This operating principle leads to the following three types of electromagnetic interference:

1. Harmonic interference: The rectifier circuit generates harmonic currents, which cause a voltage drop across the power supply system's impedance, resulting in voltage waveform distortion. This distorted voltage interferes with many electronic devices (because most electronic devices can only operate under sinusoidal voltage conditions). A common voltage distortion is a flattening of the top of the sine wave. When the harmonic current is constant, the voltage distortion is more severe under weak power supply conditions. This interference is characterized by interfering with devices sharing the same power grid, regardless of the distance between the device and the inverter.

2. Conducted radio frequency (RF) emission interference: Because the load voltage is pulsed, the current drawn by the inverter from the power grid is also pulsed. This pulsed current contains a large number of high-frequency components, forming RF interference. This interference is characterized by interfering with devices sharing the same power grid, regardless of the distance between the device and the inverter.

3. Radiation interference: RF radiation interference comes from the inverter's input and output cables.

In the above-mentioned conducted RF emission interference scenario, when there is RF interference current on the inverter's input and output cables, the cables, acting as antennas, will inevitably generate electromagnetic radiation, resulting in radiated interference. The PWM voltage transmitted on the inverter's output cable also contains abundant high-frequency components, generating electromagnetic radiation and causing radiated interference. A characteristic of radiated interference is that the interference becomes severe when other electronic devices are near the inverter. According to the basic principles of electromagnetism, three elements are necessary for electromagnetic interference: an electromagnetic interference source, an electromagnetic interference path, and a system sensitive to electromagnetic interference. To prevent interference, hardware and software anti-interference measures can be used. Among these, hardware anti-interference is the most basic and important measure, generally addressing interference from both suppression and amplification perspectives. The overall principle is to suppress and eliminate interference sources, cut off the coupling path of interference to the system, and reduce the system's sensitivity to interference signals. Specific measures in engineering include isolation, filtering, shielding, and grounding.

The main steps to solve on-site interference are:

1. Using software anti-interference measures: Specifically, this involves adjusting the inverter's carrier frequency through the inverter's human-machine interface to a suitable range. If this method is ineffective, then only hardware anti-interference measures can be taken.

2. Proper grounding: Through on-site investigation, we can see that the on-site grounding situation is not ideal. Proper grounding can effectively suppress external interference and reduce the interference of the equipment itself to the outside world, which is the most effective measure to solve the interference of frequency converter. Specifically, it is necessary to do the following:

(1) The main circuit terminals PE (E, G) of the frequency converter must be grounded. This grounding can be shared with the motor connected to the frequency converter, but it cannot be shared with other equipment. A separate grounding stake must be driven, and the grounding point should be as far away as possible from the grounding point of weak electrical equipment. At the same time, the cross-sectional area of the frequency converter grounding wire should not be less than 4mm2, and the length should be controlled within 20m.

(2) Among the grounding wires of other electromechanical equipment, the protective grounding and the working grounding should be set up separately with separate grounding electrodes, and finally connected to the electrical grounding point of the distribution cabinet. The shielding ground of the control signal and the shielding ground of the main circuit wire should also be set up separately with separate grounding electrodes, and finally connected to the electrical grounding point of the distribution cabinet.

3. Shielding interference sources Shielding interference sources is a very effective way to suppress interference. The inverter itself is usually shielded with an iron shell to prevent electromagnetic interference leakage. However, the output line of the inverter should preferably be shielded with a steel pipe, especially when the inverter is controlled by an external signal (4~20mA signal output from the controller). The control signal line should be as short as possible (generally within 20m) and must be shielded twisted pair cable, and completely separated from the main circuit line (AC380) and control line (AC220V). In addition, the circuits of electronically sensitive equipment in the system should also be shielded twisted pair cable, especially pressure signals. And all signal lines in the system must never be placed in the same conduit or cable tray as the main circuit line and control line. To ensure effective shielding, the shielding layer must be reliably grounded.

4. Reasonable wiring methods:

(1) The power lines and signal lines of the equipment should be kept as far away as possible from the input and output lines of the inverter.

(2) The power lines and signal lines of other equipment should avoid being parallel to the input and output lines of the inverter.

If the above methods are still ineffective, then continue with the following methods:

5. Interference isolation Interference isolation refers to isolating the interference source and the susceptible part from the circuit so that they do not have electrical contact. Usually, an isolation transformer is used on the power line between the power supply and the amplifier circuits such as the controller and transmitter to prevent conducted interference. The power isolation transformer can be a noise isolation transformer.

6. Set up filter equipment in the system line The function of the filter is to suppress the interference signal from the frequency converter to the power supply and motor through the power line. To reduce electromagnetic noise and loss, an output filter can be set on the output side of the frequency converter; to reduce interference to the power supply, an input filter can be set on the input side of the frequency converter. If there are sensitive electronic devices such as controllers and transmitters in the line, a power noise filter can be set on the power line of the device to prevent conducted interference.

Filters can be divided into the following categories according to their location:

(1) Input filters There are usually two types:

a. Line filter: mainly composed of inductor coils, which weakens the high-frequency harmonic current by increasing the impedance of the line at high frequencies.

b. Radiation filter: mainly composed of high-frequency capacitors, which absorbs the high-frequency harmonic components with radiation energy.

(2) The output filter is also composed of an inductor coil, which can effectively weaken the high-order harmonic components in the output current. It not only plays an anti-interference role, but also weakens the additional torque caused by the harmonic current generated by the high-order harmonics in the motor. For anti-interference measures at the output of the frequency converter, the following aspects must be noted:

a. Capacitors are not allowed to be connected to the output of the frequency converter, so as to avoid generating a large peak charging (or discharging) current at the moment when the power transistor is turned on (off), which will damage the power transistor;

b. When the output filter is composed of an LC circuit, the side of the filter connected to the capacitor must be connected to the motor side.

7. Using reactors The proportion of low-frequency harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) in the input current of the frequency converter is very high. In addition to potentially interfering with the normal operation of other equipment, they also consume a lot of reactive power, which greatly reduces the power factor of the line. Connecting a reactor in series in the input circuit is an effective way to suppress low-harmonic currents. According to the different wiring positions, there are mainly two types:

(1) AC reactors are connected in series between the power supply and the input side of the inverter. Its main functions are: a) to improve the power factor to (0.75-0.85) by suppressing harmonic currents; b) to reduce the impact of surge current in the input circuit on the inverter; c) to reduce the influence of power supply voltage imbalance.

(2) DC reactors are connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple, which is to reduce the high-order harmonic components in the input current. However, it is more effective than AC reactors in improving the power factor, which can reach 0.95, and has the advantages of simple structure and small size.

Therefore, the anti-interference measures of the inverter mainly include installing AC reactors and filters in the inverter input line, using shielded cables for the input and output lines, and grounding the shielding layer of all cables together with the protective ground of the reactor, filter, inverter and motor, and this grounding point is separated from other grounding points and kept at a sufficient distance. At the same time, the signal cable and the power cable of the inverter should not be arranged in parallel. In addition, to prevent the frequency converter from interfering with signals and control loops, separate isolated power supplies are required for the controller, instruments, and industrial computer.