Effect of B(H) curve extrapolation at high saturation levels on calculation of no-load operating conditions of IPM motors

The design of Interior Permanent Magnet (IPM) motors requires a careful sizing of the rotor bridges, with a balanced trade-off between the following conflicting targets: • containment of the leakage flux in the bridges, with respect to the flux produced by the permanent magnets (PMs), thus limiting the width of the bridges around the minimum mechanical sizing; • achievement of adequate mechanical robustness of the rotor, by adopting suited width of the bridges, to sustain the centrifugal forces on pole shoes and PMs, occurring at maximum speed. Frequently, the magnetization curves of commercial lamination data sheets are given for B data up to Bmax.test = 1.8 T, assuming that usually the design magnetic operating conditions of the main core branches do not exceed that limit. However, the accurate estimation of rotor bridges leakage requires to extend the B(H) data to significantly higher B levels, because the bridges, that behave as magnetic short-circuits, are deeply saturated: in absence of test data, this requires extending the B(H) curve of data sheets beyond Bmax.test; however, the extrapolation is a non-univocal procedure. The proposed paper develops a model of an IPM V-shape motor in no-load conditions, based on a magnetic circuit. Adopting the same extended B(H) curve, the magnetic circuit results and the results of FEM simulations are compared among them, showing a substantial agreement. Then, the magnetic model is used to analyze how the adoption of differently extended B(H) curves affects the values of the bridge leakage flux and of the air-gap flux.