If the angle of attack "220V Cooling Fan" at the entrance of a blade reaches a critical value, the first place on the blade stall, but not all of the blades simultaneously stall occurs, 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. Fan into unstable conditions district run, will produce a number of rotating stall area to the impeller. "220V Cooling Fan" blade after each time zone would be the role of a stall exciting force, thereby allowing the blade to resonate. In this case, the dynamic stress increases blade may cause blade breakage, so from the absolute reference frame outside the impeller, the stall area or rotated in the direction of rotation of the impeller, causing the mechanism of rotating stall "220V Cooling Fan", rotating stall pressure fluctuations generated by the impeller rotor excitation occurs is abnormal vibration excitation force, the excitation force size and the size of the gas molecular weight, they are proportional to the relationship, the greater the molecular weight, the greater the incentive force.