The preparation of the inverter (Variable frequency Drive, VFD) braking resistor is mainly to consume part of the energy on the DC bus capacitor through the braking resistor to avoid excessive capacitor voltage. In theory, if the energy stored in the capacitor is large, it can be released to drive the motor to avoid energy waste. However, the capacity of the capacitor is limited, and the withstand voltage of the 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 power. This release is wasted and there is no way to do it.
Bus capacitors are energy-limited buffers.
After the three-phase alternating current is fully rectified, a 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 cannot be lower than 480 volts, otherwise it will give an undervoltage alarm protection. The bus bar 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 cannot 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 immediately make the braking resistor work, the bus voltage will not drop quickly, so many VFDs use the braking unit to make the braking resistor work at about 700 volts, so that 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.
Stopping quickly requires braking resistance, but also instantaneous acceleration.
The reason why the VFD bus voltage will become high is often the VFD. Let the motor work in the electronic braking state, and let the IGBT go through a certain conduction sequence. Utilizing the large inductance of the motor, the current will not change suddenly, and a high voltage is generated instantly to charge the bus capacitor. At this time, let the motor decelerate rapidly. 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, 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 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 rapid 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 are also required to start together. In the past, I tried to use a VFD to drive a special punching machine, and the acceleration time of the VFD was required to be designed to be 0.1 seconds. At this time, it starts 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 can work normally. Analysis, because the start-up time is too short, the voltage of the bus capacitor is emptied instantly, and the rectifier is instantly charged with a large current, which causes the bus voltage to suddenly increase, causing the bus voltage to fluctuate too much, and may exceed 700V in an instant. The addition of braking resistors 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 direction of the motor are opposite, or the motor works at zero speed and outputs 100% torque. For example, when a crane falls and stops in mid-air, torque control is required in the case of unwinding and unwinding, 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, energy can be consumed in time to keep the bus voltage balanced and stable.
Many low-power VFDs, such as 3.7kW, usually have built-in braking units and braking resistors. Considering the reduction of bus capacitance, small power resistors and braking units are not so expensive.
The preparation of the inverter (Variable frequency Drive, VFD) braking resistor is mainly to consume part of the energy on the DC bus capacitor through the braking resistor to avoid excessive capacitor voltage. In theory, if the energy stored in the capacitor is large, it can be released to drive the motor to avoid energy waste. However, the capacity of the capacitor is limited, and the withstand voltage of the 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 power. This release is wasted and there is no way to do it.
Bus capacitors are energy-limited buffers.
After the three-phase alternating current is fully rectified, a 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 cannot be lower than 480 volts, otherwise it will give an undervoltage alarm protection. The bus bar 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 cannot 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 immediately make the braking resistor work, the bus voltage will not drop quickly, so many VFDs use the braking unit to make the braking resistor work at about 700 volts, so that 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.
Stopping quickly requires braking resistance, but also instantaneous acceleration.
The reason why the VFD bus voltage will become high is often the VFD. Let the motor work in the electronic braking state, and let the IGBT go through a certain conduction sequence. Utilizing the large inductance of the motor, the current will not change suddenly, and a high voltage is generated instantly to charge the bus capacitor. At this time, let the motor decelerate rapidly. 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, 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 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 rapid 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 are also required to start together. In the past, I tried to use a VFD to drive a special punching machine, and the acceleration time of the VFD was required to be designed to be 0.1 seconds. At this time, it starts 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 can work normally. Analysis, because the start-up time is too short, the voltage of the bus capacitor is emptied instantly, and the rectifier is instantly charged with a large current, which causes the bus voltage to suddenly increase, causing the bus voltage to fluctuate too much, and may exceed 700V in an instant. The addition of braking resistors 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 direction of the motor are opposite, or the motor works at zero speed and outputs 100% torque. For example, when a crane falls and stops in mid-air, torque control is required in the case of unwinding and unwinding, 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, energy can be consumed in time to keep the bus voltage balanced and stable.
Many low-power VFDs, such as 3.7kW, usually have built-in braking units and braking resistors. Considering the reduction of bus capacitance, small power resistors and braking units are not so expensive.
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