Research On Soft Starter For AC Motor

The Three-phase asynchronous motor has been widely used in fields such as industry and agriculture as well as electric drive due to its simple structure and reliable operation. However, featured by high starting current and low starting torque, its start performance is far from satisfaction. Especially for the high-power motor, without taking any steps to improve its start performance, high impulse current will significantly impact power grid and other electric devices. In essence, the traditional starting devices belong to step-down starting mode with several voltage levels, they fail to realize smooth change of starting voltage and current and make secondary impulse current inevitable, besides, the existence of mechanical contact needs more maintenance. In order to improve its start performance, a soft starter based on DSP F2812 is designed and followed by detailed discussion of its control circuit.The current state of soft starter is introduced and various of different start modes are compared firstly. Then the most commonly used theory for soft starter-phase control voltage regulation theory is discussed according to mathematical model of asynchronous motor. Different trigger methods are introduced. The reliability and feasibility of soft starter is tested by comparing the simulation results of start voltage, current and torque under direct and soft starting conditions.In chapter three, all function modules of the control circuit are discussed in detail, which include the minimum system design of DSP, zero-crossing detection circuit, voltage and current amplitude detection, trigger circuit for thyristor, communication module, switching circuit and protection circuit. A reliable hardware circuit system is crucial for software design.Chapter four mainly analysis the software design for the control system, which consist of main program, fault diagnose subprogram, capture subprogram, AD sampling subprogram, PWM pulse generation subprogram as well as current-limit start mode subprogram that combines the PI control method. In the end, parts of the experimental results are showed, proving that the soft starter can effectively eliminate impulse current and provide better start performance.