Abstract
Disclosed is a belt/disc sander with speed adjusting apparatus, comprising a speed adjusting apparatus arranged inside the belt/disc sander, which speed adjusting apparatus being provided for controlling the variation of rotational speed of the motor driving the sand belt or sand disk, and a speed adjusting knob provided outside the belt/disc sander. The speed adjusting apparatus includes a control circuit, a current sensor, a driving circuit, and a switching circuit, wherein the current sensor detects and feeds the current of the motor back to the control circuit. The control circuit may, in response to an input of the speed adjusting knob, control the frequency variation of square wave signals output from driving circuit. The switching circuit transforms the square wave signal into alternate current signal outputs to drive rotation of the motor. Accordingly, a user may adjust the rotational speed of the belt/disc sander by the speed adjusting knob, and the control circuit obtains the current rotational speed of the motor from the detection result of the sensor, and by comparing with the set rotational speed of the speed adjusting knob, decides whether the current rotational speed of the motor matching with the setting of the speed adjusting knob, for modifying the rotational speed of the motor.