Analysis of slotting models for the calculation of no-load rotor losses in PM machines

The magnetic flux density distribution in the airgap of electric machines is essential for accurate prediction of no-load eddy-current power losses. The effect of slotting can be modelled using a simplified single-slot model or a complete multiple-slots model. Until now there has not been a clear and justified criterion to choose between the two models. In this paper we propose a criterion based on the conformal transformations used to calculate the magnetic field distribution of single-slot and multiple-slots models. The computational implementation of both methods produced a graph that clearly shows which method to use as a function of the normalised geometrical parameters of the machine. The paper presents a case study of a high speed machine whose proportions fall within the multiple-slot model range, according to the criterion proposed. It is shown that using a single slot model in this case results in significant errors in the estimation of rotor losses. Better agreement with FEA results is achieved when a multiple-slot model is used.