The inverter has a brake resistor, which mainly consumes part of the energy of the DC bus capacitor to prevent the capacitor voltage from being too high. In theory, if the energy stored in the capacitor is large, it can be released to drive the motor to avoid energy waste. However, the capacity of the capacitor is limited, and the voltage of the capacitor is also limited. When the bus capacitor voltage reaches a certain level, it will damage the capacitor, and sometimes it will also damage the IGBT, which requires the system to handle it in time. Dynamic resistance will release electrical energy, and this release is wasteful, which is also a way out.
The bus capacitor is a buffer with limited energy. The three-phase AC power is connected to the capacitor after being fully rectified. When the load is fully loaded, the normal bus voltage is about 1.35 times the input voltage, that is, 380*1.35=513 volts. This voltage fluctuates in real time, but the minimum value cannot be lower than 480 volts, otherwise it will trigger the undervoltage alarm protection. The bus capacitor is usually a group of 450V electrolytic capacitors connected in series, with a theoretical withstand voltage of 900V. If the bus voltage exceeds this value, the capacitor will explode directly, so the bus voltage cannot reach 900 volts under any circumstances.
In practice, a three-phase 380V IGBT is rated for 1200V and usually needs to be operated below 800V. It is important to consider that if the voltage is increased, there will be inertia problems. That is, if you activate the brake resistor immediately, the bus voltage will not drop quickly. Many converters are designed to activate at around 700V. The brake unit allows the brake resistor to start working, which can reduce the bus voltage to avoid further upstream pulses.
