I. Introduction
The permanent magnet (PM) machines exhibit several key features, such as high torque density and high efficiency over switched reluctance machines and induction machines [1]. However, due to limited resources and unstable price of the rare-earth PM materials, the magnet-less or even no PM machines are becoming more and more desirable as alternative solutions [2]. One of them is synchronous reluctance (SynRM) machine, in which the stator can be exactly the same as the induction machine and the rotor is simple and robust only formed by steel laminations [3]. Hence, there is no copper loss in the rotor compared with induction machine and lower vibration and noise than that of switched reluctance machine. This machine can be dated back to 1923 and are still the subject of research recently due to the rise of cost of rare-earth PM material. The rotor has various topologies in order to increase the saliency ratio. In terms of lamination orientation, the commonly employed two topologies are shown in Fig. 1(a) and (b), viz. axially laminated rotor and transversally laminated rotor [4]. The large saliency ratio can be achieved by employing axially laminated rotor structure to have high power density and high power factor. However, this rotor structure has some disadvantages over transversally laminated rotor structure, in terms of mechanical stress and manufacturing complexity. Therefore, the transversally laminated rotor will be employed for investigation in the paper. Typical rotor structures of synRM machine.