Speed control of the motor using TRIAC:
This basic phase triggering circuit uses the triac in series with the motor across an AC sinusoidal supply. The variable resistor, VR1 is used to control the amount of phase shift on the gate of the triac, which in turn controls the amount of voltage applied to the motor by turning it ON at different instants during the AC cycle. The triac’s triggering voltage is derived from the VR1 – C1 combination via the Diac (The diac is a bidirectional semiconductor device that helps to provide a sharp trigger current pulse to fully turn-ON the triac). At the start of each cycle, C1 charges up via the variable resistor, VR1. This continues until the voltage across C1 is sufficient to trigger the diac into conduction which in turn allows capacitor, C1 to discharge into the gate of the triac turning it “ON”. Once the triac is triggered into conduction and saturates, it effectively shorts out the gate triggering phase control circuit connected in parallel across it. Therefore, there is no control over the conduction of triac for the remainder of the halfcycle. At the end of this half-cycle, the triac current falls to zero and triac is commutated naturally. In the next half-cycle, the VR1 – C1 triggering process starts again. However, because the triac requires different amounts of gate current in each switching mode of operation, for example Ι+ and ΙΙΙ–, a triac is therefore asymmetrical meaning that it may not trigger at the exact same point for each positive and negative half cycle.