1. Introduction
The increasing popularity of frequency converters has provided good production and process benefits for industrial automation control in various industries. But with the continuous improvement of automation level, the degree of power pollution caused by automation equipment is also increasing, and the corresponding interference on automatic control systems is also becoming stronger. The requirements for power filtering, purification, and obtaining relatively stable green power supply are also increasing.
There are clear laws and regulations on the design and application of electromagnetic compatibility (EMC or EMI) internationally, which have clear regulations on the interference and interference of electronic devices, as well as the harmonic content of power supplies. Due to the relatively slow development of automation in electronic equipment in our country, there is still no certain understanding of the pollution of its harmonic content on the power grid. Therefore, this understanding has not yet developed to a legal level. However, in some industrial production scenarios with relatively high levels of automation, the significance of electromagnetic compatibility has become relatively clear. Some electronic devices are very sensitive to electromagnetic interference and cannot function properly.
2. Introduction to Automatic Control System in Fermentation Workshop
The fully automatic DCS control system monitors the temperature, pressure, pH value (pH value) of each fermentation tank in all aspects, and automatically operates the feeding and discharging during the fermentation process, including sugar addition, phenylacetic acid, ammonia addition, etc. There are three sensors for detection, which send electrical signals to the microcomputer control system. The microcomputer control system sends out a pulse signal (+5V) in a timely manner based on the detected voltage (or current) value, controls the opening and closing of the solenoid valve (the working voltage of the solenoid valve is+24V), and realizes the supply of incoming and outgoing materials. In this way, each fermentation tank has six solenoid valves, three sensors, and two detection instruments for three input and output controls, all of which are remotely monitored on a microcomputer and fully displayed on a large screen wall. There are a total of 18 fermentation tanks in the entire workshop. It can be imagined that this type of system has a huge structure, complex control, dense wiring, and must be carefully and meticulously considered during installation.
3. Analysis and Solution of Electromagnetic Interference Problems
Due to the fact that the user has already tried out a frequency converter and has a certain understanding of energy-saving by replacing the pulley and adjusting the motor speed, they have decided to install our company's frequency converter to achieve the goal of energy conservation and efficiency improvement.
When we first installed the frequency converter, we did not consider the severity of electromagnetic compatibility. After the frequency converter started up, it interfered with the original control system and sent alarm signals to multiple fermentation tanks on the microcomputer. Later, we added input and output reactors, but the problem was not solved. Later, we observed the user's original frequency converter and found that he used a dedicated power filter for the input and output terminals of the frequency converter produced by Shanghai Feijing Electric Co., Ltd., which has been running normally for a year. Therefore, the pharmaceutical factory suggested that our company purchase a power filter of the same model.
The input power filter of the frequency converter is composed of a high permeability ferrite core and iron powder core, coupled with a certain capacitor, to form an LC filter. It filters out the high-order harmonics (within a certain frequency band) generated by the frequency converter, so that electrical equipment working in the same or adjacent power grid is not disturbed and can work normally. The schematic diagram is shown in Figure 1:
Figure 1 Schematic diagram of input filter circuit
The output power filter of the frequency converter adopts inductance (L) filtering to suppress the conducted interference of the frequency converter output and reduce the low-frequency radiation interference on the output line, reducing the electromagnetic noise of the directly driven motor and significantly reducing the copper and iron losses of the motor. The schematic diagram is shown in Figure 2.
Figure 2 Schematic diagram of output filter circuit
After purchasing this type of filter, we went to the site for debugging. Due to limited on-site contact with this type of equipment, technical personnel were not adequately prepared. Although filters were added, the filtering effect was still not ideal, and there was still interference during heavy loads. The DCS system could not function properly, and the frequency converter could not operate. So we conducted a specific analysis of the problem.
Reasons for frequency converter interference:
Figure 3 Main circuit diagram of frequency converter
The main circuit of a frequency converter is generally in the AC-DC-AC mode (see Figure 3). The external input of a 380V/50Hz power supply is uncontrollably rectified into a DC voltage signal through a three-phase bridge, which is then filtered by a filtering capacitor and inverted into a variable frequency AC signal by a high-power transistor switching element. In the rectifier circuit, the waveform of the input current is an irregular rectangular wave, which is decomposed into fundamental waves and various harmonics according to the Fourier series. The higher-order harmonics will interfere with the input power supply system. In the inverter output circuit, the output current signal is a pulse waveform modulated by a PWM carrier signal. For GTR high-power inverter components, the PWM carrier frequency is 2-3kHz, while the PWM maximum carrier frequency of IGBT high-power inverter components can reach 15kHz. Similarly, the output circuit current signal can also be decomposed into fundamental waves containing only sine waves and other harmonics, while higher-order harmonic currents directly interfere with the load. In addition, high-order harmonic currents also radiate into space through cables, interfering with adjacent electrical equipment.
The main transmission channels of frequency converter interference:
When the frequency converter is working, as a powerful interference source, its interference paths are generally divided into radiation, conduction, electromagnetic coupling, secondary radiation, and radiation while conducting. The main approach is shown in Figure 4:
Figure 4 Main propagation pathways of frequency converter interference
From the above figure, it can be seen that the radiation interference generated by the frequency converter has a strong impact on the surrounding wireless receiving equipment. Conducted interference causes electromagnetic noise in the directly driven motor, resulting in a significant increase in copper and iron losses. At the same time, conducted interference and radiation interference have a significant impact on the electronic sensitive devices connected to or adjacent to the power input terminal.
In response to these two debugging situations and the interference and interference caused by the frequency converter, the engineer analyzed and summarized the working principle and wiring situation, and concluded that the main interference was caused by high-frequency harmonics generated at the input end of the frequency converter. After installing the frequency converter, the input line of the frequency converter is in the original power line slot, while the output line is not in the line slot and is relatively close to the motor. Furthermore, the original wiring system is not very reasonable. The distance between the power cable tray and the control cable tray is relatively close, only 20cm. According to regulations, it should not be less than 50cm, and the two cable trays should run parallel. These are all taboos. The grounding wire of the frequency converter is also not very reasonable. It is connected to the wiring groove of the power line. The function of the wire groove is to support the power line and to shield it. The interference of the frequency converter is then transmitted to the wire groove through the grounding wire. The high-order harmonics generated by the frequency converter are radiated to the power and signal lines of other devices (especially sensitive sensor signal lines) through the input and ground wires of the frequency converter. It is emphasized that our frequency converter and DCS control system are not powered by the same transformer, which can eliminate direct conducted interference, interfering with the normal operation of the control system.
Analyzing these issues, as the original wiring system has become standardized, it is almost impossible to move again. Therefore, the idea of changing the wiring of power and signal lines should be ruled out. The ground wire of the frequency converter can be taken separately, and a ground wire can be directly connected to the ground wire of the electrical control cabinet in the distribution room. The input end of the frequency converter should be further filtered to solve the theoretical problem.
After parking the original fermentation tank on site, we added a set of common mode and differential mode magnetic rings on top of the original filter. We set two differential mode rings on each input and output phase line, and two common mode magnetic rings on the three input phase lines. We also connected the ground wire to the ground in the distribution room. After processing in this way, the machine started up and ran normally with the motor unloaded, without any interference alarms.
When operating under load, there is an interference alarm in tanks 305 and 307. Change the ground wire to the ground wire of the transformer controlling tank 307 (which has already used a frequency converter and runs its output line in the wire slot). Tank 305 no longer interferes with the alarm, but tank 307 still experiences interference alarms every few minutes. Analysis suggests that this may be due to the superposition of common mode interference generated by two frequency converters, or it may be caused by the ground wire being placed in the power line slot and running for a long time. Therefore, a ground wire filter is installed on the ground wire, But the effect is not very good either. Later, the ground wire was removed (after measurement, the leakage current of the entire frequency converter was very small, which did not cause harm to the human body, so the ground wire could be removed). The effect was better, but the alarm phenomenon also appeared intermittently. This analysis suggests that it is not caused by the ground wire, but rather due to insufficient filtering measures at the input end, high-frequency interference was not completely filtered out. Therefore, the machine is shut down. Two differential mode rings are added to each input phase line, and three common mode rings are placed on the three input phase lines. This way, the machine can start up and operate normally, and the entire system will no longer experience interference. The processed block diagram of the system is shown in Figure 5.
Figure 5 System Block Diagram
4. Conclusion
This experiment has made an attempt to solve the compatibility (i.e. electromagnetic compatibility) problem between automatic control and frequency converters in the fermentation industry. This type of automatic control system is widely used in the pharmaceutical industry. We have made efforts and also received training, which has taken us a step forward in dealing with electromagnetic compatibility issues. The application of frequency converters in the pharmaceutical manufacturing industry is bound to achieve significant breakthroughs. With the increasing expansion of China's frequency converter market, the significance of electromagnetic compatibility will become more extensive, and its application prospects will be very optimistic.
