1. Introduction Currently, AC variable frequency speed control systems widely employ simple energy-consuming braking, which suffers from drawbacks such as energy waste, severe resistance heating, and poor rapid braking performance. However, regenerative braking is a highly effective energy-saving method for frequent braking of asynchronous motors, avoiding environmental and equipment damage during braking. It has achieved satisfactory results in industries such as electric locomotives and oil extraction. With the continuous emergence of new power electronic devices, their increasing cost-effectiveness, and growing public awareness of energy conservation and consumption reduction, it has broad application prospects. Energy regenerative braking devices are particularly suitable for applications with large motor power (e.g., 100kW or more), large moment of inertia (gd²), repetitive short-time continuous operation, significant deceleration from high to low speed, short braking time, and the need for strong braking. To improve energy saving and reduce energy loss during braking, it is essential to recover deceleration energy and feed it back to the power grid to achieve energy-saving effects. 2. Regenerative Braking Principle In a variable frequency speed control system, the motor speed reduction and stopping are achieved by gradually decreasing the frequency. At the instant the frequency decreases, the synchronous speed of the motor decreases accordingly. However, due to mechanical inertia, the rotor speed of the motor remains unchanged, and its speed change has a certain time lag. At this point, the actual speed may exceed the given speed, resulting in a situation where the motor's back electromotive force (e) is higher than the DC terminal voltage (u) of the inverter, i.e., e > u. In this case, the motor becomes a generator, not only not requiring grid power but also able to supply power to the grid. This achieves both good braking effect and the conversion of kinetic energy into electrical energy, thus recovering energy by supplying power to the grid – a double benefit. Of course, an energy feedback device unit is required for automatic control to achieve this, and its principle block diagram is shown in Figure 1. In addition, the energy feedback circuit should also include AC and DC reactors, RC absorbers, electronic switches, etc. [align=center]Figure 1 Block diagram of inverter regenerative braking circuit[/align] As is well known, the bridge rectifier circuit of a general-purpose inverter is three-phase uncontrollable, thus it cannot achieve bidirectional energy transfer between the DC circuit and the power supply. The most effective way to solve this problem is to use active inverter technology, with the rectifier section using a reversible rectifier, also called a grid-side converter. By controlling the grid-side converter, the regenerated electrical energy is inverted into AC power with the same frequency and phase as the grid, which is fed back to the grid, thereby achieving braking[sup][2][/sup]. Previously, active inverter units mainly used thyristor circuits. This type of circuit can only safely operate under a stable grid voltage (grid voltage fluctuation not greater than 10%) where failures are unlikely. This is because during generator braking operation, if the grid voltage braking time is greater than 2ms, commutation failure may occur, damaging the components. Furthermore, this method, when used in deep control, suffers from low power factor, high harmonic content, and commutation overlap, which can cause grid voltage waveform distortion. It also involves complex control and high cost. With the practical application of fully controlled devices, chopper-controlled reversible converters have been developed, employing PWM control. Thus, the structure of the grid-side converter is identical to that of the inverter, both using PWM control. From the above analysis, it is clear that the key to truly achieving energy feedback braking in the frequency converter lies in the control of the grid-side converter. The following section focuses on the control algorithm for the grid-side converter using fully controlled devices and PWM control. 3. Control Algorithm The control algorithm for grid-side converters typically employs the vector control algorithm shown in Figure 2. In Figure 2, v_dc, v_*, and Δv_dc represent the measured value, setpoint, and control error of the DC bus voltage, respectively; i_d, i_*, and Δi_d represent the measured value, setpoint, and control error of the d-axis of the grid-side inverter, respectively. i[sub]q[/sub], i[sup]＊[/sup][sub]q[/sub], and △i[sub]q[/sub] represent the measured value, setpoint value, and control error of the q-axis current of the grid-side converter, respectively; △v[sup]＊[/sup][sub]d[/sub], v[sup]＊[/sup][sub]d[/sub], and v[sup]＊[/sup][sub]q[/sub] represent the setpoint values ​​of the d-axis output voltage deviation, d-axis output voltage, and q-axis output voltage of the grid-side converter, respectively; e[sub]abc[/sub], v[sup]＊[/sup][sub]abc[/sub], and i[sub]abc[/sub] represent the grid potential, the instantaneous setpoint value of the grid-side converter output voltage, and the three-phase instantaneous value of the output current, respectively; e and φ represent the amplitude and phase of the grid potential, respectively. [align=center] Figure 2. Block diagram of vector control algorithm for grid-side converter of energy feedback inverter[/align] The vector control algorithm obtains the given value of d-axis current by using the difference between the measured DC bus voltage and the given value through the PI regulator; then, based on the phase of the