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An improved analytical model for predicting the rotor eddy current loss in brushless machines equipped with surface-mounted permanent magnets is presented. It is formulated in polar co-ordinates and based on the calculation of the two-dimensional electromagnetic field in the airgap/magnet regions, with due account of the eddy current reaction field. It enables the eddy current loss in the permanent magnets and the retaining sleeve, if fitted, to be calculated, and caters for motors having either overlapping or non-overlapping stator windings, as well as any specified load condition. The analysis accounts for both time and space mmf harmonics, but neglects the influence of stator slotting. The model is applied to a brushless DC traction machine in which the rotor loss is due predominantly to time harmonics in the armature reaction field which result from commutation events. The predicted rotor loss is compared with the loss deduced from thermometric measurements and from an analytical magnetostatic model which neglects the eddy current reaction field. Good agreement between predictions and measurements is achieved over the complete operating speed range.
Date of Publication: 7 Nov. 2004