Electromagnetic Compatibility of Central Control System in Coal Washery
This text introduces the example analysis and solution of inverter for slag-slurry pump interfering auto control system in coal washery factory. It can solve the problem of mistake action led by inverter interfering center control system effectively, when the suited power line filter are added in the inverter input and output termination and active filters are added in the input termination of PLC power supply. The problem of Electromagnetic Compatibility were solved completely after whole machine debugging and trial.
EMI Interference Suppression Method Electromagnetic Compatibility
Increasingly wider use of inverter brings favorable production and technical benefits for automatic control of many fields. But 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 the interference 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.
2. Introduction of central control system in coal washery
The central control system is adopted in order to improve the output and production efficiency by coal washery, and IPC control the start and stop of motor through PLC, the current is4~20mA from IPC to PLC to inverter, the main 4 pumps are controlled by inverter(technical request), which are 2 sets of 355KW inverter and 2 sets of 132KW inverter. The motor stops working automatically during the process of equipment debugging, including 4 sets of motors controlled by 4 sets of inverter, the reason is PLC`s wrong action; some spots appear on the screen of surveillance systems, and the images jitter acutely. After examination it is found that the harmonics current of inverter exceeds standard badly, PLC`s wrong action is led by the conduction interference of inverter, and the problem of surveillance systems is led by radiation interference. In order to solve the problem of electromagnetism interference, should consider the 3 factors of electromagnetic compatibility first.
3. Three factors of electromagnetic compatibility
3.1 Electromagnetic disturbance source:
The electromagnetic energy launched by any form of nature or electric device can injure the man or other biology in the same environment, or lead electromagnetic hazards to other devices, subsystems or systems resulting in performance degradation or failure.
3.2 Coupling path: the pathway or agent of transmitting disturbance
3.3 Victim: It means the man or other biology injured and components,devices,subsystems or systems suffered electromagnetic hazards resulting in performance degradation or failure, when they suffer the electromagnetic energy launched by electromagnetic disturbance source
If the electromagnetic compatibility problems occur, there must be 3 factors : electromagnetic disturbance source, coupling path, victim. Therefore, in the face of the electromagnetic compatibility problems, we should start with these 3 factors. If one of the three factors is eliminated, the problem of electromagnetic compatibility can be solved.
4. Analysis and solution of electromagnetic interference
4.1 Why does the inverter generate interference
The main circuit of inverter generally is AC~DC~AC mode(Fig 1), the external input frequency power with 380V/50Hz is uncontrollably commuted to DC voltage signal by the threephase 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 highpower 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. Another high harmonic currents bring radiation to space through the cables to disturb nearby electrical equipment.
4.2 Main rout of transmission of inverter interference in coal washery
When inverter work as a powerful source of interference, its routs of interference are generally divided into radiation, conduction, electromagnetic coupling, the second radiation and conduction-radiation etc. The main rout is shown as Fig.2 according to the analysis of scene line laying.
From Fig2 we can see that the radiation interference generated by inverter have a strong impact to the surrounding equipment, the conduction interference makes the direct`drive motor produce electromagnetic noise, and make the copper loss, iron loss increase highly. Meanwhile, the conduction interference and radiation interference have great impact to the PLC and IPC connected by power input termination.
Rout the power input line, output line of inverter separately with other cable lines, remove the earth line of inverter from switch room and connected them to new ground, install active filter in PLC input termination to suppress the harmonic and high frequency interference from electric network, and observe the state of electronic running, occasionally the motor will stop automatically. After analysis the reason is that the interference may not be filtered completely. Install filter for inverter in the input and output terminal of one inverter(the inverter which is connected to the same transformer with PLC), install EMI ferrite core and differential mode inductors in the input termination of PLC, the motor will run normally with no load when it start up. In order to avoid the strength of interference after loading, install filters for inverter and EMI ferrite core in the input and output terminal of other 3 inverters, install power line filter in all power supply of cameras, install EMI ferrite core in the video signal cable and video collection signal cable of all cameras, install super insulation transformer and high performance power line filter in the input termination of chief power supply of surveillance center to prevent the direct conduction interference. Upon adopted the above measures, the equipment of coal washery run normally(with load), and the surveillance systems show normally generally.
The application of automatic control system is more and more widely, how to make all equipment work properly and not disturb each other, laying lines and grounding are also very important in addition to enhance anti-jamming capacity of equipment.