This triac based AC motor speed controller circuit is designed for controlling the speed of AC motors like drill machines, fans, vacuums, (2000W) etc. The speed of the motor can be controlled by changing the setting of potentiometer. The setting of potentiometer determines the phase of the trigger pulse that fires the triac. The circuit incorporates a self-stabilizing technique that maintains the speed of the motor even when it is loaded.
When the motor of the drill machine is slowed down by the resistance of the drilled object, the counter-EMF of the motor also decreases. This results to a voltage increase in resistance and capacitance value causing the triac to be triggered earlier and the speed increases accordingly.