The structure diagram of the general self-synchronous brushless DC motor inverter and drive is shown in Figure 1. The drive system shown in the figure is usually used for voltage source inverters (VSI). The counterpart of the voltage source inverter is the current source inverter (CSI). The reason why VSI is more widely used is that its cost, weight, dynamic performance, and ease of control are superior to CSI. The difference in weight and cost between the two inverters is due to the fact that the VSI uses capacitors for DC coupling, while the CSI requires a bulky reactor between the rectifier and the inverter. VSI is also different from CSI in dynamic response capabilities. Because the function of the large reactor is to meet the needs of the CSI as a constant current source for a larger commutation overlap angle, to prevent the rapid change of the current in the motor windings, and to inhibit the high-speed servo operation of the motor. This will increase the size of the damper in the drive system. For the constant current control and constant torque control performance expected by CSI, in VSI, it can also be approximated by hysteresis current control in its internal current control loop.
The term "self-synchronization" refers to the correct turn-on sequence of the tubes required for the stator phase current pulse to be consistent with the opposite potentials of the motor, and the drive circuit's requirements for instant rotor position information.