Investigation of the Torque Ripple in 6-pole Synchronous Reluctance Motors with Asymmetrical Rotor Structures

Abstract:
For the use of transversally laminated synchronous reluctance motors (SynRM) in applications with relatively large shaft outer diameters, like in motor spindles, the use of pole counts above four can be reasonable. Since for the use in motor spindles the torque ripple should not exceed certain limits in this paper the reduction of the torque ripple of a 6-pole SynRM by the use of an asymmetric rotor geometry is discussed to avoid an expensive skewing of the motor. Therefore in a first step, analytic considerations on the positioning of the flux barriers in two possible asymmetric rotor geometries for a 6-pole rotor are carried out. In a second step, example geometries are analyzed in a finite-element (FE) model and compared to a fully optimized symmetric design with a relative torque ripple of 8:1% in the considered operation point. The two considered asymmetric rotor geometries are defined by an alternating arrangement around the rotor circumference of two, respectively, three pole geometries with different flux barrier positions. As a result, it is shown that it is possible to reduce the torque ripple significantly only by variation of the flux barrier positioning in the different pole geometries of the asymmetric designs. In a FE analysis, starting from a symmetric design with a very high relative torque ripple of 53:3% to get a manufacturable rotor geometry for all tested asymmetric designs, a reduction to a value of 2:9% for the rotor with two, respectively, 1:9% for the rotor with three pole geometries is possible. But it is also shown that there are some limitations and disadvantages compared to the fully optimized symmetric geometry, especially for the rotor geometry with three different pole geometries.