Patent ID: 6147615
Filing Date: 2000-11-14
Classification: H02H

Abstract:
A speed detecting device for a direct current motor, comprising:a coupling circuit adapted to be connected electrically to a power input line of the motor and to generate a current-coupled pulse output that corresponds to current flowing through the power input line;a one-shot circuit connected electrically to said coupling circuit so as to receive the current-coupled pulse output therefrom, said one-shot circuit being triggered by the current-coupled pulse output to generate a pulse train signal in a form of a series of constant-width pulses, where spacing between adjacent ones of the constant-width pulses varies according to instantaneous operating speed of the motor;a converter circuit connected electrically to said one-shot circuit so as to receive the pulse train signal therefrom, said converter circuit generating a test signal having a magnitude that varies in accordance with the spacing between the adjacent ones of the constant-width pulses to provide an indication of a detected average operating speed of the motor;a comparator connected electrically to said converter circuit so as to receive the test signal therefrom, said comparator comparing the test signal with a predetermined reference signal that corresponds to a preset minimum operating speed of the motor, and generating an alarm activating signal when the test signal is less than the reference signal; andan alarm device connected electrically to said comparator for generating an alarm output upon receiving the alarm activating signal from said comparator.