Study On Three-Phase Asynchronous Motor Soft-start Strategy And Soft-Starter Device Design

The smooth starting and braking of three-phase asynchronous motor is the most important factors not only to improve motor's lifetime but also to decrease impacting effect above the power grid and linked equipments.For a long time,the researching on motor's dragging has always been an important project among the manufacturers and electro-mechanical engineers.Firstly,this paper investigates the current status of Soft-starter and start control strategy, and analysis three new asynchronous motor soft-starting methods:Phase-shifting voltage control based on Thyristor,Alternative voltage chopping control based on high-power IGBT and Three-phase full-bridge converter based on PWM frequency conversion control.All of the methods are realized and compared results are provided.Working state of phase-shifting control circuit,pulse trigger method of Thyristor,the approximate linear input-output relationship of chopping control device and its harmonic suppression principle are discussed. For three-phase full-bridge converter based on PWM frequency control,the generation of PWM controlled wave's method,fundamental-wave equivalent signal's changing regularity is analyzed.Secondly,on the basis of previous discussion a three-phase asynchronous motor's electronic soft-starter is designed.System structure,hardware design and software flow control are described in detail,which provides a new method for soft-starter.The soft-starter takes double AT89C52 microcontroller as the core,Phase-shifting voltage control based on Thyristor and chopping control based on high-power IGBT as the starting control methods. System's LCD device can simultaneously display the line-voltage,line-current,rotating speed and power factor of the motor,etc.From signal's generation,transmission,conversion to procession,the design of soft-starter system includes hardware and software two parts.The circuit design includes:data-acquisition,over-current protection,fault detection, human-machine interface circuits,etc.System's software design includes:voltage-regulation start,current-limit start,master-slaver controller parallel communication program, analog/digital conversion program,fault and protection program design,etc.Finally,Experiment and simulation results proved that:using three kinds of reduced-voltage starting methods can improve motor's starting performance effectively.By comparing 3 starting methods above,electronic soft-starter system is proved to be practical, and realizable.