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Under what circumstances does the inverter need to prepare a braking resistor

The preparation of the variable-frequency Drive (VFD) braking resistor is mainly to consume a part of the energy on the DC bus capacitor through the braking resistor to avoid the capacitor voltage being too high. In theory, if the energy stored in the capacitor is very large, it can be used to release it to drive the motor to avoid wasting energy. However, the capacity of the capacitor is limited, and the withstand voltage of the capacitor is also limited. When the voltage of the bus capacitors reaches a certain level, the capacitors can be damaged, some even damage the IGBT. Therefore, it is necessary to brake the resistor in time to release the power. This release is wasted and there is no way to do it.

The bus capacitors are energy-limited buffers.

After the three-phase alternating current is fully rectified, connect the capacitor. 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 cannot be lower than 480 volts, otherwise it will give undervoltage alarm protection. The busbar is generally composed of two sets of 450V electrolytic capacitors in series, and the theoretical withstand voltage is 900V. If the bus voltage exceeds this value, the capacitor will explode directly, so the bus voltage will not reach 900V anyway.


In fact, the withstand voltage of a three-phase IGBT with a 380-volt input is 1200 volts, and it is often required to work within 800 volts. Considering that if the voltage rises, there will be an inertia problem, that is, if you make the braking resistor work immediately, the bus voltage will not drop quickly, so many VFDs use the braking unit to make the braking resistor work at about 700 volts, thus Reduce bus voltage to avoid rising.


Therefore, the core of the braking resistor design is to consider the withstand voltage of the capacitor and the 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.


A quick stop requires braking resistance, as well as instant acceleration.


The reason for the high VFD bus voltage is often the VFD. Let the motor work in the electronic braking state, and let the IGBT go through a certain turn-on sequence. Using the large inductance of the motor, the current will not change suddenly, and a high voltage will be generated instantaneously to charge the bus capacitor. At this time, let the motor decelerate quickly. At this time, 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 requirement, the braking resistor is not needed in this case. Even if a braking resistor is installed, 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 the braking resistor and braking unit for fast braking in the case of heavy load deceleration, in fact, if the requirements of heavy load starting time and very fast starting time are met, the braking unit and braking resistor need to cooperate to start. I have tried to drive a special punching machine with a VFD before, and the acceleration time of the VFD is required to be designed to be 0.1 seconds. At this time, it starts up at full load, although the load is not very heavy. However, because the acceleration time is too short, the bus voltage fluctuates greatly at this time, and overvoltage or overcurrent will also occur. Later, an external braking unit and braking resistor were added, and the VFD could work normally. Analysis is because the start-up time is too short, the bus capacitor voltage is instantly hollowed out, and the rectifier is charged with a large current, causing the bus voltage to suddenly increase, causing the bus voltage to fluctuate too much, which may exceed 700V in an instant. The addition of the braking resistor can eliminate this fluctuating high voltage in time, and the VFD can work normally.


There is also a special case, in the case of vector control, the torque and speed of the motor are in opposite directions, or the motor works at zero speed and outputs 100% torque. For example, the crane stops in mid-air after losing weight. Torque control is required in the case of pay-off and pay-off, so that the motor needs to work in the generator state, and the continuous current will reversely charge the bus capacitor. Through the braking resistor, the energy can be consumed in time and the bus voltage can be kept balanced and stable.