Three major interference problems of the inverter

- Aug 15, 2018-

1. The interference of the inverter to the microcomputer control board

In the control system using the inverter, the microcomputer or PLC is often used for control. During the system design or modification process, the interference of the inverter to the microcomputer control board must be paid attention to. Because the user's own design of the microcomputer control board generally has poor process level and does not comply with EMC international standards, the conduction and radiation interference generated after the inverter is used often causes the control system to work abnormally, so it is necessary to take necessary measures.

(1) Good grounding. The grounding wire of the strong electric control system such as the motor must be reliably grounded through the grounding busbar, and the shielding ground of the microcomputer control board is preferably grounded separately. For some serious interference situations, it is recommended to connect the sensor and I/O interface shielding layer to the control ground of the control board.

(2) Adding EMI filter, common mode inductor, high frequency magnetic ring, etc. to the input power of the microcomputer control board, the cost is low. It can effectively suppress conducted interference. In addition, in the case of severe radiation interference, such as the presence of GSM or PHS station, a metal mesh shield can be added to the microcomputer control board for shielding.

(3) Adding EMI filter to the inverter input can effectively suppress the conducted interference of the inverter to the power grid. Adding input AC and DC reactors L1 and L2 can improve the power factor and reduce harmonic pollution. The overall effect is good. . When the distance between some motors and the inverter exceeds 100m, it is necessary to add an AC output reactor L3 to the inverter side to solve the leakage current protection caused by the distribution parameters of the output wires to the ground and reduce the external radiation interference. An effective method is to use a steel pipe to thread or shield the cable and reliably connect the steel casing or cable shield to the ground. Please note that when the AC output reactor L3 is not added, if the steel pipe is threaded or shielded, the distributed capacitance of the output to the ground is increased, and overcurrent is likely to occur. Of course, in practice, only one or several methods are generally adopted.

(4) Electrically shield and isolate the analog sensor detection input and analog control signals. In the design process of the control system composed of the inverter, it is recommended not to use the analog control as much as possible, especially if the control distance is greater than 1M and the installation is across the control cabinet. Because the inverter generally has multi-speed setting and switching frequency input and output, it can meet the requirements. If it is necessary to use analog control, it is recommended to use a shielded cable and ground the remote point on the sensor side or the inverter side. If the interference is still severe, DC/DC isolation measures are required. Standard DC/DC modules can be used, or V/F conversion, aperture isolation, and frequency setting input can be used.

2, the inverter itself anti-interference problem

When there is a high-frequency impact load near the power supply system of the inverter, such as electric welding machine, electroplating power supply, electrolysis power supply or power supply with slip ring, the inverter itself is easy to be protected due to interference. Users are advised to take the following measures:

(1) Add an inductor and a capacitor to the input side of the inverter to form an LC filter network.

(2) The power line of the inverter is directly powered from the transformer side.

(3) A separate transformer may be used where conditions permit.

(4) When using external switch quantity control terminal control, shielded cable is recommended when the connection line is long. When the control circuit and the main circuit power supply are buried in the trench, except for the control cable, the shielded cable must be used. The main circuit must be shielded by the steel pipe to reduce mutual interference and prevent the inverter from malfunctioning.

(5) When using external analog control terminal control, if the connection line is within 1M, use shielded cable to connect and implement one point of grounding on the inverter side; if the line is long and the site interference is serious, it is recommended to be in the inverter. Side-mounted DC/DC isolation module or V/F conversion, using frequency command given mode for control.

(6) When using external communication control terminal control, it is recommended to use shielded twisted pair and ground the shield on the inverter side (PE). If the interference is very serious, it is recommended to connect the shield to the control power ground (GND). For the RS232 communication mode, pay attention to the control line as far as possible not to exceed 15m. If you want to lengthen, you must reduce the communication baud rate. When the distance is about 100m, the baud rate of normal communication is less than 600bps. For RS485 communication, terminal matching resistors and the like must also be considered. For high-speed control systems using fieldbus, the communication cable must be a dedicated cable and multi-point grounding to improve reliability.

3. Grid quality issues

In high-frequency impact loads such as electric welding machines, electroplating power supplies, electrolysis power supplies, etc., the voltage often flickers; in a workshop, there are hundreds of inverters, such as capacitive rectification loads, when working, the harmonics of the grid are very large. There is a serious pollution to the quality of the power grid, and it also has a considerable destructive effect on the equipment itself. If it is not able to operate normally continuously, it will cause damage to the input circuit of the equipment. The following measures can be taken:

(1) In high-frequency impact loads such as electric welding machines, electroplating power supplies, electrolysis power supplies, etc., it is recommended that users increase the reactive power compensation device to improve the power factor and quality of the power grid.

(2) In the workshop where the inverter is concentrated, it is recommended to use centralized rectification and DC common bus power supply. Users are advised to use a 12-pulse rectification mode. The utility model has the advantages of small harmonics and energy saving, and is particularly suitable for occasions where frequent braking, electric operation and power generation operation are simultaneously performed.

(3) A passive LC filter is installed on the input side of the inverter to reduce input harmonics, improve power factor, lower cost, high reliability and good effect.

(4) The input PFC device is installed on the input side of the inverter, which has the best effect but high cost.

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