With the rapid development and improvement of electrical transmission technology (frequency conversion speed regulation technology, motor vector control technology, etc.), the mechanical structure of the main transmission system of high-speed CNC machine tools has been greatly simplified, basically eliminating pulley transmission and gear transmission. The spindle of the machine tool is directly driven by an internal electric motor, which shortens the length of the main transmission chain of the machine tool to zero, achieving "zero transmission" of the machine tool. This type of transmission structure combines the spindle motor with the machine tool spindle, making the spindle components relatively independent from the transmission system and overall structure of the machine tool. Therefore, it can be made into a "spindle unit", commonly known as an "electric spindle" (Motor Spindle). Due to the current use of AC high-frequency motors, electric spindles are also known as "High Frequency Spindles". Due to the lack of intermediate transmission links, it is sometimes referred to as a "Direct Drive Spindle".

Electric spindle has the advantages of compact structure, light weight, low inertia, low vibration, low noise, and fast response. In addition, it has high speed, high power, simplifies machine tool design, and is easy to achieve spindle positioning. It is an ideal structure in high-speed spindle units.