaped iron core; and a rotor rotatably disposed inside the stator, the rotor comprising a rotor shaft, a yoke integrally mounted on the rotor shaft, a plurality of magnet holes formed at equa

What is claimed is: 1. An electric motor comprising a stator with a plurality of coils wound around a ring-shaped iron core; and a rotor rotatably disposed inside the stator, the rotor comprising a rotor shaft, a yoke integrally mounted on the rotor shaft, a plurality of magnet holes formed at equal pitches near a periphery of the yoke, and a plurality of plate-like permanent magnets disposed in respective ones of the magnet holes, wherein the yoke has a plurality of air holes, equal in number to the number of permanent magnets and provided between the rotor shaft and the permanent magnets, for accelerating heat release of the rotor; and wherein with a center of the rotor shaft as C, one of each two adjacent permanent magnets as M1, a point of intersection of a perpendicular line-drawn from the center C to the permanent magnet M 1 and the periphery of the yoke as P1, a permanent magnet adjacent to the permanent magnet M1 as M2, a point of intersection of a perpendicular line drawn from the center C to the permanent magnet M2 and the periphery of the yoke as P2, and a point of intersection where a radial line passing through the center C and a midpoint between the adjacent permanent magnets M1, M2 intersects with the periphery of the yoke as Q, when a circle of a radius r passing through the intersection point P1 and the intersection point P2 is drawn around the intersection point Q, the air holes lie outside of the circle in symmetric relation relative to respective ones of the perpendicular lines. 2. An electric motor according to claim 1; wherein the air holes each have a contour having an angled portion having an apex that points toward a mid-portion of a respective one of the permanent magnets. 3. An electric motor according to claim 2; wherein the apex of each air hole contour lies on a respective one of the perpendicular lines. 4. An electric motor comprising: a stator with a plurality of coils wound around a ring-shaped iron core; and a rotor rotatably disposed inside the stator, the rotor comprising a rotor shaft, a yoke integrally mounted on the rotor shaft, a plurality of magnet holes formed at equal pitches near a periphery of the yoke, and a plurality of plate-like permanent magnets disposed in respective ones of the magnet holes; wherein the yoke has a plurality of air holes, two associated with each permanent magnet and provided between the rotor shaft and the permanent magnets, for accelerating heat release of the rotor; and wherein with a center of the rotor shaft as C, one of each two adjacent permanent magnets as M1, a point of intersection of a perpendicular line drawn from the center C to the permanent magnet M 1 and the periphery of the yoke as P1, a permanent magnet adjacent to the permanent magnet M1 as M2, a point of intersection of a perpendicular line drawn from the center C to the permanent magnet M2 and the periphery of the yoke as P2, and a point of intersection where a radial line passing through the center C and a midpoint between the adjacent permanent magnets M1, M2 intersects with the periphery of the yoke as Q, when a circle of a radius r passing through the intersection point P1 and/or the intersection point P2 is drawn around the intersection point Q, the air holes lie outside of the circle with the two air holes associated with each permanent magnet in symmetric relation relative to the permanent magnet. 5. An electric motor according to claim 4; wherein the two air holes associated with each permanent magnet each have a contour having an angled portion having an apex that points toward a mid-portion of the permanent magnet. 6. An electric motor according to claim 5; wherein the two air holes associated with each permanent magnet are disposed on opposite sides of a respective one of the perpendicular lines. 7. An electric motor comprising: a stator having a ring-shaped core on which are wound a plurality of coils; and a rotor rotatably disposed within the ring-shaped core, the rotor comprising a rotor shaft, a yoke connected to the rotor shaft, and a plurality of plate-like permanent magnets connected to the yoke and circumferentially spaced from one another about a peripheral portion of the yoke, the permanent magnets each having opposed faces interconnected by sides and being disposed in side-by-side equidistantly-spaced relation so that an inward face of each permanent magnet faces the rotor shaft, the yoke having a plurality of air holes circumferentially spaced from one another in a region between the rotor shaft and the permanent magnets, the air holes each having a contour which includes an angled portion having an apex that points toward a mid-portion of one of the inward faces of the permanent magnets. 8. An electric motor according to claim 7; wherein the number of air holes is the same as the number of permanent magnets, and the apexes of the air hole contours lie on respective imaginary lines that pass through the center of the rotor shaft and are perpendicular to the inward faces of respective ones of the permanent magnets. 9. An electric motor according to claim 7; wherein there are two air holes associated with each permanent magnet, and the apexes of the air hole contours of the two air holes lie on opposite sides of an imaginary line that passes through the center of the rotor shaft and is perpendicular to the inward face of the associated permanent magnet.