With the popularization and promotion of frequency converter in the field of motor speed regulation, rapid stop or instantaneous deceleration is required in many application fields. At this time, due to the presence of mechanical inertia, the actual speed of the motor will be higher than the synchronous speed output of the inverter, and the motor will work in the generation state, or lift/lift/uncoil in the following cases: the output torque of the motor is opposite to the actual speed, and the motor is also working in the generation state. When the electric energy generated by the motor exceeds the storage capacity of the DC bus capacitor, the rectifier unit on the grid side of the inverter will not be able to feed back to the grid. When the capacity or feedback capacity is insufficient, the power on the DC link should be consumed as soon as possible. A simpler approach is to use brake units and electronic resonators. Here is a quick way to calculate the power of electronic resonators.

electronic resonators calculation of power

Because the braking time of the drive system is usually short, the temperature increase of the braking resistance cannot reach its rated temperature increase during a short braking period, and the rest time after braking is longer. Its temperature can be reduced to room temperature. Therefore, it is possible to choose electronic resonators with a power rating less than the power consumption when the power supply is switched on. When the power of electronic resonators provided in the Siemens selection manual does not meet the field operating conditions, it can be calculated by formula (1) :

electronic resonators calculate power

In the formula, the brake resistance PB0 -- has a higher power.

In the formula, the DC bus voltage of UD -- is usually 760V.

In the formula, the resistance of the brake resistance RB --.

The resistance value of the brake resistor RB can be found in the manual and is determined by the frequency converter.

Calculate the power of electronic resonatorsPB0 according to formula (1). When the greater power of the braking resistance obtained during the calculation exceeds the rated power of the inverter, the rated power of the inverter is used as the greater braking power. Resistors can be attached to a circuit for a long time. However, its working time under actual working conditions is very short, and its actual power value may be less than the power consumption value. Therefore, the principle of determining the power of electronic resonators is to reduce the power value as much as possible under the premise that the temperature rise of the resistance does not exceed the rise in its rated temperature. In actual selection, the electronic resonators power is calculated according to formula (2).

electronic resonators calculate power

Where B is the power correction factor of the external brake resistance.

Determination of power correction coefficient of electronic resonators

(1) infrequent braking load. The number of braking is small, and after one braking, the load will no longer brake, such as the secondary speed load. For this load, the size of the correction factor depends on the time required for each braking.

When each braking time tB is less than 10 s, B=7 can be used.

When each braking time tB exceeds 100 s, B=1;

When each braking time is between the two, i.e. 10stB100s, the value of B can be calculated according to the relationship shown in the figure below.

(2) Frequent braking load. Many machines require repeated braking, such as lifting machines, planers, etc.

For this type of load, the correction factor depends on the relationship between each brake time tB and the time interval tC(tB/tC) between each brake, and their relationship is called the brake duty cycle. In actual production, because the brake duty cycle often changes, it can only be averaged. In this case, the correction factor B can be determined as follows:

When tB /tC0.01, B=5;

When tB/tC0.15 is used, B=1;

When 0.01tB/tC0.15 is available, the value of B can be calculated according to the relationship shown in the following figure.

Because the rated power of electronic resonators is much less than the actual power consumed, and the activation time of the brake resistance is difficult to accurately estimate, braking will occur if the activation time exceeds the preset activation time in actual operation. The motor resistance overheats and is damaged. Therefore, the selection of models should be appropriate to increase the margin. Secondly, overheat protection should be added to the brake resistance. Overheat protection can use a thermal relay, or you can design your own overheat protection circuit.

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