first of all thanks for the replies.
Second,for mr@awesome academy a buck converter delivers an output voltage that is lower than the input voltage depending on the duty cycle. for a duty cycle varying from 0 to 1 , i will have différent RPMs.
knowing this , i began my design but différent calculations like copper thickness, junction temperature rise,inductor filter size etc were not realistic at all. i began researching and now i am considering a 2 phase synchronous buck converter wich, so far, has offered me better results (calculations). I hope that i cleared some confusion for you , and thanks again .