Patent ID: 6949006

Claim:
A belt-disc sander having a speed adjuster for controlling the rotation speed of a motor of said belt-disc sander, said speed adjuster comprising: a drive circuit for generating a sinusoidal pulse-width-modulation (SPWM) signal; a converter coupled with said drive circuit for receiving said SPWM signal and generating a rotating speed control signal for driving said motor; a current sensor coupled with said motor for feedback of the current of said motor; a speed adjust knob for generating a speed adjust signal; and a control circuit coupled between said drive circuit and said speed adjust knob for controlling the rotating speed of said motor; whereby said control circuit controls a frequency variation of said SPWM signal generated by said drive circuit in response to said speed adjust signal of said speed adjust knob, said rotating speed control signal output by said converter is adjusted according to the frequency variation of said SPWM signal for adjusting the rotating speed of said motor, and said control circuit determines whether the rotating speed of said motor conforms to that set by said speed adjust knob, thereby correcting the frequency variation of said SPWM signal generated by said drive circuit.