Sensorless Direct Slip Position Estimator of a DFIM Based on the Air Gap $pq$ Vector—Sensitivity Study

This paper presents a new sensorless method for the direct estimation of the slip position of the wound-rotor induction machine. The main purpose of this study is to implement the control of the doubly fed induction machine (DFIM). The method computes directly the slip position as the difference between the position of the air gap power vector (q_e, p_e) and the position of the measured rotor current. The method does not need any information about the stator or rotor fluxes, and so, it is only indirectly dependent of the flux dynamics. Because the slip position is directly obtained, it is not necessary the knowledge of the stator flux to implement field orientation. The method is sensitive to errors in the stator inductance parameter. A general analytic expression for the estimation error is presented. The sensitivity becomes important when the rotor current is small. To overcome the sensitivity problem, an open-loop on-line adjustment is proposed and tested with acceptable results. Experimental results confirm that the method is appropriate for the vector control of the DFIM.