I asked this question before. Those ah pek repairing electrical hardware dunno the concept behind. Those Master in Electronic student never study before. Finally I got a not so confirm answer from my aircon friend, he said fan speed is controlled by varying the frequency.
Now come the next question..
If a fan speed have 1, 2 and 3 selection, 3 being the fastest.
Is the power consumption heavier on speed 3 than on speed 1?
Nobody knows yet