Thanks for your answers. The motor is 1-Phase AC... So I don't know if "universal" also applies, not too sure on the terminology... Anyway, I've read online that a transformer is the best way to control these Fan/Vent motors, since phase chopping causes hum and potentially other problems.
How do you think PWM compares to switching taps via relais in terms of power dissipation? The controller will sit in a weatherproof box (no airflow). I'd assume rectifying then switching 230V*sqrt2 DC would be dissipating more power than magnetic losses + contact resistance. But I could very well be wrong (read below quote
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Looking at off-the-shelf solutions (ebay), there are some cheaper fan controls, but those are likely phase controllers, and I'd still have to hack in a PID for temp control. (The fan will be part of a homemade AC unit, but I don't want it to be too loud). So with a transformer, some relais, µC and temp probe, I should still be cheaper off than buying fan control with temperature feedback.
Am I looking in the wrong category?
Yes, Dummy! Turns out there are transformers for (but nor limited to) this very application, called autotransformers. (Spartransformator, for anyone speaking German having the same problem...)
The very first google result for "Spartranformator Stufen" (Autotransformer taps in English) is a datasheet for these things stating these are specifically for controlling fans/vents.
Don't they (farnell/digikey) stock such transformers?
I found some, most being over-spec (read: too expensive) for the job. Ebay has some for around 40€. Not as cheap as I imagined, but still cheap enough...