Suppliers of frequency converter supporting equipment remind you that according to the basic principles of electromagnetics, electromagnetic interference (EMI) must have three elements: electromagnetic interference source, electromagnetic interference path, and system sensitive to electromagnetic interference. To prevent interference, hardware anti-interference and software anti-interference can be used. Among them, hardware anti-interference is the most basic and important anti-interference measure for application systems. Generally, interference is suppressed from two aspects: anti-interference and prevention. The general principle is to suppress and eliminate interference sources, cut off the coupling channels of interference to the system, and reduce the sensitivity of system interference signals. Specific measures in engineering can include isolation, filtering, shielding, grounding, and other methods.
1. The so-called interference isolation refers to isolating the interference source from the susceptible parts of the circuit, so that they do not have electrical contact. In variable frequency speed control transmission systems, isolation transformers are usually used on the power lines between the power supply and amplifier circuits to prevent conducted interference. Noise isolation transformers can be used for power isolation transformers.
2. The purpose of setting up filters in the system circuit is to suppress interference signals transmitted from the frequency converter through the power line to the power supply from the motor. To reduce electromagnetic noise and losses, an output filter can be installed on the output side of the frequency converter; To reduce power interference, an input filter can be installed on the input side of the frequency converter. If there are sensitive electronic devices in the circuit, a power noise filter can be installed on the power line to prevent conducted interference. In the input and output circuits of a frequency converter, in addition to the lower harmonic components mentioned above, there are also many high-frequency harmonic currents that will propagate their energy in various ways, forming interference signals to other devices. Filters are the main means used to attenuate high-frequency harmonic components. According to different usage locations, it can be divided into:
(1) There are usually two types of input filters:
a、 Line filters are mainly composed of inductive coils. It weakens higher frequency harmonic currents by increasing the impedance of the circuit at high frequencies.
b、 Radiation filters are mainly composed of high-frequency capacitors. It will absorb high-frequency harmonic components with radiated energy.
(2) The output filter is also composed of inductive coils. It can effectively weaken the high-order harmonic components in the output current. Not only does it have an anti-interference effect, but it can also weaken the additional torque caused by high-order harmonic currents in the motor. For anti-interference measures at the output end of the frequency converter, the following aspects must be noted:
a、 The output terminal of the frequency converter is not allowed to be connected to a capacitor, in order to avoid generating a large peak charging (or discharging) current at the moment when 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 connected to the capacitor must be connected to the motor side.
3. Shielding interference sources is the most effective way to suppress interference. Usually, the frequency converter itself is shielded with an iron shell to prevent electromagnetic interference leakage; It is best to shield the output line with steel pipes, especially when controlling the frequency converter with external signals. The signal line should be as short as possible (generally within 20m), and the signal line should be shielded with dual cores and completely separated from the main power line (AC380V) and control line (AC220V). It must not be placed in the same piping or trunking, and the surrounding electronic sensitive equipment lines should also be shielded. To ensure effective shielding, the shielding cover must be reliably grounded.
4. Proper grounding can effectively suppress external interference in the system and reduce the interference of the equipment itself to the outside world. In practical application systems, the chaotic connection of the system power neutral line (neutral line), ground line (protective grounding, system grounding), and control system shielding ground (control signal shielding ground and main circuit wire shielding ground) greatly reduces the stability and reliability of the system.
For frequency converters, the correct grounding of the main circuit terminals PE (E, G) is an important means to improve the noise suppression ability of the frequency converter and reduce its interference. Therefore, it must be highly valued in practical applications. The cross-sectional area of the grounding wire of the frequency converter should generally not be less than 2.5mm2, and the length should be controlled within 20m. It is recommended that the grounding of the frequency converter be separated from the grounding points of other power equipment and not shared.
5. 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 possibly 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. Inserting a reactor in series into the input circuit is an effective method to suppress lower harmonic currents. According to the different wiring positions, there are mainly two types:
(1) The reactor is connected in series between the power supply and the input side of the frequency converter. Its main functions include:
a、 By suppressing harmonic currents, the power factor is increased to (0.75-0.85);
b、 Weaken the impact of surge current in the input circuit on the frequency converter;
c、 Weaken the impact of power supply voltage imbalance.
(2) The DC reactor is connected in series between the rectifier bridge and the filtering capacitor. Its function is relatively simple, which is to weaken the high-order harmonic components in the input current. But it is more effective than AC reactors in improving power factor, reaching 0.95, and has the advantages of simple structure and small size.
6. Reasonable wiring
For interference signals propagated through induction, they can be weakened through reasonable wiring. The specific methods include:
(1) The power and signal lines of the equipment should be kept away from the input and output lines of the frequency converter;
(2) The power and signal lines of other devices should avoid being parallel to the input and output lines of the frequency converter;