Speed-Control for Small Asynchronous Induction Motor

On the circuit diagram below show us a schematic which is used to set the speed of a low-power induction motor, such as those which can be found in fan applications.

Noise coming from TRIAC commutations can be filtered using capacitors C1, C2 and C4. C2 has to be from X2 technology, whereas C1 and C4 capacitors have to be from Y2 technology. Here’s the circuit diagram:

Synchronization across the TRIAC

[Source: STMicroelectronics Application Note]