Calculation of power and size of braking resistor

power

After selecting the resistance 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 cumbersome and is related to many factors. The instantaneous power consumed by the braking resistor is the power value that the braking resistor can dissipate if it works uninterrupted for a long time. However, the braking resistor does not work uninterrupted. This selection involves a lot of waste. You can choose the braking resistor. Usage rate, which specifies the short-time working ratio of the braking resistor. Braking resistor usage rate. In actual use, the power of the braking resistor can be selected according to the above formula, or the usage rate that the braking resistor can withstand can be calculated based on the resistance and power of the selected braking resistor, so as to set it correctly and avoid overheating of the braking resistor. And damaged.

Size calculation

First estimate the braking torque

Braking torque = [(motor inertia moment + motor load measurement converted to motor inertia measurement) (speed before braking - speed after braking)] / 375 * deceleration time - load torque. Under normal circumstances, when braking the motor, there is a certain loss inside the motor, which is about 18%-22% of the rated torque. Therefore, if the calculated result is less than this range, there is no need to connect a braking device.

Then calculate the resistance value of the braking resistor

The resistance of the braking resistor = the square of the operating voltage of the braking element/(0.1047*(braking torque-20% motor rated torque) motor speed before braking) During the operation of the braking unit, the voltage of the DC bus The rise and fall depends on the constant RC, R is the resistance of the braking resistor, and C is the capacity of the electrolytic capacitor inside the frequency converter. The braking unit operating voltage here is generally 710V.

Then select the braking unit

When selecting a braking unit, the maximum operating current of the braking unit is the only basis for selection. The calculation formula is as follows: braking current instantaneous value = braking unit DC bus voltage value/braking resistor value. Finally, calculate the braking resistor. The nominal power of the braking resistor is a short-time working system. Therefore, according to the characteristics and technical indicators of the resistor, we know that the nominal power of the resistor will be less than the power consumption when power is on. Generally, it can be obtained by the following formula: Nominal power of the braking resistor Power = braking resistor derating coefficient, average power consumption during braking, braking utilization rate %.

Braking characteristics

The advantage of energy-consuming braking (resistance braking) is that the structure is simple, but the disadvantage is that the operating efficiency is reduced, especially during frequent braking, a large amount of energy will be consumed, and the capacity of the braking resistor will increase.