Simulation study of heavy motor soft starter based on discrete variable frequency

For conventional motor soft starter, the starting torque would be seriously decreased when reduce the starting voltage, so it is limited to light load, However, many overloading start-up is required in practical applications. According to the relationship between three-phase asynchronous motor electromagnetic torque and power frequency, when keeping the size of voltage unchanged, the torque can be increased by reducing the power frequency. Keeping the main circuit of original motor soft starting system unchanged, the motor rotation is controlled by changing the thyristor trigger angle, converting the frequency which add up in the stator and utilizing the fundamental components. This paper presents a method of improving starting torque based on discrete variable frequency and the three-phase asynchronous motor soft starter model is established using MATLAB. The simulation results illustrate discrete variable frequency softer starter has many advantages in current, speed and torque compared to full-voltage starter and current limit starter.