Study On The Soft Starting And The Intelligent Braking Of Three-phase Induction Motor

There is a big current surge in the case of direct starting of an induction motor. Traditional reducing voltage starting method can't resolve the problem completely. A general soft starter of the three-phase induction motor is presented in this dissertation, which can solve the starting problem. The main power circuit configuration includes three sets of back-to-back thyristors to connect the three-phase power supply and the three-phase induction motor. The principle of the soft starter is reducing the voltage applied to the stator of the induction motor by control the triggering angle of the thyristors in order to limit the starting current surge.The soft starting control system of the three-phase induction motor is constituted of the main control system employing the Digital Signal Processor (TMS320F240) and the monitor system employing the single chip microprocessor (89C52). The main control system includes the synchronous signal generating circuit, the testing circuit of thyristor cut-off voltage and the trigger circuit of the thyristors etc.The AC voltage regulated circuit with thyristors is analyzed in detail. The closed-loop control system of the three-phase induction motor soft starter is designed, with the use of power factor angle as the feedback signal in the system. And the simulation model of the closed-loop system is founded in MATLAB (SIMULINK) enviroment. The contrast research of the open-loop soft start control system and the closed-loop soft start control system is carried out.The research on intelligent braking of three-phase induction motors is carried out, including the design of dynamic braking and plug braking. Simulation model for it is built, and the ideal simulation results are obtained. In addition, the motor soft stopping and its application is studied.The method of reducing the power supply frequency in discrete steps to increase the starting torque of induction motors is proposed, without changing the main circuit of conventional motor soft starter. The sequence of the voltage applied to the stator of the three-phase induction motor and the triggering strategy of the thyristors are studied. Simulation model is founded, and the simulation results have proved that the discrete frequency control method can be applied to the soft starter of the induction motor with heavy loads.