Minimising rotor losses in high-speed high-power permanent magnet synchronous generators with rectifier load

In an early stage of the design of a high-speed 1400 kW synchronous generator with permanent magnet excitation and loaded by a rectifier, it became apparent that rotor losses are a major problem. The stator currents cause asynchronous components in the air-gap field. Analysis shows that a modified polyphase system reduces the number of these components. An approximate solution for the rotor losses caused by the asynchronous field components has been derived. The formulae show the effects of machine dimensions and harmonics and the effect of a conducting shield in the rotor. The main purpose of the study is to have a tool for making an early choice among several stator winding configurations. A modified nine-phase system, combined with a shield around the permanent magnet rotor, is a prospective option.