Motor Speed Control triac schematic

Double-time-constant schematic diagram for Incandescent Light Dimming, Heat Control,and Motor Speed Control

The schematic shown is a doubletime-constant circuit which has improved performance.This circuit uses an additional RC network to extend the phase angle so that the Triac can be triggered at small conduction angles. The additional RC network also minimizes any hysteresis effect.

Source: Power Thyristor application note, Littelfuse