The inverter is equipped with a braking resistor, mainly to consume part of the energy on the DC bus capacitor through the braking resistor to avoid the voltage of the capacitor being too high. Theoretically, if the capacitor stores more energy, it can be used to release it to drive the motor to avoid energy waste, but 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 extent, it may damage the capacitor, and some may damage the IGBT, so it needs to release the power through the braking resistance in time, this release is wasted in vain, and it is a practice that has no way.
The bus capacitor is a buffer and has a limited amount of energy
After the three-phase alternating current is rectified, the capacitor is accessed, and when the full load is running, the normal voltage of the bus is about 1.35 times, 380*1.35=513 volts, of course, this voltage will be shaken in real time, but the minimum can not be lower than 480 volts, otherwise it will be undervoltage alarm protection. The bus capacitor is generally composed of two groups of 450V electrolytic capacitors in series, and the theoretical withstand voltage is 900V, if the bus voltage exceeds this value, the capacitor will directly burst out, so the bus voltage can not reach 900V in any case.
In fact, the withstand voltage value of the three-phase 380 volt input IGBT is 1200 volts, which often requires the operation to be within 800 volts, considering that if the voltage rises, there will be an inertia problem, that is, you immediately let the braking resistor work, and the bus voltage will not be reduced quickly, so many inverters are planned to pass through the braking unit at about 700 volts to let the braking resistor start to operate, so that the bus voltage is reduced to avoid continuing to rush up.
Therefore, the core of braking resistance planning is to take into account the withstand voltage of capacitors and IGBT modules, so as to avoid these two important equipment being damaged by the high voltage of the busbar, if these two types of components are broken, the inverter will not be able to operate normally.
Rapid stopping requires braking resistance, and instant acceleration is also required
The reason why the bus voltage of the inverter will become higher is that the inverter makes the motor work in the electronic braking condition, so that the IGBT goes through a certain conduction sequence, and the motor is a large inductor current can not be abruptly changed, and the high-voltage alternating bus capacitor is instantly charged, so that the motor can quickly reduce the speed down. If there is no braking resistor at this time, the energy of the bus will be consumed in time, and the bus voltage will continue to increase and the safety of the inverter will be threatened. If the load is not very heavy, and there is no requirement for quick stopping, there is no need to use the brake resistor in this case, even if you install the brake resistor, the operating threshold voltage of the brake unit is not triggered, and the brake resistor will not be put into operation.
In addition to the need to increase the braking resistance and braking unit to brake quickly in the case of large-load deceleration, in fact, if the fit is heavier, the starting time is always required to be very fast, and the braking unit and braking resistor are also required to cooperate with the start, in the past I have tried to use the inverter to drive a special punch, It is required to plan the acceleration time of the inverter to 0.1 seconds, at this time the full load starts, although the load is not very heavy, but because the acceleration time is too short, at this time, the bus voltage is very shaken, and it will also present the situation of overvoltage or overcurrent, and then the external braking unit and braking resistance are added, and the inverter can operate normally. Analyzed, because the starting time is too short, the voltage of the bus capacitor is instantly hollowed out, and the rectifier is instantly charged with a large current, causing the bus voltage to suddenly become high, so that the voltage of the bus is too shaken, and it may exceed 700 volts in an instant.
There is also a special situation, is the vector control occasion, the torque and speed of the motor in the opposite direction, perhaps the operation in the zero speed 100% torque output of the occasion, such as the crane dropped a heavy object and stopped in mid-air, the occasion of rewinding and unwinding requires torque control, all need to let the motor work in the generator condition, the continuous current will be backcharged into the bus capacitor, through the braking resistance, it can consume the energy in time, keep the bus voltage balance stable.
Many small inverters, such as 3.7KW, often have built-in braking units and braking resistors, which should be due to the reason for the reduction of bus capacitance, while the low-power resistors and braking units are not so expensive.
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