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In a variable frequency speed control system, the deceleration and shutdown of the motor are achieved by gradually reducing the frequency. At the moment when the frequency decreases, the synchronous speed of the motor drops accordingly, while due to mechanical inertia, the rotor speed of the motor remains unchanged. When the synchronous speed is less than the rotor speed, the phase of the rotor current almost changes by 180 degrees, and the motor changes from the electric state to the power generation state. At the same time, the torque on the motor shaft becomes the braking torque, causing the motor speed to drop rapidly, and the motor is in the regenerative braking state. The electrical energy regenerated by the motor is fully rectified by the rectifier diode and then fed back to the DC circuit. Since the electrical energy in the DC circuit cannot be fed back to the power grid through the rectifier bridge, it is only absorbed by the capacitor of the frequency inverter itself. Although other parts can consume electrical energy, there is still a short-term charge accumulation in the capacitor, forming a "pumped voltage", which increases the DC voltage. The braking resistor is a carrier used to consume the regenerative energy of the motor in the form of heat energy. It includes two important parameters: resistance value and power capacity. When the motor or other inductive load driven by a frequency inverter stops, it is generally achieved through energy consumption braking. That is, the kinetic energy of the motor and the magnetic energy in the coil after stopping are consumed through another energy-consuming component, thereby achieving a rapid stop.
In engineering, the two most commonly used types are corrugated resistors and aluminum alloy resistors. The corrugated resistor adopts a vertical corrugated surface, which is conducive to heat dissipation and reduces parasitic inductance. It also uses a high flame-retardant inorganic coating to effectively protect the resistance wire from aging and extend its service life. Aluminum alloy resistors are easy to install tightly and attach heat sinks. They have an attractive appearance, a high heat dissipation aluminum alloy shell, a fully sealed structure, and possess extremely strong vibration resistance, weather resistance and long-term stability. It is small in size, high in power, easy and stable to install, and has an attractive appearance. It is widely used in highly harsh industrial environments.

When the frequency inverter decelerates, the output frequency of the frequency inverter decreases. However, when the motor changes from high speed to low speed, the motor changes from an electric state to a power generation state. The electricity generated is returned to the DC bus through the IGBT switch. Therefore, during braking, the DC bus voltage will rise. When using the braking resistor, it should be used in conjunction with the braking unit. When the braking unit is engaged, if the DC bus voltage is detected to be too high, it will control the IGBT inside the braking unit to conduct (similar to the frequency conversion output, intermittently conducting) for discharge. When the voltage drops below the set value, the triggering will stop. The calculation of the resistance value of the braking resistor: The selection of the braking resistor is not only limited by the maximum allowable current of the dedicated energy consumption braking unit for frequency inverters, but also has no clear corresponding relationship with the braking unit. Its resistance value is mainly selected based on the size of the required braking torque, and the power is determined according to the resistance value of the resistor and the usage rate. There is an unbreakable principle for the selection of the braking resistor's resistance value: it should be ensured that the current Ic flowing through the braking resistor is less than the maximum allowable current output capacity of the braking unit, that is: R > 800/Ic. Among them: 800 - The maximum DC voltage that may occur on the DC side of the frequency inverter. Ic - The maximum allowable current of the braking unit. To fully utilize the capacity of the dedicated braking unit for the selected frequency inverter, the resistance value of the braking resistor is usually selected to be close to the minimum value calculated by the above formula, which is the most economical and can also obtain the maximum braking torque.

Calculation of braking torque or braking resistance (380V series) : 92% R=780/ Motor KW n100% R= 700/ Motor power n110% R=650/ Motor KW n120%r =600/ Motor KW (for motors greater than 7.5KW) n400 / Motor KW (for motors less than 7.5KW)  

Note: ①The smaller the resistance value, the greater the braking torque and the greater the current flowing through the braking unit.

② The working current of the braking unit must not exceed its maximum allowable current; otherwise, the device will be damaged.

③ The braking time can be selected manually;

④ Small-capacity frequency converters (≤ 7.5KW) are generally equipped with built-in braking units and braking resistors. The calculation method for resistance power: Resistance power = Motor power * (10%- 15%) General load W(Kw)= Motor power * 10% Frequent braking (more than 5 times per minute) W(Kw)= Motor power * 15% Long-term braking (more than 4 minutes each time) W(Kw)= Motor power * The selection of a 20% general braking resistor should ensure that the braking current Is does not exceed the rated current Ie of the frequency converter, and the maximum power Pmax of the braking resistor should be less than 1.5 times the power of the frequency converter, and then multiplied by the overload factor. The overload factor is related to the deceleration time and the continuous braking time.