As we know from the working principle of the frequency converter , changing the frequency of the motor's operating power supply requires a rectification-->inversion process. The braking resistor is located after rectification, as shown in the figure below between resistors ⑧ and ⑨:
So what is the function of the braking resistor?
In the example below:
When the motor is in the deceleration phase, the motor begins to feed energy back to the frequency converter, i.e., P-brake;
Then the DC-side voltage begins to rise. When the voltage rises to a certain threshold, the braking chopper (BRC) turns ON, and the feedback energy begins to be released onto the braking resistor, i.e., Pv.
As excess energy is dissipated as heat through the braking resistor, the DC side voltage begins to decrease. When it drops to a certain threshold, the braking chopper (BRC) is in the OFF state, and the braking resistor no longer works.
The above explains the working principle and process of the braking resistor.
Generally, due to different design philosophies among manufacturers, the design of the capacitors on the DC side may vary.
Some products have large capacitance, which allows them to absorb more energy during operation. When the operating conditions are not very harsh, they may not need a braking resistor to work properly.
Some products have small capacitors that cannot absorb feedback energy. In this case, it is essential to add a braking resistor. For example, if SEW's MDX61B or MC07B does not have a braking resistor, and alarms F04 or F07 occur, it is very likely because there is no braking resistor.
The function of braking resistor
1. Protect the frequency converter from the hazards of regenerative electrical energy.
During rapid stopping, the motor generates a large amount of regenerative energy due to inertia. If this regenerative energy is not consumed in time, it will directly affect the DC circuit of the frequency converter. At best, the frequency converter will report a fault; at worst, it will damage the frequency converter. The braking resistor effectively solves this problem and protects the frequency converter from the harm of the motor's regenerative energy.
2. Ensure the stable operation of the power supply network.
The braking resistor directly converts the regenerative electrical energy generated during the rapid braking process of the motor into heat energy. This prevents the regenerative electrical energy from being fed back into the power grid, thus avoiding voltage fluctuations in the power grid and ensuring the stable operation of the power grid.
The main purpose of equipping a frequency converter with a braking resistor is to dissipate some of the energy stored in the DC bus capacitor, preventing the capacitor voltage from becoming too high. Theoretically, if the capacitor stores a lot of energy, it can be released to drive the motor, avoiding energy waste. However, the capacitor's capacity and voltage rating are limited. When the voltage of the bus capacitor reaches a certain level, it may damage the capacitor, and in some cases, it may even damage the IGBT. Therefore, it is necessary to release the energy through the braking resistor in a timely manner. This release is a wasteful measure and is a last resort.
The bus capacitor acts as a buffer, with a limited capacity for energy.
After all three-phase AC power is rectified and connected to capacitors, the normal bus voltage under full load is approximately 1.35 times the rated voltage, 380 * 1.35 = 513 volts. This voltage will naturally fluctuate in real time, but it must not fall below 480 volts; otherwise, an undervoltage alarm will activate. The bus capacitor is typically composed of two sets of 450V electrolytic capacitors connected in series, with a theoretical withstand voltage of 900V. If the bus voltage exceeds this value, the capacitors will explode. Therefore, the bus voltage must never reach such a high voltage as 900 volts.
In reality, the withstand voltage of a three-phase 380V input IGBT is 1200V, and it is often required to operate below 800V. Considering that if the voltage rises, there will be an inertia problem, that is, even if you immediately activate the braking resistor, the bus voltage will not drop quickly. Therefore, many frequency converters are designed to activate the braking resistor through the braking unit at around 700V to reduce the bus voltage and prevent it from continuing to rise.
Therefore, the core of braking resistor design is to consider the voltage withstand capability of the capacitor and IGBT module, so as to prevent these two important components from being damaged by the high voltage of the bus. If these two types of components fail, the frequency converter will not be able to work properly.
Rapid stopping requires braking resistors, and instantaneous acceleration also requires them.
The reason why the inverter bus voltage rises is often because the inverter operates the motor in an electronic braking state. This involves the IGBTs conducting in a specific sequence, utilizing the fact that the motor's large inductive current cannot change abruptly to generate a high voltage that charges the bus capacitor, causing the motor to quickly decelerate. If there is no braking resistor to dissipate the energy from the bus in time, the bus voltage will continue to rise, threatening the safety of the inverter.
