Bearingless Induction Motor Design with the DPNV Winding and Reduced Axial Length Pole-Specific Rotor

This paper investigates the design of bearingless induction motor that uses a dual pur- pose no voltage (DPNV) winding and pole-specific rotor. The investigation is motivated by a previous study that showed that by integrating these two technologies into a bearing- less induction motor, a high performance machine can be created for industrial compressor applications (over 95% effciency, over 15 kNm=m3 torque density, and less than 2.5% cur- rent for supporting the rotor weight). The use of parallel pole-specific rotor significantly reduces the rotor axial length, improving rotor dynamics. However, implementing the DPNV winding is diffcult because it results in an asymmetric winding for two preferred pole combinations of the induction machine. This paper investigates these asymmetries and the challenges that they pose for the bearingless machine performance. Complex winding factors are introduced as a tool to explore the physics behind each design in terms of sub-harmonics and winding asymmetry. Transient FEA is used to obtain steady state suspension force vectors as a function of suspension current phase angle for five selected optimal designs. The results are visualized as polar plot, where relevant performance measures are proposed. It is found that a symmetric winding (with or without sub-harmonics) has a circular force trajectory, while asymmetric winding causes an undesired elliptical force trajectory that degrades the machines force rating. Moreover, it is found that asymmetric sub-harmonics alone can also lead to this elliptical force trajectory. It is revealed by further studying the force error angle trajectory that the force error angle is generally a function of suspension current phase angle, which has implications for the optimization of these machines. In conclusion, this paper shows that the asymmetric DPNV winding can result in a high performance design, which enables higher performance pole combinations for bearingless induction motors.