Operating Envelopes of the Variable-Flux Machine With Positive Reluctance Torque

Variable-flux interior permanent magnet synchronous motors (VFIPMSMs) find growing attention for electrified transportation applications, especially in the area of electric vehicles. While a prior focus was on optimizing the magnetization requirement to not oversize the inverter, and improving the machine power over a wide operating range, this paper aims to investigate and compare different possible operating envelopes of the VFIPMSM from the drive point of view. Demagnetizing the low-coercive magnets via only a short <inline-formula> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula>-axis current pulse eliminates the need of continuously applying a negative <inline-formula> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula>-axis current in the flux-weakening region; hence, lower copper loss and improved motor efficiency are expected. In this paper, this has been investigated and compared with the utilization of continuous negative <inline-formula> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula>-axis current in the flux-weakening region considering the nonlinear demagnetization characteristics of the low-coercive magnets. The latter scheme has been seen to improve the high-speed output characteristics and to extend the speed range. Although a constant-power–speed range with VFIPMSMs is not feasible due to the irreversible demagnetization of low-coercivity magnets, an improvement of high-speed output power is shown to be feasible via saliency manipulation with the latter scheme. A VFIPMSM with a positive-reluctance torque (inverted saliency <inline-formula> <tex-math notation="LaTeX">$L_{q} < L_{d}$ </tex-math></inline-formula>) is used for experimental validation.