sin αst

ROTOR
sidering the output from the rotor, and thus ignoring the efficiency of the generator, the
capacity factor increases to 35%.
6.3.2 Self-starting capability
For the wind turbine to start there must be a starting torque present, which is sufficient to
overcome frictional resistance in the bearings of the generator. Using the BEM theory,
described in appendix B.1, it is not possible to accurately calculate the torque at standstill.
It is however possible to calculate the torque at low rotational speeds and analyse the
tendency [33].
Figure 6.23 shows the rotor torque at low rotational speeds on basis of the rotor design
tool. The underlying calculations are made for a wind speed of 4 m/s, equivalent to the cut-
in wind speed mentioned in section 7.1.
64
Figure 6.23: Rotor torque at low rotational speeds and wind speed of 4 m/s
Extrapolation of the curve on figure 6.23 yields a starting torque of 100 Nmm at 0 rpm,
which is compared to the required starting torque of the bearings, which is 79.1 Nmm (att.
4). From this it is made probable that the wind turbine will self-start, as expected of a
HAWT design.