The vertical axis wind turbine with straight blades attracts our attention from a point of view that it is not influenced by the wind direction and can be expected to have the high efficiency. However, it is pointed out that the control is difficult because the self start is weak and the rotational speed increases rapidly by the velocity change in the natural wind. It is considered that the transition response characteristics of the wind turbine are an important factor when the most suitable control of the turbine is demanded, because these response characteristics to the wind velocity change are affected by the magnitude of the moment of inertia of the rotor. In this study, the moment of inertia dependency on the transient responses of the wind turbines in the abrupt wind speed change is investigated by wind tunnel tests. A device which can change the moment of inertia of the rotor without changing the shape of the rotor is developed. In the experiments, the moment of inertia of the rotor is changed into 0.1 kgm2, 0.15 kgm2, 0.20 kgm2, 0.25 kgm2, 0.30 kgm2. In order to obtain the time constant, transient response of the rotational speed is compared with a theoretical expression obtained by assuming a first order lag system. The time constant increases lineally as the moment of inertia increases.
