The motor system driven by the inverter is more and more applied due to its obvious energy saving effect, convenient adjustment, simple maintenance and network. However, the interference caused by the special working mode of the inverter can not be ignored. Inverter interference mainly includes: First, non-linear rectifying devices such as thyristors or rectifier diodes are commonly used in inverters, and the harmonics generated by them will cause conduction interference to the grid, causing grid voltage distortion (voltage distortion rate is represented by THDv, inverter The THDv caused by harmonic generation is about 10~40%), which affects the power quality of the power grid. Second, the output part of the inverter is generally a switching device such as IGBT, which will generate strong output on the output line while outputting energy. Electromagnetic radiation interferes with the normal operation of surrounding electrical appliances.
2 Harmonics and electromagnetic radiation damage to the power grid and other systems
(1) Harmonics cause additional harmonic losses in electrical components in the power grid, reducing the efficiency of power transmission and transformation and electrical equipment.
(2) Harmonics can be transmitted to other electrical appliances through the power grid, affecting the normal operation of many electrical equipment. For example, harmonics will cause mechanical vibration of the transformer, causing local overheating, insulation aging, shortened life, and even damage; Conducted harmonics can interfere with the normal operation of the internal software or hardware of the electrical equipment.
(3) Harmonics can cause local series or parallel resonance in the power grid, thus amplifying the harmonics.
(4) Harmonics or electromagnetic radiation interference may cause malfunction of the relay protection device, making the electrical meter measurement inaccurate or even unable to work normally.
(5) Electromagnetic radiation interference causes disturbances such as control signals and detection signals near the output wires of the inverter to be interfered. In severe cases, the system cannot obtain correct detection signals or disorder the control system.
Generally speaking, the influence of the inverter on the system with large grid capacity is not very obvious, which is why the harmonics are not valued by most users. However, for systems with small system capacity, the interference caused by harmonics cannot be ignored.
3 International and national standards on harmonics
The current relevant standards mainly include: international standard IEC61000-2-2, IEC61000-2-4, European standard EN61000-3-2, EN61000-3-12, International Electrotechnics recommended standard IEEE519-1992, Chinese national standard GB/ T14549-93 "Power Quality Sharing Grid Harmonics". The following is a brief introduction:
(1) International standards
The IEC61000-2-2 standard applies to utility grids, and the IEC61000-2-4 standard applies to plant-level power grids. These two standards specify the degree of harmonics allowed without causing damage to the grid. They specify the maximum allowable voltage distortion rate. THDv.
The IEC61000-2-2 standard stipulates that each harmonic voltage at the public access point of the grid contains a THDv of about 8%.
The IEC61000-2-4 standard is divided into three levels. The first type of harmonic-sensitive occasions (such as computers, laboratories, etc.) THDv is 5%; the second category is for the public access point of the grid and a part of the factory access point THDv is 8%; the third category is mainly for in-plant The entry point THDv is 10%.
The above two standards also stipulate the amplitude of the harmonic current allowed by the electrical equipment. The former is mainly for 16A or less, and the latter is mainly for 16A to 64A.
The IEEE 519-1992 standard is a proposed standard with the goal of limiting a single THDv to less than 3% and a total THDv of less than 5%.
(2) Domestic standards
GB/T14549-93 stipulates that the utility grid harmonic voltage (phase voltage) limit is 380V (220V) grid voltage total THDv is 5%, each harmonic voltage content rate is 4%, evenly 2% .
From the above criteria, the general single-time voltage distortion rate is 3 to 6%, and the total voltage distortion rate is acceptable in the range of 5 to 8%.
4 Ways to reduce the influence of inverter harmonics on other equipment
(1) Increase the AC / DC reactor
After using the AC/DC reactor, the THDv of the incoming current is reduced by about 30% to 50%, which is about half of the harmonic current of the reactor.
(2) Multiphase pulse rectification
Multi-phase rectification can be used when the conditions are met or the harmonics required to be generated are relatively small. The 12-phase pulse rectification THDv is about 10%~15%, and the 18-phase pulse rectification THDv is about 3%~8%, which meets the strict standards of EN61000-3-12 and IEEE519-1992. The disadvantage is that special transformers and rectifiers are needed, which is not conducive to equipment modification and the price is high.
