Wide-Speed-Range Sensorless Control of Surface-Mounted PMSMs with Compensated I/F Startup, Adaptive Transition, and Variable-Step Flux Observation

Sensorless control of surface-mounted permanent magnet synchronous motors (SPMSMs) over a wide speed range is challenged by weak low-speed back electromotive force, limited transient performance of fixed-gain flux observers, and discontinuities during the transition between startup and observer-based control. This study proposes an integrated sensorless control framework combining a current- and frequency-compensated I/F startup strategy, a load-adaptive current-amplitude transition, and a variable-step nonlinear flux observer. The observer adaptively adjusts its gain according to the evolution of the objective function to balance convergence speed and estimation stability, while load-based current compensation and active-power-based frequency compensation improve low-speed load adaptability. The transition gain is further adjusted according to load conditions to coordinate current reduction and control-mode switching. Experiments on an SPMSM drive validate zero-speed startup, I/F-to-closed-loop transition, forward/reverse operation, speed variations up to 2000 rpm, and load disturbances. Under a load step from 0.4 to 0.6 N·m, conventional I/F control lost synchronism, whereas the proposed strategy maintained stable operation. Under medium-to-high-speed load disturbances, the variable-step observer consistently reduced transient speed- and rotor-position-estimation deviations compared with the conventional observer. The results demonstrate coordinated sensorless operation and improved dynamic performance over the tested wide speed range.