Formation of "24V DC Cooling Fan" rotating stall: the fan impeller, the size is in accordance with the rated air flow (CFM) designed, when the normal air flow (CFM) working gas into the impeller direction of the wind blade inlet install angle consistent, "24V DC Cooling Fan" gas may smoothly into the impeller when the flow rate of gas into the impeller is less than the rated flow rate of the gas into the impeller is reduced radially, relative velocity of the gas flow enters the impeller in a direction to reduce the angle of small, and thus "24V DC Cooling Fan" and blade inlet angle is not consistent with the installation. The fan blades in the processing and installation, due to various reasons can not have a blade shape and the same mounting angle.