During rapid stop, the motor generates a large amount of regenerative power due to inertia, which directly acts on the middle DC part of the inverter. If it does not consume this part of the power, it will cause the inverter to fault or the inverter will be damaged. Braking resistor is used to consume this part of the energy. Each inverter has a brake unit (small power is the braking resistor, high-power transistor GTR and its drive circuit), low-power is built-in, high-power is external, please according to the inverter manual Configure the braking resistor.
Braking unit and braking resistor matching
A, first estimate the braking torque
= ((Motor inertia + motor load measured to motor measured moment of inertia) * (pre-brake speed - brake speed) / 375 * deceleration time - load torque
Under normal circumstances, the motor brake, the motor there is a certain loss of about 18% -22% of the rated torque, so calculated results in less than this range, then do not need to connect the brake;
B, then calculate the resistance of the braking resistor
= Squared value of braking element operating voltage / (0.1047 * (braking torque -20% of rated motor torque) * motor speed before braking)
In the braking unit work, the DC bus voltage rise and fall depends on the constant RC, R is the resistance of the braking resistor, C is the capacity of the inverter electrolytic capacitors. Braking unit operating voltage here is generally 710V.
C, then make the selection of brake unit
During the selection of brake unit, the maximum working current of the brake unit is the only choice based on the following formula:
Braking current instantaneous value = braking unit DC bus voltage / braking resistance value
D, the final calculation of braking resistor nominal power
Since the braking resistor is short-time working system, according to the characteristics of resistance and technical indicators, we know that the nominal power of the resistance will be less than the power consumption when power is on, generally obtained by the following formula: Braking resistor nominal power = braking Resistance derating factor X Average power consumption during braking X Braking utilization%
Braking features The advantage of energy-consuming braking (resistive braking) is that it has a simple construction with the disadvantage of a reduction in operating efficiency, in particular in the case of frequent braking, and that the capacity of the braking resistor is increased.
Design Calculation Method of Inverter Braking Resistor
1, the number of braking resistor box roughly calculated as: Motor power (KW) /11.2 (integer upper limit).
2, the choice of braking unit power is generally the size of the power inverter (1 ~ 2) times;
3, the braking resistor power is greater than the motor power KW / 2. (According to the formula Pb = 8Q * v * η)
4, the braking resistor size selection formula 700 / motor power KW (using multiple braking unit parallel operation, each braking unit configured resistor resistance is not less than 700 / motor power KW; the minimum resistance value according to Check the configuration table);
5, first determine the size of the drive power inverter size;
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