Inverters still have some shortcomings in operation, which leads to a shortened service life and a corresponding increase in the need for repairing their components, thus increasing maintenance costs.
Today, we will analyze the application environment, power grid quality, and electromagnetic interference of frequency converters, and put forward some issues that should be noted and improvement suggestions. I believe these will be helpful to everyone.
In practical applications of frequency converters, many people install them directly in industrial sites to reduce costs. These sites typically present challenges such as high dust levels, high temperatures, and high humidity. Some industries also encounter issues like metal dust and corrosive gases. Appropriate countermeasures must be developed 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) Most frequency converters lack special treatment to prevent moisture and mold growth on their internal printed circuit boards and metal structural components. If exposed to harsh working environments for extended periods, these metal components are prone to corrosion. The conductive copper busbars experience accelerated corrosion at high temperatures, and corrosion can damage the fine copper wires on the microcomputer control board and drive power supply board. Therefore, for applications involving humid environments or those containing corrosive gases, the internal design of the frequency converter must meet specific requirements.
6) 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.
Electromagnetic interference
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. Because some frequency converters' microcomputer control boards do not conform to international EMC standards, conducted and radiated interference can occur after using the frequency converter, often leading to abnormal control system operation. The following methods can be used as a reference.
1) Installing an EMI filter at the inverter input 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 a good overall effect. 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.
One method is to use steel conduit or shielded cable to reliably connect the conduit casing or cable shielding layer to the ground. However, without adding an AC output reactor, using steel conduit or shielded cable increases the distributed capacitance of the output to ground, making it prone to overcurrent.
2) 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 a remote point grounding on 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 V/F conversion with optical isolation, followed by frequency setting input.
3) 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.
4) Proper 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 layers of sensors and I/O interfaces to the control ground of the control board.
Power Grid Quality
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 methods can be used to address these issues.
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.
Starting from the problems encountered in actual frequency converter application systems, this paper addresses the impact of adverse factors on frequency converters in practical applications, including external interference, operating environment, and power grid quality. It then summarizes solutions and improvement suggestions that can effectively extend the service life of frequency converters and have certain reference value in practical engineering applications.
Of course, usually one or more of these methods are used.
