"Micro Cooling Fan" impeller structure, dimensions are in accordance with the rated air flow design, when the cooling fan in normal air volume of work, the gas enters the impeller direction and amount of wind blade inlet installation angle consistent with a gas can smoothly into the impeller, when into the "Micro Cooling Fan" gas flow impeller is less than the rated flow, the radial velocity of the gas into the impeller is reduced, the gas flow into the direction of the relative angular speed of the impeller is reduced, and thus the installation angle of the blade inlet is not consistent. Leaves "Micro Cooling Fan" in the processing and the fan installation process, due to various reasons can not have a blade shape and the same mounting angle. Therefore, when the operating conditions change the direction of flow leaving the deviation occurs in the angle of attack of each blade can not import the same.