最佳答案：In 1895, German flooding manufactures the world's first DC electric drill. The outer casing is m
In 1895, German flooding manufactures the world's first DC electric drill. The outer casing is made of cast iron to drill 4mm holes on the steel plate. The three-phase power frequency (50 Hz) is then appeared, but the motor speed can not exceed 3000R / min. In 1914, a single stroke exchang motor driven electric tool, the motor speed reached 10,000r / min. In 1927, an intermediate frequency power tool with 150 ~ 200 Hz has occurred.
It has the advantages of high speed of single-phase stroke motor speed, and has a simple and reliable advantage of three-phase worker motor structure, but is limited by the use of intermediate current currents.
