Foreword: With the development of China's economy and the progress of science and technology, the application of AC variable frequency speed control is becoming increasingly widespread. Among various speed control methods, AC variable frequency speed control technology has been recognized both domestically and internationally as the most ideal and promising method. When factories and equipment use AC speed control, harmonic interference is generated on both the power supply side and the motor side of the frequency converter. On the one hand, when the frequency converter is running, it is necessary to prevent it from being subjected to external electromagnetic interference; on the other hand, it is also necessary to prevent the generation of high-order harmonics that interfere with other external equipment, i.e., the so-called "EMC".
Generally, electrical equipment must be able to suppress both high-frequency and low-frequency interference. High-frequency interference mainly includes electrostatic discharge, pulse interference, and electromagnetic fields at emission frequencies; while low-frequency interference mainly refers to power supply voltage fluctuations, undervoltage, and frequency instability. Frequency converters typically operate in industrial environments where high electromagnetic interference (EMI) may exist; they are both noise sources and potential noise receivers.
The input and output currents of frequency converters contain many high-order harmonic components, which propagate their energy in various ways, creating interference signals for other equipment. Generally speaking, the propagation methods of interference signals are as follows:
1) Circuit coupling method: This refers to the propagation of interference signals through the power supply network. Since the input current is non-sinusoidal, when the inverter has a large capacity, it will cause distortion in the network voltage, affecting the operation of other equipment. This is the main propagation method of inverter input current interference signals.
2) Inductive Coupling: When the inverter's input or output circuit is very close to the circuits of other devices, the inverter's high-order harmonic signals will be coupled into these devices through induction. There are two types of induction: Electromagnetic induction: induction occurs through inductance. This is the main propagation method for current interference signals. Electrostatic induction: induction occurs through inter-line capacitance. This is the main propagation method for voltage interference signals.
3) Aerial radiation : This refers to radiation into the air as electromagnetic waves, which is the main propagation mode for high-frequency harmonic content.
