Types and Causes of Signal Interference in Frequency Converter Systems

Frequency converter (hereinafter referred to as -VFD) is widely used in today’s small and medium industrial environments, mainly because of its energy saving and some industrial control system applications. So do you know the types and causes of signal interference in the inverter system?

1. Cyclic interference of VFD system signal

In the industrial production process, it is necessary to use various automatic instruments, control systems and actuators to realize monitoring and control. The signal transmission between them includes small signals ranging from tens of volts to millivolts and milliamperes, and even large signals such as low-frequency DC signals and high-frequency pulse signals of thousands of volts and hundreds of amperes. After forming a system, it is often found that the transmissions between instruments and equipment interfere with each other. The system is unstable or even malfunctions. In addition to the performance reasons of various instruments and equipment such as anti-electromagnetic interference, another important reason for this situation is that all instruments and equipment need to be grounded according to requirements and purposes. For example, for safety reasons, the chassis needs to be grounded. In order for the circuit to work properly, the system needs a common reference point, and the shield also needs to be grounded to suppress interference. However, there is a potential difference between the reference points of the instrument and equipment (that is, the common place of each equipment is different), which leads to this situation. When the circulating current is large, there will be a high common-mode noise voltage, which will be coupled to the signal line through the distributed parameters or directly connected to the level signal line, which will cause a large series-mode interference and even damage the interface circuit and equipment.

2. Natural interference of VFD system signal

Lightning is a major natural interference source, and the interference generated by lightning can be transmitted thousands of kilometers away. The time-domain waveform of lightning interference is a large spike superimposed on a background of random pulses. Cosmic noise is produced by ionizing radiation, which varies throughout the day. Solar noise varies dramatically with solar activity. Noise in nature mainly interferes with communication, and lightning energy spikes can cause damage to many devices, which should be avoided or reduced to reduce the degree of damage and loss.

3. Human interference

The root cause of electromagnetic interference is the change of voltage or current in the conductor, that is, a larger dv/dtdi/dt.dv/dt or di/dt can cause the conductor to generate electromagnetic radiation. On the one hand, people can use this feature to realize specific functions, such as wireless communication, radar or other functions. On the other hand, dv/dt or di/dt in conductors will produce accompanying electromagnetic radiation when electronic equipment is working. No matter what the subjective purpose is, it pollutes the electromagnetic environment objectively. In addition, some large-scale equipment (motors, VFD, etc.) are frequently switched on and off during the production process of factories and enterprises, which will also cause some capacitive and inductive interference, which will affect the normal display or collection of instruments. Where there is a sudden change in voltage and current, there will definitely be electromagnetic interference. Digital pulse circuits are typical sources of interference. With the wide application of electronic technology, electromagnetic pollution will become more and more serious.