(3) Passive filter
After adopting the passive filter, the THDv in the incoming line can be reduced to 5%~10% at full load, meeting the requirements of EN61000-3-12 and IEEE519-1992, and the technology is mature and the price is moderate. Suitable for THDv <30% under all loads. The disadvantage is that the power factor is reduced at light loads.
(4) Output reactor
It is also possible to use a method of adding an AC reactor between the frequency converter and the motor, the main purpose of which is to reduce the electromagnetic radiation generated by the line during the energy transmission of the output of the frequency converter. The reactor must be installed closest to the inverter to minimize the lead distance from the inverter. If an armored cable is used as the connection between the inverter and the motor, this method can be omitted, but the cable must be reliably grounded at the inverter and the motor end, and the grounding pin should be grounded as it is and cannot be twisted into a rope. Or identify, can not be extended with other wires, the inverter side should be connected to the ground terminal of the inverter, and then the inverter is grounded.
5 Method for reducing or weakening the interference of the inverter harmonics and electromagnetic radiation to the equipment
The method described above is to reduce the influence of the external equipment when the inverter is working, but it does not eliminate the external interference of the inverter. If you want to further improve the immunity of other equipment to the harmonics and electromagnetic radiation of the inverter, especially in the inverter ( Different brands may produce different levels of interference. The methods commonly used in places with severe interference are usually the following:
1) Use an isolation transformer
The use of an isolation transformer is primarily a response to conducted interference from the power supply. Using an isolation transformer with an isolation barrier, most of the conducted interference can be blocked before the isolation transformer. At the same time, it can also have the function of power supply voltage conversion. Isolation transformers are commonly used to control the conducted interference of instruments, PLCs, and other low-voltage, low-power electrical equipment in control systems.
2) Use filter modules or components
At present, there are many filter modules or components specially used for anti-conduction interference. These filters have strong anti-interference ability, and also have the function of preventing the conduction of the electric device itself to the power supply, and some also have the peak voltage absorption function. There are many benefits to various types of electrical equipment.
A commonly used structure of a two-hole magnetic core filter. There is also a single-hole magnetic core filter, which has a weaker filtering capability than the two-hole, but the cost is lower.
3) Select low-voltage equipment such as meters with switching power supply
Generally, the switching power supply has strong resistance to power conduction interference, because the relevant filters are also used inside the switching power supply. Therefore, when selecting the power supply equipment of the control system or selecting the control electric appliance, try to use the type of switching power supply.
4) Make the signal line anti-interference
The signal line bears the transmission task of the detection signal and the control signal. It is undoubted that the quality of the signal transmission directly affects the accuracy, stability and reliability of the entire control system. Therefore, it is necessary to do the anti-interference of the signal line.
The interference on the signal line is mainly electromagnetic radiation from space, and there are two kinds of normal interference and common mode interference.
(1) Suppression of normal interference
Normal interference refers to the interference signal superimposed on the measurement signal line. Most of the interference is a high frequency alternating signal, and the source is generally coupled interference. The methods to suppress normal interference are:
a Connect the RC filter or the double T filter to the input loop.
b Try to use double integral A/D converter. Due to the characteristics of this integrator, it has certain functions of eliminating high frequency interference.
c The method of converting the voltage signal into a current signal and transmitting it has a very strong inhibitory effect on the normal interference.
(2) Suppression of common mode interference
Common mode interference refers to the interference signal shared on the signal line. Generally, it is caused by the potential difference between the ground of the signal under test and the ground of the control system. The period and amplitude of the interference on the two signal lines. Basically equal, so the above method cannot be eliminated or suppressed. The suppression method for common mode interference is as follows:
a Differential amplifier with dual differential inputs with high common-mode rejection ratio.
b Stranding the input wires, twisted twisted pairs can reduce the common mode interference, which changes the direction of the electromagnetic induction e of the wires, so that the inductances cancel each other out.
c Using optical isolation, common mode interference can be eliminated.
d When using a shielded wire, the shield is grounded only at one end. Because if both ends are grounded, interference will occur due to the ground potential difference flowing through the shield. Therefore, interference can be prevented by grounding at one end.
Whether it is to suppress normal interference or suppress common mode interference, the following should be done:
(1) The input line should be as short as possible.
(2) Avoid wiring and power line when wiring, separate the signal line from the power line, place the signal line in the shielded metal tube, or separate the power line and signal line by more than 40cm.
(3) In order to avoid signal distortion, attention should be paid to impedance matching for signals transmitted over a longer distance.
5) In the control system using MCU, PLC, computer, etc. as the core, when software is compiled, software filtering of the detection signal and output control part can be appropriately added to enhance the anti-interference ability of the system itself.
6 Conclusion
The distribution parameters of the interference are very complicated. Therefore, in the anti-interference, appropriate measures should be taken, considering the effect, but also considering the price factor, and also depends on the site conditions. As long as the measures adopted can solve the problem, often too many anti-interference measures may cause additional interference, and the specific situation is specifically solved.
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RJ45 port editing
1. Origin:
2. The name RJ stands for the registered jack and is the USOC (universal service ordering codes) code of bell system. USOC is a series of registered sockets and their wiring mode, which are developed by bell system to connect the user's equipment to the public network. FCC regulations control the application of this purpose. The FCC (Federal Communications Commission) issued a document on behalf of the U.S. government to specify RJ11.
3. RJ11 is the common name of the connector developed by Western Electric Co. Its shape is defined as a 6-pin connecting device. Originally called wexw, where x means "active", contact or needle. For example, we6w has all six contacts, numbered from 1 to 6. The we4w interface only uses four pins, the outermost two contacts (1 and 6) are not used, and we2w only uses the middle two pins. For RJ11, the information source is contradictory. It can be a 2 or 4-core 6-pin connector. What's more confusing is that RJ11 is not only used to represent the 6-pin connector, it also refers to the 4-pin version.
4. RJ45 and RJ11: different standards, different sizes
5. Due to the different sizes of the two (RJ11 is 4 or 6-pin, RJ45 is an 8-pin connection device), it is obvious that the RJ45 plug cannot be inserted into the RJ11 socket. However, it is physically feasible (RJ11 plug is smaller than RJ45 jack), which makes people think that they should or can work together. It's not. It is strongly recommended not to use RJ11 plug for RJ45 socket.
â’ because RJ11 is not internationally standardized, its size, insertion force, insertion angle, etc. are not in accordance with the international standard connector design requirements, so interoperability cannot be guaranteed. They even cause damage to both. Since the RJ11 plug is smaller than the RJ45 socket, the plastic parts on both sides of the plug will damage the metal pin of the inserted socket.
RJ - 45 port is our most common port, it is our common twisted pair Ethernet port
Because twisted pair is mainly used as transmission medium in Fast Ethernet, RJ-45 port can be divided into 10Base-T network RJ-45 port and 100base TX network RJ-45 port.
Among them, the RJ-45 port of 10Base-T network is usually identified as "eth" in the router, while the RJ-45 port of 100base TX network is usually marked as "10 / 100btx". This is mainly due to the fact that most products of Fast Ethernet router still adopt 10 / 100Mbps bandwidth adaptive.
The left figure shows the RJ-45 port of 10Base-T network, while the right figure shows the RJ-45 port of 10 / 100base-tx network. In fact, the two RJ-45 ports are exactly the same in terms of the port itself, but the corresponding network circuit structure in the ports is different, so they can not be connected casually.
Definition of RJ45 interface pin signal
Ethernet 10 / 100Base-T interface:
1 TX + tranceive data +
2 TX - tranceive data -
3 RX + receive data +
4 N / C not connected
5 N / C not connected
6 RX - receive data
7 n / C not connected
8 N / C not connected
Ethernet 100base-t4 interface:
1 TX_ D1 + tranceive data +
2 TX_ D1 - tranceive data
3 RX_ D2 + receive data +
4 BI_ D3 + bi directional data +
5 BI_ D3 bi directional data
6 RX_ D2 - receive data
7 BI_ D4 + bi directional data +
8 BI_ D4 bi directional data
Note: RJ45 interface adopts differential transmission mode, TX + and TX - are a pair of twisted pair, which can reduce interference when twisted together.
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