Because of the blades curvature, the blades experience less drag force (Fconvex) when moving against the wind than the blades
when moving with the wind (Fconcave). Hence, the half cylinder with concave side facing the wind will experience more drag
force than the other cylinder, thus forcing the rotor to rotate. The differential drag causes the Savonius turbine to spin. For
this reason, Savonius turbines extract much less of the wind's power than other similarly sized lift type turbines because
much of the power that might be captured has used up pushing the convex half, so savonius wind turbine has a lower
efficiency.
Similarly, the three blade savonius wind turbine is constructed from three half cylinders, they are arranged at (120o
) relative
to each other as shown in figure (2).