The main purpose of equipping a frequency converter with a braking resistor is to dissipate some of the energy stored in the DC bus capacitor, preventing the capacitor voltage from becoming too high. Theoretically, if the capacitor stores a lot of energy, it can be released to drive the motor, avoiding energy waste. However, the capacitor's capacity and voltage rating are limited. When the voltage of the bus capacitor reaches a certain level, it may damage the capacitor, and in some cases, it may even damage the IGBT. Therefore, it is necessary to release the energy through the braking resistor in a timely manner. This release is a wasteful measure and is a last resort.
The bus capacitor acts as a buffer, with a limited capacity for energy.
After all three-phase AC power is rectified and connected to capacitors, the normal bus voltage under full load is approximately 1.35 times the rated voltage, 380 * 1.35 = 513 volts. This voltage will naturally fluctuate in real time, but it must not fall below 480 volts; otherwise, an undervoltage alarm will activate. The bus capacitor is typically composed of two sets of 450V electrolytic capacitors connected in series, with a theoretical withstand voltage of 900V. If the bus voltage exceeds this value, the capacitors will explode. Therefore, the bus voltage must never reach such a high voltage as 900 volts.
In reality, the withstand voltage of a three-phase 380V input IGBT is 1200V, and it is often required to operate below 800V. Considering that if the voltage rises, there will be an inertia problem, that is, even if you immediately activate the braking resistor, the bus voltage will not drop quickly. Therefore, many frequency converters are designed to activate the braking resistor through the braking unit at around 700V to reduce the bus voltage and prevent it from continuing to rise.
Therefore, the core of braking resistor design is to consider the voltage withstand capability of the capacitor and IGBT module, so as to prevent these two important components from being damaged by the high voltage of the bus. If these two types of components fail, the frequency converter will not be able to work properly.
Rapid stopping requires braking resistors, and instantaneous acceleration also requires them.
The reason why the inverter bus voltage rises is often because the inverter operates the motor in electronic braking mode. This involves the IGBTs conducting in a specific sequence, utilizing the motor's large inductive current, which cannot change abruptly, to instantly generate high voltage to charge the bus capacitor. This forces the motor to decelerate quickly. If there is no braking resistor to dissipate the energy from the bus in time, the bus voltage will continue to rise, threatening the inverter's safety. If the load is not heavy and there is no requirement for rapid stopping, a braking resistor is not needed. Even if a braking resistor is installed, it will not operate unless the braking unit's operating threshold voltage is triggered.
Besides the need for additional braking resistors and braking units in high-load deceleration applications for rapid braking, braking units and braking resistors are also necessary for applications with relatively heavy loads and very short start-up times. I previously tested using a frequency converter to drive a special punch press, requiring the converter's acceleration time to be designed to be 0.1 seconds. Even with a relatively light load, the extremely short acceleration time caused severe bus voltage fluctuations, leading to overvoltage or overcurrent. Adding an external braking unit and braking resistor resolved the issue, allowing the frequency converter to operate normally. The reason is that the extremely short start-up time instantly depletes the bus capacitor's voltage, while a large current rushes into the rectifier, causing a sudden spike in bus voltage. This severe voltage fluctuation can momentarily exceed 700 volts. The braking resistor effectively eliminates this high-voltage fluctuation, allowing the frequency converter to operate normally.
Another special case is vector control, where the motor's torque and speed are in opposite directions, or when it is operating at zero speed and 100% torque output. For example, when a crane drops a heavy object and stops in mid-air, or when torque control is required during winding and unwinding, the motor needs to operate in generator mode. The continuous current will be fed back into the bus capacitor. Through the braking resistor, this energy can be consumed in time to maintain the balance and stability of the bus voltage.
Many small frequency inverters, such as 3.7KW, often have built-in braking units and braking resistors, probably because the bus capacitor is reduced in size. And low-power resistors and braking units are not that expensive.
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