When the motor is in the working mode of the motor, electric energy is transferred from the power grid to the motor through the frequency converter, and then the frequency converter is converted into mechanical energy to drive the load, so that the load has kinetic energy or potential energy. When the load releases this energy to change the movement state, the motor is driven by the load, enters the working mode of the generator, and converts mechanical energy into electrical energy. Feedback to the pre-stage converter. This feedback energy is called regenerative braking energy, which can be fed back to the grid through the inverter or consumed in the braking resistor of the inverter's DC bus (energy consumption braking). The inverter has four common braking methods.
1. Energy consumption braking
The energy consumption braking method absorbs the regenerative electric energy of the motor through chopping and braking resistors, and uses the braking resistors in the DC circuit to achieve rapid braking of the inverter.
Advantages of energy consumption braking:
Simple structure, no pollution to the power grid (compared to the feedback mechanism), and low cost;
Disadvantages of energy consumption braking:
Low operating efficiency, especially when frequent braking, will consume a lot of energy, and the capacity of the braking resistor will increase.
2. Feedback brake [frequency converter energy feedback device: IPC closing and energizing].
The feedback braking method uses active frequency converter technology to convert regenerative electrical energy to AC power that is returned to the grid at the same frequency as the grid to achieve braking.
Inverter dedicated energy feedback braking unit
The implementation of energy feedback braking requires voltage in-phase control, feedback current control, etc..
Advantages of feedback braking:
It can operate in four quadrants, and power feedback can improve the efficiency of the system;
Disadvantages of feedback braking:
(1)This feedback braking method can only be used when the grid voltage is stable and is not prone to failure (the grid voltage fluctuation is not greater than 10%). Since the power grid voltage fault time is greater than 2ms when the generator brake is running, commutation faults may occur and damage the device.
(2) During feedback, the grid harmonic pollution;
(3) The control is complicated and the cost is high.
3. DC braking
Definition of DC braking:
DC braking generally means that when the output frequency of the inverter is close to zero, the motor speed drops to a certain value, and the inverter becomes DC in the stator winding of the asynchronous motor, forming a static magnetic field. At this time, the motor is in an energy-consuming braking state, and the rotor cuts off the static magnetic field to generate braking torque when rotating, so that the motor generates braking torque. Stop quickly.
It can be used for irregular rotation caused by external factors that require accurate parking or braking the motor before starting..
Elements of DC braking:
The value of the DC braking voltage is basically the set value of the braking torque. Obviously, the inertia of the towing system is large, and the DC braking voltage value is correspondingly large. Generally, the rated output of a frequency converter with a DC voltage of about 15-20% is about 15-20%. The voltage is about 60-80V, using the percentage of braking current;
The DC braking time, that is, the time for the DC current to enter the stator winding, should be slightly longer than the actual shutdown time.
As the working frequency of the frequency converter decreases, the initial frequency of DC braking starts to change from energy consumption braking to DC braking, which is related to the load braking time requirements. If there is no strict requirement, set the starting frequency of DC braking as low as possible.
4. Common DC bus feedback brake
The principle of the common DC bus feedback braking method is to feed back the regenerative energy of the motor A to the common DC bus, and then the motor B consumes the regenerative energy.
The common DC bus feedback braking method can be divided into two methods: common DC balanced bus feedback braking method and common DC loop bus feedback braking method.