US Patent 7425785 Permanent magnet motor

In a permanent magnet motor consisting of a rotor 1, which is supported on a rotating shaft 2 and has twelve permanent magnets 3 arranged at fixed intervals in the circumferential direction in the peripheral edge portion thereof, and a stator 4, which is disposed around the rotor 1 with a gap being provided with respect to the rotor 1 and has nine magnetic poles 4a, each having a coil wound thereon, arranged at fixed intervals in the circumferential direction so as to face towards the outer peripheral surface of the rotor 1, a value obtained by dividing the radius of the rotor 1 by the radius of curvature of a permanent magnet surface facing towards the stator is set at a value such as to decrease cogging torque.