Influence and compensation of the stator flux on the direct flux control sensorless technique for PMSMs

Several techniques exploiting machines anisotropies have been proposed in literature for sensorless control of synchronous machines. Among them, the direct flux control (DFC) technique allows estimating the electrical rotor position by exploiting the zero-sequence voltage of the machine. This work aims at analysing the effects of magnetic saturation on the application of the DFC technique to permanent magnet synchronous machines. This effect is present when stator flux is controlled during machine operation, leading to an error in the estimation of the electrical rotor position. Starting from an analytical model of the machine phase inductances, a complete mathematical description of the DFC technique is derived and presented. The proposed mathematical model shows how the estimated electrical rotor position is biased when operating under load conditions. Moreover, the result of the analysis of the DFC technique allows to refine the obtained estimate and to reduce the electrical rotor estimation error. Experimental results on a test motor are provided in order to verify and support the proposed mathematical model and technique.