In 1895, the German overtone made the world's first DC drill. The housing is made of cast iron and can drill a 4mm hole in the steel plate. Then 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 excitation motors appeared, and the motor speed reached more than 10000r/min. In 1927, an intermediate frequency power tool with a power supply frequency of 150-200Hz appeared. It not only has the advantages of high speed of a single-phase series motor, but also has the advantages of a simple and reliable structure of a three-phase power frequency motor, but it needs to be powered by an intermediate frequency current. , use is restricted.
