Research Of Discrete Frequency Soft Start Based On Thyristor

With the wide application of asynchronous motors, people begin to focus more andmore attention on some of the motor’s own problems. In order to improve the startingperformance of asynchronous motors, the research for motor’s soft start based on powerelectronics technology for starting is presented.The discrete frequency soft-tarter based on thyristor is a novel all-digital kind ofasynchronous motors control device, compared with the traditional motor start means, it hasmuch more superiority, which includes adjustable output voltage, low surge current andhigh starting torque, simple operation, narrow space demanding, and etc. As thedevelopment of discrete frequency soft starting technology, the functions of soft starter aremore and more powerful and eventually become equivalent replacement of converter whenthere is no need for speed modulation. This paper referred to the existing functions of theABB’s soft starter, and independently developed a set of soft-start equipment.The discrete frequency technology based on SCR is used in this system. At the sametime on decreasing electric source’s frequency, the motor’s stator voltage is adjusted bycontrolling SCR, the discrete frequency soft starting is accomplished. Start modes of thedevice are variable, and stop modes include the free stop and the soft stop. In addition, theequipment also has Ethernet communication module and Profibus communication module.This paper researches the discrete frequency soft-tarter system based on dsPIC SCM,and analyses its hardware part and software structure. The hardware circuit includes themain circuit module, the main control board module, the driver board module and the LCDboard module; the software part consists of main program module, the initializationprogram module, the synchronization signal capture interrupt program module, the PWMinterrupt sub program module, ADC interrupt service sub program module and thecurrent-limit start program module. Details of the synchronization signal, the generation ofPWM single and the PI control algorithm which fit for current-limit start mode are fullydiscussed. The traditional electronic soft start system and the discrete frequency soft startsystem are set up and tested by MATLAB, the simulation results indicate that discretefrequency method successfully minimize current amplitude while offering adequate startingtorque, thus superiority of the discrete frequency soft start control mode is confirmed.