1 Introduction
The speed change of AC induction asynchronous motor drives is an epoch-making technological advancement in the field of electrical transmission in the 20th century. With the wide application of frequency converters, frequency converters have increasingly become the largest source of electromagnetic pollution in the field of factory automation. It can often be seen that a dozen sets of inverters are installed in a device-intensive factory. Inverter's nonlinear equivalent load of DC-AC inverter makes inverter not only pollute factory power supply system in many system integration projects, but also interferes directly with automation engineering projects, causing measurement and control system misalignment failure and seriously undermining the stability of large-scale system. Sex, even the frequency converter itself is subject to interference, causing "bootstrap" type speed control failures. Although international standards rigorously regulate the electrical equipment EMC (IEC61000 series electromagnetic compatibility design), and China’s National Bureau of Quality and Technical Supervision has decided to adopt “equivalent†in China, at the same time, China's national standard power quality quality public power grid harmonics GB/T 14549 -93 has been in force for 14 years, but the rapid development of the country's economy and technology has made pollution control of power electronic switching devices without delay.
The author of this article is from the front line of automation integration engineering. In recent years, customer service often encounters the interference problem of the frequency converter, causing the device to malfunction, making the factory's production line unable to run, and the cause of this type of problem is also difficult to find, after consulting relevant information, and then dealing with the problem in the work Some of the experiences to specifically talk about the source of the inverter interference, the way of transmission and some of the different situations of dealing with interference problems in practical applications, the hope is different from the dogmatic preaching of textbooks.
2. Inverter interference analysis
Inverter interference problems are generally divided into self-interference of the frequency converter; electromagnetic waves generated by external equipment interfere with the frequency converter; and frequency converters interfere with other weak electrical equipment in three types of situations.
The inverter itself is a source of interference. As we all know, the inverter consists of two parts, the main loop and the control loop. The main loop of the inverter is mainly composed of a rectifier circuit, an inverter circuit and a control circuit. The rectifier circuit and the inverter circuit are composed of power electronics. Device composition, power, electronic devices have non-linear characteristics, when the inverter is running, it must perform fast switching action, and thus generate high-order harmonics, so that the inverter output waveform contains a large number of high-order harmonics in addition to the fundamental wave. Regardless of the type of interference, higher harmonics are the main cause of interference from the frequency converter. The inverter itself is a source of harmonic interference, so it will affect the equipment on the power supply side and the output side. Compared with the main loop, the control loop of the inverter is a small energy, weak signal loop, and is vulnerable to interference from other devices. Therefore, when the inverter is installed and used, it must take anti-jamming measures on the control circuit.
3. Analysis and Treatment of Inverter Interference Cases
3.1 How to determine the frequency converter interference problem
Inverter interference problems are mainly reflected in the operation of the motor. For example, the motor suddenly stops during operation, the motor runs quickly and slowly, the running speed is unstable, the motor cannot be stopped, the button does not play any role, etc. These are all examples of the frequency converter being affected by interference.
3.2 The third way to ground
The general treatment method of the interference problem is to ensure good grounding. The general requirements of the grounding terminal are as follows: The grounding terminal is grounded in the “third way†(individual grounding). The shorter the grounding wire, the better, and the grounding must be good. Control circuit line Use shielded wire, and the shielding of the shield is suspended at the far end, and the near end is grounded. According to the product requirements, reasonable wiring, strong and weak separation, keep a certain distance, avoid the parallel wiring of the drive power line and the signal line, and disperse the wiring; Resistant to wireless interference filters, inverter input and output anti-interference filters or reactors; To take measures to prevent electromagnetic induction, even the inverter can be shielded with a metal iron box; Properly reduce the carrier frequency; If you use communication, RS485 communication Use twisted pair cable. Below, I will make specific analysis of different interference situations in actual use.
3.3 Three-phase five-wire system power supply
Have encountered such a situation, the inverter has been running, press the stop button does not work, after inspection found that the ground wire of the inverter is only connected with the neutral line of the transformer, and the neutral line of the transformer is not connected to the earth, After the neutral line of the transformer is grounded, the inverter returns to normal. Nowadays, many small factories usually do not pay attention to the connection of the ground wire. When the machine is shipped from the factory, the ground wire and the neutral wire are strictly separated according to the standards stipulated by the national electrician law. The neutral terminal of the power distribution cabinet has dedicated terminals, and the ground wire has special grounding screws. Because the user came from the transformer three phase lines and a neutral line, only the neutral line is connected to the "N" terminal, and the ground line is not connected with the neutral line, although the control line uses a shielded line, the shielding layer also The grounding screw was received, but it was not connected to the earth. It did not provide a shielding effect, causing the drive to stop without interference due to the uncontrolled motor. After the neutral and ground cables in the power distribution cabinet are connected, they will return to normal, and the ground in the distribution cabinet can also be directly connected to the earth. Many users adopt the method of connecting the ground wire to the neutral wire. However, there are drawbacks in using this method. If the neutral wire is disconnected, starting one action of the machine tool may cause the machine tool to be charged and pose a safety hazard to the human body. This interference belongs to the interference type of the drive itself.
3.4 Interference Cases of External Equipment to Frequency Converters
(1) Phenomenon. Occasionally, the motor cannot be stopped. After checking that the shield is properly grounded, reducing the carrier frequency does not work. Inverter input and output side magnetic loop filters do not work.
(2) Analysis. The distribution cabinet where the inverter is installed is too close to the power distribution room. The distribution room distribution cabinet has a large current flowing through it. There is a strong magnetic field around the current, which interferes with the normal operation of the inverter and keeps the distribution cabinet away from the power distribution. After the room is back to normal, this is outside equipment interference to the inverter.
3.5 Inverter's interference with external equipment
(1) Phenomenon. After starting the inverter, the motor does not operate.
(2) Analysis. The frequency converter is given the operating frequency by the external 4-20ma, 4-20MA DC signal is sent by the transmitter, see the display board, the frequency display is 0.00. The ammeter was used to measure the output of the transmitter and no output was found. After a 102 capacitor is connected to the output terminal of the transmitter and then started, the device returns to normal, indicating that the signal source has been disturbed. In engineering practice, a simple signal line shunt capacitor solves the big problem is a practical and often practical method. This is the inverter's interference with external devices.
4 Conclusion
With the rapid development of industrial automation, the use of frequency inverters is becoming more and more common. The interference problems of frequency inverters will also encounter many problems. Many end users often do not know how to solve them when they encounter this problem. I hope this article can Give them some help.