1, the inverter on the computer control panel interference
In the use of inverter control system, the use of more computer or PLC control, the system design or transformation process, we must pay attention to the inverter on the computer control panel interference problems. As the user-designed microcomputer control panel generally poor level of technology, does not meet the EMC international standards, the use of frequency converter, the resulting conduction and radiation interference, often lead to unusual control system work, and therefore need to take the necessary measures.
(1) Good grounding. Electricity and other high-voltage control system grounding wire must be grounded through the grounding of reliable, computer control panel shielding, the best separate ground. For some serious interference occasions, it is recommended that the sensor, I / O interface shield and the control panel connected.
(2) input power to the computer control panel to install EMI filters, common mode inductors, high-frequency magnetic ring, low cost. Can effectively inhibit conduction interference. In addition, in serious radiation interference occasions, such as the presence of GSM, or PHS station, the computer control panel can be added metal mesh shield shielding.
(3) Add EMI filter to the inverter input, which can effectively suppress the transmission interference of the inverter to the power grid. Adding input AC and DC reactors L1 and L2 can increase the power factor and reduce the harmonic pollution, so the comprehensive effect is good . When the distance between some motor and frequency converter exceeds 100m, AC output reactor L3 needs to be added on the inverter to solve the leakage current protection and reduce the external radiation interference caused by the distribution parameter of the output lead. An effective method is to use steel pipe threading or shielded cable method, and the steel pipe shell or cable shield and the earth connected reliably. Please note that if the AC output reactor L3 is not added, if the steel pipe is used to threading or shielded cables, the distribution capacitance of the output to ground is increased, and the overcurrent may easily occur. Of course, only one or several methods are generally adopted in practice.
(4) Electrical shielding and isolation of analog sensor test inputs and analog control signals. In the control system composed of the inverter design process, it is recommended not to use analog control, especially when the control distance is greater than 1M, cross-control cabinet installation. Because the frequency changer generally has the multistage speed hypothesis, the switching frequency quantity input output, may satisfy the request. If it is not necessary to use analog control, it is recommended to use shielded cable, and the remote side of the sensor or inverter to achieve a little grounding. If the interference is still serious, DC / DC isolation measures need to be implemented. You can use the standard DC / DC module, or use V / F conversion, optical isolation and then use the frequency setting input method.
2, the inverter itself anti-interference problems
When the inverter power supply system near the existence of high-frequency impact loads such as welding machine, electroplating power supply, electrolytic power supply or the use of slip ring power supply, the inverter itself prone to interference due to protection. Recommended users to adopt the following measures:
(1) Add inductor and capacitor on the input side of the inverter to form LC filter network.
(2) The inverter power cable is supplied directly from the transformer side.
(3) A separate transformer may be used where permitted.
(4) In the use of external switch control terminal control, the connection line is longer, it is recommended to use shielded cable. When the control circuit and the main circuit power are buried in the trench, in addition to the control line must be shielded cable, the main circuit line must be used steel pipe threading, reducing mutual interference, to prevent the inverter malfunction.
(5) In the use of external analog control terminal control, if the connection line within 1M, with shielded cable connection, and implementation of the inverter side of the ground can be; if the line is longer, the scene of serious interference occasions, it is recommended in the inverter Side installation of DC / DC isolation module or by V / F conversion, the frequency command given mode 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. If the interference is very serious, it is recommended to connect the shield to the control ground (GND). For RS232 communication mode, pay attention to the control circuit as much as possible not to exceed 15m, if you want to lengthen, you must reduce the communication baud rate, 100m or so, the normal communication baud rate is less than 600bps. For RS485 communication, you must also consider the terminal matching resistance. For high-speed control system using fieldbus, communication cables must use special cables and use multi-point grounding to improve reliability.
3, power quality problems
In high-frequency impact load such as welding machine, electroplating power supply, electrolytic power supply and other occasions, the voltage often flicker; in a workshop, there are hundreds of frequency converter and other capacitive rectification load at work, the power grid harmonic is very large, The quality of the grid is very serious pollution, the device itself has a considerable role in the destruction, ranging from not be able to continue normal operation, while causing the equipment input circuit damage. Can take the following measures:
(1) In the high-frequency impact load such as welding machine, electroplating power supply, electrolysis power supply and other occasions, users are advised to increase the static and static compensation device to improve the grid power factor and quality.
(2) In the workshop where frequency converter is more concentrated, it is recommended to adopt centralized rectification and direct current common bus power supply. Users are advised to use 12-pulse rectifier mode. The advantage is that the harmonic is small, energy saving, especially for frequent braking, electric operation and power generation running at the same time.
(3) Addition of passive LC filter on the input side of inverter to reduce input harmonics, improve power factor, lower cost, high reliability and good effect.
(4) Install the active PFC device on the input side of the inverter, the effect is best, but the cost is higher.