Under what circumstances does the inverter use the brake resistance?
The inverter has a braking resistance, which mainly consumes the energy of some DC bus capacitors through the braking resistance to avoid the capacitor voltage being too high.
In theory, if the capacitor has more energy storage, the motor can be driven by release to avoid energy waste, but the capacitor capacity is limited and the capacitor voltage is limited.
When the voltage of the bus capacitor reaches a certain level, the capacitor will be damaged, and some will damage the IGBT and need to pass the system in time.
Dynamic resistance releases electric energy. This release is a waste and a way out.
Bus capacitor is a buffer containing limited energy.
After the three-phase AC is completely rectified, connect the capacitor.
When the load is full, the normal voltage of the bus is about 1. 35 times, 380*1. 35 = 513 volts.
This voltage will fluctuate in real time, but the minimum value cannot be lower than 480 volts, otherwise it will be under pressure alarm protection.
Bus capacitors are usually a series of 450V Electrolytic capacitors connected in series. The theoretical withstand voltage is 900 V.
If the bus voltage exceeds this value, the capacitor will explode directly, so the bus voltage cannot reach 900 volts under any circumstances.
In fact, The withstand voltage value of the three-phase 380-volt IGBT is 1200 volts, which usually needs to work within 800 volts.
Considering that if the voltage rises, there will be inertia problems.
That is to say, you immediately let the brake resistor work while the bus voltage does not decrease quickly.
Many converters are designed to be turned on at a voltage of about 700 volts.
The over-brake unit allows the brake resistor to start working, which reduces the bus voltage to avoid further upstream pulses.
Therefore, the core of the brake resistance design is to consider the voltage problem of the capacitor and IGBT module to avoid the two important devices being damaged by the high voltage of the bus. If these two components are damaged, the inverter will not work properly.
The reason why the braking resistance is stopped quickly and the instantaneous acceleration also requires the bus voltage of the frequency converter to rise is that the frequency converter makes the motor work in the electronic braking state and makes the IGBT pass through a certain conduction sequence.
With this motor, the large inductance current will not change suddenly, and the high voltage bus capacitor can be charged instantly. At this point, the speed of the motor can be rapidly reduced.
If the bus energy is consumed in time without braking resistance, the bus voltage will continue to rise, threatening the safety of the converter.
In addition to the need to increase the braking resistance and for fast braking units, in the past, I tried to drive a dedicated punch with a frequency converter. The acceleration time of the inverter needs to be designed to be 0. 1 second.
At this point, the full load starts, although the load is not large, because the acceleration time is too short, the bus voltage fluctuation is very strong.
Overvoltage or overcurrent can also occur, and then external brake units and brake resistors can be added, and the inverter can work normally.