In the direct-driven wind turbines， the permanent magnet synchronous generator（PMSG） usually is driven with sensorless control strategies. Considering the operation features of PMSG in wind turbines， the fundamental excitation based position estimators are feasible. A novel estimator of this kind is proposed， which is composed of an extended-flux observer and a phase-lock-loop（PLL） control to get in phase with the observed flux. The proposed observer， where the parameters of PMSG are utilized in the simplest manner， has the minimum order without any approximation on its mathematical model. Based on the derivation and discussion on structure and stability of the proposed extended-flux observer， the design process of the observer is presented. Experiments on a 11 kW PMSG validate the performance of the proposed sensorless control strategy.