measured grid voltage, the measured output current of the grid-side converter is synchronously transformed to obtain the measured values ​​of d-axis current and q-axis current. After PI regulation, the d-axis quantity is added to the grid voltage amplitude to obtain the given values ​​of d-axis voltage and q-axis voltage, which are then output after synchronous inverse transformation. The advantages of this algorithm are high control accuracy and good dynamic response; the disadvantages are that there are many coordinate transformations in the control algorithm, the algorithm is more complex, and the computing power requirement of the control processor is higher[sup][3][/sup]. Equipment with lower processing power of the control system processor can also use a simplified current control algorithm, as shown in Figure 3. [align=center] Figure 3. Block diagram of current control algorithm for grid-side converter of energy feedback inverter[/align] As can be seen from Figure 3, it adopts a current tracking PWM rectifier configuration. This simplified algorithm directly multiplies the d-axis current setpoint by the three-phase sinusoidal reference value obtained by looking up the phase of the measured grid voltage in a table to obtain the setpoint value of the three-phase output current. Then, it performs simple PI regulation to obtain the setpoint value of the three-phase output voltage and outputs it. Since this algorithm omits coordinate transformation calculations, it has lower computational requirements for the control processor. On the other hand, due to the inherent characteristics of the PI regulator, its control of AC quantities has a certain steady-state error, resulting in a lower power factor than the standard vector control algorithm. During dynamic processes, the DC bus voltage fluctuation is relatively large, and the probability of DC bus voltage surges during rapid dynamic processes is relatively high. 4. Regenerative Braking Characteristics Strictly speaking, the grid-side converter cannot be simply called a "rectifier," as it can function as both a rectifier and an inverter. Due to the use of self-turn-off devices, the magnitude and phase of the AC current can be controlled through appropriate PWM modes, making the input current close to a sine wave and ensuring that the system's power factor is always close to 1. When the regenerative power returned from the inverter during motor deceleration and braking increases the DC voltage, the phase of the AC input current can be made opposite to the phase of the power supply voltage to achieve regenerative operation and feed the regenerative power back to the AC grid. The system can still maintain the DC voltage at a given value. In this case, the grid-side converter operates in active inverter mode. This facilitates bidirectional power flow and provides a fast dynamic response. Furthermore, this topology allows the system to fully control the exchange of reactive and active power between the AC and DC sides, achieving an efficiency of up to 97%, significant economic benefits, and heat loss that is only 1% of that of regenerative braking. It also does not pollute the grid, has a power factor of approximately 1, and is environmentally friendly. Therefore, regenerative braking can be widely applied to energy-saving operation in PWM AC drive applications, especially suitable for applications requiring frequent braking and where the motor power is also relatively high. The energy-saving effect is significant, averaging about 20% depending on the operating conditions. The only drawback of regenerative control is the complexity of the control system structure. 5. Conclusion In summary, it can be seen that the advantages of energy feedback systems far outweigh those of regenerative braking and DC braking. By using regenerative braking to feed regenerated electricity back to the grid, energy consumption can be reduced and electricity costs saved. Therefore, at a time when many parts of China are experiencing power shortages due to rapid economic development, promoting and applying regenerative braking devices has significant energy-saving implications. Consequently, in recent years, many organizations have requested the configuration of energy feedback devices based on the characteristics of their equipment. However, previously, only a few companies such as ABB and Siemens could provide these products, leaving a near-complete gap in the domestic market. Therefore, accelerating the research and development and production of related products in China is of great practical significance. About the Authors: Liu Yongfeng (1983-) Male, Master's student, research direction: power electronics and electric drives. Xie Jihua (1964-) Male, Associate Professor, mainly engaged in teaching and research in power electronics, motor control, intelligent instruments and network security. References [1] Zhang Xuanzheng, Zhang Jinyuan. Experience in the application of frequency converters. Beijing: China Electric Power Press, 2006 [2] Zhou Zhimin, Zhou Jihai, Ji Aihua. Practical technology of frequency converter power supply. Beijing: China Electric Power Press, 2005 [3] Yi Peng. Technical principle and application of high voltage high power frequency converter. Beijing: People's Posts and Telecommunications Press, 2008 [4] Chen Guocheng. PWM inverter technology and application. Beijing: China Electric Power Press, 2007 [5] Li Fangyuan. A brief discussion on load power generation and frequency conversion braking. Electrical Engineering Technology, 2003 (7): 38-41 [6] Wu Zhongzhi, Wu Jialin. Frequency converter application manual (2nd edition). Beijing: Machinery Industry Press, 2002