1 Introduction In recent years, with the continuous improvement of the functions of the inverter itself, the AC speed control technology has made great progress. How to fully develop the inverter's own functions under different application conditions and effectively reduce the cost of equipment modification has become an important issue. Compared with the usual control method, the design scheme of realizing the accurate stop of the AC drag system by using the DC braking function of the inverter eliminates the expensive dedicated brake unit/brake resistor of the inverter, which effectively reduces the equipment transformation cost and works. Stable and reliable, high control accuracy. 2. Theoretical analysis of VVVF energy braking The braking modes provided by general-purpose inverters mainly include: energy braking, regenerative braking, rectification feedback and so on. Under the condition of large moment of inertia, the general mode recommended by the inverter manufacturer is the external braking resistor and the regenerative braking mode of the braking unit. In some cases, DC braking can be used. This design idea is basically accepted by most users in the field, and has achieved good results in actual use. However, the solution needs to purchase a special brake unit/brake resistor provided by the inverter manufacturer, which invisibly increases the transformation cost. The so-called "DC braking" generally means that when the output frequency of the inverter is close to zero and the speed of the motor is reduced to a certain value, the inverter changes to the stator winding of the asynchronous motor to form a static magnetic field, and the motor is in the state. In the state of braking, the rotating rotor cuts the static magnetic field to generate braking torque, and the motor is quickly stopped. During the braking process of the motor, since the output frequency of the inverter is gradually reduced, the synchronous magnetic field speed in the stator winding is lower than the rotor speed, and the motor is in the regenerative braking process. At this time, the kinetic energy stored in the rotating system is converted into the form of electric energy heat loss. In the rotor circuit of the asynchronous motor, in order to prevent the regenerative braking formed during the deceleration of the motor and the energy feedback during the DC braking process, the inverter and the motor are damaged, and the dedicated braking unit/brake resistor needs to be serially connected. . The mechanical characteristic curve of the general AC motor brake. Let point A be the normal working point. The synchronous rotating magnetic field speed of the motor is: For the synchronous motor speed, the power frequency is the motor pole pair. During the braking process of the normal motor, the motor decelerates first, the synchronous rotating magnetic field speed of the motor is lower than the rotor speed, and the operating point jumps from point A of curve 1 to point B of curve 2 at the same speed, that is, transition from the first quadrant to The second quadrant is called the jump of the characteristic at the same speed, and the motor obtains the braking torque T in the opposite direction to enter the generating braking state. The dragging system rapidly decelerates along the curve 2 in Fig. 1, when it is lower than a certain speed. Thereafter, a direct current is input to the stator winding to form a fixed magnetic field, and a braking torque is generated. In this process, the motor will eventually stop after regenerative braking and energy braking. Theoretically, if the speed of the synchronous magnetic field of the motor can be controlled to slowly decrease, and the characteristic jump of the motor at the same speed, the characteristic curve is maintained in the first quadrant, as shown by the dashed line group 3 in FIG. Turning to the second quadrant, the drag system can effectively avoid the regenerative braking process during the speed reduction process. As shown in Figure 1, when the motor speed is less than the critical speed nh, the DC is connected to brake, and the magnitude and time of the connected DC are controlled accordingly. Theoretically, the motor only experiences a limited energy braking phase, Will overheat. The good internal and external characteristics of the inverter can ensure the above conditions are met. However, there are some necessary preconditions for adopting this method. First, the system cannot start/stop frequently, otherwise it will cause the DC circuit of the inverter to malfunction. Secondly, the working conditions of lifting heavy objects such as hoists and elevators are not suitable. Again, the system slowdown time should not be too short, that is, the speed reduction cannot be too fast, otherwise the working point will enter the second quadrant to generate a regenerative braking process, causing the motor to overheat. 3. Conclusion Theoretical analysis can prove that the design idea is completely reasonable. In practice, the frequency converter adopts DC braking and cooperates with appropriate DC braking time. The braking effect of the motor generated by the starting frequency and braking level is also obvious. gree , https://www.greegroups.com