On-site, interference from the frequency converter was frequent and severe, even rendering the control system unusable. The operating principle of the frequency converter inherently generates strong electromagnetic interference.

A frequency converter consists of a rectifier circuit and an inverter circuit. The input AC power is rectified and smoothed by the rectifier circuit, converting it into DC voltage. The inverter then converts the DC voltage into pulse voltages of varying widths (called pulse width modulation, PWM). This PWM voltage is used to drive the motor, allowing adjustment of its torque and speed. This operating principle leads to the following three types of electromagnetic interference:

1. Harmonic Interference: The rectifier circuit generates harmonic currents. These harmonic currents create a voltage drop across the impedance of the power supply system, causing 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. 

1. **Harmonic Current Interference:** Voltage distortion is more severe under weak power supply conditions when the harmonic current is constant. This type of interference affects equipment sharing the same power grid, regardless of the distance between the equipment and the inverter.

2. **Radio Frequency Conducted Emission Interference:** Because the load voltage is pulsed, the current drawn from the grid by the inverter is also pulsed. This pulsed current contains a large number of high-frequency components, forming radio frequency interference. This type of interference also affects equipment sharing the same power grid, regardless of the distance between the equipment and the inverter.

3. **Radio Frequency Radiation Interference:** Radio frequency radiation interference originates from the inverter's input and output cables. In the case of conducted emission interference, when there is radio frequency interference current on the inverter's input and output cables, the cables act as antennas, inevitably generating electromagnetic radiation and causing 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. Radiated interference is characterized by increased severity when other electronic devices are near the inverter.

According to the basic principles of electromagnetism, three elements are necessary for electromagnetic interference to occur: an electromagnetic interference source, an electromagnetic interference path, and a system sensitive to electromagnetic interference. To prevent interference, both hardware and software anti-interference measures can be employed. Hardware anti-interference is the most basic and crucial measure, generally addressing interference from both the 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 engineering measures include isolation, filtering, shielding, and grounding. The following are the main steps for resolving on-site interference:

1. Employing 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 hardware anti-interference measures must be implemented.

2. Proper Grounding: On-site investigations reveal that the grounding conditions are often less than ideal. Proper grounding effectively suppresses external interference and reduces the equipment's own interference with the outside world, making it the most effective measure for resolving inverter interference. Specifically, the following points should be observed:

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

(2) Among the grounding wires of other electromechanical equipment, protective grounding and working grounding should be separately installed with separate grounding electrodes, and finally connected to the electrical grounding point of the distribution cabinet. The shielding ground of control signals and the shielding ground of main circuit conductors should also be separately installed 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 method for suppressing interference. Inverters are typically shielded with a metal casing to prevent electromagnetic interference leakage. However, the inverter's output lines should ideally be shielded with steel conduits, especially when the inverter is controlled by external signals (4-20mA signals output from the controller). These control signal lines should be as short as possible (generally within 20m) and must use shielded twisted-pair cable, completely separated from the main circuit lines (AC380V) and control lines (AC220V). Furthermore, shielded twisted-pair cable is required for the wiring of electronically sensitive equipment in the system, especially for pressure signals. All signal lines in the system must never be run in the same conduit or cable tray as the main circuit lines and control lines. For effective shielding, the shielding layer must be reliably grounded.

4. Specific methods for reasonable wiring include:

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

(2) Power and signal lines of other equipment should avoid running parallel to the inverter's input and output lines. If the above methods are ineffective, continue with the following:

5. Interference Isolation

Interference isolation refers to isolating the interference source from the susceptible parts of the circuit, preventing them from being electrically connected. This is usually achieved by using an isolation transformer on the power line between the power supply and amplifier circuits such as controllers and transmitters to prevent conducted interference. A noise isolation transformer can be used for the power supply isolation transformer.

6. Installing Filters in the System Circuit

The function of equipment filters is to suppress interference signals from the inverter being conducted through the power line to the power supply and motor. To reduce electromagnetic noise and losses, an output filter can be installed on the inverter output side; to reduce interference to the power supply, an input filter can be installed on the inverter input side. If there are sensitive electronic devices such as controllers and transmitters in the circuit, a power noise filter can be installed on the power line of these devices to prevent conducted interference. Filters can be classified according to their location:

(1) Input Filters

There are usually two types:

a. Line Filters: mainly composed of inductors, which weaken high-frequency harmonic currents by increasing the impedance of the line at high frequencies. b. Radiation filter: mainly composed of high-frequency capacitors, it absorbs high-frequency harmonic components with radiant energy.

(2) Output filter is also composed of inductor coils.

It 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 high-order harmonics in the motor. For anti-interference measures at the inverter output, the following aspects must be noted:

a. Capacitors are not allowed to be connected to the inverter output to avoid generating a large peak charging (or discharging) current at the moment the power transistor is turned on (off), which could 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. Use reactors

The proportion of low-frequency harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) in the inverter input current is very high. Besides potentially interfering with the normal operation of other equipment, they also consume a large amount of reactive power, greatly reducing the power factor of the line. Introducing reactors in series within the input circuit is an effective method for suppressing lower harmonic currents. Depending on the wiring location, there are two main types:

(1) AC Reactor: 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 currents in the input circuit on the inverter;

c. To reduce the impact of power supply voltage imbalance.

(2) DC Reactor: Connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple: to reduce high-order harmonic components in the input current. However, it is more effective than AC reactors in improving the power factor, reaching 0.95, and has advantages such as simple structure and small size.

Therefore, the main anti-interference measures for inverters include installing AC reactors and filters in the inverter's input section, using shielded cables for both input and output lines, and grounding all cable shielding layers together with the protective grounds of the reactor, filter, inverter, and motor, ensuring that this grounding point is separate from other grounding points and maintains a sufficient distance. Furthermore, signal cables and inverter power cables should not be laid parallel to each other.

In addition, to prevent the inverter from interfering with signals and control circuits, separate isolated power supplies are required for the controller, instruments, and industrial computer.