Therefore, when "220V AC 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 "220V AC Cooling Fan" blade stall occurs simultaneously, this phenomenon be continued, so that caused the stall clogging zone along with the impeller rotating in the opposite direction to promote, which produce so-called "rotating stall" phenomenon. Fan into unstable conditions district run, "220V AC Cooling Fan" will produce a number of rotating stall area to the impeller. Each time through the blade stall area would be the role of a exciting force, thereby allowing the blade to resonate.