Suppliers of energy feedback devices for frequency converters remind you that in traditional frequency control systems composed of general-purpose frequency converters, asynchronous motors, and mechanical loads, when the potential load transmitted by the motor is lowered, the motor may be in a regenerative braking state; Or when the motor decelerates from high speed to low speed (including parking), the frequency may suddenly decrease, but due to the mechanical inertia of the motor, it may be in a regenerative power generation state. The mechanical energy stored in the transmission system is converted into electrical energy by the motor and sent back to the DC circuit of the inverter through the six freewheeling diodes of the inverter. At this time, the inverter is in a rectified state. At this point, if no measures are taken to consume energy in the frequency converter, this energy will cause the voltage of the energy storage capacitor in the intermediate circuit to rise. If the braking is too fast or the mechanical load is a hoist, this energy may cause damage to the frequency converter, so we should consider how to utilize this energy.
In general frequency converters, there are two commonly used ways to handle regenerative energy: (1) dissipating it into a "braking resistor" artificially set in parallel with a capacitor in the DC circuit, which is called the power braking state; (2) If it is fed back to the power grid, it is called feedback braking state (also known as regenerative braking state). There is another braking method, namely DC braking, which can be used in situations where accurate parking is required or when the brake motor rotates irregularly due to external factors before starting.
With the development of frequency conversion technology, the design and application of frequency converter braking, especially the new braking method of "energy feedback braking", have the advantages of "feedback braking" and high operating efficiency, as well as the advantages of "energy consumption braking", which has no pollution to the power grid and high reliability.
Energy consumption braking
The method of using the braking resistor set in the DC circuit to absorb the regenerative electrical energy of the motor is called energy consumption braking, which has the advantage of simple construction; No pollution to the power grid (compared to feedback control), low cost; The disadvantage is low operating efficiency, especially during frequent braking, which will consume a large amount of energy and increase the capacity of the braking resistor.
Generally, in general frequency converters, low-power frequency converters (below 22kW) are equipped with a built-in brake unit, which only requires an external brake resistor. High power frequency converters (above 22kW) require external brake units and brake resistors.
Feedback braking
To achieve energy feedback braking, conditions such as voltage control at the same frequency and phase, feedback current control, etc. are required. It adopts active inverter technology to invert regenerated electrical energy into AC power of the same frequency and phase as the power grid and return it to the grid, thereby achieving braking. The advantage of feedback braking is that electric energy feedback improves the efficiency of the system. Its disadvantage is that: (1) this feedback braking method can only be used under stable grid voltage that is not prone to faults (grid voltage fluctuation 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 feedback, there is harmonic pollution to the power grid. (3) The control is complex and the cost is high.
New braking method (capacitor feedback braking)
The energy feedback technology uses IGBT as the rectifier bridge, and the IGBT functional module can achieve bidirectional flow of energy, while using high-speed DSP chips to generate PWM control pulses. On the one hand, it can reverse the stored electrical energy in the capacitor to the power grid; On the other hand, the input power factor can also be adjusted to eliminate harmonic pollution to the power grid,
During power consumption, the DSP of the rectification control unit generates 6 high-frequency PWM pulses to control the conduction and cutoff of the 6 IGBTs on the rectification side. The conduction and cutoff of IGBT work together with reactors to generate a sine current waveform that is consistent with the phase of the input voltage, thus eliminating the harmonics generated by the rectifier bridge and eliminating harmonic pollution to the power grid.
When in the power generation state, energy is fed back to the DC bus through the diode on the inverter side, and as it accumulates, the voltage on the DC bus also increases. When it exceeds a certain value, the energy feedback part on the rectifier side starts, reversing the DC power into AC power. After adjusting the phase and amplitude, it is transmitted back to the AC power grid to achieve energy-saving effects.