In this research, a new estimation method for the sensorless control of a permanent magnet synchronous motor (PMSM) in the low speed range is proposed using a speed observer and an opened phase voltage. An approximate linear equation of the opened phase voltage, which varies depending on the rotor position, is calculated using the least squares method with constant time intervals. This estimated position is used as the input of the speed observer instead of the actual position information. The position detection method based on the opened phase voltage can be applied to a non-saliency and closed shelf motor in a low speed range where back-EMF (Electro-Motive Force)-based methods are difficult to apply. In the proposed method, the current waveform is satisfactorily controlled in a sinusoidal waveform during the period when the opened phase voltage is not measured. Compared to conventional methods, it improves the energy efficiency and reduces the torque ripple. In this paper, we present the proposed position estimation method and the experimental results.