If the load is not very heavy and there is no requirement for rapid stopping, there is no need to use a braking resistor. Even if you install a braking resistor, the braking resistor will not work if the operating threshold voltage of the braking unit is not triggered.
Besides the need for additional braking resistors and braking units in high-load deceleration applications for rapid braking, braking units and braking resistors are also necessary for applications with relatively heavy loads and very short start-up times. I previously tested using a frequency converter to drive a special punch press, requiring the converter's acceleration time to be designed to be 0.1 seconds. Even with a relatively light load, the extremely short acceleration time caused severe bus voltage fluctuations, leading to overvoltage or overcurrent. Adding an external braking unit and braking resistor resolved the issue, allowing the frequency converter to operate normally. The reason is that the extremely short start-up time instantly depletes the bus capacitor's voltage, while a large current rushes into the rectifier, causing a sudden spike in bus voltage. This severe voltage fluctuation can momentarily exceed 700 volts. The braking resistor effectively eliminates this high-voltage fluctuation, allowing the frequency converter to operate normally.
Another special case is vector control, where the motor's torque and speed are in opposite directions, or when it is operating at zero speed and 100% torque output. For example, when a crane drops a heavy object and stops in mid-air, or when torque control is required during winding and unwinding, the motor needs to operate in generator mode. The continuous current will be fed back into the bus capacitor. Through the braking resistor, this energy can be consumed in time to maintain the balance and stability of the bus voltage.
Many small frequency inverters, such as 3.7KW, often have built-in braking units and braking resistors, probably because the bus capacitor is reduced in size. And low-power resistors and braking units are not that expensive.
Selection of braking resistor
The selection of braking resistor is limited by the maximum allowable current of the inverter-specific energy-saving braking unit, and there is no clear correspondence between the braking resistor and the braking unit. Its resistance value is mainly selected according to the required braking torque.
The power rating is determined based on the resistor value and utilization rate. There is an inviolable principle in selecting the braking resistor value: the current IC flowing through the braking resistor must be less than the maximum allowable current output capacity of the braking unit, i.e., R > 800/Ic.
Where: 800—the maximum DC voltage that may occur on the DC side of the frequency converter.
Ic—The maximum permissible current of the braking unit.
To fully utilize the capacity of the selected inverter-specific braking unit, the most economical approach is to choose a braking resistor value close to the minimum calculated by the above formula, while also achieving the maximum braking torque. However, this requires a larger braking resistor power rating. In some cases, a large braking torque is not necessary. In such cases, a more economical approach is to choose a larger braking resistor value, thereby reducing the braking resistor power rating and the cost of purchasing a new braking resistor. The trade-off is that the braking unit's capacity is not fully utilized.
Calculation of braking resistor
After selecting the resistance value of the braking resistor, the power value of the braking resistor should be determined. The selection of the power of the braking resistor is relatively complicated, as it is related to many factors.
The instantaneous power consumed by the braking resistor is calculated using the following formula: P_instantaneous = 7002/R
The braking resistor power value calculated using the above formula represents the power dissipated by the braking resistor during long-term, uninterrupted operation. However, braking resistors do not operate continuously, making this selection wasteful. In this product, the utilization rate of the braking resistor can be selected, specifying its short-time operating ratio. The actual power consumed by the braking resistor is calculated using the following formula:
P amount = 7002 / R × rB%
rB%: Braking resistor utilization rate.
In practical use, the power of the braking resistor can be selected according to the above formula, or the utilization rate that the braking resistor can withstand can be calculated by reverse calculation based on the selected braking resistor resistance value and power, so as to set it correctly and avoid the braking resistor from overheating and being damaged.
Calculation of power used in braking resistor
The braking resistor utilization rate specifies the efficiency of the braking resistor to prevent it from overheating and being damaged, which would affect the braking effect of the braking unit. A lower utilization rate means less heat generation and less energy consumption, resulting in poorer braking performance. Simultaneously, the capacity of the braking unit is not fully utilized.
Theoretically, 100% utilization of the braking resistor maximizes the use of the braking unit's capacity and yields the most significant braking effect. However, this comes at the cost of a higher braking resistor power, which users should consider. Given a fixed braking resistor value and power, for slow-decelerating, high-inertia loads, a lower resistor utilization rate will achieve better results. For loads requiring rapid shutdown, a higher braking resistor utilization rate is preferable.
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