A fan includes a casing having a bottom wall and a shaft mounted inside the casing
for rotatably supporting a rotary blade assembly comprised of blades inside the
casing and above the bottom wall with the blade assembly being axially movable
with respect to the shaft. A levitation device includes magnets mounted to the
blades of the blade assembly and a magnetic field generator mounted in the bottom
wall and opposing the magnets of the blades. The magnet field generator is controlled
by a control circuit to selectively generate a magnetic field that interacts with
the magnets of the blades to induce expelling force acting upon the blade assembly
to axially move the blade assembly a predetermined distance away from the bottom
wall. The magnitude of the expelling force can be changed by controlling the magnetic
field generator whereby the distance between the blades and the bottom wall can
be adjusted.
