In the traditional variable frequency speed control system, which consists of a general-purpose inverter, an asynchronous motor, and a mechanical load, the motor may enter a regenerative braking state when it is driven downward. Similarly, when the motor decelerates from high to low speed (including stopping), a sudden drop in frequency can occur. Due to the inertia of the motor, it might still be in a regenerative state. The mechanical energy stored in the transmission system is converted into electrical energy by the motor and sent back to the DC link of the inverter through its six freewheeling diodes. At this point, the inverter operates in rectification mode. If no energy dissipation measures are taken, the energy can cause the voltage of the intermediate circuit capacitor to rise. In cases where the braking is too fast or the load involves a hoist, this could potentially damage the inverter, making it crucial to manage this energy effectively.
There are two common methods for handling regenerative energy in general-purpose inverters: dynamic braking and feedback braking. Dynamic braking involves dissipating the regenerative energy through a brake resistor connected in parallel with the DC bus capacitor, while feedback braking returns the energy to the grid using active inverter technology. Another method, DC braking, is used for precise parking or irregular rotation caused by external factors before starting.
Recent advancements have introduced a new braking technique known as "capacitor feedback braking," which combines the benefits of four-quadrant operation and high efficiency, while avoiding grid pollution and ensuring reliability.
Dynamic braking is a simple and cost-effective method that does not pollute the power grid. However, it suffers from low efficiency, especially when frequent braking occurs, leading to increased energy consumption and larger brake resistor requirements. Small inverters (below 22 kW) typically have built-in brake units, while larger ones require external components.
Feedback braking allows energy to be returned to the grid, improving overall system efficiency. However, it requires stable grid voltage, has complex control, and can introduce harmonic distortion. Additionally, during grid faults, commutation failure may occur, risking device damage.
The new "capacitor feedback braking" method utilizes a reversible chopper circuit. This configuration allows bidirectional energy flow, enabling energy to be fed back to the power source. The main circuit includes a rectifier bridge, electrolytic capacitors, a reactor, and IGBT modules. The CPU monitors voltages in real time and controls the charging and discharging processes to maintain safe operating conditions.
One key challenge in this system is selecting the right reactor. It must handle high currents during fault conditions and use materials like ferrite cores to optimize performance. Control challenges include managing the voltage on the electrolytic capacitor and preventing overvoltage in the DC link.
This braking method has found applications in mining equipment, lifting systems, and other industries requiring efficient energy management. Companies such as Shandong Fengguang Electronics have adopted this technology, successfully implementing it in mine hoisting systems. The technology is now running smoothly in coal mines in Shandong and Shanxi, marking a significant advancement in China's industrial automation sector. With growing demand, this application holds great potential for future development.
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