Start by trying to block off the supply partially somehow and you will see that the amperage will decrease.
That is correct block off either the supply or return it doesnt matter. Fan motors are designed to do work, if they are not loaded they will not build sufficient speed to cool themselves.
Ever take a blower out of a replacement furnace and try and run it?
Sometimes they will overheat etc, block off the supply they build speed and drop amperage and run fine.