The supplier of the frequency converter braking unit reminds you that the frequency converter is equipped with a dynamic resistor mainly to consume a part of the energy on the DC bus capacitor through the braking resistor, in order to avoid excessive voltage of the capacitor. In theory, if a capacitor stores a lot of energy, it can be used to release it to drive a motor and avoid energy waste. However, the capacity of a capacitor is limited, and its withstand voltage is also limited. When the voltage of the bus capacitor reaches a certain level, it may damage the capacitor, and some may even damage the IGBT. Therefore, it is necessary to release the electricity through a braking resistor in a timely manner. This release is a waste of time and is an unavoidable solution.
Bus capacitor is a buffer zone that can hold limited energy
After all three-phase AC power is rectified and connected to capacitors, the normal voltage of the bus during full load operation is approximately 1.35 times, 380 * 1.35=513 volts. This voltage will naturally fluctuate in real time, but the minimum cannot be lower than 480 volts, otherwise it will trigger an undervoltage alarm protection. Bus capacitors are generally 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 capacitor will directly explode, so the bus voltage cannot reach such a high voltage of 900V no matter what.
In fact, the withstand voltage value of IGBT with three-phase 380 volt input is 1200 volts, which often requires operation within 800 volts. Considering that if the voltage increases, there will be an inertia problem, that is, if you immediately make the braking resistor work, the bus voltage will not decrease quickly. Therefore, many frequency converters are designed to start working at around 700 volts through the braking unit to lower the bus voltage and avoid further upward charging.
So the core of designing braking resistors is to consider the voltage resistance of capacitors and IGBT modules, in order to avoid these two important components from being damaged by the high voltage of the bus. If these two types of components are damaged, the frequency converter will not work properly.
Quick parking requires a braking resistor, and instant acceleration also requires it
The reason why the bus voltage of the frequency converter increases is often due to the frequency converter causing the motor to operate in an electronic braking state, allowing the IGBT to pass through a certain conduction sequence, utilizing the large inductance current of the motor that cannot suddenly change, and instantly generating high voltage to charge the bus capacitor. At this time, the motor is quickly slowed down. If the braking resistor does not consume the energy of the bus in a timely manner at this time, the bus voltage will continue to rise, posing a threat to the safety of the frequency converter.
If the load is not very heavy and there is no requirement for quick stopping, there is no need to use a braking resistor in this situation. Even if you install a braking resistor, the working threshold voltage of the braking unit will not be triggered, and the braking resistor will not be put into operation.
In addition to the need to increase braking resistance and braking unit for rapid braking in heavy load deceleration situations, in fact, if it meets the requirements of heavy load and very fast starting time, the braking unit and braking resistance also need to be coordinated for starting. In the past, I tried to use a frequency converter to drive a special punch press, and the acceleration time of the frequency converter was designed to be 0.1 seconds. At this time, when starting at full load, although the load is not very heavy, because the acceleration time is too short, the bus voltage fluctuation is very severe, and overvoltage or overcurrent situations may occur. Later, an external braking unit and braking resistance were added, and the frequency converter can work normally. In analysis, it is because the start-up time is too short, and the voltage of the bus capacitor is instantly emptied. The rectifier instantly charges a large current, causing the bus voltage to suddenly increase. This results in severe voltage fluctuations on the bus, which may exceed 700 volts in an instant. With the addition of a braking resistor, this fluctuating high voltage can be eliminated in a timely manner, allowing the frequency converter to operate normally.
There is also a special situation in vector control, where the torque and speed directions of the motor are opposite, or when working at zero speed with 100% torque output. For example, when a crane drops a heavy object and stops in mid air, or when rewinding, torque control is required. The motor needs to work in the generator state, and the continuous current will be back charged into the bus capacitor. Through the braking resistor, this energy can be consumed in a timely manner to maintain the balance and stability of the bus voltage.
Many small frequency converters, such as 3.7KW ones, often have built-in braking units and braking resistors, probably due to the consideration of reducing the bus capacitor, while low-power resistors and braking units are not that expensive.