DCS Automatic Control System and The Electromagnetic Compatibility(EMC) of Inverter


This article introduces how the inverter used in fermenter disturbs the DCS automatic control system and the processing methods in the pharmaceutical factory. Through mounting the matched power line filters on input and output of inverter, effectively resolve the electromagnetic interference(EMI) made by inverter when it is working. After debugging and testing for several times, the EMCbetween DCS automatic control system and inverter carry out in the Fermentation workshop of the pharmaceutical factory.

Key Words:

EMI interference    Suppression Method    Electromagnetic Compatibility

1. Introduction

The application of inverter is wider increasingly, which offer favorable production and technical benefit for industry automatic control of many fieldsBut along with the increasing of automation extent, the power pollution from automatic equipment is more and more., and the interference of automation system is more and more stronger., so the requirements of filtering and cleaning power to get relative steady green power is higher and higher.

There are specific law and statute for the EMI/EMC design and application, and for theinterference from electric equipment and interference to electric equipment,the content of power harmonic in the world. The development of electric equipment automation is slow relatively in our country, and we have little cognition about the power pollution from harmonic content, so this cognition has not reached to the extent of law. But electromagnetic compatibility is more and more important in some occasions of high degree of industry automation, Some electronic equipment are very sensitive to electromagnetic interference, which make them not work normally.

We have met this problem when we mount the inverter equipment in Hua Xing Medicines Factory ,the problem have already solved through filtering for many times.Here we talk about the methods for solving this problem for the business community jointly to explore.

2. Introduction for Fermentation workshop Automatic Control System

The NO.107 workshop of penicillin fermentation in Hua Xing Medicines Factory, adopts the DCS automatic control system which is designed and manufactured by Beijing Biochemical Engineering Company Ltd.to all-round monitor for temperature, pressure, acidity (PH value)of each fermentation tank,as well as carrying out automatic operation on the fermentation process of feeding, the material, including sweetened, acid, ammonia etc.,there are three sensors for testing,which emit signals to computer control system.The computer control system timely emit pulse signal(+5V) based on tested voltage or current to control opening and closing of electromagnetic valve (its operating voltage is +24V), to achieve the materials supply. In this way ,each fermentation tank has three items controlled by the material systems such as six electromagnetic valves, three sensors and two measuring instruments. All the three items are on the implementation of all long-distance monitoring in computers,what’s more ,all the data are displayed on a large full-screen wall.There are a total of 18 fermentation tanks in the full-workshop. This system can be imagined such a huge structure, complex to control, dense wiring, therefore it must be very careful and meticulous to consider when installed.

3. Analysis and solutions of electromagnetic interference

The users have got a certain awareness after trying to use one set of inverter according to replace the pulley, adjust motor speed,so they decide to install our inverter to achieve the purpose of energy efficiency.

When firstly installing the inverter without considering seriousness of EMC, after the inverter start-up the original control system of interference ,many sets of fermentation tank in computer issued a warning signal, but later the problem was not solved even if mounting input and output reactors. Afterwards, we observed that the users adopted the inverter input and output series filters manufactured by Jinan Filtemc Equipment Co.,Ltd, so the inverter had already normal operation for one year,hereby they suggested us to purchase the same type power line filters.

The inverter input power line filter adopt a high permeability of the ferrite cores and iron core, it match a capacitor to be the LC filter which filter out the high harmonics (in a band)made by inverter, as a result that these electrical equipment work either close or in the same power grid could work well and far away from interference . Its schematic as shown in Figure 1.

Figure 1  Input filter circuit schematics

Inverter output power line filter used inductance (L) filtering to suppress the conduction interference output from inverter and reduce the low`frequency radiation interference, lower the motor and electromagnetic noise of direct drive, sharply reduce the electrical losses of copper and iron loss. Its schematic shown in Figure 2.

Figure 2 Output filter circuit schematics


After purchase such filter, we were at the scene to debug. As less contact with this filter at the scene,the technician didn’t prepare completely, although used filter,but the filtering effect is not ideal,there was still interference in the heavy load condition, the DCS system couldn’t work, and the inverter still couldn’t run. Upon that we do issue a specific analysis.

