The following are the main steps to resolve on-site interference:
1. Use 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 hardware anti-interference measures must be taken.
2. Ensure proper grounding: Our on-site investigation revealed that the grounding conditions were 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 terminal E of the frequency converter must be grounded. This grounding can be shared with the motor driven by the frequency converter, but it cannot be shared with other equipment. A separate grounding electrode must be installed, 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) The protective grounding and working grounding of other electromechanical equipment should be provided 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 provided 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 and preferably fitted with ferrite cores, wound parallel to each other 3-4 times. This helps suppress high-order harmonics, especially when the frequency converter is controlled by an external signal (4-20mA signal 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 lines and signal lines of the equipment should be kept as far away as possible from the input and output lines of the frequency converter; (2) The power lines and signal lines of other equipment should avoid being 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 line 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 line of these devices to prevent conducted interference. Filters can be classified according to their location of use:
(1) There are usually two types of input filters: a) Line filter: mainly composed of inductors, which 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 high-frequency harmonic components with radiating energy.
(2) The output filter is also composed of an inductor coil. It can effectively reduce the high-order harmonic components in the output current. It not only provides anti-interference but also reduces 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 inverter to avoid generating a large peak charging (or discharging) current at the moment the inverter tube is turned on (off), which may damage the inverter tube; b. When the output filter is composed of an LC circuit, the side of the filter where the capacitor is connected 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: Connected in series between the power supply and the input side of the frequency converter. 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 frequency converter; c) to 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, which is to reduce the high-order harmonic components in the input current. However, it is more effective than AC reactor in improving the power factor, which can reach 0.95, and has the advantages of simple structure and small size.
Figure 1 shows a typical solution for resolving inverter interference.
As shown in the figure, 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.
Disclaimer: This article is a reprint. If there are any copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
