Induction motor has a simple structure, easy to manufacture, use and maintain, reliable, and lightweight, relatively low cost and so on. At the same time its high efficiency, good operating characteristics, and range from no load to full load, which run nearly constant speed, to meet most industrial and agricultural machinery and household appliances transmission requirements. With the development of power electronic components and AC variable speed technology, asynchronous motor and inverter are combined into an AC speed control system of economy and performance have DC drive systems can and Mother America, and to use and easy to maintain, Therefore, applications are increasingly widespread.