This type of device is simple in construction and has a high capacity to compensate automatically for the unbalance of a rotor above the critical speed. Below the critical speed, however, the displacement of balls introduces an unbalance, and a violent self-excited vibration takes place in the neighbourhood of the critical speed. To cope with these problems, this paper deals with the theoretical and experimental investigations on the static balance and the transient response at a constant acceleration of an automatic balancer using three or more balls placed in the race which is partitioned into equal sectors. It is shown that partitioning the race is very effective against the unbalance and the self-excited vibration. It is found that, when the acceleration of the rotor surpasses a certain value, the effect of decreasing its maximum amplitude near the critical speed is reduced rapidly.