Introduction: In 1914, electric tools that were driven by single-phase series-excited motors appeared, with motor speeds of more than 10,000 r/min. In 1927, medium-frequency electric tools with a power supply frequency of 150 to 200 Hz appeared. It not only has the advantage of high speed of single-phase series-excited motors,but also with simple structure and reliability of three-phase industrial frequency motors, but has its limits because they need to be powered by medium-frequency current.