What is the role of the braking resistor in the inverter?

From the working principle of the frequency converter, we know that changing the working power frequency of the motor requires the process of rectification-->inversion. The brake resistor is located after rectification, as shown in the resistor between ⑧ and ⑨ in the figure below:

So what is the role of the brake resistor?

In the example below:

When the motor is in the deceleration stage, the motor starts to feed back energy to the inverter, which is called P-brake.

Then the DC side voltage starts to increase. When the voltage reaches a certain threshold, the brake chopper (BRC) is in the ON state. At this time, the feedback energy begins to be released to the brake resistor, that is, Pv

Since the excess energy is consumed in the form of heat through the braking resistor, the DC side voltage begins to decrease. When it drops to a certain threshold, the brake chopper (BRC) is in the OFF state and the braking resistor no longer works.

The above is the working principle and process of the braking resistor.

Generally speaking, due to different design concepts of various manufacturers, there may be differences in the design of capacitors on the DC side.

Some products have large capacitance and can absorb more energy when working. When the working conditions are not very harsh, they may not need a braking resistor to work normally.

Some products have small capacitance and cannot absorb feedback energy. In this case, it is necessary to add a brake resistor. For example, if SEW's MDX61B or MC07B does not have a brake resistor and alarms F04 or F07, it is most likely because there is no brake resistor.

The role of brake resistor

1. Protect the inverter from the harm of regenerative power

When the motor stops quickly, it will generate a lot of regenerative electric energy due to inertia. If this part of regenerative electric energy is not consumed in time, it will directly act on the DC circuit part of the inverter. In the mildest case, the inverter will report a fault. In the worst case, the inverter will be damaged. The emergence of braking resistors has solved this problem well and protected the inverter from the harm of motor regenerative electric energy.

2. Ensure the smooth operation of the power supply network

The braking resistor converts the regenerated electric energy during the rapid braking process of the motor directly into heat energy, so that the regenerated electric energy will not be fed back to the power supply network and will not cause grid voltage fluctuations, thereby ensuring the smooth operation of the power supply network.

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 capacitor voltage being too high. Theoretically, if the capacitor stores a lot of 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 voltage resistance 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 even damage the IGBT, so it is necessary to release the electricity through the braking resistor in time. This release is a waste of energy and is a helpless approach.

The bus capacitor is a buffer zone with limited energy storage capacity.

After the three-phase AC power is fully rectified, the capacitor is connected. When the bus voltage is fully loaded, it is about 1.35 times the normal voltage, 380*1.35=513 volts. This voltage will of course fluctuate in real time, but the minimum cannot 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 connected 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 can never reach such a high voltage of 900 volts.

In fact, the withstand voltage of the IGBT with a three-phase 380V input is 1200V, and it is often required to work within 800V. Considering that if the voltage increases, there will be an inertia problem, that is, if you immediately let the braking resistor work, the bus voltage will not drop quickly. Therefore, many inverters are designed to let the braking resistor start working through the braking unit at around 700V, so as to reduce the bus voltage and avoid further increase.

Therefore, the core of brake resistor design is to consider the voltage resistance of capacitors and IGBT modules to prevent these two important components from being damaged by the high voltage of the bus. If these two types of components are damaged, the inverter will not be able to work properly.

Rapid stopping requires a brake resistor, as does instant acceleration.

The reason why the inverter bus voltage becomes high is that the inverter often makes the motor work in the electronic braking state, and makes the IGBT go through a certain conduction sequence. The motor has a large inductance and the current cannot change suddenly, and it instantly generates high voltage to charge the bus capacitor, which makes the motor slow down quickly. If there is no braking resistor to consume the energy of the bus in time, the bus voltage will continue to rise and threaten the safety of the inverter.

If the load is not very heavy and there is no requirement for quick stopping, there is no need to use a braking resistor. Even if you install a braking resistor, the braking unit's operating threshold voltage is not triggered and the braking resistor will not work.

In addition to the need to add brake resistors and brake units for fast braking in heavy load deceleration situations, in fact, if the requirements are relatively heavy and the start-up time is very fast, brake units and brake resistors are also needed to cooperate with the start-up. In the past, I tried to use a frequency converter to drive a special punching machine, requiring the acceleration time of the frequency converter to be designed to be 0.1 seconds. At this time, the full load start-up, although the load is not very heavy, but because the acceleration time is too short, the bus voltage fluctuates very strongly at this time, and overvoltage or overcurrent will occur. Later, an external brake unit and brake resistor were added, and the frequency converter can work normally. Analysis shows that it is because the start-up time is too short, the voltage of the bus capacitor is instantly emptied, and the rectifier is instantly charged with a large current, causing the bus voltage to suddenly increase. In this way, the bus voltage fluctuates too much and may exceed 700 volts in an instant. With the addition of a brake resistor, this fluctuating high voltage can be eliminated in time, allowing the frequency converter to work in a normal state.

There is also a special situation, which is the vector control situation. The torque and speed of the motor are in opposite directions, or it works at zero speed and 100% torque output. For example, a crane drops a heavy object and stops in mid-air, or torque control is required for reeling and unwinding. In these cases, the motor needs to work in the generator state. The continuous current will be reversed into the bus capacitor, and the energy can be consumed in time through the braking resistor to keep the bus voltage balanced and stable.

Many small inverters, such as 3.7KW ones, often have built-in brake units and brake resistors. This is probably because the busbar capacitance is reduced, and low-power resistors and brake units are not that expensive. Return to Sohu to see more.

Selection of brake resistor

The selection of braking resistor is limited by the maximum allowable current of the inverter-specific energy-consuming braking unit and has no clear corresponding relationship with the braking unit. Its resistance value is mainly selected according to the required braking torque.

The power is determined by the resistance value and utilization rate of the resistor. There is an inviolable principle in selecting the resistance value of the brake resistor: the current IC flowing through the brake resistor should be less than the maximum allowable current output capacity of the brake unit, that is: R > 800/Ic

Among them: 800 is the maximum DC voltage that may appear on the DC side of the inverter.

Ic —— The maximum allowable current of the braking unit.

In order to make full use of the capacity of the selected inverter-specific brake unit, it is usually most economical to select a brake resistor value close to the minimum value calculated by the above formula, and at the same time, the maximum brake torque can be obtained. However, this requires a larger brake resistor power. In some cases, a large brake torque is not required. At this time, a more economical way is to select a larger brake resistor value, which can also reduce the power of the brake resistor, thereby reducing the cost of purchasing the brake resistor. The cost of this is that the capacity of the brake unit is not fully utilized.

Calculation of brake resistance

After selecting the resistance value of the brake resistor, the power value of the brake resistor should be determined. The selection of the brake resistor power is relatively complicated and it is related to many factors.

The instantaneous power consumed by the braking resistor is calculated as follows: P instantaneous = 7002 /R

The braking resistor power value calculated by the above formula is the power value that the braking resistor can dissipate in long-term uninterrupted operation. However, the braking resistor does not work uninterruptedly, and this selection is very wasteful. In this product, the utilization rate of the braking resistor can be selected, which stipulates the short-term working ratio of the braking resistor. The actual power consumed by the braking resistor is calculated as follows: