Abstract: In some special applications, the free-running induction motor needs to be put into operation after short power interruption. In the case of speed-sensorless control, the initial speed of the motor is needed to realize the restarting procedure. Therefore, a single-current closed-loop DC injection method is introduced to obtain the initial speed of induction motor in a short time. The characteristics of free-running induction motor under power interruption are analyzed, the mathematical model of induction motor under signal-current closed-loop DC injection is established, and the method of measuring feedback current frequency to estimate initial speed is analyzed. At the same time, the switching strategy from single-current closed-loop DC injection to full-order adaptive observer is proposed. Finally, the effectiveness of the proposed method is demonstrated by simulation.