The inverter is equipped with dynamic resistance, which mainly consumes a part of the energy on the DC bus capacitor through the braking resistance to avoid the capacitor voltage being too high. In theory, if the capacitor stores more energy, it can be used to release it to drive the motor, avoiding energy waste. However, the capacity of the capacitor is limited, and the withstand voltage of the capacitor is also limited. When the voltage of the bus capacitor is high to a certain level, the capacitor may be damaged, and some IGBTs may also be damaged. Therefore, it is necessary to discharge through the braking resistor in time. This release is wasted, there is no way.

Recycling the inverter

After the three-phase alternating current is fully rectified, it is connected to the capacitor. When the bus is running at full load, the normal voltage of the bus is about 1.35 times, 380*1.35=513 volts. Of course, this voltage will fluctuate in real time, but the minimum cannot be lower than 480 volts, otherwise it will be under-voltage alarm protection. The bus capacitor is generally composed of two sets of 450V electrolytic capacitors connected in series, and the theoretical withstand voltage is 900V. If the bus voltage exceeds this value, the capacitor will explode directly, so the bus voltage will not reach a voltage as high as 900 V anyway.

In fact, the withstand voltage value of IGBTs with three-phase 380-volt input is 1200 volts, and it is often required to work within 800 volts. Consider that if the voltage rises, there will be an inertia problem, that is, if you let the braking resistor work immediately, the bus voltage will not drop very quickly. Therefore, the design of many inverters is to let the braking resistor work through the braking unit at about 700 volts, thereby reducing the bus voltage and avoiding further agitation.

Therefore, the core of the braking resistor design is to consider the withstand voltage of the capacitor and the IGBT module to prevent these two important devices from being damaged by the high voltage of the bus. If these two parts are broken, the inverter will not work properly.