This paper proposes a novel sensorless I/f control with efficiency optimization by controlling the phase difference between phase voltage and current. The conventional sensorless filed oriented control scheme require high performance digital processing and complicated calculation and are not suitable in applications to low cost fan motor drivers. Hence, simpler control method such as V/f drives which consist of the stabilization and the high efficiency control have been proposed. The proposed I/f control scheme combines power factor control and current loop control to achieve low distortion current as well as high efficiency motor torque control over wide speed control range. Compared with the V/f control scheme, the I/f control scheme can achieve high quality current waveform under larger motor parameters uncertainty. An optimal power factor angle as a function of speed is constructed based on the fan motor torque-speed characteristics as well as the motor parameters. A power factor angle detector and regulator is used to achieve optimal torque generation. Simulations and experimental results have been obtained to show the effectiveness of the proposed scheme.