Patent ID: 7222044

Claim:
A rotating speed detecting apparatus, comprising: a bearing sensor configured to detect rotation of a motor and output detection pulses whose frequency depending on a rotating speed of said motor; and a calculating unit configured to receive said detection pulses from said bearing sensor, wherein said detection pulses include a first pulse and a second pulse output next to said first pulse, and said calculating unit calculates a revolution of said motor from a pulse interval between said first pulse and said second pulse, wherein said calculating unit classifies rotation status of said motor into a plurality of stages, said plurality of stages include: a low-speed stage in which said calculating unit fixes said revolution to a predetermined value or less; and a mid- to high-speed stage in which said calculating unit calculates said revolution based on said pulse interval, wherein when said second pulse is not output during a first overflow time after said first pulse is output in said mid- to high-speed stage, said calculating unit changes said rotation status from said mid- to high-speed stage to said low-speed stage.