New brake method for inverters - Solutions - Huaqiang Electronic Network

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1 Introduction

In traditional variable frequency systems composed of general-purpose inverters, asynchronous motors, and mechanical loads, when the motor is driven downward, it enters a regenerative braking state. Similarly, when the motor decelerates rapidly, such as in vehicle braking, the frequency drops abruptly. However, due to the mechanical inertia of the motor, it may still be in a regenerative state. The mechanical energy stored in the transmission system is converted into electrical energy by the motor and fed back to the six freewheeling diodes of the inverter, which then operates in a rectified state. If no energy dissipation measures are taken, this energy can cause the DC bus capacitor voltage to rise, potentially damaging the inverter, especially in cases of rapid braking or heavy lifting applications. Therefore, it's essential to manage this energy effectively.

There are two common methods for handling regenerative energy in general-purpose inverters: (1) Energy dissipation through a brake resistor connected in parallel with the DC bus capacitor, known as energy consumption braking; and (2) Feedback of the energy back to the power grid, called feedback or regenerative braking. Another method is DC braking, used for precise parking or irregular stopping before motor startup.

Many experts have discussed the design and application of inverter braking, particularly in recent years, with increasing focus on energy feedback braking. This article introduces a new braking method that combines the advantages of four-quadrant operation from feedback braking with high efficiency and the benefits of energy braking, such as no grid pollution and high reliability.

2 Energy Braking

Energy braking involves using a brake resistor in the DC loop to absorb the regenerative energy from the motor. As shown in Figure 1, this method has a simple structure, no grid pollution, low cost, and is suitable for low-power inverters. However, its main disadvantage is low efficiency, especially when frequent braking occurs, leading to higher energy consumption and larger resistor sizes.

Figure 1: Schematic diagram of energy consumption braking

3 Feedback Braking

Feedback braking requires voltage equal-frequency control and feedback current control. It uses active inverter technology to convert the regenerative energy back to AC power at the same frequency as the grid, as shown in Figure 2. This method allows four-quadrant operation, improving system efficiency. However, it has several drawbacks: it requires stable grid voltage (within 10% fluctuations), may cause harmonic pollution, and involves complex control with higher costs.

Figure 2: Schematic diagram of feedback braking

Figure 3: Four-quadrant motion map

4 New Braking Method (Capacitive Feedback Braking)

4.1 Main Circuit Principle

The main circuit schematic is shown in Figure 4. The rectification part uses an uncontrolled rectifier bridge (VD1–VD6), the filter uses electrolytic capacitors (C1, C2), and the delay loop uses a contactor or thyristor (T1). The charging and feedback loop includes an IGBT module (VT1, VT2), reactor L, and a large electrolytic capacitor C. The inverter part uses another IGBT module (VT5–VT10), and the protection circuit includes IGBTs and power resistors.

During motor generation, the CPU monitors the AC input and DC link voltage Ud. When Ud exceeds the corresponding value, the CPU turns off VT3, allowing VT1 to pulse and charge the capacitor C. The reactor L and capacitor C work together to keep the capacitor within safe limits. If the voltage becomes too high, the safety circuit activates resistance braking to consume excess energy.

When the motor is running, the CPU pulses VT3, causing the reactor L to generate a negative voltage, enabling energy feedback from the capacitor to the DC link. By monitoring the capacitor voltage and DC link voltage, the CPU controls the switching frequency and duty ratio of VT3 to prevent excessive Ud.

4.2 System Challenges

(1) Reactor Selection

Reactor selection is critical. In case of a fault, the motor may accelerate freely, generating high regenerative energy. The reactor must handle this current, so a large diameter is required. For the feedback loop, a ferrite core is preferred over silicon steel to reduce losses, though large ferrite cores may be expensive and hard to find.

Therefore, it’s recommended to use separate reactors for charging and feedback loops.

(2) Control Difficulties

In the DC circuit, the voltage U4 is typically above 500 VDC, while the electrolytic capacitor C is rated for only 400 VDC. Controlling the charging process is more complex than energy braking. The voltage drop across the reactor must be carefully managed to ensure the capacitor remains within safe limits.

During feedback, it's crucial to prevent the capacitor current from raising the DC link voltage too high, triggering overvoltage protection.

4.3 Main Applications and Examples

This new braking method—capacitive feedback braking—offers significant advantages and has been adopted by many users based on their equipment needs. Despite technical challenges, few foreign companies have developed similar solutions. Only Shandong Fengguang Electronics Co., Ltd. has implemented this method in mine hoisting machines. Currently, this type of inverter is operating successfully in coal mines in Ningyang, Shandong, and Taiyuan, Shanxi, filling a gap in China’s market.

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