When "Small Cooling Fan" operating conditions deviate the flow direction changes resulting in each blade angle of attack can not be imported identical. If a blade angle of attack at the entrance reaches a critical value, the first place on the blade stall, but not all "Small Cooling Fan" blade stall occurs simultaneously, this phenomenon be continued, so that caused the stall clogged area along with the impeller rotating in the opposite direction to promote, which produce the so-called "rotating stall" phenomenon. "Small Cooling Fan" into the zone run unstable conditions will produce a number of rotating stall area to the impeller.