Coil excited pseudo direct drive electrical machines

A Pseudo Direct Drive (PDD) equipped with a coil excited high-speed (HS) rotor is presented. An analytical model which predicts the flux density in the air spaces and permanent magnets (PM) of the PDD is presented, and it is shown, that a good agreement exists between the analytical and finite element (FE) predictions. Furthermore, the model is employed to investigate the effects of the key design parameters on the performance of a coil excited PDD for a 10MW wind turbine application and an optimised design is proposed. It is shown that shear stress in excess of 100kPa can be achieved, and that compared to a PM excited PDD a reduction in PM mass can be realized. It is also shown that the efficiency over the operating range of the wind turbine can be maximized by adopting an appropriate control strategy of the HS rotor excitation currents.