The study explores the process of dynamic balancing of the impeller of an axial fan VО 06-300 (Ukraine) by two automatic ball balancers. The computer simulation of the dynamics of the fan in the absence and presence of automatic balancers has confirmed the qualitative results of a previously conducted field full-scale experiment. Thus, the presence of automatic balancers reduces the following: – the mean square value of the vibration velocity in the segment from the rotor start to the beginning of automatic balancing, – the vibration velocity values at two resonant peaks when the rotor is running down, and – the peaks of the vibration velocities in the section of the start of automatic balancing (74 times in the 3D modelling; 5.4 times in the field experiment). The computer simulation of the dynamics of the axial fan with the «on» and «off» gravity forces has allowed determining the following: – the effect of gravity on the accuracy of balancing the impeller decreases rapidly with increasing the cruising speed of the impeller, – when increasing the forces of viscous resistance to the motion of the balls, the effect of gravity on the accuracy of the rotor balancing increases; – at low speeds of rotation (15 r/s), the impeller can be balanced not better than by accuracy class G 2.5, but at the rated speed of 25 r/s, it is balanced according to accuracy class G 1. Herewith, the residual vibration velocities that are caused only by gravity decrease with increasing the rotor speed. The residual vibration velocities that are caused only by the eccentricities of the raceways increase directly proportionally to the rotor speed. Therefore, fast-turning rotors need a more precise installation of automatic balancers. It is recommended to reduce the eccentricity of the raceway of the automatic balancer at least 2.5 times in relation to the maximum permissible value. Residual vibration velocities in the automatic balancing mode (up to 3 mm/s) on the test fan are mostly caused by gravity. The probable causes of residual vibration velocities are eccentricities of the raceways of the automatic balancers, standstill of the balls (lack of reaction to small unbalances), etc. Therefore, residual vibration velocities can be reduced at the stages of manufacturing and installing automatic balancers into a fan.