In recent years, the development of the industrial era, the application of energy feedback technology is becoming more and more common. In elevators, mine lifts, port cranes, factory centrifuges, oil field pumps and many other occasions, will be accompanied by changes in load potential and kinetic energy. For example, when lifts, cranes and other mechanical discharge of heavy goods, the energy will be reduced, and when the centrifuge equipment is down, the kinetic energy will be reduced. According to the law of energy conservation, we know that energy will not disappear from the air, so where did this part of the energy go? The answer is that it is converted into renewable electricity by the motor. In fact, in equipment using variable frequency control, this part of the electricity is usually wasted by converting the brake resistor to heat.
If there is a device that uses this part of renewable electricity to return to the grid, then it can save this part of electricity, and play the effect of saving energy consumption. Energy feedback device is such a product. It uses power electronics conversion technology, its main role is to use the renewable electricity generated by the above equipment in the process of operation, and convert to synchronous AC power back to the grid, to play the effect of saving electricity.
In the traditional frequency control system consisting of general frequency inverters, asynchronous motors and mechanical loads, when the bit energy load driven by the motor is discharged, the motor may be in a regenerative power generation braking state; Or when the motor slows down from high speed to low speed (including stopping), the frequency may drop, but due to the mechanical inertia of the motor, the motor may be in a state of regenerative power generation, and the stored mechanical energy in the transmission system is converted into electricity by the motor, which is returned to the DC circuit of the inverter through the six continuous current diodes of the inverter.
In general frequency converters, there are two most commonly used methods of processing renewable energy:
(1) dissipated into the "braking resistance" in parallel with the capacitor artificially set in the DC circuit, called the dynamic braking state;
(2) to make it back to the grid, it is called the feedback braking state (also known as regenerative braking state). There is also a braking method, that is, DC braking, can be used in situations requiring accurate parking or irregular rotation of the motor brake before starting due to external factors.
Energy Brake
Using the braking resistance set in the DC circuit to absorb the renewable electricity of the motor is called energy consumption braking. Its advantages are simple construction, no pollution to the grid (compared with feedback manufacturing), and low cost; The disadvantage is low operating efficiency, especially when frequent braking will consume a lot of energy and the capacity of the braking resistance will increase.
In general, in the general frequency converter, the small power frequency converter (below 22kW) has a brake unit built-in, only need to add brake resistance. The high power frequency converter (above 22kW) requires an external brake unit, brake resistance.
Feedback Brake
To achieve energy feedback braking requires voltage and frequency and phase control, feedback current control and other conditions. It is the use of active reversal technology, to reverse the renewable electricity to the grid with the same frequency and phase AC power back to the grid, thus achieving braking.
The advantage of feedback braking is that it can run four quadrants, and electric energy feedback improves the efficiency of the system. Its disadvantages are:
(1) This feedback braking method can only be used under a stable grid voltage that is not easy to fail (grid voltage fluctuation is not greater than 10%). Because when the power generation brake is running, the grid voltage failure time is greater than 2ms, it may occur phase change failure, damage the device.
(2) In the feedback, there is harmonic pollution to the grid.
(3) Complex control, high cost.
With the rapid progress of research and application of frequency converters at home and abroad, especially universal frequency converters have been widely used in industrial production, energy feedback technology will be more and more reused.