In the field, the interference of the inverter appears more and more serious, and even the control system can not be put into use. The working principle of the inverter 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 smooth wave circuit, and then the DC voltage is converted into a pulse voltage of a different width by the inverter (referred to as a pulse width modulation voltage, PWM). . By using this PWM voltage to drive the motor, it is possible to adjust the torque and speed of the motor. This principle of operation leads to the following three types of electromagnetic interference:
1, harmonic interference
The rectifier circuit generates a harmonic current that produces a voltage drop across the impedance of the power supply system, causing distortion of the voltage waveform that interferes with many electronic devices (since most electronic devices only work) Under sinusoidal voltage conditions, the 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 a weak power supply, which is characterized by interference with equipment using the same power grid, regardless of the distance between the equipment and the frequency converter;
2. RF conducted emission interference
Since the load voltage is pulsed, the inverter draws current from the grid as a pulse. This pulse current contains a large amount of high-frequency components to form radio frequency interference. This interference is characterized by the use of equipment using the same grid. Interference, regardless of the distance between the device and the frequency converter;
3. Radio frequency interference
The RF emissions interfere with the input and output cables from the drive. In the case of the above-mentioned radio frequency conducted emission interference, when there is radio frequency interference current on the input and output cable of the frequency converter, since the cable is equivalent to the antenna, electromagnetic wave radiation is inevitably generated, and radiation interference is generated. The PWM voltage transmitted on the output cable of the inverter also contains abundant high-frequency components, which will generate electromagnetic wave radiation and form radiation interference. The characteristic of radiated interference is that the interference phenomenon becomes serious when other electronic devices are close to the inverter.
According to the basic principles of electromagnetics, electromagnetic interference 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 most important anti-interference measure. Generally, the anti-and anti-discharge is used to suppress 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 isolated, filtered, shielded, grounded, etc. in the project. The following are the main steps to resolve the on-site interference:
1. Adopt software anti-interference measures
Specifically, the carrier frequency of the inverter is lowered by the man-machine interface of the inverter, and the value is lowered to an appropriate range. If this method does not work, then only the following hardware anti-jamming measures can be taken.
2. Perform proper grounding
Through the specific investigation on the spot, we can see that the grounding situation at the site is not ideal. The correct grounding can not only effectively suppress external interference, but also reduce the interference of the equipment itself to the outside world. It is the most effective measure to solve the inverter interference. Specifically, it is to do the following:
(1) The main circuit terminal PE (E, G) of the inverter must be grounded. The grounding can be shared with the motor of the inverter, but it cannot be shared with other equipment. The grounding pile must be ground separately and the connection should be made. The location 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 grounding conductor of the inverter should be no less than 4mm2, and the length should be controlled within 20m.
(2) In the grounding of other electromechanical equipment, the protective grounding and working grounding shall be separately provided with grounding poles and finally 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 conductor should also be separately grounded and finally connected to 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 by an iron casing, so that the electromagnetic interference can be prevented from leaking, but the output line of the inverter is preferably shielded by a steel pipe, especially when an external signal (4~20 mA signal is output from the controller) is used to control the inverter. The control signal line is required to be as short as possible (generally within 20m), and shielded twisted pair must be used and completely separated from the main circuit line (AC380) and the control line (AC220V). In addition, shielded twisted pairs, especially pressure signals, are required for electronically sensitive equipment lines in the system. And all the signal lines in the system must not be placed in the same piping or trunking as the main circuit and control lines. For effective shielding, the shield must be reliably grounded.
4, reasonable wiring
The specific methods are:
(1) The power and signal lines of the equipment should be as far as possible from the input and output lines of the inverter.
(2) The power and signal lines of other equipment should be avoided in parallel with the input and output lines of the inverter.
If the above measures still do not work, then continue the following:
5, interference isolation
The so-called interference isolation means that the interference source and the susceptible part are isolated from the circuit so that they do not have electrical connection. An isolation transformer is usually used on the power line between the power supply and the amplifier circuit such as the controller and the transmitter to avoid conducted interference. The power isolation transformer can be applied with a noise isolation transformer.
6, set the filter in the system line
The purpose of the device filter is to suppress interference signals from the drive through the power line to the power supply and the motor. In order 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 placed on the power line of the device to avoid conducted interference. Filters can be divided into: depending on where they are used:
(1) Input filter
There are usually two types:
a. Line filter: It is mainly composed of an inductor coil, which weakens the harmonic current with higher frequency by increasing the impedance of the line at high frequency.
b. Radiation filter: It is mainly composed of high-frequency capacitors, which will absorb harmonic components with high radiant energy at high frequency points.
(2) The output filter is also composed of an inductor coil
It can effectively attenuate higher harmonic components in the output current. It not only acts as an anti-interference, but also weakens the additional torque caused by harmonic currents generated by higher harmonics in the motor. For the anti-jamming measures at the output of the inverter, you must pay attention to the following aspects:
a, the output of the inverter is not allowed to access the capacitor, so as to avoid a large peak charging (or discharge) current when the power tube is turned on (off), damage the power tube;
b. When the output filter is composed of an LC circuit, the side of the filter that is connected to the capacitor must be connected to the motor side.
7, using a reactor
The lower frequency harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) in the input current of the inverter account for a high proportion, and they may interfere with other In addition to the normal operation of the equipment, because they consume a large amount of reactive power, the power factor of the line is greatly reduced. Serializing the reactor in the input circuit is an effective way to suppress lower harmonic currents. There are two main types depending on the wiring location:
(1) AC reactor
Connected in series between the power supply and the input side of the frequency converter. Its main functions are:
a, by suppressing the harmonic current, the power factor is increased to (0.75-0.85);
b. weaken the impact of the surge current in the input circuit on the inverter;
c. Attenuate the effects of power supply voltage imbalance.
(2) DC reactor
Connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple, which is to weaken the higher harmonic components in the input current. However, it is more effective than AC reactor in improving the power factor, up to 0.95, and has the advantages of simple structure and small size.
Therefore, the anti-interference measures of the inverter mainly include the installation of AC reactors and filters in the incoming part of the inverter, the shielded cables for the incoming and outgoing lines, the shielding layers of all cables and the reactors, filters, inverters and motors. The protective ground is commonly grounded, 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 inverter from interfering with the signal and the control loop, the controller, the instrument and the industrial computer need to be powered by separate isolated power sources.
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