Robust Sensorless Control of Electrically Excited Synchronous Machines Considering Inductance Uncertainties

For electrically excited synchronous machines (EESMs) used in automotive applications, the flux linkages are highly nonlinear due to magnetic saturation and cross-coupling effects. Therefore, the inductances strongly depend on the actual current operating point. For sensorless control and/or observer design, the inductances are usually read from lookup tables (LUTs) according to the estimated
$(d,q)$
-axes currents, which are affected by errors in the estimated angular position, meaning inductance mismatches can arise despite having accurate inductances stored in the LUTs. These inaccurate inductances will themselves additionally deteriorate position estimate, as inductance evaluation and angle estimate are coupled and influence each other. In this article, the impact of this coupling effect on the stability of the sensorless control scheme is investigated. The conditions for stable convergence but also for potentially new (undesired) stabilizing points are analyzed, and a compensation method is proposed to overcome this problem and to ensure global convergence/stability of the observer. Experimental results on an EESM test bench confirm the stability analysis and the effectiveness of the proposed method.