Kabushiki Kaisha Toshiba

Shigeya Tanimoto

Toshihiko Futami

Mayumi So

In a permanent magnet motor, a stator includes an annular stator core and a plurality of stator windings wound on the stator core so that a plurality of phases are composed. A rotor is rotatably mounted with a predetermined air gap between it and an inner peripheral face of the stator core. The rotor includes a rotor core having a plurality of slots and a plurality of permanent magnets embedded in the slots respectively. Each permanent magnet having a generally arc-shaped cross section is embedded in the slot so that its convex portion is directed toward a rotor axis. Each permanent magnet is magnetized so that directions of easy magnetization are concentrated on a point C at a side of center of curvature thereof. The relation between a distance L between the point C and a center line passing the center of each permanent magnet in the direction of the thickness thereof and a mean radius of curvature R of the permanent magnets is shown by the expression, 1.0.times.R.ltoreq.L.ltoreq.7.5.times.R.

Permanent magnet motor

A position detector detects the position of a rotor of a brushless motor. A driving signal generator generates a driving signal at a timing corresponding to the detected position. While the driving signal is received, a driving circuit sequentially ON/OFF-operates switching elements of an inverter circuit. At this time, a rotational speed detector detects a rotational speed of the brushless motor. A rotational speed controller controls the duty of ON/OFF operations of the switching elements, performed by the driving circuit, through a chopper signal generator in accordance with the difference between a detected rotational speed and a designated rotational speed. If the detected rotational speed does not reach the designated rotational speed even when the duty becomes a preset value or more, the rotational speed controller generates a phase shift signal for advancing the phase of the driving signal from that of a driving signal in a normal operation, and supplies it to the driving signal generator. Upon this phase shift operation, in the brushless motor, a magnetic flux effectively acting from the rotor to armature windings is reduced as compared with a magnetic flux in a normal operation. Thus, the rotational speed of the brushless motor can be reliably set to a desired value without decreasing the operation efficiency.

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Chita, Japan

Shigeya Tanimoto