Accuracy and robustness in sensorless speed control of non-salient PMSMs at low-speed and standstill is a critical issue, due to the low amplitude of the back-EMF. The usually adopted open-loop control does not provide robustness against load torque, inertia variations and random initial position, making the extension of closed-loop operating range towards zero an important goal. Where no relevant anisotropy is present in the considered class of machines, magnetic saliency cannot be exploited, but some methods based on the detection of torque produced by signal injection can be adopted, both for initial position detection and closed-loop control Besides the adopted estimation technique, adding a constant direct-axis current has been shown experimentally to improve the feedback control capability. An analytical description of this effect has been obtained, yielding a means for the design of the proper d-axis current value. A complete theoretical analysis, validated by simulations, is reported to show the features of the proposal. Experimental tests on a general-purpose industrial drive demonstrate the feasibility of full-speed range control, thanks to a proper integration of low-frequency injection and direct-axis current stabilization at low-speed and fundamental back-EMF at medium-high speeds.
