【Daily Question 20090223】Some frequency converters have a resistor box connected to them externally; what is its purpose?

Braking resistor! Used for short-term parking, or those with continuous regenerative braking energy, such as when a train derails while moving downhill!

The poster is probably referring to the braking resistor and the braking unit. Its function is as follows: the braking process of the inverter is achieved by reducing the frequency. During this process, due to inertia, the rotor speed of the motor will be greater than the synchronous speed; at this point the motor operates in a regenerative braking mode. The energy generated is fed back into the DC circuit of the inverter, where it is consumed by the capacitor in the inverter itself. If the inverter is not able to consume all of this energy, an external braking resistor is required to facilitate the consumption of that energy. The braking unit is designed to activate a power-consuming circuit when the voltage in the DC circuit exceeds a specified limit, allowing the DC current to be dissipated as heat through braking resistors.

In motors powered by an inverter, when they are operating in deceleration mode, the kinetic energy of the motor is converted into electrical energy, which is then sent to a resistor through another circuit of the inverter for consumption. Slow down the speed. Therefore, each inverter is equipped with a corresponding resistor, and there must be no breaks in the connection between the inverter and the resistor.

Braking resistor of the inverter. When power supply is interrupted, the inverter circuit of the frequency converter conducts in reverse, feeding this remaining electrical energy back into the DC bus of the frequency converter. As a result, the voltage on the DC bus rises. When it reaches a certain level, the braking resistor of the frequency converter comes online, allowing this electrical energy to be dissipated as heat through the resistor, while simultaneously maintaining the voltage on the DC bus at a normal level. When the DC bus voltage of a three-phase inverter is normal, it is usually around 530 V; the maximum value should not exceed 760 V (this value may vary slightly depending on the brand). If this limit is exceeded, the inverter may get damaged, and therefore it will enter protection mode at 760 V (displaying OU). If, for some reason during operation, the motor’s operating frequency exceeds the frequency set by the inverter, the motor enters a regenerative mode (i.e., it generates electricity). This can occur when lowering a load with a crane, slowing down a load with high inertia, or due to external forces pulling on the motor. Since the rectification section of conventional inverters is usually of the non-controllable type, the energy generated cannot be fed back into the power grid; it can only be used to charge the capacitors in the inverter’s DC section. The function of the braking resistor is to dissipate this energy in the resistor, thereby maintaining the voltage of the DC bus at a normal level. Since a torque is generated during regeneration in the opposite direction to the electric torque, it exerts a braking effect; therefore, this state is also known as regenerative braking mode, and such a resistor is called a braking resistor.

When the frequency converter slows down, the voltage in the DC circuit rises. If the load has high inertia and the deceleration time is insufficient, and no braking unit or braking resistor is used, the frequency converter will be recharged, resulting in overvoltage protection； By adding a braking unit and braking resistors, the kinetic energy of the load is converted into thermal energy generated by the resistors, preventing the voltage on the DC circuit bus from rising sharply and thus protecting the inverter components. :hug: