In the drive system composed of the power grid, frequency converter, motor, and load, energy can be transmitted bidirectionally. When the motor is in the motor operating mode, electrical energy is transmitted from the power grid to the motor via the frequency converter and converted into mechanical energy to drive the load, so the load has kinetic energy or potential energy. When the load releases this energy to change its motion state, the motor is driven by the load instead, entering the generator operating mode and converting mechanical energy into electrical energy, which is then fed back to the previous - stage frequency converter. These feedback energies are called regenerative braking energies, which can be fed back to the power grid through the frequency converter or consumed in the braking resistor on the DC bus of the frequency converter (dynamic braking). There are four common braking methods for frequency converters.

  1. Dynamic Braking

  The dynamic - braking method uses a braking unit and a braking resistor. The braking resistor set in the DC circuit is used to absorb the regenerative electrical energy of the motor, achieving rapid braking of the frequency converter.

  Advantages of dynamic braking:

  Simple structure, no pollution to the power grid (compared with regenerative braking), and low cost.

  Disadvantages of dynamic braking:

  Low operating efficiency, especially when braking frequently, a large amount of energy will be consumed and the capacity of the braking resistor will increase.

  2. Regenerative Braking

  The regenerative - braking method adopts active - inverter technology to invert the regenerative electrical energy into AC power with the same frequency and phase as the power grid and send it back to the power grid, thus achieving braking.

  Frequency - converter - specific energy - regenerative braking unit

  To achieve energy - regenerative braking, conditions such as voltage control with the same frequency and phase and feedback - current control are required.

  Advantages of regenerative braking:

  It can operate in four quadrants, and the energy feedback improves the system efficiency.

  Disadvantages of regenerative braking:

  (1) This regenerative - braking method can only be adopted under a stable power - grid voltage where failures are not likely to occur (the power - grid voltage fluctuation is not greater than 10%). Because during the power - generation braking operation, if the power - grid voltage failure time is greater than 2ms, commutation failure may occur and components may be damaged.

  (2) During the energy - feedback process, there is harmonic pollution to the power grid.

  (3) Complex control and high cost.

  3. DC Braking

  Definition of DC braking:

  DC braking generally means that when the output frequency of the frequency converter is close to zero and the motor speed drops to a certain value, the frequency converter supplies DC power to the stator windings of the asynchronous motor, forming a static magnetic field. At this time, the motor is in the dynamic - braking state, and the rotating rotor cuts this static magnetic field to generate a braking torque, making the motor stop quickly.

  It can be used in cases where accurate parking is required or to brake the motor due to irregular rotation caused by external factors before starting.

  Elements of DC braking:

  The DC - braking voltage value essentially sets the size of the braking torque. Obviously, the greater the inertia of the drive system, the larger the DC - braking voltage value should be. Generally, the DC voltage is about 15 - 20% of the rated output voltage of the frequency converter, which is approximately 60 - 80V, and some use the percentage value of the braking current.