With the rapid development and improvement of electrical transmission skills (frequency conversion and speed regulation skills, motor vector control skills, etc.), the mechanical structure of the main transmission system of high-speed CNC machine tools has been greatly simplified, and belt wheel drive and gear drive have basically been eliminated. The machine tool spindle is directly driven by a built-in motor to a high speed electric spindle, which shortens the length of the main drive chain of the machine tool to a zero speed electric spindle and completes the "zero drive" of the machine tool., This "spindle unit" spindle motor and machine tool spindle are "integrated into one" transmission structure, which makes the spindle components relatively independent from the transmission system and the whole structure of the machine tool, also known as "electric spindle". At present, the electric spindle mainly adopts AC high-frequency motor, so it is also called "high-frequency spindle". Because there is no intermediate transmission link, it is sometimes called "direct drive spindle".