Extending the service life of frequency converters is a major concern in the industry. This paper analyzes the application environment, electromagnetic interference and anti-interference, and power grid quality of frequency converters, and proposes issues to be aware of when using frequency converters and corresponding improvement suggestions. It is believed that this will have a significant effect on improving the service life of frequency converters.
1. The impact of electromagnetic interference on frequency converters
In modern industrial control systems, microcomputer or PLC control technology is often used. During system design or modification, it is crucial to pay attention to the interference of the frequency converter on the microcomputer control board. External interference sources affecting the frequency converter often stem from user-designed microcomputer control boards, which are generally of poor manufacturing quality and do not meet international EMC standards. The conducted and radiated interference generated after using the frequency converter frequently leads to abnormal operation of the control system. Therefore, the following necessary measures must be taken.
1) Good grounding. The grounding wire of high-voltage control systems such as motors must be reliably grounded through a grounding busbar. The shielding ground of the microcomputer control board should be grounded separately. In some cases with severe interference, it is recommended to connect the shielding layer of sensors and I/O interfaces to the control ground of the control board.
2) Adding EMI filters, common-mode inductors, and high-frequency magnetic rings to the input power supply of the microcomputer control board can effectively suppress conducted interference. Additionally, in situations with severe radiated interference, such as when there are GSM or PHS base stations nearby, a metal mesh shield can be added to the microcomputer control board for shielding.
3) Adding an EMI filter to the inverter input terminal can effectively suppress conducted interference from the inverter to the power grid. Adding input AC and DC reactors can improve the power factor and reduce harmonic pollution, resulting in good overall performance. In some cases where the distance between the motor and the inverter exceeds 100m, an AC output reactor needs to be added to the inverter side to address leakage current caused by the distributed parameters of the output conductors to ground and to reduce radiated interference to the outside. An effective method is to use steel conduit or shielded cable, and reliably connect the steel conduit shell or cable shielding layer to the ground. It is worth noting that without adding an AC output reactor, using steel conduit or shielded cable increases the distributed capacitance of the output to ground, which can easily lead to overcurrent. Of course, in practical applications, one or more of these methods are usually used.
4) Electrically shield and isolate analog sensor inputs and analog control signals. In the design of control systems composed of frequency converters, it is recommended to avoid analog control as much as possible, especially when the control distance is greater than 1m and the system is installed across control cabinets. This is because frequency converters generally have multi-speed settings and switching frequency input/output, which can meet the requirements. If analog control must be used, it is strongly recommended to use shielded cables and implement remote grounding at either the sensor side or the frequency converter side. If interference is still severe, DC/DC isolation measures are required. This can be achieved using standard DC/DC modules, or by using optical isolation for V/F conversion followed by frequency setting input.
2. Impact of the work environment
In practical applications of frequency converters, most domestic customers, except for a few with dedicated machine rooms, install them directly in industrial sites to reduce costs. These sites typically present challenges such as high dust levels, high temperatures, and high humidity, as well as metal dust and corrosive gases, as seen in industries like aluminum. Therefore, appropriate countermeasures must be implemented based on the specific site conditions.
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 should be installed vertically, and large components that may block the exhaust and intake should be avoided directly above and below it.
3) The minimum distance between the upper and lower edges of the frequency converter and the top, bottom, partition, or large components that must be installed in the control cabinet should be greater than 300mm.
4) If a special user needs to remove the keyboard during use, the keyboard hole on the inverter panel must be strictly sealed with tape or replaced with a dummy panel to prevent a large amount of dust from entering the inverter.
5) When using frequency converters in dusty environments, especially those with a lot of metallic dust or flocculent matter, the control cabinet should be completely sealed and specially designed with air inlets and outlets for ventilation; the top of the control cabinet should have a protective net and a protective top cover with an air outlet; the bottom of the control cabinet should have a base plate, an air inlet, and a cable inlet, and be equipped with a dustproof net.
6) Most inverter manufacturers do not perform special moisture- and mildew-proofing treatments on the printed circuit boards and metal structural components inside. If the inverter is exposed to harsh operating environments for a long time, the metal structural components are prone to corrosion. The conductive copper busbars will experience accelerated corrosion under high-temperature operation, and corrosion will cause damage to the fine copper wires on the microcomputer control board and drive power supply board. Therefore, for applications in humid environments and those containing corrosive gases, the internal design of the inverter must have basic requirements. For example, the printed circuit board must be coated with conformal coating, and structural components must undergo processes such as nickel-chromium plating. In addition, other proactive, effective, and reasonable measures must be taken to prevent moisture and corrosive gases.
3. The impact of power grid quality on frequency converters
In applications with impact loads such as welding machines, electric arc furnaces, and rolling mills, voltage flicker frequently occurs. In a workshop, when multiple frequency converters and other capacitive rectifier loads are operating, the harmonics they generate severely pollute the power grid and cause considerable damage to the equipment itself. This can range from preventing continuous normal operation to damaging the equipment's input circuits. The following measures can be taken.
1) In situations involving impact loads such as welding machines, electric arc furnaces, and rolling mills, it is recommended that users add static compensation devices to improve the power factor and quality of the power grid.
2) In workshops with a high concentration of frequency converters, centralized rectification and a common DC bus power supply are recommended. Users are advised to use a 12-pulse rectification mode. The advantages are low harmonics and energy saving, making it particularly suitable for frequent starts and stops, and situations where the motor operates in both motoring and generating modes.
3) Adding a passive LC filter to the input side of the frequency converter reduces input harmonics, improves the power factor, and has high reliability and good effect.
4) Adding an active PFC device to the input side of the frequency converter has the best effect, but the cost is higher.
