1, the frequency converter on the microcomputer control board interference
In the use of frequency converter control system, more microcomputer or PLC control, in the system design or transformation process, must pay attention to the frequency converter on the microcomputer control board interference. Due to the user's own design of the microcomputer control board is generally poor process level, does not meet the international standards of EMC, in the use of frequency converters, the resulting conducted and radiated interference, often leading to abnormal control system operation, so it is necessary to take the necessary measures.
(1) Good grounding. Motor and other strong power control system grounding wire must be reliably grounded through the grounding busbar, microcomputer control board shielding ground, it is best to separate grounding. For some serious interference occasions, it is recommended that the sensor, I / O interface shielding layer and the control board control ground connected.
(2) To the microcomputer control board input power supply with EMI filters, common mode inductors, high-frequency magnetic ring, etc., low cost. Conducted interference can be effectively suppressed. In addition, in the radiation interference in serious occasions, such as the presence of GSM, or small smart phone stations around, you can add a metal mesh shielding microcomputer control board shielding treatment.
(3) to the frequency converter input EMI filter, can effectively inhibit the frequency converter on the grid conduction interference, add input AC and DC reactor L1, L2, can improve the power factor, reduce harmonic pollution, the overall effect is good. In some occasions where the distance between the motor and the inverter is more than 100m, it is necessary to add AC output reactor L3 on the inverter side to solve the problem of leakage current protection caused by the output conductor to ground distribution parameters and to 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 earth reliably. Please note that when the AC output reactor L3 is not added, the use of steel pipe threading or shielded cable increases the distributed capacitance of the output to ground, which is prone to overcurrent. Of course in practice only one or more of these methods are generally taken.
(4) Electrical shielding and isolation of analog sensor detection inputs and analog control signals. In the process of designing the control system composed of frequency converters, it is recommended that analog control should not be used as much as possible, especially when the control distance is greater than 1M and installed across the control cabinet. Because the frequency converter generally have multi-step speed setting, switching frequency volume input and output, can meet the requirements. If you have to use analog control, it is recommended to use shielded cable, and in the sensor side or inverter side to achieve the remote end of a point of grounding. If the interference is still serious, DC/DC isolation measures need to be realized. Standard DC/DC module can be used, or the use of V / F conversion, optical coupling isolation and then use the frequency setting input method.
2, the inverter itself anti-interference problems
When the power supply system near the frequency converter, there are high-frequency shock loads such as welding machines, electroplating power supply, electrolytic power supply or the use of slip ring power supply occasions, the frequency converter itself is prone to protection due to interference. Users are recommended to adopt the following measures.
(1) Add inductors and capacitors on the input side of the inverter to form an LC filter network.
(2) The power line of the inverter is supplied directly from the transformer side.
(3) Where conditions permit, a separate transformer can be used.
(4) When external switching control terminals are used for control, it is recommended that shielded cables be used when the connection line is long. When 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 threaded with steel pipe shielding to reduce interference with each other and prevent inverter malfunction.
(5) in the use of external analog control terminal control, if the connection line within 1M, the use of shielded cable connection, and the implementation of the inverter side of a point of grounding can be; if the line is longer, the scene of serious interference occasions, it is recommended that the inverter side of the addition of DC / DC isolation module or the use of the V / F conversion, the use of frequency command to give the mode of control.
(6) In the use of external communication control terminal control, it is recommended to use shielded twisted-pair cable, and the inverter side of the shield ground (PE), if the interference is very serious, it is recommended to connect the shield to the control power ground (GND). For RS232 communication, pay attention to the control line try not to exceed 15m, if you want to lengthen, you must subsequently reduce the communication baud rate, in 100m or so, the baud rate can be normal communication is less than 600bps. for RS485 communication, you must also consider the terminal matching resistor and so on. For the use of fieldbus high-speed control system, the communication cable must be a special cable, and the use of multi-point grounding in order to improve reliability.
3,Grid quality problems
In high-frequency shock loads such as welding machines, electroplating power supply, electrolytic power supply and other occasions, the voltage often flicker; in a workshop, there are several hundred sets of inverters and other capacitive rectifier loads in the work, the harmonics of the grid is very large for the quality of the grid has a very serious pollution of the equipment itself is also quite destructive, or not be able to run continuously and normally, or cause damage to the input circuit of the equipment. The following measures can be taken.
(1) In high-frequency impact loads such as welding machines, electroplating power supply, electrolytic power supply and other occasions, it is recommended that the user to increase the reactive static complementary devices to improve the power factor and quality of the power grid.
(2) In the workshop where inverter is concentrated, it is recommended to adopt centralized rectification and DC common bus power supply mode. It is recommended that users adopt 12-pulse rectification mode. The advantages are, small harmonics, energy saving, especially suitable for frequent starting and braking, electric operation and power generation operation at the same time.
(3) Add passive LC filter on the input side of the inverter to reduce the input harmonics and improve the power factor, with low cost, high reliability and good effect.
(4) Add active PFC device on the input side of frequency converter, which is the best effect, but the cost is higher.