Research on high torque soft starter of induction motors based on discrete frequency

Soft starters have been widely used in fans and pumps drives. It is difficult to start high-power heavy-duty induction motor for traditional soft starter. To solve the problem of soft starter for large power and heavy load in conventional soft starters, the soft starter based the theory of discrete frequency is studied in this paper, which can make motor start with high torque and little current. Using discrete frequency makes the start frequency of motor from 5.25Hz to 50Hz step by step, which can make start torque larger. This paper presents a method of the calculation of optimal triggering angle which can make the torque be the biggest and positive was produced. The simulation waveform by MATLAB is similar to the real waveform by experiment. Control hardware circuit is simple and dependable. The experiment of the high torque soft starter which has a DSP as microcomputer is completed. The experimental results show that discrete frequency soft starter can start motor smoothly with high starting torque and small starting current. The advantages of discrete frequency soft starting proposed in this paper are stable start process, simple structure, low starting current and high starting torque. The result of load test is identical that conforms the theory is correct and valid.