Development Of Motor Soft-Starter With The Reactive Power Compensation

When starting the large capacity three-phase asynchronous motors directly with full voltage, large starting shock currents and low power factors will produce.These will influence the motors, the power grid and the other equipment in the same grid. For reducing these damages, this dissertation designs and develops a soft-start device with reactive power compensation function.This dissertation compares several common soft-start methods used for the current AC asynchronous motors and analyses their advantages and disadvantages respectively via consulting document literatures related with soft-start technology and reactive power compensation. Finally, the scheme for soft-start based on the variable reactor is chosen and the reactive power compensation is added for this scheme’s insufficient. Based on this, the domestic and foreign research status and development trend of the reactive power compensation are analyzed, and the reactive power compensation scheme is proposed in the soft-start process.According to the requirements of the reactive power compensation, the topology structure of the soft-start device with the reactive power compensation is proposed. And the working principle of this device’s topology structure is detailed with the soft-start and the reactive power compensation in two parts. Among them, this soft-start adopts the step-down soft-start based on the variable reactor, and the reactive compensation is realized by controlling the reactive power compensation device to switch according to the power factor. And then, based on the device’s topology structure, the main circuit and operating circuit are designed and the overall scheme of the control system is determined. According to the overall design of the control system, the hardware selection of the control system is carried out.According to the requirements of the control system overall scheme and the hardware selection, The hardware with PLC as a controller is designed, including the switch input channel, switching output channel, power factor detection circuit and pulse trigger circuit, and the working principles of these circuits are analyzed.Based on the hardware design of the control system and respond to Siemens200 series PLC controller as well as STEP7-Microwin programming environment, the software design is finished. The program function modules are designed at first, including fault alarm and symbol table. And then, the principal PLC programs are finished, including main program, soft-start subprogram, the reactive power compensation subprogram and touch screen software design. Finally, the program flow charts and the touch screen interface design are given.Finally, according to the design of this dissertation, the prototype of the control cabinet is developed, and the experiment platform is built in the laboratory. The soft-start control cabinet with the reactive power compensation function is conducted debug and experimental research. Based on the analysis of the experimental data, when adopting eight-steps soft-start, the starting current is greatly reduced, only2-3times of the rated current, realizing the starting current no higher than3.5times of the stator rated current in the starting process. In the aspect of the reactive compensation, the power factor has increased from0.2~0.3to0.8above after switching the reactive power compensation, having realized the reactive power compensation in the starting process.