Motor Y has a star connection. The star connection of motor Y has a small starting current and a small power winding for a three-phase brushless DC motor. Like other types of motors, the brushless DC motor body is also composed of two major components: the stator and the rotor. The rotor refers to the part of the motor that can rotate during operation. It usually consists of a rotating shaft, permanent magnets, a yoke and other components. Its main function is to generate sufficient magnetic induction intensity in the air gap of the motor, and interact with the energized stator winding to generate an induced potential to drive itself. The stator refers to the part that does not move when the motor is running. It is mainly composed of silicon steel punched sheets with windings distributed in their slots, as well as components such as casings, end covers, and bearings. The so-called "winding" refers to the sum of some coils connected according to certain rules. After the winding is energized, it interacts with the magnetic field generated by the rotor magnet, generating a force or induced electromotive force that drives the rotor to drive the load to rotate together. After the rotor magnet rotates, its magnetic field lines in turn cut the stator winding, generating an induced electromotive force in the stator winding, which in turn affects the balance relationship of the electromotive force in the motor. It can be seen that the interaction between the energized winding and the magnetic field is the main medium for electromechanical energy conversion inside the motor. Only by understanding the distribution and action of the magnetic field in the motor can we accurately analyze the induced potential and the magnitude and direction of the induced electromotive force generated by the winding, so as to derive the induced potential balance equation and electromotive force balance equation of the motor. However, without the specific structure and connection method of the windings, it is difficult to explain the basic process of electromechanical energy conversion in the motor. Basic issues such as induced electromotive force, circuit parameters, and electromagnetic induced electromotive force will also feel empty or unclear. In this chapter, some basic issues of winding structure will be discussed in conjunction with the basic performance requirements of brushless DC motors. In order to analyze the winding issues concisely, first some necessary descriptions and simplifications are made on the magnetic circuit and air gap flux of the brushless DC motor.