At the scene, the interference of the inverter has more interference, and it is more serious, and even causing the control system to not be put into use. The working principle of the inverter is destined to produce strong electromagnetic interference.
The inverter includes a rectifier circuit and a pulse voltage waveform generating circuit. The input AC electrical electrical electriforation is transformed into a DC voltage through the inverter to convert the DC voltage into a different width pulse voltage (referred to as pulse width modulation) by the inverter. Voltage, PWM). With this PWM voltage drive motor, you can adjust the purpose of adjusting motor torque and speed. This working principle leads to the following three electromagnetic interference:
1. Harmonic interference: The rectifier circuit generates a harmonic current. This harmonic current generates a voltage drop in the impedance of the power supply system, resulting in distortion of the voltage wave type, and the voltage of this distortion is interfered with many electronic devices (because large Some electronic devices can only work under sinusoidal voltage conditions), and common voltage distortion is the top of the sine wave. When the harmonic current is constant, the voltage distortion is more severe in the case of weak power, and this interference is characterized by interfere with the device using the same grid, and the distance between the device and the frequency converter is independent;
2, radio frequency conduction emission interference: Since the load voltage is pulsed, the inverter is also pulsed from the grid, which contains a large number of high-frequency components, forming radio frequency interference, and this interference characteristic is Use the device of the same power grid to form interference, and is independent of the distance between the device and the inverter;
3, RF radiation interference: RF radiation interference from the input cable and output cable from the inverter. In the case where the radio frequency conduction interference is transmitted, when there is a radio frequency interference current on the input cable of the inverter, electromagnetic wave radiation is generated due to the cable equivalent to the antenna, and radiation interference is generated. The inverter outputs the PWM voltage transmitted on the cable, and also contains rich high frequency components, which generate electromagnetic waves radiation to form radiation interference. The radiation interference is characterized in that the interference phenomenon becomes serious when other electronic devices are close to the inverter.
According to the basic principle of electromagnetic properties, the electromagnetic interference must have three elements: electromagnetic interference sources, electromagnetic interference pathways, sensitive systems for 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 most important anti-interference measures, generally starting from anti-and putting two aspects to suppress interference, and the overall principle is to suppress and eliminate interference sources, cut off interference to systemically coupled channels, reduce system interference signals Sensitivity. Specific measures can be used in engineering, isolation, filtering, shielding, grounding and other methods. The following is the main steps to solve the on-site interference:
1. Software anti-interference measures: Specifically, it is to lower the value of the carrier frequency of the inverter to a suitable range. If this method does not work, only the following hardware anti-interference measures can be taken.
2. Conduct the right ground: Through the specific survey of the site, we can see that the grounding situation on the scene is not ideal. The correct ground can either effectively suppress foreign interference, and can reduce the interference of the equipment itself to the outside world is the most effective measure to solve the most effective measures of the inverter interference. Specifically, it is to do the following:
(1) The main circuit terminal E of the inverter must be grounded, the grounding can be commonly formed with the motor belt, but cannot be co-accepted separately with other devices, and the grounding point should be as far as possible away from weak electricity The grounding point of the device. At the same time, the cross-sectional area of the inverter ground wire should not be less than 4mm2, and the length should be controlled within 20m.
(2) Other electromechanical equipment The grounding of grounding and working ground should be separately set separately, and finally exchange the electrical grounding point of the power distribution cabinet. The shielding ground of the control signal and the shield of the main circuit conductor should also be separately set separately, and finally exchange the electrical grounding point of the power distribution cabinet.
3, shielding interference source: Shielding the source is a very effective way to suppress interference. Usually the inverter itself is shielded with iron shell, which can not allow its electromagnetic interference to leak, but the output line of the inverter is preferably shielded with the steel pipe, and it is preferable to add a magnetic ring, parallel around 3-4 turns, which helps to inhibit high When the harmonic, especially when the external signal (4 to 20 mA signals from the controller) controls the inverter, the control signal line is required as short as possible (generally 20m), and must be shielded twisted pair, and The main circuit (AC380) and the control line (AC220V) are completely separated. In addition, the electronic sensitive equipment line in the system also requires shielded twisted pairs, especially pressure signals. And all signal lines in the system must never be placed in the same pipe or wire slot with the main power route and control line. In order to make the shield, the shield must be reliably grounded.
4, reasonable wiring:
Specific methods include: (1) The power cord and signal line of the device should be as far away from the input of the inverter, the output line: (2) The power cord and the signal line of other devices should be avoided and the output line is parallel; if you take After the above approach, I still can't work, then continue the following measures:
5, interference isolation: so-called interference isolation refers to the connection of the interference source and susceptible interference from the circuit, so that they do not even occur. Usually, an isolation transformer is used on the power supply line between the power supply and the controller and the transmitter, and the power isolation transformer can apply noise isolation transformers.
6. Set the filter in the system line: The function of the device filter is to suppress the interference signal from the drive through the power cord to interfere with the power supply and electric motor. In order to reduce electromagnetic noise and loss, an output filter can be set in the inverter output side; in order to reduce power supply interference, an input filter can be set on the inverter input side. If there is a sensitive electronic device such as a controller and a transmitter, such as a controller and transmitter, such as a controller and transmitter, such as a controller and transmitter, such that the power supply noise filter can be used to avoid transmitting interference. The filter can be divided into:
(1) Input filters usually have two:
A, line filter: mainly consisting of inductive coils, which weakens higher frequency harmonic current by increasing the impedance of the line at high frequency.
B, radiation filter: mainly consisting of high frequency capacitors, which has a high absorption point frequency and has a harmonic component of radiant energy.
(2) The output filter is also composed of an inductor coil. It can effectively weaken the high-end harmonic ingredients in the output current. The action of anti-interference, and the additional torque caused by the harmonic current generated by the high-hard harmonic can be performed. For anti-interference measures in the inverter output, you must pay attention to:
A. The output of the inverter does not allow access capacitors to avoid a large-scale charging (or discharge) current, damage to the inverse tube;
B, when the output filter is constituted by the LC circuit, the side of the filter is connected to the side of the capacitor must be connected to the motor side.
7. Adopt reactor: The proportion of the harmonic component (5th harmonics, 7th harmonics, 11 harmonics, 13th harmonics, etc.) in the input current of the inverter is very high. They can interfere with the normal operation of other devices, because they consume a lot of reactive power, making the power factor of the line decreased. String reactor in the input circuit is an effective method for suppressing a lower harmonic current. Based on the position of the wiring, there are main two types:
(1) AC reactor: between the input side of the power source and the inverter. Its main functions are: a, by suppressing harmonic current, increase power factors to (0.75-0.85); B, weaken the surge current in the input circuit to the impact of the inverter; C, weaken the impact of power supply voltage imbalance.
(2) DC reactor: between the rectifier bridge and the filter capacitor. Its function is a single one, which is to weake the high harmonic ingredients in the input current. However, it is effective than the power factor than the AC reactor, which can reach 0.95, and has the advantages of simple structure, small size.
Figure 1 is a typical scheme for solving the interference of inverter
As shown, the anti-interference measures of the inverter mainly include installing AC reactors and filters, pilots and wires with shielding cables, shielding and reactors, filters, and inverters. The protective ground of the motor is grounded together, and the grounding point is separated from other ground points to maintain a sufficient distance. At the same time, the power cable of the signal cable and the inverter do not arrange parallel.
In addition, in order to prevent the inverter from interfering signals and control circuits, it is necessary to supply power to the controller, instrument, and industrial control.
Reprinted from electronic technology applications.
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