Analytical prediction of rotor eddy current loss in brushless machines equipped with surface-mounted permanent magnets, Part I. Magnetostatic field model

An analytical technique for predicting the rotor eddy current loss in brushless motors equipped with surface-mounted permanent magnets is described. The analytical model, formulated in polar coordinates and based on the calculation of the two-dimensional magnetostatic field in the airgap/magnet regions, enables the eddy current loss to be calculated in the permanent magnets and the retaining sleeve, if fitted, of motors having either radial or parallel magnetised magnets and either overlapping or nonoverlapping stator windings, and can cater for any specified load condition. The technique is applied to a brushless DC traction machine, in which the rotor loss is due predominantly to time harmonics in the stator armature reaction field which resulted from commutation events, and loss predictions are compared with thermometric-based measurements. Good agreement is achieved at low speeds. However, a significant discrepancy exists at high speeds, due to the neglect of the eddy current reaction field. This is accounted for in a refined analytical model described in part II.