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About inverter braking resistor selection

Oct 17, 2020

About inverter braking resistor selection

 During the rapid stop of the motor, due to the effect of inertia, a large amount of regenerative power will be generated, which will directly act on the intermediate DC part of the inverter. If this part of the power is not consumed, the inverter will report a fault or damage the inverter. The braking resistor is used to consume this part of the electric energy. Each inverter has a braking unit (low power is braking resistor, high power is high power transistor GTR and its drive circuit), low power is built-in, high power is external, please follow the inverter manual Configure braking resistor.


   Braking unit and braking resistor selection


  A, first estimate the braking torque


  =((Moment of inertia of the motor + motor load measurement converted to the measured moment of inertia of the motor) * (speed before braking-speed after braking))/375 * deceleration time-load torque


   Under normal circumstances, when the motor is braked, there is a certain loss inside the motor, which is about 18%-22% of the rated torque, so if the calculated result is less than this range, there is no need to connect the brake device;


   B, then calculate the resistance of the braking resistor


  =The square of the operating voltage value of the braking element/(0.1047*(brake torque-20% motor rated torque)*motor speed before braking)


  During the operation of the braking unit, the rise and fall of the DC bus voltage depends on the constant RC, where R is the resistance of the braking resistor, and C is the capacity of the electrolytic capacitor inside the inverter. Here, the operating voltage of the braking unit is generally 710V.


  C, then select the brake unit


  When selecting the braking unit, the maximum working current of the braking unit is the only basis for selection, and the calculation formula is as follows:


  The instantaneous value of braking current=brake unit DC bus voltage value/brake resistance value


  D, finally calculate the nominal power of the braking resistor


Since the braking resistor is a short-time work system, 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 it is energized. Generally, the following formula can be used to obtain: nominal power of the braking resistor = braking Resistance derating factor X Average power consumption during braking X braking utilization rate%


   Braking characteristics The advantage of dynamic braking (resistance braking) is its simple structure, but its 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.


  Inverter braking resistor design calculation method


  1. The number of brake resistor boxes is roughly calculated as: motor power (KW)/11.2 (take the upper limit of the integer).


  2. The choice of braking unit power is generally (1~2) times the power of the inverter;


   3. The power of the braking resistor is greater than the power of the motor KW/2. (According to the formula Pb=8Q*v*η)


4. Braking resistance value selection formula 700/motor power KW (when multiple braking units are used in parallel operation, the resistance value of the resistor configured for each braking unit is not less than 700/motor power KW; the minimum resistance value should be in accordance with Check the relevant configuration table);


5. First determine the power of the frequency converter according to the size of the motor;

     

 

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