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In a drive system composed of the power grid - frequency converter - motor - load, energy can be transferred in both directions. When the motor is in the motor-working mode, electrical energy is transferred from the power grid to the motor via the frequency converter and then converted into mechanical energy to drive the load. As a result, the load has kinetic or potential energy. When the load releases this energy to change its state of motion, the motor is instead driven by the load and enters the generator - operating mode, converting mechanical energy into electrical energy and feedback it to the precedingstage frequency converter. This feedback energy is called regenerative braking energy and can be fed back to the power grid through the frequency converter or dissipated in the braking resistor on the DC bus of the frequency converter (dynamic braking). There are four commonly - used braking modes of frequency converters.

  1. Dynamic Braking

  1.The Energy Consumption Brakingmethod utilizes the braking unit and braking resistor to absorb the regenerative electrical energy of the motor through the braking resistor arranged in the DC circuit, thus realizing the rapid braking of the frequency converter.

  Advantages of dynamic braking:

  It has a simple structure, causes no pollution to the power grid (compared with regenerative braking), and is low in cost.

  Disadvantages of dynamic braking:

  It has 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

  2.The regenerative braking method adopts the active inverter technology to convert the regenerative electrical energy into alternating current with the same frequency and phase as that of the power grid and then send it back to the power grid, thus achieving braking.

  The realization of energy - feedback braking requires conditions such as voltage control with the same frequency and phase and feedback current control.

  Advantages of feedback braking:

  It can operate in four quadrants, and the feedback of electrical energy improves the efficiency of the system;

  Disadvantages of feedback braking:

  (1)This feedback braking method can only be used under stable grid voltage that is not prone to faults (with grid voltage fluctuations not exceeding 10%). Because during the operation of power generation braking, if the voltage fault time of the power grid is greater than 2ms, commutation failure may occur and the components may be damaged.

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

  (3) The control is complex and the cost is relatively high.

  3.DC Braking

  Definition of DC Braking:

  DC braking generally means that when the output frequency of the frequency converter is nearly zero and the motor speed drops to a certain value, the frequency converter changes to supply direct current to the stator winding of the asynchronous motor to form a static magnetic field. At this time, the motor is in a Energy Consumption Brakingstate. The rotating rotor cuts this static magnetic field to generate a braking torque, which enables the motor to stop quickly.

  It can be used in situations that require accurate stopping or the irregular rotation of the braking motor caused by external factors before starting.

  Keys of DC Braking:

  The value of DC braking voltage essentially sets the magnitude of the braking torque. Obviously, the greater the inertia of the drive system, the correspondingly larger the value of the DC braking voltage should be. Generally, the rated output voltage of the inverter with a DC voltage of about 15-20% is about 60-80V. Some use the percentage value of the braking current.

  The DC braking time is the time during which the direct-current is supplied to the stator winding. It should be slightly longer than the actual required shutdown time.

  The starting frequency of DC braking, when the operating frequency of the inverter drops to a certain extent, starts to switch from Energy Consumption Brakingto DC braking, which is related to the load's requirements for braking time. If there are no strict requirements, the starting frequency of DC braking should be set as small as possible;

  4.Common DC - bus Feedback Braking

  The principle of the common DC - bus feedback braking method is that the regenerative energy of Motor A is fed back to the common DC bus and then the regenerative energy is consumed by Motor B.

  The common DC – bus feedback braking method can be divided into two types: common DC - balanced - bus feedback braking and common DC - loop - bus feedback braking.