On-site, interference from the frequency converters was frequent and severe, even rendering the control system unusable. The operating principle of frequency converters inevitably leads to the generation of strong electromagnetic interference.
A frequency converter includes a rectifier circuit and an inverter circuit. The input AC power is rectified and smoothed by the rectifier circuit to convert it into DC voltage. The inverter then converts the DC voltage into pulse voltages of different widths (called pulse width modulation voltage, PWM). Using this PWM voltage to drive the motor allows for adjustment of the motor's torque and speed. This working principle leads to the following three types of electromagnetic interference:
1. Harmonic interference
Rectifier circuits generate harmonic currents, which create a voltage drop across the power supply system's impedance, causing voltage waveform distortion. This distorted voltage interferes with many electronic devices (since most electronic devices can only operate under sinusoidal voltage conditions). A common form of voltage distortion is a flattening of the sine wave's peak. With a constant harmonic current, voltage distortion is more severe under weak power supply conditions. This interference is characterized by affecting devices sharing the same power grid, regardless of the distance between the device and the inverter.
2. Radio frequency conducted emission interference
Since the load voltage is pulsed, the current drawn by the inverter from the grid is also pulsed. This pulsed current contains a large number of high-frequency components, forming radio frequency interference. The characteristic of this interference is that it will interfere with equipment using the same grid, regardless of the distance between the equipment and the inverter.
3. Radio frequency radiation interference
Radio frequency (RF) radiated interference originates from the input and output cables of the frequency converter. In the aforementioned case of conducted RF interference, when there is RF interference current in the input and output cables of the frequency converter, the cable, acting as an antenna, will inevitably generate electromagnetic radiation, resulting in radiated interference. Similarly, the PWM voltage transmitted on the output cable of the frequency converter contains abundant high-frequency components, which will also generate electromagnetic radiation, forming radiated interference. A characteristic of radiated interference is that the interference becomes severe when other electronic equipment is near the frequency converter.
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, hardware and software anti-interference measures can be used. Hardware anti-interference is the most basic and important measure, generally addressing interference from both the suppression and amplification perspectives. The overall principle is to suppress and eliminate the interference source, 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. Adopt software anti-interference measures
Specifically, this involves adjusting the inverter's carrier frequency through the inverter's human-machine interface to lower the value to an appropriate range. If this method fails, then the following hardware anti-interference measures must be taken.
2. Perform proper grounding.
Our on-site investigation revealed that the grounding situation was 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 to resolve inverter interference. Specifically, this involves the following:
(1) The main circuit terminals PE (E, G) of the inverter must be grounded. This grounding can be shared with the motor connected to the inverter, 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 the weak electrical equipment. At the same time, the cross-sectional area of the inverter 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 conductor 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 the source of interference is a very effective method for suppressing interference. While the frequency converter itself is usually shielded with a metal casing to prevent electromagnetic interference leakage, the output lines of the frequency converter should ideally be shielded with steel conduit, especially when the frequency converter is controlled by external signals (4-20mA signals output from the controller). In this case, the control signal line 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, the wiring of electronically sensitive equipment in the system also requires shielded twisted-pair cable, 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. Reasonable wiring
Specific methods include:
(1) The power cord and signal line of the equipment should be kept as far away as possible from the input and output lines of the frequency converter.
(2) Power lines and signal lines of other equipment should be kept parallel to the input and output lines of the frequency converter.
If the above methods are still ineffective, then continue with the following methods:
5. Interference isolation
Interference isolation refers to isolating the interference source from the susceptible components in the circuit, preventing them from being electrically connected. This is typically achieved by using an isolation transformer on the power line between the power supply and the amplifier circuits such as controllers and transmitters to prevent conducted interference. Noise isolation transformers can be used as power isolation transformers.
6. Install filters in the system circuitry.
The function of equipment filters is to suppress interference signals conducted from the frequency converter through the power lines to the power supply and motor. To reduce electromagnetic noise and losses, an output filter can be installed on the output side of the frequency converter; to reduce interference to the power supply, an input filter can be installed on the 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 lines of these devices to prevent conducted interference. Filters can be classified according to their location of use:
(1) Input filter
There are usually two types:
a. Line filter: mainly composed of inductors, it weakens high-frequency harmonic currents by increasing the impedance of the line at high frequencies.
b. Radiation filter: mainly composed of high-frequency capacitors, which absorbs harmonic components with high frequency and radiant energy.
(2) The output filter is also composed of inductors.
It can effectively reduce high-order harmonic components in the output current. It not only provides anti-interference, but also weakens the additional torque caused by harmonic currents generated by high-order harmonics in the motor. For anti-interference measures at the inverter output, the following aspects must be considered:
a. Capacitors are not allowed to be connected to the output terminal 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 may damage the power transistor.
b. When the output filter is composed of an LC circuit, the side of the capacitor connected inside the filter must be connected to the motor side.
7. Use reactors
In the input current of a frequency converter, low-frequency harmonic components (5th, 7th, 11th, 13th, etc.) constitute a high proportion. Besides potentially interfering with the normal operation of other equipment, they also consume a significant amount of reactive power, greatly reducing the power factor of the line. Inserting a reactor in series in the input circuit is an effective method to suppress low-harmonic currents. Depending on the wiring location, there are two main types:
(1) AC reactor
It is connected in series between the power supply and the input side of the frequency converter. Its main functions are:
a. By suppressing harmonic currents, the power factor can be improved to (0.75-0.85).
b. Reduce the impact of surge current in the input circuit on the frequency converter;
c. Reduce the impact of power supply voltage imbalance.
(2) DC reactor
It is connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple: to reduce the 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 frequency converters include installing AC reactors and filters at the input line of the frequency converter, using shielded cables for both input and output lines, and ensuring that the shielding layer of all cables shares a common ground with the protective ground of the reactor, filter, frequency converter, and motor, and that this grounding point is separate from other grounding points and maintains a sufficient distance. Additionally, signal cables and the power cables of the frequency converter should not be laid parallel to each other.
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.
