Patent ID: 7646123

Claim:
A vehicle alternator for generating electric power that is driven by an engine through a driving belt, comprising: a pulley part joined to the engine through the driving belt and directly receiving a driving force supplied from the engine; a field magnet part located in separation from the pulley part in the vehicle alternator; and a rotation speed changing part located between the pulley part and the field magnet part, joining the pulley part with the field magnet part, and configured to convert the rotation speed between the pulley part and the field magnet part and reduce a speed changing ratio “S” according to increasing the rotation speed of the engine, where the speed changing ratio “S” is obtained by dividing the rotation speed of the field magnet part by the rotation speed of the pulley part, wherein the rotation speed changing part is comprised of a sun gear as a front part of a field magnet rotating shaft in the pulley part side of the field magnet part, planetary gears rotatably engaging with the sun gear, an internal gear rotatably engaging with an outside of the planetary gears, and an electric motor as a driving mechanism configured to drive the internal gear, and the number of revolution of the electric motor is controlled by an external control unit, and the speed changing ratio “S” is controlled by the external control unit through the electric motor, the rotation speed of the electric motor is changed when the external control unit changes the speed changing ratio “S”, and the rotation speed of the internal gear is controlled by changing the rotation speed of the electric motor, and the speed changing ratio “S” is decreased according to increasing the rotation speed of the engine so that the rotation speed of the internal gear is decreased when the rotation speed of the engine is low, and increasing the rotation speed of the field magnet part is suppressed during increasing the rotation speed of the engine by increasing the rotation speed of the internal gear according to increasing the rotation speed of the engine.