What seemed strange to me is that i have seen non brushed motors being controlled with dimmer switches. I have never looked into this very closely, but maybe some day i will break out the scope and do some measurements.
What i think might be happening is they might naturally be selecting a fairly high setting as it does not take too low of a setting to get somewhat lower speeds. That might be why it seems to work. If they go too far down it may stall, but again i havent tried this myself in years now.
Back many years ago i looked into controlling a transformer with a triac. The idea was to use a fairly cheap triac to lower the output of a transformer for some rough voltage regulation. If the output filter is right, it could work. But then there is the core of the tranformer which can not stand an asymmetrical current waveform, it should be the same area above zero as below zero. If not, the core could saturate.
All i remember though from those many years ago was that it did in fact work to some degree. I was able to lower the output voltage without blowing anything up.
What i dont remember is how well it worked, like did the transformer get hot. I do remember that i did not test it for that long of a time period, so i do not know if it would have gotten hot or not.
I also took a brief look into inductors with triacs, and it seems that once a certain point is reached the current keeps the triac turned on when it shouldnt be. But gain that was long ago so i'd have to start all over again with this.
If i get some time i'll do that again.
If anyone else has some experience with this perhaps they can add some comments.