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A new sensorless control algorithm for brushless dc motors (BLDCMs) is proposed in this paper. The torque constant of a BLDCM is used as a reference signal for position detection because it is constant during the entire speed range and can be estimated by calculating the ratio of the back electromotive force (EMF) to the rotor speed. By using both a disturbance observer and the torque constant estimation error, the rotor speed can be obtained. The back EMF can be easily obtained from the voltage equation of the BLDCM. The estimated back EMF decreases simultaneously with the estimated torque constant at the commutation point. By using this phenomenon, the commutation of the phase currents can be done automatically at the drop point of the estimated torque constant. Unlike conventional back-EMF-based methods, the proposed method provides highly accurate sensorless operation even under low speeds because only the drop of the torque constant is used for position detection and current commutation. Therefore, the position accuracy is not affected by the electric parameter errors of the BLDCM. Also, this algorithm does not require an additional hardware circuit for position detection. The validity of the proposed algorithm is verified through several experiments.