In 1895, German Overtone produced the world’s first DC electric drill. The shell is made of cast iron and can drill 4mm holes in the steel plate. Subsequently, a three-phase power frequency (50Hz) electric drill appeared, but the motor speed failed to exceed 3000r/min. In 1914, electric tools driven by single-phase series motors appeared, and the motor speed reached more than 10000r/min. In 1927, an intermediate frequency electric tool with a power supply frequency of 150 to 200 Hz appeared. It not only has the advantage of high speed of single-phase series motor, but also has the advantages of simple and reliable three-phase power frequency motor. However, it needs to be supplied with intermediate frequency current. , Use is restricted.