The electric spindle technology is one of advanced technologies in CNC machine tools. (The other advance technologies include numerical control system and feed drive system. ) With the rapid development of numerical control technology and cutting tools, more and more mechanical equipments will have higher speed, higher precision, higher efficiency and higher intelligence. The electric spindle has become one of the most core functional parts for CNC machine tools with high performance. It is a trend that in the machine tool industry, the electric spindle motor will replace traditional mechanical spindle in some fields.