Types and causes of signal interference in inverter systems

Frequency conversion speed regulation is widely regarded as a speed regulation method with long-term development prospects at home and abroad due to its excellent starting, braking and speed regulation performance, high power factor, high efficiency, power saving, wide application range and many other advantages. However, since the frequency converter generally uses a variety of nonlinear components such as rectifier diodes, high-power IGBTs and thyristor switches, a large number of harmonics will be generated during use, which will interfere with the normal operation of surrounding electrical appliances. There are three main types of interference generated by the frequency converter during use, namely, interference with electronic communication equipment, electronic equipment, radio, etc. The interference generated to radio and communication equipment is mainly radiation interference, and the interference generated to electronic equipment such as automatic control devices and computers is induction interference. Once the problem of frequency converter interference is not solved in a reasonable way, not only will the system be difficult to operate normally, but it will also indirectly affect the normal operation of other electrical equipment and electronics.

1. Signal ground current interference in the inverter system

In the process of industrial production, various automatic instruments, control systems and actuators are needed to realize monitoring and control. The signal transmission between them includes small signals as weak as millivolt and milliampere level, tens of volts, and even large signals of thousands of volts and hundreds of amperes, including low-frequency DC signals and high-frequency pulse signals. After the system is formed, it is often found that the transmission between the instrument and the equipment interferes with each other, causing system instability or even misoperation. In addition to the performance reasons of each instrument and equipment itself, such as anti-electromagnetic interference, there is another very important reason for this situation. Various instruments and equipment need to be grounded according to requirements and purposes. For example, for safety, the casing needs to be connected to the earth. In order to make the circuit work properly, the system must have a common reference point. In order to suppress interference, a shielding cover is added, and the shielding cover also needs to be grounded. However, due to the potential difference between the reference points between the instrument and the equipment (that is, the common points of each device are different), the "ground loop" and "ground loop" problems are formed. When the loop current is large, a high common mode noise voltage will appear, and it will be coupled to the signal line through distributed parameters, or directly connected to the level signal line, which will generate a lot of common mode interference and even damage the interface circuit and equipment.

2. Natural interference of inverter system signal

Lightning is a major natural interference source. The interference generated by lightning can be transmitted thousands of kilometers away. The time domain waveform of lightning interference is a large spike pulse superimposed on a background of random pulses. Cosmic noise is generated by ionizing radiation and changes continuously throughout the day. Solar noise changes dramatically with solar activity. Natural noise mainly interferes with communications, while lightning energy spike pulses can damage many devices, which should be avoided or reduced to reduce the degree of damage and reduce losses.

3. Human interference

The fundamental reason for electromagnetic interference is the change of voltage or current in the conductor, that is, a large dv/dtdi/dt. dv/dt or di/dt can cause the conductor to radiate electromagnetic waves. On the one hand, people can use this feature to achieve specific functions, such as wireless communication, radar or other functions. On the other hand, when electronic equipment is working, dv/dt or di/dt in the conductor will produce accompanying electromagnetic radiation. Regardless of the subjective purpose, it objectively pollutes the electromagnetic environment. In addition, factories and enterprises often have some large equipment (motors, inverters, etc.) frequently switched on and off during the production process, which will also cause some capacitive and inductive interference, and will also affect the normal display or collection of instruments. Wherever there is a sudden change in voltage and current, there will definitely be electromagnetic interference. Digital pulse circuit is a typical interference source. With the widespread application of electronic technology, electromagnetic pollution will become more and more serious.