Suppliers of frequency converter supporting equipment remind you that in the traditional frequency control system consisting of universal frequency converters, 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 electric 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) "Braking resistance" in parallel with the capacitor artificially set in the DC circuit, called the dynamic braking state;
(2), 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.
In books and publications, many experts have talked about the design and application of inverter braking, especially recently there have been many articles about "energy feedback braking". Today, the author offers a new type of braking method, which has the advantages of four-quadrant operation of "feedback braking", high operating efficiency, and also has the advantages of "energy consumption braking" for the grid without pollution, high reliability.
Energy Brake
Using the braking resistance set in the DC circuit to absorb the renewable electrical energy of the motor is called energy consumption braking.
Its advantages are simple construction; No pollution to the grid (compared to feedback), 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, as shown in Figure 3, electric energy feedback improves the efficiency of the system. Its disadvantages are:
(1), only under stable grid voltage that is not easy to fail (grid voltage fluctuation is not greater than 10%), this feedback braking method can be used. 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.
New type of braking (capacitive feedback braking)
Main circuit principle
The rectification part uses a common uncontrollable rectification bridge for rectification, the filter circuit uses a common electrolytic capacitor, the delay circuit uses a contactor or a controllable silicon. The charging, feedback routing power module IGBT, charging, feedback resistor L and large electrolytic capacitor C (the capacity is about zero points, can be determined according to the operating system where the frequency converter is located). The inverter part consists of the power module IGBT. The protection circuit consists of IGBT, power resistor.
(1) Electric motor power generation operating status
CPU real-time monitoring of the input AC voltage and DC circuit voltage νd, decide whether to send a charging signal to VT1, once νd is higher than the input AC voltage corresponding to the DC voltage value (e.g. 380VAC-530VDC) to a certain value, the CPU shuts down VT3, through the pulse conduction of VT1 to achieve the charging process of the electrolytic capacitor C. At this time, the resistor L is divided into the electrolytic capacitor C, thus ensuring that the electrolytic capacitor C works within the safe range.
(2) Electric motor electrical operating status
When the CPU detects that the system is no longer charged, it conducts the VT3 pulse, so that the line on the resistor L becomes an instantaneous left and right negative voltage (as shown in the icon), plus the voltage on the electrolytic capacitor C can achieve the process of energy feedback from the capacitor to the DC circuit. The CPU controls the switching frequency of the VT3 and the vacancy ratio by detecting the voltage and DC circuit voltage on the electrolytic capacitor C, thus controlling the feedback current to ensure that the DC circuit voltage νd does not appear too high.
System difficulties
(1) Selection of resistor
(a) We take into account the peculiarities of the working conditions, assuming that the system has some kind of failure, leading to the free acceleration of the load of the bit contained in the motor, when the motor is in a state of power generation operation,
Renewable energy is returned to the DC circuit through six continuous current diodes, causing νd to rise, quickly making the frequency converter in a state of charge, at which time the current will be large. So the selected resistor wire diameter should be large enough to pass the current at this time.
(b), in the feedback loop, in order to make the electrolytic capacitor before the next charge to release as much electrical energy as possible, the choice of ordinary iron core (silicon steel plate) is not able to achieve the purpose, it is best to choose the iron core made of ferrous oxide material, and then look at the above consideration of the current value is so large, you can see how large this iron core, I do not know if there is such a large iron core on the market, even if there is, its price will certainly not be very low.
Therefore, I suggest that charging and feedback circuits each use an electrical resistor.
(2) Difficulties in control
(a) In the DC circuit of the frequency converter, the voltage νd is generally higher than 500VDC, and the resistance voltage of the electrolytic capacitor C is only 400VDC. It can be seen that the control of this charging process is not like the control method of energy braking (resistance braking). Its transient voltage on the resistor is reduced to, the transient charging voltage of the electrolytic capacitor C is νc = νd-νL, in order to ensure that the electrolytic capacitor works within the safety range (≤400V), it is necessary to effectively control the voltage drop νL on the resistor, and the voltage drop νL depends on the amount of inductance and the instantaneous change rate of current.
(b) In the feedback process, the electrical energy released by the electrolytic capacitor C must also be prevented from causing excessive DC circuit voltage through the resistor, so that the system appears overvoltage protection.
Main Applications and Application Examples
It is because of the advantages of this new type of braking (capacitive feedback braking) of the frequency converter, recently, many users have proposed to equip this system with the characteristics of their equipment. Because of the technical difficulty, it is not known whether there is such a braking method abroad. At present, only Shandong Fengguan Electronics Co., Ltd. has switched to this new type of mining elevator series of capacitive feedback braking from the frequency converter that used feedback braking in the past (there are still 2 in normal operation), so far, this capacitive feedback braking frequency converter has been operating normally for a long time in Shandong Ningyang Security Coal Mine and Shanxi Taiyuan, filling this gap at home.
With the expansion of the field of frequency converter applications, this application technology will be very promising, specifically, mainly used in the mine hanging cage (manned or loading), bevel well mining truck (single or double cylinder), lifting machinery and other industries. In short, the need for energy feedback devices can be used.