Soft-starter For Heavy Load Developed Based On The Variable Frequency

Three-phase AC asynchronous motors have been used widely in variousindustrial areas because of their simple structure, small volume, light weight,low cost and convenient maintenance, now, as the power source commonly usedin the transmission project. But asynchronous motors direct startup will havegreat impact on current, which usually is5~8times rated current or more. It willaccelerate the motor’s aging process and lead to a lot of energy consumption.The very high startup current will also affect the power network and make thepower voltage fluctuate． So， research on how to improve the startingperformance of asynchronous motors has very important economic value．The theory of soft-starter for the AC asynchronous motor is deduced basedon the analysis of the motor starting process．Some starting methods and severalworking modes about soft-starter are compared and analyzed． Thereduced-voltage starter or conventional soft-starter is difficult to provide enoughstart torque, unable to realize heavy load start.Soft-starter based on three-phase voltage regulating is analyzed. Thesoft-starter through controlling thyristor conduction angle, changes the outputvoltage of the soft-starter to realize motor start. It doesn’t only cut down thecurrent or torque shocks of the motor during startup effectively, but also set itswork status according to the applied conditions, i.e. have very strong flexibilityand adaptability. However, a small reduction in voltage produces a sharp declineof electromagnetic torque because torque is proportional to the square of thevoltage. This kind of soft-starter is used in light load, instead of heavy load.Unfortunately, there are many applications as heavy loads to start such as BallGrinder、Muller、Mine Crane and so on, therefore, the asynchronous motorneeds to start with heavy load. For this, the paper mainly studied how to usediscrete frequency principle to improve asynchronous motor startingperformance and tarting torque, and studied the design of hardware and software of heavy load soft-starter control system.The main contents as follows：1) Expounded traditional starting methods before thyristor turned up.2) Elaborated the principles of three-phase adjusting voltage soft-starterwith thyristors and discrete frequency conversion soft-starter, and analyzed theirharmonic voltage respectively．3) Without changing the main circuit structure of conventional thyristorsoft-starter, only used discrete variable frequency to improve starting torque.Changed voltage frequency through controlling the turn-on time of the triggerangle. At the same time, analyzed and chose the optimal trigger anglecombination was determined by three system of positive sequence asymmetry、negative sequence asymmetry and imbalance．Furthermore, this paper studiedhow to choose sub frequency and trigger angle.4) Introduced a controlling and protective system for three-phase ACasynchronous motor’s soft-starter based on ARM+CPLD as control core,AT89C52as the user interface and display processing unit, data communicationvia RS485, three anti-parallel thyristors as control element.Then analysed thehardware composing of the system, completed the hardware circuit design anddevelopment.5) In this design, algorithm operation and operation monitoring wererealized by CPU(STM32), and CPLD was combined with STM32sendingtrigger pulse control signal to extend STM32logic function, with the excellentperformance of them into full play. This meant that the CPLD resources was ingood use to get the high quality control of trigger pulse, and meet the highpower, high reliability of the control requirements. At the same time, the controlsystem can also realize soft brake and system protection, etc.