Research Of Discrete Frequency-Control Soft Start For Three-Phases Asynchronous Motor

There are lots of over-loading transport equipment in mineral industry, such as ebelt conveyer and drawing conveyer, etc. It is more difficult to restart once those equipments appear to break down. Starting directly will produce big dash current, which will shorten the service life of the electromotor. Traditional soft starter can not provide enough starting torque to carry out overloading starting. Starting by using inverter will make high cost. Aiming at those problems, a new resolution is represented in this paper and the exact scheme is to make discrete frequency-control by controlling the triggering instants of thyristors, and realizes over-loading soft starting with high start torque by using VVVF method. This resolution also immensely cuts the cost.After Analyzing discrete frequency-control technology and taking further research on how to realize three-phase frequency-control by using three groups of inversely parallel-connecting thyristors, I give the general equation of initial phase angle. Then I provide the algorithm to get the best combination after three-phase frequency-control, introduce the control strategy which can realize over-loading soft start by using discrete frequency-control technology, and point out how to realize switch among different frequency. This control strategy also leads into fuzzy control method to carry on the constant current control in the process of low frequency switch to line frequency and it realizes the real soft start.Finally, this paper sets up the system model by using MATLAB/ SIMULINK, makes simulation for this system and verifies the feasibility of this control strategy, and, by simulating experiment, compares discrete variable frequency soft start with other starting ways and verifies the advantages of the former in over-loading start area.