Analysis of Externally Excited Synchronous Motors Operation Under Joule Losses Minimization Control

The interest in rare earth-free traction motors is significantly increasing due to cost and sourcing challenges related to rare earth hard ferromagnetic materials. Among rare earth-free solutions, the externally excited synchronous motor (EESM) is currently the most studied machine topology in automotive. In this paper, an analytical solution for a Joule losses minimization control strategy for EESMs is presented. The model is based on the assumption of linear inductances and constant stator and rotor resistances, and aims at identifying the optimal current references for each speed and torque request. The operating range of the motor is divided in different regions, depending on whether the stator current, rotor current or voltage result constrained or not. While analytical solutions for Joule losses minimization have already been presented in literature, in this work the model is developed with a higher level of generality, considering both the case in which the optimal control zone at partial load is constrained by rotor excitation current limit and the case in which it is limited by stator current. In addition, also the case in which the maximum rotor current is not sufficient to enter unity power factor operation at full load is considered. A separate finite element analysis is also proposed to evaluate the effects of magnetic saturation on the optimal control strategy. The aim of the work is to provide an insight and a clear interpretation of the EESM operation once controlled with a losses minimization strategy, possibly suggesting a control-oriented design approach of such machines.