Deciphering the inverter anti-interference problem

　　In the field, the interference of frequency converters is quite frequent and serious, and even makes the control system unusable. The working principle of frequency converters 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 the rectifier circuit and the smoothing circuit, and then the DC voltage is converted into pulse voltages of different widths (called pulse width modulation voltage, PWM) through the inverter. Using this PWM voltage to drive the motor can adjust the motor torque and speed. This working principle leads to the following three types of electromagnetic interference:

　　1. Harmonic interference

　　The rectifier circuit will generate harmonic current, which will cause voltage drop on the impedance of the power supply system, resulting in voltage waveform distortion. This distorted voltage will interfere with many electronic devices (because most electronic devices can only work under sine wave voltage conditions). Common voltage distortion is that the top of the sine wave becomes flat. 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 pulsed, the inverter draws current from the grid in pulsed form. This pulsed current contains a large amount of high-frequency components, which forms radio frequency interference. The characteristic of this interference is that it will interfere with the equipment using the same grid, regardless of the distance between the equipment and the inverter.

　　3. Radio frequency radiation interference

　　RF radiation interference comes from the input and output cables of the inverter. In the above-mentioned RF conducted emission interference, when there is RF interference current on the input and output cables of the inverter, since the cables are equivalent to antennas, electromagnetic wave radiation will inevitably be generated, resulting in radiation interference. The PWM voltage transmitted on the inverter output cable also contains a lot of high-frequency components, which will generate electromagnetic wave radiation and form radiation interference. The characteristic of radiation interference is that when other electronic devices are close to the inverter, the interference phenomenon becomes serious.

　　According to the basic principles of electromagnetism, the formation of 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 important anti-interference measure. Generally, interference is suppressed from two aspects: resistance and release. The overall principle is to suppress and eliminate the interference source, cut off the coupling channel of interference to the system, and reduce the sensitivity of the system to interference signals. Specific measures in engineering can be isolation, filtering, shielding, grounding and other methods. The following content is the main steps to solve on-site interference:

　　1. Use software anti-interference measures

　　Specifically, it is to adjust the carrier frequency of the inverter through the inverter's human-machine interface to a suitable range. If this method does not work, the following hardware anti-interference measures can only be taken.

　　2. Proper grounding

　　Through the specific investigation on site, we can see that the grounding condition on site is not ideal. Correct grounding can not only effectively suppress external interference in the system, but also reduce the interference of the equipment itself to the outside world. It is the most effective measure to solve the interference of the inverter. 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 not with other equipment. A separate grounding stake must be driven, and the grounding point should be as far away from the grounding point of weak current equipment as possible. At the same time, the cross-sectional area of ​​the inverter grounding wire should be no less than 4mm2, and the length should be controlled within 20m.

　　(2) For the ground wires of other electromechanical equipment, the protective ground and working ground should be separately grounded 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 separately grounded and finally connected to the electrical grounding point of the distribution cabinet.

　　3. Shield 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 its electromagnetic interference from leaking, but the output line of the inverter is best shielded with a steel pipe, especially when the inverter is controlled by an external signal (outputting a 4~20mA signal from the controller), the control signal line is required to be as short as possible (generally within 20m), and a shielded twisted pair must be used, and it must be completely separated from the main circuit line (AC380) and the control line (AC220V). In addition, the electronic sensitive equipment lines in the system are also required to use shielded twisted pairs, especially pressure signals. And all signal lines in the system must not be placed in the same pipe or cable trough as the main circuit line and control line. In order to make the shielding effective, the shielding layer must be reliably grounded.

　　4. Reasonable wiring

　　The specific methods are:

　　(1) The power cord and signal cord of the equipment should be kept as far away from the input and output cords of the inverter 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 don’t work, try the following:

　　5. Isolation of interference

　　The so-called interference isolation refers to isolating the interference source from the part susceptible to interference in 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 the amplifier circuits such as the controller and transmitter to avoid conducted interference. The power isolation transformer can also be a noise isolation transformer.

　　6. Set up filters in the system line

　　The function of the equipment filter is to suppress the interference signal from the inverter through the power line to the power supply and 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 noise filter can be set on the power line of the device to avoid conducted interference. Filters can be divided into the following types according to their use locations:

　　(1) Input filter

　　There are usually two types:

　　a. Line filter: It is mainly composed of an inductor coil, which weakens the high-frequency harmonic current by increasing the impedance of the line at high frequencies.

　　b. Radiation filter: Mainly composed of high-frequency capacitors, it will absorb high-frequency harmonic components with radiation energy.

　　(2) The output filter is also composed of an inductor coil

　　It can effectively weaken the high-order harmonic components in the output current. It not only plays an anti-interference role, but also can weaken the additional torque caused by the harmonic current generated by the high-order harmonics in the motor. For the anti-interference measures at the output end of the inverter, the following aspects must be paid attention to:

　　a. The output end of the inverter is not allowed to be connected to a capacitor, so as to avoid generating a large peak charging (or discharging) current at the moment when the power tube is turned on (off), thereby damaging the power tube;

　　b. When the output filter is composed of an LC circuit, the side of the capacitor connected to the filter must be connected to the motor side.

　　7. Use reactor

　　The proportion of low-frequency harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) in the input current of the inverter is very high. In addition to interfering with the normal operation of other equipment, they also consume a lot of reactive power, which greatly reduces the power factor of the line. Inserting a reactor in series in the input circuit is an effective way to suppress low-harmonic current. According to the different wiring positions, there are mainly two types:

　　(1) AC reactor

　　Connected in series between the power supply and the input side of the inverter. Its main functions are:

　　a. Improve the power factor to (0.75-0.85) by suppressing harmonic current;

　　b. Reduce the impact of surge current in the input circuit on the inverter;

　　c. 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 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 installing AC reactors and filters on the incoming line of the inverter, using shielded cables for incoming and outgoing lines, and grounding the shielding layers of all cables together with the protective ground of the reactor, filter, inverter and motor, and keeping this grounding point separate from other grounding points and keeping a sufficient distance. At the same time, the signal cable and the power cable of the inverter should not be laid in parallel.

　　In addition, to prevent the inverter from interfering with signals and control loops, the controller, instrument and industrial computer need to be powered by a separate isolated power supply.