A Novel Method for Sensorless Speed Detection of Voltage-Controller-Fed IM Drive Based on Voltage Across Thyristor

AC Voltage controller-based soft starters are used in induction motor drives. A novel method for sensorless motor speed detection based on voltage across thyristor is presented in this paper. The blocked voltage across the nonconducting thyristor is the difference between the source voltage and the back EMF. The magnitude and the phase lag of back EMF keep changing as the motor accelerates. A new expression involving back emf and source voltage to express the blocked voltage across thyristor is proposed. Hence, the speed can be obtained through the expression according to the measurement of the magnitude and the phase lag. Dynamic Simulation and experimental results show that the proposed method is valid and effective.