Regenerative braking, also known as regenerative feed back braking, operates by utilizing active inversion technology to convert the regenerative energy generated during motor deceleration/stopping into AC power that matches the grid’s frequency, phase, and voltage. This energy is then directly fed back into the grid, enabling energy recycling and fundamentally resolving the DC bus “voltage pumping” issue while achieving energy savings. As a high end braking solution in variable frequency drive (VFD) systems, it supports four-quadrant motor operation and is well-suited for industrial applications involving potential energy loads or frequent braking.
Core Components
The regenerative braking system mainly consists of four parts: a regenerative unit (active inverter), a filter circuit, a detection and control circuit, and a protection circuit. Some integrated products integrate the core module into a regenerative braking cabinet, suitable for high-power frequency converter scenarios.
Regenerative Unit
The core is a high-power inverter bridge (composed of power devices such as IGBTs), which is key to converting DC power to AC power. It precisely controls the frequency, phase, and amplitude of the inverted voltage to match grid parameters.
Filter Circuit
Includes components like reactors and capacitors, used to suppress harmonics generated during inverting process, reduce harmonic pollution to the grid, and stabilize the feedback current.
Detection and Control Circuit
Real-time detection of grid voltage, frequency, phase, and inverter DC bus voltage. A closed-loop control algorithm adjusts the inverter bridge’s operating state to ensure the synchronization and stability of the power feedback.
Protection Circuit
Features protection functions such as overvoltage, overcurrent, phase loss, phase error, and grid fluctuation exceedance. When grid voltage fluctuations exceed 15% or a fault occurs, it immediately cuts off the feedback circuit to prevent commutation failure, device damage, and the spread of grid faults.
1. Working Principle
1.1. When the motor decelerates, stops, or releases potential energy (e.g., a crane lowering a load, an elevator descending), it maintains a high speed due to mechanical inertia. The synchronous speed is lower than the rotor speed, and the motor enters a generator state. The regenerated electrical energy is rectified by the freewheeling diode and fed back to the DC bus of the frequency converter, causing the bus voltage Ud to rise.
1.2. When the DC bus voltage reaches the start-up threshold of the regenerative unit, the detection circuit captures the voltage, frequency, and phase signals of the power grid in real time, and the control circuit drives the inverter bridge of the regenerative unit to work.
1.3. The inverter bridge converts the regenerative electrical energy on the DC bus into three-phase AC power with the same frequency, phase, and amplitude as the power grid. After the filter circuit filters out harmonics, the electrical energy is fed back to the power grid, achieving energy recovery.
1.4. When the DC bus voltage drops to the stop threshold of the regenerative unit, the inverter bridge stops working, and the feedback process is interrupted. If the bus voltage rises again, the above process is repeated, always stabilizing the DC bus voltage within a safe range.
2. Braking Process
2.1. Energy Generation: The motor operates in a regenerative braking state, feeding regenerative energy back to the VFD’s DC circuit, causing the DC bus voltage to continuously rise.
2.2. Threshold Trigger: When the bus voltage reaches the preset starting voltage of the feedback unit, the detection circuit initiates real-time sampling of grid parameters, and the control circuit enters the working state.
2.3. Active Inversion: The inverter bridge converts DC power into AC power conforming to grid standards based on the grid sampling signal, and after filtering, it is fed back to the grid.
2.4. Voltage Stabilization: As the regenerative energy is continuously fed back, the DC bus voltage gradually decreases. When it drops to the stop threshold, the regenerative unit ceases inversion.
2.5. Cyclic Monitoring: The system continuously monitors the bus voltage. If it reaches the startup threshold again, the inversion and feedback process repeats, dynamically balancing the bus voltage to ensure the safe operation of the VFD and motor system.
3. Braking Features
3.1. Energy Recovery, High Efficiency and Energy Saving: Regenerated electrical energy is directly fed back to the grid and can be reused. Compared with dynamic braking, there is no energy waste. In scenarios with frequent braking and large inertia/potential energy loads, the energy-saving effect is significant (the energy saving rate increases with the braking frequency);
3.2. No Heat Loss, Reduced Equipment Thermal Load: Since no resistors are used for energy dissipation, substantial heat generation is avoided. This eliminates the need for additional cooling equipment, improves the equipment operating environment, and reduces cooling costs.
3.3. Stable Braking Torque, Supports Four-Quadrant Operation: The torque output is stable during braking, enabling four-quadrant operation (motor forward rotation, reverse rotation, motoring, and braking). Suitable for cranes, elevators, mine hoists, and other loads requiring frequent forward and reverse rotation and potential energy lifting;
3.4. Low Long-Term Operating Costs: Although the initial investment is higher, the energy-saving benefits offset the equipment cost over the entire life cycle. Furthermore, there is no need to replace the braking resistor, resulting in lower maintenance costs.
4. Selection and Usage Precautions
4.1. Core Selection Basis
Grid Parameters: Confirm the grid voltage level, number of phases, and voltage fluctuation range to ensure compliance with the ±15% stability requirement;
Regenerative Power: Calculate the peak and average power of the regenerated energy based on the motor’s rated power, braking frequency, and deceleration time, and select a regenerative unit with matching power;
Load Characteristics: For potential energy loads, consider the regenerative power corresponding to the maximum discharge speed, reserving a power margin of 1.2 to 1.5 times;
Harmonic Mitigation Requirements: Select appropriate reactors and filters according to grid-side harmonic standards to ensure harmonic emissions meet industrial grid requirements.
4.2. Precautions for Use
4.2.1. During installation, configure an isolating circuit breaker between the regenerative unit and the grid for fault maintenance. Ensure the wiring to the VFD’s DC bus has sufficient wire gauge to minimize line losses.
4.2.2. Periodically inspect the power components and detection elements of the regenerative unit, replacing aging parts promptly to prevent device failures from causing grid faults.
4.2.3. If there are power grid fluctuations, an additional power grid voltage stabilizer can be configured to improve system adaptability.
4.2.4. The regenerative braking system requires separate grounding, independent of the inverter’s grounding system, to prevent interference.
5. Applicable Scenarios
Regenerative braking is suitable for industrial scenarios with stable power grids, frequent braking/reversal, and high energy-saving requirements. It is primarily compatible with potential energy loads and high-inertia loads, specifically including:
5.1. Lifting equipment: elevators, escalators, cranes, tower cranes, mine hoists, chain hoists, hydraulic lifting platforms;
5.2. High-inertia transmission equipment: large centrifuges, rolling mills, oilfield pumping units, port gantry cranes;
5.3. Frequent forward/reverse equipment: machine tool spindles, metallurgical conveying equipment, mining scraper conveyors.
Unsuitable Scenarios: Operating conditions with significant grid voltage fluctuations or severe harmonic pollution; general loads sensitive to cost and with low braking frequency (e.g., small fans, pumps, standard conveyor belts); standalone VFD systems without grid feedback capabilities.
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