The preparation of the braking resistor of the inverter is mainly to consume part of the energy on the DC bus capacitor through the braking resistor to prevent the capacitor voltage from being too high. Theoretically, if the capacitor stores a large amount of energy, it can be used to release it to drive the motor and prevent energy waste. However, the capacitance of a capacitor is limited, and the withstand voltage of a capacitor is also limited. When the voltage of the bus capacitor reaches a certain level, the capacitor may be damaged, and some even damage the IGBT. Therefore, it is necessary to brake the resistor in time to release the electricity. This exculpation is wasted, and there is no way to do it.
After the three-phase alternating current is fully rectified, the capacitor is connected. When 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 voltage cannot be lower than 480 volts, otherwise it will give undervoltage alarm protection. The bus 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 burst directly, so the bus voltage will not reach 900V anyway.
In fact, the withstand voltage of a 380 volt input three-phase IGBT is 1200 volts, and it is often required to operate within 800 volts. Considering 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 quickly, so many VFDs go through the braking unit to let the braking resistor start working at about 700 volts, and then reduce the bus voltage to prevent it from rising.
Therefore, the core of braking resistor planning is to consider the withstand voltage of the capacitor and IGBT module to prevent these two important devices from being damaged by the high voltage of the bus. If these two components are broken, the VFD will not work properly.
Rapid parking requires braking resistance, and also requires instantaneous acceleration.
The reason why the voltage of the VFD bus becomes higher is that in many cases, the VFD is the reason. Let the motor work in the electronic braking condition, and let the IGBT go through a certain conduction sequence. By using the large inductance of the motor, the current does not change suddenly, and a high voltage is generated to charge the bus capacitor instantaneously. At this time, let the motor slow down quickly. At this moment, if there is no braking resistor to consume the energy of the bus in time, the bus voltage will continue to rise, threatening the safety of the VFD.
If the load is not heavy and there is no emergency stop required, there is no need to use a braking resistor in this case. Even if a braking resistor is installed, the operating threshold voltage of the braking unit is not triggered, and the braking resistor is not put into operation.
In addition to the need to add a braking resistor and a braking unit for rapid braking in the case of heavy load deceleration, in fact, if the requirements of the heavy-load starting time and the starting time are very fast, the braking unit and the braking resistor are also required to cooperate to start. I once tried to drive a special punch machine with a VFD, and I asked the VFD to speed up to 0.1 seconds. At this point, it starts at full capacity, although the load is not very heavy. However, due to the shortness of the acceleration time, the bus voltage fluctuates greatly at this moment, and there will also be overvoltage or overcurrent. Later, an external braking unit and braking resistor were added, and the VFD was able to operate normally. The analysis is because the starting time is too short, the bus capacitor voltage is instantly hollowed out, and the rectifier is instantly charged with a large current, which causes the bus voltage to suddenly become high, so that the bus voltage fluctuates too much, and it may exceed 700V in an instant. The addition of braking resistors can eliminate this fluctuating high voltage in time, and the VFD can operate normally.
There is also a special case, in the case of vector control, the torque of the motor is in the opposite direction to the speed, or the motor is operating at zero speed and outputs 100% torque. For example, the crane is stopped in mid-air, and torque control is required in the case of pay-off and pay-off, so that the motor needs to work in the generator condition, and the continuous current will be reversely charged into the bus capacitor. After braking resistance, it can consume energy in time and insist on the balance and stability of the bus voltage.
Many low-power VFDs, such as 3.7 kW, usually have a built-in braking unit and braking resistors. Considering the reduction of bus capacitance, the low-power resistor and braking unit is not so expensive.
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