Variable leakage flux (VLF) IPMSMs for reduced losses over a driving cycle while maintaining the feasibility of high frequency injection-based rotor position self-sensing

This paper presents variable leakage flux (VLF) IPMSM rotor designs which use intentionally created leakage flux paths in order to reduce losses over a wide torque-speed operating range and produce a high peak torque. Above base speed, the high leakage flux within the rotor reduces the stator flux linkage, thus reducing both iron and copper losses. Below base speed, as the load current i_q increases, the magnet leakage flux decreases, and the magnet flux crossing the air gap (oriented in the d-axis) increases such that the machine can have a high peak torque. The effect of this intentional cross-coupling of d- and q-axis flux paths on high-frequency injection based position self-sensing capability, which is used at low speed conditions (including zero speed) is examined. A proof-of-principle machine is tested to experimentally evaluate the capability of obtaining i_q dependent variable leakage flux characteristics. This machine´s saliency angular offset for use in high frequency injection based position self-sensing is also analyzed with FEA and evaluated with experiments. A new leakage flux path structure with side-loops in the rotor is then proposed. This paper presents a suitable selection of the dimensions of this side-loop structure to balance the tradeoff between the capabilities to obtain a large variation in leakage flux along with good position self-sensing characteristics. The effect of the VLF properties on total losses over a driving cycle is then evaluated. The mechanical integrity of the side-loop rotor structure at the maximum speed under consideration is examined. Flux weakening machine designs with VLF properties (FW-VLF-IPMSM) are also analyzed to assess the loss reduction capability of VLF machines compared to conventional FW-IPM machines.