| Electronic soft starter can reduce starting current by regulating SCR trigger angle, it can restrain starting torque at the same time. Frequency-converter solves the problem in the starting with heavy loads, but increases costs. Discrete frequency-control soft starting technique that has the circuit of electronic soft starter modifies amplitude and frequency of voltage by controlling the power frequency half-wave in software, so it can meet the demand of starting current and torque. This paper studies the characteristics, principle of application and controlling strategy of the starting method.In this paper the principle of modifying amplitude and frequency of voltage has been discussed, the influence on the phase of first-harmonic of generated voltage has been analyzed, the best triggering mode of every frequency has been proposed, and starting frequencies of motors had been chosen. As the generated voltage has lot of harmonious waves, the ingredients of voltage harmonious waves on every frequency have been ascertained, and the conclusion that the first-harmonic of generated voltage plays the main torque role has been educed and simplifies the control strategy. In order to realize the better switching of different frequencies, a principle of that is first-harmonic of generated voltage continuous has been proposed. For the long period of discrete frequency, so a control strategy, which is based on the power frequency time slice, has been proposed. This method ensures the real-time control. To switch the discrete frequency at proper speed, a novel speed measuring method using residual voltage under interrupted power supply condition has been proposed. The algorithm of this method is simple and easy to program, and it also has high accuracy.The theory talked above is analyzed with simulations. At last, a simulation model of discrete frequency-control soft starting of three-phase asynchronous motors is designed. The simulation results show the feasibility of proposed scheme.
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