Effects of Continuous Rotor Skewing in Additively Manufactured Permanent Magnet Rotors

This paper describes the effects of skewing the rotor of a permanent magnet synchronous machine (PMSM) with buried magnets by torsion of the soft-magnetic rotor active part and maintaining the magnet pockets axially straight. The impact of this approach on the mechanical and the electromagnetic design of the rotor active part is presented. In the end, the approach leads to a reduced cogging torque with simultaneously decreased leakage flux. Using conventional manufacturing technologies, e.g. punching or laser cutting, skewing the laminated soft-magnetic rotor active part and maintaining the magnet pockets axial straight would lead to a variety of lamination cross sections. The presented approach can hence be realized usefully by means of Additive Manufacturing (AM) technologies. Therefore, a short review of metal-additive manufacturing in the field of electric machines and the common practice of skewing in PMSMs is given in a first step. After this, a PMSM with bar-shaped buried magnets was chosen to serve as reference. 3D electromagnetic finite element analyses (FEA) of the non-skewed reference machine, of a conventionally step-skewed rotor and of the new continuously skewed rotor concept are performed. Continuously twisting the rotor active part around the magnet pockets leads to the need of adjusting the stray paths flanking the magnet pockets. The trade-off between minimizing leakage flux and maximizing mechanical strength is pointed out and the outcome additional mechanical stress FEA is shown. The impact of the stray paths and the skewing angle on the rotor air-gap flux density distribution is investigated in detail which leads to the evaluation of the resulting cogging torque of the different rotor concepts. Finally, guidelines for designing continuously skewed PM rotors using the possibilities of AM technologies can be derived.