Slowing down cooker hood fan

slow down a fan in a cooker hood to reduce noise.

an induction motor speed can only be changed by altering the frequency.

higher powered AC machines need a ‘proper’ speed controller to vary the frequency, else they could be overloaded
domestic fans (often the shaded-pole type of induction motor) have such a high resistance winding that their stalled current is little different from their normal FLC.
Therefore, the speed control for such a fixed load as a fan impeller is easily achieved by limiting the supply voltage without fiddling around with the frequency.
Cheap fan controllers, like light dimmers, can cause buzzing from the fan
Some dimmer switches may cause the fan to get on fire