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

In the control system using frequency converters, most of them use microcomputers or PLCs for control. In the process of system design or transformation, we must pay attention to the interference of frequency converters on the microcomputer control board. Due to the poor process level of the microcomputer control board designed by the user, which does not comply with EMC international standards, the conduction and radiation interference generated after the use of the inverter often leads to abnormal operation of the control system, so necessary measures are required.

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

(2) Add EMI filter, common mode inductance, high frequency magnetic ring, etc. to the input power of the microcomputer control board, so the cost is low. Can effectively suppress conducted interference. In addition, when the radiation interference is serious, such as when there is GSM or PHS station around, you can add a metal mesh shield to the microcomputer control board to shield it.

(3) Adding EMI filter to the input of the inverter can effectively suppress the conduction interference of the inverter to the power grid, and installing input AC and DC reactors L1 and L2 can improve the power factor, reduce harmonic pollution, and have a good overall effect . In some occasions where the distance between the motor and the inverter exceeds 100m, it is necessary to add an AC output reactor L3 on the inverter 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 use steel pipe threading or shielded cable, and connect the steel pipe shell or cable shield to the ground reliably. Please note that when the AC output reactor L3 is not added, if the method of steel pipe threading or shielded cable is used, the distributed capacitance of the output to ground is increased, and overcurrent is likely to occur. Of course, in practice, generally only one or several of these methods are 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 frequency converters, it is recommended not to use analog control as much as possible, especially when the control distance is greater than 1M and it is installed across the control cabinet. Because the inverter generally has multi-stage speed setting and switching frequency input and output, it can meet the requirements. If analog control is required, it is recommended to use shielded cable and realize a remote grounding at the sensor side or inverter side. If the interference is still serious, DC/DC isolation measures need to be implemented. Standard DC/DC modules can be used, or V/F conversion, optical lotus isolation and frequency setting input can be used.

2. The anti-interference problem of the inverter itself

When there are high-frequency impact loads such as electric welding machine, electroplating power supply, electrolytic power supply or slip ring power supply near the power supply system of the inverter, the inverter itself is prone to protection due to interference. It is recommended that users adopt the following measures:

(1) Add inductance and capacitance on the input side of the inverter to form an LC filter network.

(2) The power cord of the inverter is directly supplied from the transformer side.

(3) When conditions permit, a separate transformer can be used.

(4) When using external switch control terminals for control and when the connection line is long, it is recommended to use shielded cables. When both the control line and the main circuit power supply are buried in the trench, in addition to the control line must be shielded cable, the main circuit line must be shielded through the steel pipe to reduce mutual interference and prevent the inverter from malfunctioning.

(5) When the external analog control terminal is used for control, if the connection line is within 1M, use a shielded cable to connect and implement a little grounding on the inverter side; if the line is long and the site interference is serious, it is recommended to use the inverter A DC/DC isolation module is installed on the side or V/F conversion is adopted, and the frequency command given mode is used for control.

(6) When using external communication control terminal control, it is recommended to use shielded twisted pair and ground the shield layer (PE) on the inverter side. If the interference is very serious, it is recommended to connect the shield layer to the control power ground (GND). For the RS232 communication method, pay attention to the control line as much as possible not to exceed 15m. If it is to be lengthened, the communication baud rate must be reduced accordingly. At about 100m, the normal communication baud rate is less than 600bps. For RS485 communication, terminal matching resistance must also be considered. For a high-speed control system using a field bus, the communication cable must use a dedicated cable and adopt a multi-point grounding method to improve reliability.

3. Power grid quality issues

In high-frequency impact loads such as electric welding machines, electroplating power supplies, electrolytic power supplies, etc., the voltage often flickers; in a workshop, when there are hundreds of inverters and other capacitive rectifying loads working, the harmonics of the power grid are very large. There is serious pollution to the quality of the power grid and considerable damage to the equipment itself, ranging from the inability to operate continuously and the damage to the input loop of the equipment. The following measures can be taken:

(1) It is recommended that users add reactive static compensation devices to improve the power factor and quality of the power grid in high-frequency impact loads such as welding machines, electroplating power supplies, and electrolytic power supplies.

(2) In workshops where inverters are concentrated, it is recommended to use centralized rectification and DC common bus power supply. It is recommended that users use the 12-pulse rectification mode. The advantage is that the harmonics are small and energy saving, and it is especially suitable for occasions where frequent starting and braking, electric operation and power generation operation are performed simultaneously.

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

(4) The source PFC device is added on the input side of the inverter, the effect is the best, but the cost is higher