Forming a rotating stall: "220V Cooling Fan" impeller structure, dimensions are in accordance with the rated air flow design, when the normal air flow of work, the gas enters the impeller in the direction of air flow blade inlet installation angle consistent gas can smoothly enter impeller, when the gas flow rate into the impeller is less than the rated flow, the radial velocity of the gas into the impeller is reduced, "220V Cooling Fan" gas flow into the direction of the relative angular speed of the impeller is reduced, and thus the blade inlet angle is not installed consistent. Fan blades during machining and installation, due to various reasons can not have a blade shape and the same mounting angle. Thus, changes in the operating conditions when leaving the flow direction deviates, in "220V Cooling Fan" of each blade inlet angle of attack can not be identical.