In 1895, Germany produced the world's first DC electric drill. The outer casing is made of cast iron and can be drilled with 4mm holes in the steel plate. Then there was a three-phase power frequency (50 Hz) electric drill, but the motor speed did not exceed 3000 r/min. In 1914, a single-phase series-excited motor-driven power tool appeared, with a motor speed of more than 10,000 r/min. In 1927, an intermediate frequency power tool with a power supply frequency of 150-200 Hz appeared. It has the advantages of high speed of single-phase series-excited motor and simple and reliable three-phase power frequency motor, but it needs to be powered by medium frequency current. , use is limited.
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