【Daily Question 20090314】Parameter Setting of Inverters (VIII)?

Without a braking unit, the motor will continue to run for a long time due to inertia. With a braking unit in place, according to the law of conservation of energy, the feedback energy is consumed, which means the motor’s inertial energy is used up; as a result, the speed at which the motor brakes increases significantly.

When the motor is in power generation mode, in the case of a single-quadrant inverter, the energy cannot be fed back, which causes the voltage on the DC side of the inverter to rise (voltage induction). This can easily damage the inverter; therefore, a resistor must be used to dissipate an equal amount of energy. Depending on the actual circumstances, it essentially provides a channel for energy consumption (for example, when a motor needs to be braked quickly, most of the motor’s kinetic energy is dissipated by the resistor, thereby enabling rapid braking; the smaller the resistor, the greater the braking power)

In a variable-frequency speed control drive system composed of an inverter, an asynchronous motor, and a mechanical load, when the motor slows down or the potential energy load it drives is lowered, the motor operates in a regenerative braking mode. The mechanical energy stored in the drive system is converted by the motor into electrical energy, which is then rectified by the freewheeling diodes in the inverter and fed back to the DC bus side, causing the voltage of the capacitors on the DC side to rise. When the motor brakes rapidly, the voltage across the capacitor rises very high. Such high voltages can trigger the \"braking overvoltage protection\" mechanism in the inverter, and may even damage the inverter. Therefore, it is necessary to use an appropriate braking resistor connected to the inverter’s DC bus to dissipate the energy regenerated by the motor, thereby enabling rapid braking of the motor.

When the motor is in power generation mode, in the case of a single-quadrant inverter, the energy cannot be fed back, which causes the voltage on the DC side of the inverter to rise (voltage induction). This can easily damage the inverter; therefore, a resistor must be used to dissipate an equal amount of energy. Based on the actual situation, it essentially provides a channel for energy consumption (for example, when a motor needs to be braked rapidly, most of its kinetic energy is dissipated by resistors; therefore, it is necessary to select appropriate braking resistors to be connected to the inverter’s DC bus in order to dissipate the energy regenerated by the motor, thus achieving rapid braking of the motor). This enables rapid braking; the smaller the resistance, the greater the braking power

For those equipped with motors of 7.5KW or more, there is usually no internal energy-consuming braking circuit, and an external braking resistor must be used for energy-consuming braking. For systems of 7.5 kW or less, some are equipped with braking resistors internally, but the braking current is insufficient and the braking time does not meet the requirements; in such cases, it is necessary to connect braking resistors in parallel externally. The resistance value and power of the braking resistor are determined not only by the deceleration time but also by the frequency of decelerations and the nature of the load; selecting an appropriate resistance value is quite difficult. Generally, a thermal relay is also installed in the braking resistor box for protection purposes. As for the case where an external resistor and an internal resistor are connected in parallel, since the value of the internal resistance is usually unknown, it is common practice to disconnect the wiring of the internal resistor before connecting the external resistor, so that the internal braking resistor is not connected as well. This prevents the parallel resistance value from becoming too low when both the internal and external resistors are connected, which could damage the internal braking unit.

Without a braking unit, the motor will continue to run for a long time due to inertia； With the addition of the braking unit, according to the law of conservation of energy, the feedback energy is consumed; in other words, the inertial energy of the motor is used up, which results in a significantly faster braking speed for the motor.

When high-power equipment stops, the motor remains in a generating mode due to inertia, and energy is fed back toward the power source. However, most frequency converters do not have devices for energy feedback; therefore, a braking unit must be used to dissipate this energy as heat. Yet, if the braking unit is not chosen appropriately, it may experience excessive current levels that can lead to its destruction. This post was last edited by faner on 2009-3-14 18:25]

For those equipped with motors of 7.5KW or more, there is usually no internal energy-consuming braking circuit, and an external braking resistor must be used for energy-consuming braking. For systems of 7.5 kW or less, some are equipped with braking resistors internally, but the braking current is insufficient and the braking time does not meet the requirements; in such cases, it is necessary to connect braking resistors in parallel externally. The resistance value and power of the braking resistor are determined not only by the deceleration time but also by the frequency of decelerations and the nature of the load; selecting an appropriate resistance value is quite difficult. Generally, a thermal relay is also installed in the braking resistor box for protection purposes. As for the case where an external resistor and an internal resistor are connected in parallel, since the value of the internal resistance is usually unknown, it is common practice to disconnect the wiring of the internal resistor before connecting the external resistor, so that the internal braking resistor is not connected as well. This prevents the parallel resistance value from becoming too low when both the internal and external resistors are connected, which could damage the internal braking unit.