Interference in RS485 Communication of Frequency Converter and Its Solutions
Abstract
RS485 communication technology is used in the control system of frequency converters to realize the transmission of communication data. However, due to the high speed and large capacity of communication data, and the wide channel for data transmission, interference signals will be generated and received, which will cause communication errors. In this paper, the common interference signals and their influence on the communication system will be discussed, and methods for protecting the communication system against these interference signals are proposed.
Introduction
Frequency converter is the basis of modern motor control system and the key equipment in world industry. It is the core of motor intelligent system. The application of frequency converter technology has greatly improved the motor performance and the realization of automation, intelligence and energy saving. In the step-up machine, frequency converter is usually used to control the speed and torque output of the motor by changing the output frequency and voltage of the frequency converter in pulse duty cycle modulation.
The communication control system of frequency converter adopts RS485 communication technology. This technology is used to realize communication and network control of related equipments. It uses two-wire differential signal transmission to reduce crosstalk between the transmission lines, improve signal noise margin and make the transmission more reliable.
Interference Sources
The underlying integrated circuits and the surrounding electromagnetic environment are the main sources of interference for RS485 communication. The common interference sources are as follows:
1) The electromagnetic emission of each integrated circuit. After the digital signal is transformed into an analog signal, its frequency spectrum will leak out from the transmitting end.
2) Electrostatic discharge (ESD) can cause interference noise in the receiving end.
3) Radio frequency interference (RFI) can be induced by motor electromagnetic interference or transmitted through power or other signal lines. The induction of interference signals into the receiving circuit has a great influence on the receiving signal.
4) Switching noise, which is generated in the process of signal switching, is a strong interference signal in the system.
5) Poor shielding of the practical application circuit board and noise generated by other components may also introduce interference signals into the communication circuit.
The Influence of Interference
Interference signals may interfere with the communication system by causing signal errors, loss of synchronization, data errors, excessive noise and signal distortions, which will affect the accuracy and reliability of communication data transmission and can even cause system communication failure.
Protection against Interference
In order to ensure the stability and reliability of the frequency conversion communication system and reduce the risk of communication interruption and data errors, it is necessary to take measures to protect and improve the communication system from interference.
1) Strengthen shielding measures. The most effective way to prevent interference is to shield the communication system and its modules with materials that have good shielding performance, such as steel plate and aluminum foil.
2) Protect the circuit with varistor and filter. If a surge or noise appears in the communication line and damages the circuit, the varistor can play a good protective role. Using filter capacitors to filter out high-frequency interference signals is also effective in preventing interference.
3) Adopt the surge protection circuit. The surge protection circuit can effectively reduce the amplitude of the interference signal and retain the effective signal, thus protecting the communication line.
4) Increase the shielded wire length. The shorter the wire is, the more easily the interference signal will be induced on the line.
5) Add a signal line terminal resistor. A terminal resistor can be added to each signal line at both ends of the communication line, which can limit the current flowing in the signal line, reduce the interference on the line and reduce the interference from the line to other equipment.
Conclusion
In the control system of frequency conversion, RS485 communication technology is used to realize communication and network control of related devices. The interference signals generated in the electrical and electromagnetic environment can interfere with the RS485 communication system and cause data errors. Strengthening shielding measures, using protection—filtering components, using surge protection circuits and increasing the length of the shielded wire can effectively protect the communication system from interference and ensure stable and reliable communication.
