Considerations on torque and power factor characteristics of synchronous reluctance machines based on their magnetic energy properties

This paper describes the torque and power factor characteristics of SynRMs (synchronous reluctance motors) considering their magnetic energy and co-energy properties. Since the inductance of SynRM varies drastically due to the magnetic saturation effect, it is not easy for us to understand their characteristics, MTPA (maximum torque per ampere) points, and maximum power factor points, quantitatively applying the frame based on equivalent circuit models. In this study, we focused on the fact that the model constructed by the magnetic energy and co-energy gives more simple expressions to the torque and power factor compared to the equivalent circuit model. A SynRM that consists of a stator, same as a benchmark model (D-model) provided by IEEJ, and a rotor with 3 layer flux-barrier was simulated by the electromagnetic field analysis, and its magnetic energy and co-energy were calculated. As a result, three states (I, II and III) were defined depending on degrees of the magnetic saturation level. Then, the output torque per square ampere becomes maximum on a boundary region between state II and III. Further, the current phase of the MTPA point moves to the leading phase side with increasing the current magnitude on state III and the phase of the maximum power factor point is larger than that of the MTPA point. It was clarified that the model constructed by the magnetic energy and co-energy are suitable for discussions on the SynRM characteristics taking account of the magnetic saturation. © 2019 The Institute of Electrical Engineers of Japan.