Speed control accuracy and robustness of sensorless drives based on non-salient PMSMs in the low-speed region are critical issues due to the low amplitude of back-EMF. Although open-loop control is commonly adopted up to a sufficiently high speed, it results in low accuracy and poor robustness against load torque and inertia variations or random initial rotor position conditions. Thus, extension of the closed-loop operating range towards zero is strongly desired. An integrated approach based on low-frequency current injection technique and a more general speed control stabilization technique based on reactive current superimposition is shown in this paper, allowing both initial position detection and closed-loop speed control at low-speed. Analytical development, simulation and experimental investigation results will be reported, considering an actual sensorless industrial drive system.
