Analysis of the three major interference problems of frequency converters.
1. Interference of the frequency converter to the microcomputer control board
In control systems using frequency converters, microcomputers or PLCs are often employed for control. During system design or modification, it is crucial to pay attention to the interference from the frequency converter to the microcomputer control board. Because user-designed microcomputer control boards are generally of poor manufacturing quality and do not meet international EMC standards, the conducted and radiated interference generated after using the frequency converter often leads to abnormal operation of the control system. Therefore, necessary measures must be taken.
(1) Good grounding. The grounding wire of the high-voltage control system such as the motor must be reliably grounded through the grounding busbar. The shielding ground of the microcomputer control board should preferably be grounded separately. In some cases with severe interference, it is recommended to connect the shielding layer of the sensor and I/O interface 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 is cost-effective and can effectively suppress conducted interference. In addition, 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 can effectively suppress conducted interference from the inverter to the power grid. Adding input AC and DC reactors L1 and L2 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 L3 needs to be added to the inverter side to solve the leakage current protection caused by the distributed parameters of the output conductor to ground and reduce external radiated interference. An effective method is to use steel pipes or shielded cables, and reliably connect the steel pipe shell or cable shielding layer to the ground. Please note that if steel pipes or shielded cables are used without adding an AC output reactor L3, the distributed capacitance of the output to ground will increase, which can easily lead to overcurrent. Of course, in practice, only 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 1 meter 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 recommended to use shielded cables and ground at a remote point on either the sensor side or the frequency converter side. If interference is still severe, DC/DC isolation measures are required. Standard DC/DC modules can be used, or a V/F converter with optocoupler isolation and frequency setting input can be employed.
2. Inverter's own anti-interference problem
When there are high-frequency impact loads such as welding machines, electroplating power supplies, electrolytic power supplies, or in applications using slip ring power supplies near the inverter's power supply system, the inverter itself is prone to triggering its protection due to interference. Users are advised to take the following measures:
(1) Add an inductor and a capacitor to the input side of the frequency converter to form an LC filter network.
(2) The power supply of the frequency converter is directly supplied from the transformer side.
(3) Where conditions permit, a separate transformer may be used.
(4) When using external switch control terminals, it is recommended to use shielded cables when the connection lines are long. When both the control lines and the main circuit power supply are buried in the trench, in addition to the control lines which must be shielded cables, the main circuit lines must be shielded by steel pipes to reduce mutual interference and prevent the inverter from malfunctioning.
(5) When using external analog control terminals, if the connection line is within 1M, a shielded cable can be used for connection and the inverter side can be grounded at one point; if the line is long and there is severe interference on site, it is recommended to install a DC/DC isolation module on the inverter side or use a frequency command given mode after V/F conversion for control.
(6) When using external communication control terminals, it is recommended to use shielded twisted-pair cables and ground the shielding layer on the inverter side (PE). If interference is severe, it is recommended to connect the shielding layer to the control power ground (GND). For RS232 communication, the control line should not exceed 15m as much as possible. If it needs to be lengthened, the communication baud rate must be reduced accordingly. At around 100m, the baud rate for normal communication is less than 600bps. For RS485 communication, the terminal matching resistor must also be considered. For high-speed control systems using fieldbus, dedicated cables must be used for communication, and multi-point grounding must be employed to improve reliability.
3. Power grid quality issues
In high-frequency impact loads such as welding machines, electroplating power supplies, and electrolytic power supplies, voltage flicker frequently occurs. In a workshop with hundreds of frequency converters and other capacitive rectifier loads operating, the harmonics in the power grid are very high, causing serious pollution to the grid quality and considerable damage to the equipment itself. This can range from preventing continuous normal operation to damaging the equipment's input circuit. The following measures can be taken:
(1) In high-frequency impact loads such as welding machines, electroplating power supplies, and electrolytic power supplies, it is recommended that users add reactive power compensation devices to improve the power factor and quality of the power grid.
(2) In workshops where frequency converters are concentrated, it is recommended to adopt centralized rectification and DC common bus power supply. It is suggested that users use the 12-pulse rectification mode. The advantages are low harmonics and energy saving, making it particularly suitable for situations involving frequent starting and braking, and simultaneous electric and generator operation.
(3) Adding a passive LC filter to the input side of the frequency converter reduces input harmonics, improves the power factor, has low cost, 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.
