How can I slow down an 230v electrical fan motor??? Answered

I expect the cheapest way to fix this is to just buy a smaller, more quiet, fan.

Induction motors, typically, are not easy to drive over a wide range of speeds, not without adding hardware, like a variable frequency driver, that would be more expensive than the motor itself.

In contrast, other kinds of fans, like the fans they put in desktop computers,

https://en.wikipedia.org/wiki/Computer_fan

https://en.wikipedia.org/wiki/Computer_fan_control

These are cheap, ubiquitous, and easy to drive over a wide range of speeds. The only disadvantage of these little computer fans, is that they are small.

I mean if you really do need something like 40 watts of air moving power, it would be huge number of little computer fans, since the typical power consumption for one of these little fans, like the square-shaped, 80 mm, or 120 mm sized ones, is around 1 or 2 watts each.

But they are easy to drive, over a wide range of speed. A typical computer fan is powered by a DC voltage source, in the range from 0 to 12 VDC, and the speed at which it turns is roughly proportional to the voltage, or the time-averaged voltage if using PWM.