Under different operating conditions, there are still some unsatisfactory aspects of the frequency converter, resulting in the shortening of its service life and the corresponding increase of 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 grid quality 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 the modern industrial control system, the 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 to the microcomputer control board. Due to the external interference source, the general process level of the microcomputer control board designed by the user is poor, which does not conform to 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 the motor and other strong current control system must be reliably grounded through the grounding busbar, and the shielding ground of the microcomputer control board shall be separately grounded. For some serious interference occasions, it is recommended to connect the sensor, I / 0 interface shielding layer and the control ground of the control board.

2) Adding EMI filter, common mode inductor and high frequency magnetic ring to the input power of the microcomputer control board can effectively suppress the conducted interference. In addition, in the case of serious radiation interference, such as GSM or PHS base station around, the computer control board can be shielded by adding metal mesh shield.

3) Adding EMI filter to the input of the inverter can effectively suppress the conducted interference of the inverter to the power grid. Adding input AC and DC reactors can improve the power factor, reduce harmonic pollution and achieve 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 steel pipe threading or shielded cable, and connect the steel pipe shell or cable shield layer with the earth reliably. It is worth noting that when no AC output reactor is added, if the method of steel pipe threading or shielded cable is adopted, the distributed capacitance of output to ground is increased, and overcurrent is easy to occur. Of course, one or several methods are generally adopted in practical application.

4) The analog sensor detection input and analog control signal are electrically shielded and isolated. In the design process of the control system composed of frequency converter, it is recommended not to use analog control as much as possible, especially when the control distance is greater than 1 m and 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 it is not necessary to use analog control, it is recommended to use shielded cable and realize remote one-point grounding at the sensor side or converter side. If the interference is still serious, DC / DC isolation measures need to be implemented. The standard DC / DC module can be used, or the V / F conversion optical isolation can be used, and then the frequency setting input method can be used.

2. Impact of working environment

In the practical application of frequency converter, in order to reduce the cost, most domestic customers install the frequency converter directly on the industrial site except a few special machine rooms. Generally, there are dust, high temperature and high humidity problems in the work site, as well as metal dust and corrosive gas in the aluminum industry. Therefore, corresponding countermeasures must be made according to the site conditions.

1) The frequency converter shall be installed inside the control cabinet.

2) It is better to install the frequency converter in the middle of the control cabinet; the frequency converter should be installed vertically, and avoid installing large components that may block the exhaust and inlet air.

3) The minimum distance between the upper and lower edge of the frequency converter and the top and bottom of the control cabinet, or the partition board, or the large components that must be installed, shall be greater than 300 mm.

4) If a special user needs to remove the keyboard in use, the keyboard hole of the converter panel must be sealed with tape or replaced with a fake panel to prevent a large amount of dust from entering the converter interior.

5) When the frequency converter is used in the place with multi dust, especially in the place with multi metal dust and floccules, the overall requirements are that the control cabinet shall be sealed as a whole, and the air inlet and outlet shall be specially designed for ventilation; the top of the control cabinet shall be equipped with a protective net and a protective top cover air outlet; the bottom of the control cabinet shall be equipped with a base plate, an air inlet and an air inlet hole, and a dust-proof net shall be installed.

6) Most of the internal printed boards and metal structural parts of the inverter manufacturers have not been specially treated against moisture, mildew and mildew. If the inverter is in a bad working environment for a long time, the metal structural parts are prone to rust. Under the condition of high temperature operation, the conductive copper bar will be more severely corroded. For the small copper conductor on the microcomputer control board and the driving power board, corrosion will cause damage. Therefore, for the application in humid and corrosive gas containing occasions, there must be basic requirements for the internal design of the frequency converter used, for example, the printed circuit board must be treated with three anti paint spraying, and the structural parts must be treated with nickel plating and chromium plating. In addition, other active, effective and reasonable measures should be taken to prevent moisture and corrosion.

3. Influence of power grid quality on frequency converter

In the impact load such as electric welding machine, electric arc furnace, rolling mill and other occasions, the voltage often flickers; in a workshop, there are multiple frequency converters and other capacitive rectifier loads in operation, the harmonic generated by them has a very serious pollution to the quality of the power grid, and also has a considerable destructive effect on the equipment itself, light is not able to continue normal operation, heavy is the damage to the input circuit of the equipment. The following measures can be taken.

1) It is suggested that the user should add the static reactive compensation device to improve the power factor and quality of the power grid.

2） In the workshop where the frequency converter is concentrated, it is suggested to adopt centralized rectification and DC common bus power supply mode. 12 pulse rectification mode is recommended. It has the advantages of small harmonic and energy saving, especially suitable for frequent starting and braking. The motor is in the situation of both electric operation and power generation operation.

3) The input side of the converter is equipped with a passive LC filter, which can reduce the input harmonic and improve the power factor.

4) The input side of the converter is equipped with active PFC device, which has the best effect but high cost.

4. conclusion:

Starting from the problems in the practical application system of frequency converter, and from the interference of external factors, use environment, power grid 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 to the problems and suggestions for improvement, which can play a good role in prolonging the service life of the frequency converter The application in international engineering has certain reference value.