In the application site, the interference of the inverter occurs more and more seriously, and even the control system cannot be put into use. The working principle of the frequency converter is destined to generate strong electromagnetic interference.
The inverter includes a rectifier circuit and an inverter circuit. The input AC power is converted into a DC voltage through a rectifier circuit and a smoothing circuit, and then the DC voltage is converted into a pulse voltage of different widths through the inverter (called pulse width modulation voltage, PWM) . Using this PWM voltage to drive the motor can adjust the torque and speed of the motor. This working principle causes the following three types of electromagnetic interference:
1. Harmonic interference
The rectifier circuit produces harmonic currents, which create a voltage drop across the impedance of the power supply system, resulting in a distorted voltage waveform. This distorted voltage interferes with many electronic devices (because most electronic devices only work Under sine wave voltage conditions), a common voltage distortion is the flattening of the top of the sine wave. When the harmonic current is constant, the voltage distortion is more serious in the case of weak power supply. The characteristic of this interference is that it will interfere with the equipment using the same power grid, regardless of the distance between the equipment and the inverter;
2. Radio frequency conducted emission interference
Since the load voltage is pulse-shaped, the current drawn by the inverter from the power grid is also pulse-shaped. This pulse current contains a large amount of high-frequency components, causing radio frequency interference. interference, regardless of the distance between the device and the inverter;
3. Radio frequency radiation interference
Radio frequency radiated interference comes from the input and output cables of the frequency converter. In the case of the above-mentioned radio frequency conduction emission interference, when there is radio frequency interference current on the input and output cables of the inverter, since the cable is equivalent to an antenna, electromagnetic wave radiation will inevitably be generated, resulting in radiation interference. The PWM voltage transmitted on the output cable of the inverter also contains rich high-frequency components, which will generate electromagnetic wave radiation and cause radiation interference. The characteristic of radiated interference is that when other electronic equipment is close to the frequency converter, the interference phenomenon becomes serious.
According to the basic principles of electromagnetics, the formation of electromagnetic interference must have three elements: the source of electromagnetic interference, the path of electromagnetic interference, and the system sensitive to electromagnetic interference. To prevent interference, hardware anti-jamming and software anti-jamming can be used. Among them, hardware anti-jamming is the most basic and important anti-jamming measure. Generally, the anti-jamming and releasing are two aspects to suppress the interference. The general principle is to suppress and eliminate the interference source, cut off the coupling channel of the interference to the system, and reduce the system interference signal. sensitivity. Specific measures can be adopted in engineering, such as isolation, filtering, shielding, grounding and other methods. The following are the main steps to resolve field disturbances:
1. Adopt software anti-interference measures
Specifically, it is to lower the carrier frequency of the inverter through the man-machine interface of the inverter, and adjust the value to an appropriate range. If this method does not work, then only the following hardware anti-jamming measures can be taken.
2. Conduct correct grounding
Through the specific investigation on the site, we can see that the grounding situation at the site is not ideal. The correct grounding can not only effectively suppress the external interference, but also reduce the interference of the equipment itself to the outside world, which is the most effective measure to solve the interference of the inverter. Specifically, do the following:
1. The main circuit terminals PE (E, G) of the inverter must be grounded. The ground can be grounded with the motor brought by the inverter, but cannot be grounded with other equipment. A grounding stake must be built separately, and the grounding point must be grounded. It should be kept as far away as possible from the grounding point of weak current equipment. At the same time, the cross-sectional area of the grounding wire of the inverter should not be less than 4mm2, and the length should be controlled within 20m.
2. In the ground wires of other electromechanical equipment, the protective grounding and the working grounding should be separated and set up with separate grounding electrodes, and finally merged into the electrical grounding point of the power distribution cabinet. The shielding ground of the control signal and the shielding ground of the main circuit wires should also be separately set up with separate grounding electrodes, and finally merged into the electrical grounding point of the power distribution cabinet.
3. Shielding interference sources
Shielding interference sources is a very effective way to suppress interference. Usually the inverter itself is shielded with an iron shell to prevent the leakage of its electromagnetic interference, but the output line of the inverter is best shielded with steel pipes, especially when the inverter is controlled by an external signal (output 4~20mA signal from the controller), It is required that the control signal line be as short as possible (usually within 20m), and must be shielded twisted pair, and be completely separated from the main circuit line (AC380) and the control line (AC220V). In addition, the electronic sensitive equipment lines in the system also require the use of shielded twisted pairs, especially pressure signals. And all the signal lines in the system must not be placed in the same piping or trunking with the main circuit lines and control lines. For shielding to be effective, the shield must be grounded reliably.
Fourth, the specific methods of reasonable wiring are:
1. The power line and signal line of the equipment should be kept away from the input and output lines of the inverter as far as possible.
2. The power lines and signal lines of other equipment should not be parallel to the input and output lines of the inverter.
If the above methods still do not work, then continue with the following methods:
5. Isolation of interference
The so-called interference isolation refers to the isolation of 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 amplifier circuits such as controllers and transmitters to avoid conducted interference. The power isolation transformer can be applied to a noise isolation transformer.
6. Set the filter in the system circuit
The function of the equipment filter is to suppress the interference signal conducted from the inverter through the power line to the power supply and the motor. To reduce electromagnetic noise and loss, an output filter can be set on the output side of the inverter; to reduce interference to the power supply, an input filter can be set on the input side of the inverter. If there are sensitive electronic devices such as controllers and transmitters in the line, a power supply noise filter can be installed on the power line of the device to avoid conducted interference. Filters can be divided into:
1. Input filter
(1) Line filter: It is mainly composed of an inductive coil, which weakens the harmonic current with higher frequency by increasing the impedance of the line at high frequency.
(2) Radiation filter: It is mainly composed of high-frequency capacitors, which will absorb harmonic components with high frequency points and radiated energy.
2. The output filter is also composed of an inductive coil
It can effectively weaken the high-order harmonic components in the output current. It not only plays the role of anti-interference, but also weakens the additional torque caused by the harmonic current generated by the higher harmonic in the motor. For the anti-interference measures at the output of the inverter, the following aspects must be paid attention to:
1. Capacitors are not allowed to be connected to the output end of the inverter, so as to avoid the generation of a charging (or discharging) current with a large peak value at the moment when the power tube is turned on (off), which will damage the power tube;
2. When the output filter is composed of LC circuit, the side of the filter connected to the capacitor must be connected to the motor side.
Seven, the use of reactors
In the input current of the inverter, the proportion of harmonic components with lower frequencies (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) is very high, and they may interfere with other In addition to the normal operation of the equipment, the power factor of the line is greatly reduced because they consume a large amount of reactive power. Adding a reactor in series in the input circuit is an effective method to suppress lower harmonic currents. According to the different wiring positions, there are mainly the following two types:
1. AC reactor
It is connected in series between the power supply and the input side of the inverter. Its main functions are:
(1) Increase the power factor to (0.75-0.85) by suppressing the harmonic current;
(2) Weakening the impact of the surge current in the input circuit on the inverter;
(3) Weakening the influence of unbalanced power supply voltage.
2. DC reactor
It is connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple, that is, to weaken the high-order harmonic components in the input current. However, it is more effective than the AC reactor 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 adding AC reactors and filters to the incoming line of the inverter, and using shielded cables for incoming and outgoing lines. The protective grounds are grounded together, and the grounding point is separated from other grounding points to maintain 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, in order to prevent the frequency converter from interfering with the signal and control loop, it is necessary to use a separate isolated power supply for the controller, instrument and industrial computer.
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