Eddy-current losses in the segmented surface-mounted magnets of a PM machine

A gas-turbine driven, high-speed, high-efficiency generator system intended for use in series-hybrid vehicles is developed. It consists of a permanent-magnet generator with surface-mounted magnets and a six-pulse controlled rectifier. The stator currents of the rectifier-loaded generator contain time harmonics which cause eddy-current losses in the magnets. These losses can be so high that they result in demagnetisation of the magnets. To reduce these losses, the magnets may be segmented. The aim of the paper is to model the eddy-current losses in such segmented magnets. This is done by incorporating magnet loss resistances in the equivalent circuits of the permanent-magnet machine. The model is verified by means of locked-rotor tests. To show the usefulness of the model, the losses in the magnets of a rectifier-loaded permanent-magnet machme are calculated. The eddycurrent losses in the magnets can be decreased by increasing the number of magnet segments: these losses are proportional to the square of the magnet width.