The reason why inverter generates interference:

Fig3 Main circuit of inverter

The main circuit of inverter generally is AC~DC~AC mode(Fig 3), the external input frequency power with 380V/50Hz is uncontrollably commuted to DC voltage signal by the three`phase bridge, and inverter to variable frequency AC signal by the filter capacitor filtering and high`power transistor switching components. In rectifier circuit, the waveform of input current is irregular rectangular wave, and the waveform is divided into fundamental and harmonics by Fourier Series, the high harmonics will interfere with the importation of power supply system. In the loop of inverter output, the output current signal is pulse waveform modulated by PWM carrier wave signal. For GTR high`power inverter components, the PWM carrier wave frequency is 2~3kHz, and highest PWM carrier frequency of IGBT inverter components can be up to 15kHz. Similarly, the output loop current signal can be divided into fundamental only with sine wave and other harmonic, while the high harmonics currents directly interfere load. Besides high harmonic currents bring radiation to space through the cables to disturb nearby electrical equipment.

The main route of transmission of inverter interference

While inverter works, as a powerful source of interference, its channels include radiation, conduction, electromagnetic coupling, the second radiation and conduction-radiation etc. Shown as Fig.4

Fig 4 The main interference channels of inverter


From Fig4 we can see that the radiation interference caused by inverter have a strong influence on around the wireless reception equipment,the conduction interference makes the direct-drive motor produce electromagnetic noise, making copper losses and core loss in a substantial increase ,at the same time both of conduction interference and radiation interference have greatly influence on all the sensitive electronic equipment which is connected or in close to input of power supply.

Aim at the twice debugging situation and the interference made by inverter with the way, we have analyzed and summarized with the engineers of the manufacturers.Moreover we contact with the engineers of Beijing Biochemical Engineering Company Ltd. Through telephone communication for many times to learn its working principles and wiring situation, after analyzing we think that the interference mainly caused by the high-frequency harmonics brought by inverter input.

First, the input line of inverter not the output one stay inside of the original driving force trunking after installing the inverter, and the inverter too close to motor. Furthermore, it’s not reasonable that the distance between driving force trunking and controlling trunking is only 20cm,but in accordance with the regulations should not be less than 50 cm.What’s less, it’s relative taboo if these two wire tray are laid in parallel. Besides, it’s not reasonable to link the ground line of inverter with the trunking of power line which has two functions such as supporting power line and shielding function, so the interference is transferred from the ground line to trunking.The high frequency harmonic made by inverter radiate to power lines and signal lines of the other equipment through input line and ground line,especially the more sensitive signal lines of sensors then interfere the control system to work. Here need stress that our inverter and DCS control system are not powered from the same transformer, therefore  you can rule out the possibility of direct transmission interference.

Obviously it’s impossible to change the stereotyped line route system, the idea of changing the line route of power lines and signal lines should be put away, but the problem could be solved through directly linking the ground line of inverter with the ground line of electric control cabinet in power distribution room and strengthen filtering effect on input of inverter.

At the scene after the fermentation tank stopping work,we take the following steps such as add one set of common mode and differential mode magnetic ring in addition,add two sets of differential mode magnetic ring on each input and output line, cover two common mode magnetic rings along three phase-lines of input, and link the ground line with the ground line of power distribution cabinet. Through the above steps and start-up operation, the inverter can run under normal circumstances in the condition of empty load of motor without the interference alarm phenomenon

When running, 305 and 307 tanks appear interference alarm phenomena.Change the ground line to the ground line of transformer controlling 307 tank which has already used inverter and its input line inside the trunking,then the 305 tank doesn’t have interference alarm phenomena, but the 307 tank still have this phenomena in a few minutes interval. Through analysis this phenomena, we guess the reason is that  it maybe common mode interference made by two inverters overlay, as well as maybe the longer line in the dynamic wire casing. Even if add filter on ground line, but the effect is still not good. Afterwards, the effect is better after tearing down the ground line but the alarm phenomena still appear  intermittently.Through analysis ,this phenomena is caused by not taking  enough filtering measures for input of inverter to filter high-frequency interference ,but not by the ground line. So after taking these followings measurements : add two sets of differential mode magnetic ring on each input phase-line, cover three common mode magnetic rings along three phase-lines of input. Through these measurements, the entire system do never have the interference phenomena, inverter works well. The diagram of processed system is shown as Figure5.

Figure 5 System diagram


4. Conclusion

This trial is one attempt for solving the EMC problem between automatic control and inverter EMC in the fermentation industry.The engineers of Beijing Biochemical Engineering Company Ltd.have said that this type automatic control system is popular in the pharmaceutical industry. We paid but also get trained, this case brings us a step forward in dealing with the problem of electromagnetic compatibility. The application of inverter must create a larger breakthrough in the pharmaceutical manufacturing industry. As the inverter market growing, the significance of EMC is widespread and its application will be very optimistic.

The fermenter department in New Scenery company can provide a perfectly frequency energy-saving solution. And the Electromagnetic Compatibility Engineering department in FILTEMC can provide technical support and project reform services for inverter manufacturers free of charge.

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