3-D Effects of Rotor Step-Skews in Permanent Magnet-Assisted Synchronous Reluctance Machines

Abstract

Permanent magnet assisted synchronous reluctance machines (PMA SynRM), stemming from the combination of synchronous reluctance and interior PM machine topologies, have recently gained increasing popularity as a viable candidate for electric vehicle traction applications due to concerns regarding the price volatility of rare-earth magnet materials. Their attractiveness can be attributed to their distinct torque production mechanism, which essentially relies on the reluctance torque component enabling the use of less expensive ferrites or bonded NdFeB magnet solutions. A drawback often associated with this machine topology is high torque ripple emanating from its intrinsically high salient nature [1]. Among several machine design techniques available to mitigate the high torque ripple, rotor step-skew is known to be one of the most effective and frequently incorporated solutions. When a step-skew is performed, it causes variation of magnetic flux density along the axial (skew) direction, and thus influences the machine's electromagnetic behavior [2]. Numerous methods are available in literature to account for the effects of skew using (either) 2D or 3D numerical finite element analysis (FEA), analytical, or semi-analytical approaches, the trade-off in each case being computational time versus accuracy. Due to the prohibitive computational time and numerical memory costs of 3D FEA, the prediction of a skewed machine's electromagnetic performance typically relies on 2D domain-based techniques, such as multi-slice 2D FEA [3] or more computationally efficient semi-analytical techniques [2] exhibiting comparable accuracy. A common limitation associated with these techniques is their inability to capture certain 3D phenomena, namely the end-effects and axial interaction between skewed steps, since they explicitly rely on the 2D domain [4]. The significance of such 3D effects is influenced by the axial length to airgap diameter ratio (viz. end-effect), the number of skewed steps, the skew angle and the structure of the machine topology under consideration. The quantification, and most importantly experimental verification, of the 3D effects associated with skewed PMA SynRMs has received limited attention in literature to date.