Large direct drive generators: the induction machine as an alternative to permanent magnet machines

Abstract

Design of direct drive power take off in renewable energy generators tends to focus on force density, naturally leading to the use of relatively expensive permanent magnet machines. Elsewhere in the industrial landscape, induction machines traditionally rule as they are low cost and robust. In this study, the design of a generator for use in an open centred tidal turbine is presented as a case study to directly compare induction and permanent magnet machines. Mechanical constraints in the turbine design enforce a large magnetic gap in the generator, which inherently limits the power density and power factor. A comparison between the permanent magnet design and the induction design in terms of performance and economy has been presented.