Design of intelligent soft-start controller for induction motor

Abstract

In soft-start controller, the output voltage of the thyristor is not only related to the trigger angle, but also associated with the power factor angle, which varies with the speed of the motor during the soft-start process. It's known that the maximal torque of induction motor is approximate in direct proportion to the square of the voltage of the stator. Accordingly, a closed-loop control system is designed in which the voltage and power factor angle are used as the feedbacks, which is a solution to the true soft starter. The main hardware circuits are provided, such as the circuit for checking synchronous signal, the circuit for checking phase order, and the circuit for measuring power factor angle. Then the control strategies adopted are also described in detail. The experiments prove that the proposed solution of the soft-start controller has better starting performances than ever. Moreover, it has smaller in-rush current and smaller mechanical impact, so it can also avoid current oscillation during soft starting.