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The blocked voltage of non-conducting thyristors contains information of the motor operation. The voltage amplitude is an indication of motor speed, while the phase angle of the voltage across it is an index of power factor angle named the measurable power factor angle. Thus a new approach to speed detection in three-phase induction motors without additional speed sensors is presented. This method utilizes the characteristics of thyristor voltage controller and the motor running state under the asymmetric power supply to find the relationship between the induction voltage of one phase of the motor without power supply and the motor speed, hence the relationship between the RMS voltage across the non-conducting thyristor and the motor speed. In addition, an analysis of the rule of power factor angle vs. speed in the three-phase induction motor is presented. Through analyzing the cause of phase current oscillation during soft starting, the power factor angle closed-loop control is introduced to induction motor soft starters. It is pointed out that the thyristor firing angle is composed of two parts. One is obtained from the predetermined voltage control rule, and the other is the increment of dynamic regulating angle with the variation of power factor angle. Experimental results show that the proposed closed-loop control method can avoid current oscillation during soft starting and have good performance.