This paper proposes a new V/f control method for permanent magnetic synchronous motors (PMSMs) without a position sensor. The proposed method uses state feedback control based on an n-t coordinate system, and controls rotational speed and the voltage amplitude. The t-axis is a tangent line of a constant voltage ellipse, and the n-axis is a normal line of the ellipse. The t-axis current is utilized to place the poles of the transfer function at the desired position at low-speed and high-speed conditions. The effectiveness of the proposed method was verified by simulation and experimental results. K E Y W O R D S coordinate system, PMSM, state feedback control, V/f control Electr Eng Jpn. 2019;206:55-65.