Selection and calculation formula of braking resistance of servo motor

When the servo motor is braked, the servo motor is in the power generation state. This means that the energy will be returned to the DC bus of the servo drive. Because the DC bus contains capacitance, the DC bus voltage will rise. The voltage increase depends on the kinetic energy of the motor at the beginning of braking and the capacity of the capacitor on the DC bus. If the braking kinetic energy is greater than the capacitance on the DC bus, and there is no other driver on the DC bus to accommodate the energy, the driver will consume the energy through the braking resistance, or feed it back to the power supply.
The way to save energy efficiency is to add as many capacitors on the DC bus as possible or connect the capacitors of each driver in parallel. Therefore, we need to consider the following points:
1. Always connect the largest drive to the power supply.
2. Ensure that the capacitance of the DC bus connected to the power supply is not within the maximum allowable range of operation.
3. The power of all connected drives shall not exceed the allowable power of servo drives connected to the power supply.
4. The braking energy shall not exceed the maximum power of the braking resistor.
The theoretical formula is as follows:

The main points related to model selection are as follows:
1. The size of the shunt capacitor on the DC bus circuit: the capacitance on the DC bus circuit cannot be increased infinitely. Considering that the increase of the capacitance will increase the charging current during charging, the size of the shunt capacitor is determined by the resistance on the charging circuit or the controllable rectifier circuit. The maximum allowable external shunt capacitor shall be specified by the manufacturer.
2. At present, the DC bus of servo driver mostly adopts the mode of multiple voltages with withstand voltage of 400V in series. When the circuit voltage is close to 800V (750v~780v), the braking unit is connected and the braking resistance is put into use. When connecting the external capacitance, it is necessary to consider the parallel resistance to make the voltage distributed in the external capacitance as equal as possible.

3. Selection parameters of braking resistance: common braking resistance includes ripple resistance and aluminum alloy resistance. The former is cheap but its overload capacity is not high, while the latter is slightly higher and has better overload capacity. The three most important parameters of braking resistance are resistance value, continuous operation power and maximum power.
4. The better the braking resistance, the better the braking effect. The allowable passing current of the braking unit determines the minimum resistance value of the braking resistance. Therefore, this parameter shall be determined by the manufacturer. The actual selected resistance is usually slightly greater than the minimum allowable resistance.
5. The continuous power and maximum power of braking resistance are calculated by the above formula. If the calculation is not accurate, the continuous power can be selected by referring to 1/3~1/2 of the total power.