Field-weakening in interior PM machines is performed by using negative rf-axis current to oppose the magnet flux and hence reduce the total fundamental rf-axis flux at high speeds. Due to the rotor saliency, the rf-axis current produces large amplitude harmonic airgap flux density components which can cause large harmonic stator iron losses. Reducing the magnet flux density reduces the required rf-axis stator mmf to oppose it and hence reduces the field-weakening iron losses. However reducing the magnet flux density also reduces the magnet torque and hence increases the stator current requirement, particularly at lower speeds. This paper examines this trade-off for an automotive integrated starter/alternator using a graphical interior PM machine parameter plane approach.
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