The supplier of the frequency converter braking unit reminds you that in the drive system composed of the power grid, frequency converter, motor, and load, energy can be transmitted in both directions. When the motor is in the electric motor working mode, electrical energy is transmitted from the grid to the motor through the frequency converter, converted into mechanical energy to drive the load, and the load therefore has kinetic or potential energy; When the load releases this energy in order to change the motion state, the motor is driven by the load and enters the generator working mode, converting mechanical energy into electrical energy and feeding it back to the front-end frequency converter. These feedback energies are called regenerative braking energies, which can be fed back to the grid through a frequency converter or consumed in the braking resistors on the DC bus of the frequency converter (energy consumption braking). There are four commonly used braking methods for frequency converters.
1. Energy consumption braking
The energy consumption braking method uses a chopper and braking resistor, and utilizes the braking resistor set in the DC circuit to absorb the regenerative electrical energy of the motor, achieving rapid braking of the frequency converter.
The advantages of energy consumption braking:
Simple construction, no pollution to the power grid (compared to feedback control), and low cost;
Disadvantages of energy consumption braking:
The operating efficiency is low, especially during frequent braking, which will consume a large amount of energy and increase the capacity of the braking resistor.
2. Feedback braking
The feedback braking method adopts active inverter technology to invert the regenerated electrical energy into AC power of the same frequency and phase as the power grid and return it to the power grid, thereby achieving braking.
Inverter specific energy feedback braking unit
To achieve energy feedback braking, conditions such as voltage control at the same frequency and phase, feedback current control, etc. are required.
The 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:
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.
Secondly, there is harmonic pollution to the power grid during feedback;
The three controls are complex and costly.
3. DC braking
Definition of DC braking:
DC braking generally refers to when the output frequency of the frequency converter approaches zero and the motor speed decreases to a certain value, the frequency converter changes to introduce DC into the stator winding of the asynchronous motor, forming a static magnetic field. At this time, the motor is in an energy consuming braking state, rotating the rotor to cut the static magnetic field and generate braking torque, causing the motor to quickly stop.
It can be used in situations where accurate parking is required or when the brake motor rotates irregularly due to external factors before starting.
Elements of DC braking:
The DC braking voltage value is essentially the setting of the braking torque. Obviously, the greater the inertia of the driving system, the higher the DC braking voltage value should be. Generally, the rated output voltage of a frequency converter with a DC voltage of around 15-20% is about 60-80V, and some use the percentage of the braking current;
The DC braking time refers to the time it takes to apply DC current to the stator winding, which should be slightly longer than the actual required downtime;
The starting frequency of DC braking, when the operating frequency of the inverter drops to a certain extent, starts to switch from energy consumption braking to 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. Shared DC bus feedback braking
The principle of the shared 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 circuit bus feedback braking;