A VFD braking solution is used to handle the regenerative energy generated during motor deceleration or stopping, preventing excessive DC bus voltage and ensuring safe and efficient system operation. Common solutions include dynamic braking and regenerative braking, which can be selected based on load characteristics, energy efficiency, and cost requirements.

In a variable frequency speed control system, motor deceleration and shutdown are achieved by gradually reducing the frequency. At the instant the frequency decreases, the synchronous speed of the motor drops accordingly, while the rotor speed remains unchanged due to mechanical inertia. When the synchronous speed w_1 becomes lower than the rotor speed w, the phase of the rotor current shifts by nearly 180 degrees, causing the motor to transition from motoring to generating mode. Simultaneously, the torque on the motor shaft becomes braking torque T_e, rapidly reducing the motor speed, placing the motor in a regenerative braking state. The regenerated electrical energy P from the motor is fed back into the DC circuit through full-wave rectification via the freewheeling diodes. Since the electrical energy in the DC circuit cannot be fed back to the grid through the rectifier bridge, it is absorbed only by the inverter’s own capacitor. Although other parts can consume electrical energy, the capacitor still accumulates charge for a short period, forming a “pumped voltage,” causing the DC voltage Ud to rise. Excessive DC voltage can damage various components. Therefore, measures must be taken to handle this regenerated energy. Our company offers the following two solutions.

Solution A: Dynamic Braking
The This method consumes regenerative energy through a braking resistor. Its working principle involves using a chopper (also known as a braking unit) to control the braking resistor in absorbing energy within the DC circuit, thereby achieving rapid braking. This solution features a simple structure, low cost, and high braking torque, and it does not cause pollution to the power grid. However, it cannot recover regenerative energy. It is suitable for scenarios sensitive to cost or with low requirements for grid stability, such as in standard centrifuges or planing machines.

Solution B: Regenerative Braking
This method inverts the regenerative energy into AC power at the same frequency and phase to feed it back to the grid. By employing active inversion technology, it enables energy recycling, improves system efficiency, and supports four-quadrant operation. However, it requires a stable grid voltage (with fluctuations not exceeding 15%); otherwise, commutation failure may easily occur. There is also a risk of harmonic pollution, and the control complexity and cost are relatively high. It is suitable for applications that require frequent braking and have a stable grid supply, such as potential energy loads in cranes and elevators.‌