Sensorless quasi-standstill and very low-speed position detection in non-salient PMSMs based on current injection and back-EMF observer

Accuracy and robustness of sensorless speed control of non-salient PMSMs in the low-speed region and at start-up is a critical issue, due to the low amplitude of the back-EMF which prevents estimation and then closed-loop control in the zero speed range. This problem is usually overcome by adopting an open-loop control (e.g. constant amplitude rotating current space vector), which is normally able to start the motor up to an enough high speed, where sensorless closed-loop control is possible. Open-loop startup is however not robust against load torque and inertia variations and random initial rotor position conditions. Also the extension of the closed-loop operating range towards zero is strongly desired. Standard techniques relying on magnetic saliency are not applicable (i.e. no relevant anisotropy is present in the considered machine), and some methods based on the detection of torque production by signal injection can be adopted. The methods proposed and studied in this paper belongs to this last class. Two different application cases will be considered: an initial high-accuracy position detection at quasi-standstill and a position and speed tracking for closed-loop sensorless speed control. A complete theoretical analysis is reported to demonstrate the features of the proposal and highlight the dependence on certain design parameters. Then extensive simulation and experimental investigations based on an industrial drive system are included to prove the feasibility of the method and validity of the theoretical analysis.