The AnalysisSolution of EMC Problem for Automatic Control SystemWind Tunnel Labo


Abstract:

This text introduces the example analysis and solution of inverter for some wind tunnel automatic control system. After we installed matched filters in the input and output termination of inverter and the circuit of other control parts, and adopted series methods of shielding the whole power supply system and grounding well, the electromagnetic interference made by inverter was solved effectively. After the several debugging and examination of whole machine, the electromagnetic compatibility of automatic control system of wind tunnel laboratory was come true.

Key Words:

Frequency Converter    Measurement System     EMI Interference  Suppression Method    Electromagnetic Compatibility

The application of wind tunnel tests takes an important role in social production. With the development of the production field and technology, the demand for wind tunnel with wide dynamic range speed, high`speed precision, fast dynamic response, and other relevant requirements, with the rapid development of electronic technology, various type of capacity and frequency inverter, the successful manufacture of rectifying device and the development of computer technology, control theory, the application of AC adjustable speed transmission is expanding continuously in frequency converter system. How to maintain the normal work of frequency converter control system is the actual problem faced by technical personnel who are engaged in industrial automation, and is the guarantee to improve the accuracy of wind tunnel test data.

1. Wind tunnel working principle

Wind tunnel is a kind of pipeline test equipment which can generate and control airflow to simulate the air flow around the aircraft or object, and can measure the effect of airflow against object as well as observe the physical phenomena. The motor and fan system are installed in the driving force of the wind tunnel, and it rolls with AC frequency converter, and drives paddle. The power energy is inverted into mechanical energy of paddle, the mechanical energy of paddle is inverted into the pressure energy of air medium which produce the flow of medium in the pump of wind tunnel. The flow is finished to become symmetrical steady airflow, so that can meet the flow fields for test.

2. Wind tunnel control system

2.1 Wind speed control

The frequency control system is composed of inverter, PID regulator, supporting low`voltage electrical appliance and pressure sensor as shown in Fig 1, the pressure sensor takes charge of detecting the pressure difference of system., and transforms the pressure signal to resistance signal as feedback, then input them into PID regulator. After comparing to the given signal, we will find if the output control signals whose deviation is got by optimization PI algorithm control the output frequency of inveter, the wind speed can be kept invariable.

2.2 The angle of control

The angle of test model is always changes in the course of wind tunnel tests, which can continuously measure the corresponding test value of different angles. It can save time and enhance the efficiency and accuracy, and realize all automation.

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. The analysis and solution of EMC problem for inverter control system of wind tunnel laboratory

4.1 The principle how inverter generates 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 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 according to Forrier 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. Another high harmonic currents bring radiation to space through the cables to disturb nearby electrical equipment.

4.2 Main ways and prevention of the transmitting of interference

4.2.1 Radiation interference

Radiation interference is the electric potential in the cable by the electrostatic capacity coupling of the loop of controlling cables and electric appliance around. Radiation interference is relative with layout of scene equipment and the size of electromagnetism field made by equipment, especially frequency. Generally, it is protected by setting shielded cable and the part shield of control systems and high-pressure relief devices.

4.2.2 Interference from bad connection

Interference of bad connection is the interference caused by the change of resistance in the cable when the electric contact of the control cable of inverter is badly connected to relay contact. The following methods can be adopted to prevent interference, such as parallel contacts, gold-plated contacts or closed relays, and tightening the cable connections regularly.

4.2.3 Conducted interference along power line

The normal power supply of control system come from power network. Because the cover range of power network is wide, it will suffer the electromagnetic interference from space radiation then produce inductive voltage on lines and useless clutter. Especially the change of the inside of power network, the switching operation surge, the start and stop of large electric equipment, the harmonics caused by AC~DC rigging, transient state impact of short circuit of power network etc. are spread to the surround of power supply through the input lines. The power supply of common equipment is supplied by outside system, and is installed super isolated transformers or filters, and is shield earthing to prevent interference.

4.2.4 Grounding interference

Wrong grounding may cause serious interference signal, which make system not work normally. The interference on control system caused by the chaos of grounding system is mainly that the loop current caused by the uneven distribution of potential sites and the potential difference between different locations effects the normal work of system. In addition, shielding layer, grounding line and the earth can form closed loop, the induced current can appear in the shielding layer on the effect of variable magnetic field, then disturb the signal loop through the coupling between shielding layer and core line. If the system ground and other grounding are handled confusedly, the ground loop current will cause unequal potential distribution, which will effect the malfunction of the logical circuit of control system and analog circuit. The jamming margin of operating logical voltage of control system is low, the distributed interference of logical earth potential easily effect the logical operation and data storage, and cause data confusion, program running out of truck or crash. The distribution of simulated earth potential will lead to the decrease of measurement accuracy, which will cause serious distortion and wrong action to monitor and control signal.

