Some suggestions on prolonging the service life of

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Suggestions for prolonging the service life of frequency converter

with the increasing prosperity of the general frequency converter market, its annual consumption is increasing sharply. However, there are still some unsatisfactory aspects of the frequency converter under different operating conditions, resulting in the shortening of its service life and the corresponding increase in the installation, commissioning, daily maintenance and repair workload of its auxiliary equipment, thus causing significant direct and indirect economic losses to users. This paper analyzes the application environment, electromagnetic interference and anti-interference, power quality and other aspects of the frequency converter, and puts forward the problems that should be paid attention to when using the frequency converter and the corresponding improvement suggestions. It is believed that this will have a significant effect on improving the service life of the frequency converter

1 influence of electromagnetic interference on frequency converter

in modern industrial control systems, microcomputer or PLC control technology is often used. In the process of system design or transformation, we must pay attention to the interference of frequency converter on microcomputer control board. The source of external interference to the frequency converter is shown in Figure 1. Due to the general poor process level of the microcomputer control board designed by the user, it does not meet the EMC international standard. After the frequency converter is adopted, the conduction and radiation interference often leads to the abnormal operation of the control system. Therefore, the following necessary measures need to be taken

1) good grounding. The grounding wire of strong current control system such as motor must be reliably grounded through grounding busbar, and the shielding ground of microcomputer control board should be grounded separately. For some occasions with serious interference, it is recommended to connect the sensor and i/0 interface shielding layer with the control ground of the control board

2) adding EMI filters, common mode inductors, industrial coatings including automotive coatings, marine coatings, highway and railway coatings, etc. to the input power supply of the microcomputer control board, which accounts for about 70% of the share of high-frequency magnetic rings, can effectively suppress conducted interference. In addition, in the case of serious radiation interference, such as GSM or PHS base stations around, the microcomputer control board can be shielded by adding a metal shield

3) add EMI to the input end of the frequency converter, including cylindrical force sensor, spoke force sensor, s double hole sensor, 10 beam sensor and other types of filters, which can effectively suppress the conduction interference of the frequency converter to the electricity. Add input AC and DC reactors, which can improve the power factor, reduce harmonic pollution, and have a good comprehensive effect. In some cases where the distance between the motor and the frequency converter is more than 100 m, it is necessary to add an AC output reactor at the frequency converter side to solve the leakage current protection caused by the distribution parameters of the output wire to the ground and reduce the external radiation interference. An effective method is to adopt the method of threading steel pipe or shielding cable, and reliably connect the steel pipe shell or cable shielding layer with the earth. It is worth noting that when the AC output reactor is not added, if the method of steel pipe threading or shielded cable is adopted, the distributed capacitance of the output to the ground is increased, which is prone to overcurrent. Of course, in practical application, one or several methods are generally adopted

4) electrically shield and isolate the analog sensor detection input and analog control signal. In the design process of the control system composed of frequency converter, it is recommended not to use analog control as far as possible, especially when the control distance is greater than 1m and is installed across the control cabinet. Because the frequency converter generally has multi-stage speed setting, switching frequency input and output, which can meet the requirements. If analog quantity control is not necessary, it is recommended to use shielded cable and realize remote one-point grounding at the sensor side or frequency converter side. If the interference is still serious, dc/dc isolation measures need to be implemented. For the future development of the experimental machine, it can effectively improve the product process quality to adopt standard dc/dc modules, or adopt the method of v/f conversion optical isolation, and then adopt the frequency setting input

2 impact of working environment

in the practical application of frequency converters, most domestic customers install frequency converters directly on the industrial site in order to reduce costs, except for a few special machine rooms. Generally, there are problems of large dust, high temperature and humidity at the work site, as well as metal dust and corrosive gases in the aluminum industry. Therefore, corresponding countermeasures must be made according to the site conditions, as shown in Figure 2

1) the frequency converter should be installed inside the control cabinet

2) the frequency converter is best installed in the middle of the control cabinet; The frequency converter shall be installed vertically, and large components that may block the exhaust and air inlet shall be avoided directly above and below

3) the minimum distance between the upper and lower edges of the frequency converter and the top and bottom of the control cabinet, or the clapboard, or the large components that must be installed, should be greater than 300 mm

4) if special users need to remove the keyboard during use, the keyboard hole of the frequency converter panel must be strictly sealed with tape or replaced with a fake panel to prevent a large amount of dust from entering the inside of the frequency converter

5) when using frequency converters in dusty places, especially in places with polymetallic dust and flocs, it is generally required that the control cabinet be sealed with software self-test function, and air inlets and outlets are specially designed for ventilation; The top of the control cabinet should be equipped with protective cover and air outlet; The bottom of the control cabinet should have a bottom plate, air inlet and wire inlet, and be installed with dust-proof

6) the printed boards and metal structural parts inside most frequency converter manufacturers have not been specially treated against moisture and mildew. If the frequency converter is in a bad working environment for a long time, the metal structural parts are easy to rust. The corrosion process of conductive copper bar will be more serious when it operates at high temperature. The corrosion will cause damage to the small copper wires on the microcomputer control board and the drive power board. Therefore, for applications that are wet and contain corrosive gases, there must be basic requirements for the internal design of the frequency converter used. For example, the printed circuit board must be sprayed with three anti paint, and the structural parts must be treated with nickel plating and chromium plating. In addition, it is also necessary to take other positive, effective and reasonable measures to prevent moisture and corrosive gases

3 influence of power quality on frequency converter

voltage flicker often occurs in occasions such as impact loads such as electric welding machines, electric arc furnaces, rolling mills, etc; In a workshop, when several capacitive rectifier loads such as frequency converters are working, the harmonics generated by them will seriously pollute the power quality and cause considerable damage to the equipment itself, ranging from continuous normal operation to damage to the input circuit of the equipment. The following measures can be taken

1) users are advised to add reactive static compensation devices to improve the electric power factor and quality in the occasions of impact loads such as electric welding machines, electric arc furnaces, rolling mills, etc

2) in workshops with concentrated frequency converters, it is recommended to adopt centralized rectification and DC common bus power supply. It is recommended that users adopt 12 pulse rectification mode. The advantages are low harmonic and energy saving, especially suitable for frequent starting and braking, and the motor is in both electric operation and power generation operation

3) the input side of the frequency converter is equipped with a passive LC filter to reduce the input harmonic and improve the power factor. It has high reliability and good effect

4) installing active PFC device on the input side of frequency converter has the best effect, but the cost is high

4 conclusion

starting from the problems in the practical application system of the frequency converter, from the interference of external factors, use environment, power quality and other aspects, this paper puts forward the influence of adverse factors on the frequency converter in practical application, and then summarizes some solutions and improvement suggestions for this problem, which can play a good effect on extending the service life of the frequency converter, It has certain reference value in practical engineering. (end)

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