This paper analyzes the impact of the numbers of stator slots and rotor layers on the optimal design of synchronous reluctance (SyR) machines. Eighteen SyR machine examples have been designed by means of a multi-objective optimization algorithm and finite element analysis so to maximize torque and minimize torque ripple. Twelve, twenty-four and forty-eight slot stators are considered, associated to rotors with fourpoles and one to six flux barriers per pole. The results of the comparative analysis show that high numbers of slots and layers are beneficial for maximizing the torque and the power factor, and that torque ripple and iron loss minimization require precise matches between the slots and the layers, which are not necessarily the same for the two purposes. Finally, for some slot/layer combinations the optimization algorithm produces nonconventional barrier distributions, very promising is some cases. A fast finite element evaluation is used for the evaluation of thousands of candidate machines during the optimization, whereas an accurate transient with motion finite element analysis stage is used for the off-line characterization of the final designs.
An important barrier to the adoption and acceptance of synchronous reluctance (SyR) machines in different applications lies in their non-standardized design procedure. The conflicting requirements incurring at high speeds among electromagnetic torque and structural and thermal limitations can significantly influence the machine performance, leading to a real design challenge. Analytical models used for design purpose lack in accuracy and force the designer to heavily rely on finite element analysis (FEA), at least during the design refinement stage. This becomes even more computationally expensive as the speed increases, as the evaluation of the rotor structural behaviour is required. This work presents a computationally efficient hybrid analytical-FE design process able to consider all the main limiting design aspects of SyR machine incurring at high speed, namely structural and thermal. As a vessel to investigate the proposed design routine accuracy, several high speed SyR machines have been designed for a wide range of operational speeds (up to 70krpm). The thermal and mechanical factors limiting the high speed operation are deeply analyzed aiming at maximize the mechanical output power. The proposed design approach is then validated by comparison against experimental measurements on a 5kW-50krpm SyR prototype. Index Terms-Analytical design, finite element analysis, high speed, iron bridges, iron losses, structural rotor design, synchronous reluctance machines.
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