Abstract: Described the application scope of low- voltage variable frequency speed governor in coal preparation plant,introduced the design principle of main circuit and control circuit as well as other issues in circuit design. According to the non- standardized situation of variable frequency device in coal preparation plant in China,put forwards a design method of variable frequency device circuit,and the application result shows that this design method increased the safety,normative and practicability of variable frequency device,and provides a convenient of routine maintenance and unified purchasing spare parts for users.