Study Of Heavy Soft-starter Based On Discrete Variable Frequency

Traditional soft starter reduces the input voltage by changing the SCR trigger angle tomake the motor starting current reduced, however, the size of the starting torque is limitedsimultaneously, thus it is often limited to be used only in light-load applications. While inheavy-duty starting needed applications, discrete frequency-control soft starting technique,which can maintain the original structure of the traditional electronic soft starter, modify theamplitude and frequency of voltage by controlling the power frequency half-wave on software,can meet the motor starting current and torque requirements under the premise of notincreasing the cost.Aiming at the problem described above, the discrete variable frequency theory which canbe used to boost the starting torque and improve the starting performance is analyzed and theheavy load soft starter control system is deep researched in this thesis.The output torque of the motor is inversely proportional to the power supply frequency,so without changing the traditional soft start system on the main circuit structure, transistorscan be selectively controlled to be turned on and off, thus some half-wave frequency is notturned on while other half-wave frequency is turned on, then the voltage applied to the motorstator is discrete, so that the frequency of the fundamental wave changes and the startingtorque is increased by reducing the voltage frequency. However, it is necessary to detect thereal-time speed of the motor in a discrete variable frequency soft start process. So, in differentapplications of conventional soft starter, the motor speed measurement methods based on theresidual voltage of power failure are selected according to their own characteristics. Thismethod not only avoids the trouble caused by tachometer speed sensor but also can obtain thespeed information of the motor only according to the characteristics of the soft startcontroller’ main circuit. What’s more, a set of experimental device about discretefrequency-control soft starting system is designed based on dsPIC30F6014which has ahigh-speed data processing capability and rich peripheral hardware resources, and theexperiment result verifies the feasibility of discrete frequency. The simulation results show that, the speed estimation method based on the residualvoltage under interrupted power supply condition has a strong anti-interference ability, a fastcalculation speed, an accurate estimation result, and a good practicability. It not only providesa certain reference value for the speed detection of AC asynchronous motors but also hasobvious advantages in the speed estimation of AC asynchronous motors. Meanwhile, theexperimental results of the experimental device designed show that the starting current ofheavy load starting is reduced and the torque’s starting requirement is improved by using thisdiscrete frequency soft starter. In short, this control system works well. And the feasibility ofthe discrete frequency soft start is verified once again.