4.3 The specific analysis and solution for system interference

Figure 1 Control system schematics

There are operating voltages of power supply DC10V, AC110V, AC220V for sensors, step motor, step motor drive, control equipment and supporting low voltage electrical appliance. In the system, consumers request to observe the speed of wind tunnel, working temperature, rotate speed of motor etc. from the control panel intuitively. So choose temperature sensors and the SCM wind speed shows developed by himself and angle change system to measure and display the wind speed, temperature and angle, the motor speed with pointer type dial gauge.

In the practical work we find that when the inverters start adjustable speed, the wrong show will appear on the digital rotate speed meter, and the speed directly jump to about 580r/m. Meanwhile the step motors vibrate, hit gear, and take their own perspective. When the inverters stop working, the control system and rotate speed meter are back to normal. Obviously, the reason is the interference for power supply made by high`frequency converter harmonic component of inverter. Generally, the best solution is to install inverter input and output filters in the power supply of control circuits. The electrical principle diagram of whole system is as Fig.1. After the  power line filters are installed in inverter input and output termination, the step motor is back to work normally, the impact of rotate speed control is smaller and shows 180r/m when starting inverter. But subsequently the former problems appear again when rotate speed meter are rectified. And the new problem appear, for example all the pulse signal offered by control circuit to the step motor driver are absorbed with no reason and the step motor is out of control. There are no phenomena such as short`circuit dot and other wrong connection etc after cutting power to inspect circuit. After the detailed observation at the scene we found that the step motor had the vibrating malfunction when inverter run at some frequency band in the course of system speeding up gradually. Moreover, when the outer console line of inverter was cut, the malfunction was still here. After further analysis of the inverter principle, we knew that for AC~DC~AC inverter the power network supply power to inverter through 3~phase rectifier bridge, power supply current can be decomposed of fundamental and 6 K ± 1 harmonics (K = 1,2,3…) component and a series of harmonic components according to Fourier series, and the harmonic content was reduced with the increase of inductance of input reactor and DC power filter reactor. According to the introduction of relative data, generally the 5, 7,11, 13 harmonic still can take 40% and 35%, 25% and 20% after the reactor is loaded. Namely there is still harmonic in the lines after the filters are installed in inverter. The output line of inverter is very long in the power distribution cabinet, there are signal control line and signal line of rotate speed meter near the inverter. When the inverter starts, we find that there are still induced voltage on the signal line after measurement, which is the interference made by the the coupling of high frequency signal on the power line of the load of inverter, it is space radiation interference. After knowing well about the reason of malfunction, we take further anti-interference measurements for the control display system according to the main way of the source of interference at scene, such as the interference made by inverter, the interference made by the line hybrid in control cabinet and space electromagnetic interference etc .

First of all, we adopt small`sized signal filters to filter the signal line of rotate speed meter. In the process of debugging the rotate speed meter, we increase the performance of signal filter gradually to get the best interference suppression performance, and solve the problem of wrong show in the rotate speed meter effectively. For the interference on the signal line of step motor, we use oscilloscope to observe the waveform of signal line, and find there are much useless high frequency vibration signal and clutter wave on the square wave. After we series the signal filters on the pulse signal lines, we find the waveform shown by oscilloscope is square wave and the problems of hitting tooth and vibrating disappear when the pulse signal lines are connected to the step motor. It should be noted that in the power distribution cabinet of inverter the given analog signal and corresponding indicator should be carried on strict metal shielding, and the signal shielding line, metal shielding layer should be carried on strict independent grounding, the signal line and control line should be shielded and isolated with power line by metal pipe, the spatial location should be relatively vertical, laying lines in parallel is prohibited. The above measurements can prevent the electromagnetic interference from space effectively.

After taken the above measures, the whole system is back to normal work, which mean that the above measures are fully effective and workable. At the same time further practice indicates, when  we solve the problem of interference of rotate speed meter, in order to weak the interference signal spread through lines, we do not series the filters in the normal order, but make use of the resonance characteristics of inductance and capacity to connect alternately and contrarily, so we could get the best results.

5. Conclusion

To sum up, make full use of the three factors of electromagnetic compatibility in solving the EMC problem for inverter control system in wind tunnel laboratory. The design of advance anti`interference is necessary, and the wiring and connection of system should be careful. Meanwhile, the mutual interference between all the electrical equipment should be fully considered, especially the interference made by high harmonics on control circuit and measurement signal as well as radiation interference from space in the measurement for small signal. In addition, make sure to isolate the control ground with power electric ground and protective ground, keep grounding properly, and create green electromagnetic environment.





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