## Turbine data: Generator |

You have a choice of different modes in which to control the generator: **specified rotation** or **specified torque**.

If you choose the **specified rotation** mode, you impose the rotation of the generator, about the turbine's $z$-axis, in the direction of the rotation sense, relative to the turbine. You can do this either by setting the generator angular velocity, either to a constant value or by supplying and selecting an external function which calculates the whole rotational motion (i.e. the generator angle and its first two derivatives).

Note: | If the imposed motion consistent with solver option is checked (implicit integration scheme only), then only the generator angle is needed. The returned derivatives will be ignored and instead will be determined by the solver. |

A guide to using external functions to model turbine controllers is available at https://github.com/Orcina-Ltd/turbine-controllers. This includes example Python controllers and an example of using external functions to wrap a Bladed style control DLL.

In the constant value case, to avoid a discontinuity between zero angular velocity in the static state and the specified value, the generator shaft rotation is ramped during the build-up period.

In the **specified torque** mode, a moment is applied to the generator shaft, in the direction of the rotation sense. Again, you may set this torque to be a constant value or to be calculated by an external function which you define. In this mode, the generator shaft rotation is a calculated degree of freedom of the OrcaFlex model.

Note: | For a turbine, with anticlockwise rotation sense, the sign convention for generator rotation/torque, for which a positive value defines an anticlockwise rotation/ torque about the turbine's $z$-axis, is contrary to the usual OrcaFlex convention. |

The initial value for rotor angle, in the direction of the rotation sense. The rotor angle is defined as the angle of the blade 1 root $z$-axis, projected onto the turbine $xy$-plane, relative to the turbine $x$-axis and about the turbine $z$-axis.

Note: | For a turbine, with anticlockwise rotation sense, the sign convention for initial rotor angle, for which a positive value defines an anticlockwise rotation about the turbine's $z$-axis, is contrary to the usual OrcaFlex convention. |

The number of turns the generator shaft makes when the main rotor shaft turns once. If a negative ratio is specified, the generator shaft and main rotor shaft will rotate in opposite directions.

The mass moment of inertia about the generator shaft.