On-site, the frequency converter interference appears more and more serious, and even lead to the control system can not be put into use. The working principle of frequency converter is doomed to produce strong electromagnetic interference.

The frequency converter includes a rectifying circuit and an inverter circuit. The input alternating current is converted into a direct current voltage by a rectifying circuit and a plain wave circuit, then through the inverter DC voltage into different width of the pulse voltage (called pulse width modulation voltage, PWM) . Using this PWM voltage to drive the motor, the motor torque and speed can be adjusted. This principle leads to three types of electromagnetic interference:

1. Harmonic interference

The rectifier circuit produces a harmonic current, which produces a voltage drop on the impedance of the power supply system, resulting in distortion of the voltage wave pattern, this voltage distortion interferes with many electronic devices (because most devices work only at sinusoidal voltages) . A common voltage distortion is the flattening of the top of the sinusoidal wave. When the harmonic current is constant, the voltage distortion is more serious in the case of weak power supply. The characteristic of this kind of interference is that it will interfere with the equipment using the same power grid, and it has nothing to do with the distance between the equipment and the inverter

2. RF conducted emission interference

Since the load voltage is pulse-shaped, so the inverter from the grid to draw current is pulse-shaped, this pulse current contains a large number of high-frequency components, radio-frequency interference, the characteristic of this kind of interference is that it will interfere with the equipment using the same power grid, and is independent of the distance between the equipment and the inverter

3. Radio frequency interference

RF radiation interference comes from the input and output cables of the converter. In the case of RF conducted transmit interference, if there is RF interference current on the input and Output Cable of inverter, the cable is equivalent to antenna, it will produce electromagnetic wave radiation and radiation interference. The PWM voltage transmitted on the output cable of the inverter also contains rich high-frequency components, which can produce electromagnetic wave radiation and form radiation interference. The characteristic of radiated interference is that it becomes serious when other electronic equipment is near the inverter.

According to the basic principle of electromagnetism, electromagnetic interference must have three elements: electromagnetic interference source, electromagnetic interference path, electromagnetic interference sensitive system. In order to prevent interference, hardware anti-interference and software anti-interference can be used. Among them, hardware anti-jamming is the most basic and important anti-jamming measures, generally from anti-jamming and put two aspects to suppress interference, the general principle is to restrain and eliminate the interference source, cut off the coupling channel to the system and reduce the sensitivity of the interference signal. Concrete measures can be used in engineering isolation, filtering, shielding, grounding and other methods. The following are the main steps in addressing on-site interference:

1. Adopt software anti-interference measures

Specifically, through the inverter man-machine interface to adjust the frequency of the frequency converter carrier, the value is reduced to an appropriate range. If this approach does not work, then the following hardware anti-jamming measures can only be taken.

2. Ground correctly

Through the site of the specific investigation we can see that the field grounding situation is not very ideal. The correct grounding is not only the system can effectively suppress external interference, but also reduce the external interference of the equipment itself, is the most effective measure to solve the inverter interference. Specifically, the following:

(1) PE (E, G) of the main circuit terminal of the inverter must be grounded, which can be grounded with the motor of the inverter, but not with other equipment, and must be grounded by a single pile, and the earthing point should be as far away from the earthing point of the weak electric equipment as possible. At the same time, the frequency converter grounding wire should not be less than 4mm2 section, length should be controlled within 20m.

(2) in the ground wire of other electromechanical equipment, the protective ground and the working ground should be separately set the ground pole, and remit into the electrical ground point of the distribution cabinet finally. The shielding ground of the control signal and the shielding ground of the main circuit conductor should also be set separately and remitted to the electrical grounding point of the distribution cabinet.

3. Shield the source of interference

Shielding jamming source is a very effective method to suppress interference. In general, the frequency converter itself is shielded with iron shell to prevent the leakage of electromagnetic interference, but the output line of the frequency converter is preferably shielded with steel tube, especially when the frequency converter is controlled by external signal (output 4 ~ 20mA signal from the controller) , the control signal line is required to be as short as possible (generally less than 20m) and must be Shielded Twisted Pair and completely separated from the main circuit line (AC380) and the control line (AC220V) . In addition, the electronic sensitive equipment circuits in the system also require the use of Shielded Twisted Pair, especially pressure signals. And all the signal lines in the system must not be placed with the main circuit lines and control lines in the same pipe or line slot. For shielding effectiveness, the shielding layer must be reliably grounded.

 4. Reasonable cabling methods include:

(1) the power line and signal line of the equipment should be as far away from the input and output lines of the inverter as possible.

(2) the power and signal lines of other equipment should be avoided parallel with the input and output lines of the inverter.

If the above approach does not work, then continue with the following:

5.Isolation of interference

The so-called interference isolation, refers to the interference from the circuit and vulnerable to interference from the part isolated, so that they do not have electrical contact. Usually, an isolation transformer is used on the power line between the power supply and the Electronic Amplifier such as the controller and the transmitter to avoid conduction interference. Noise Isolation Transformers can be used for power isolation transformers.

6. Set the filter in the system circuit

The function of the device filter is to suppress the interference signal from the inverter through the power line conduction interference to the power supply and the motor. In order to reduce the electromagnetic noise and loss, the output filter can be set at the output side of the inverter, and the input filter can be set at the input side of the inverter to reduce the interference to the power supply. If there are sensitive electronic devices such as controller and transmitter in the circuit, a power noise filter can be set on the power line of the device to avoid conduction interference. Filters can be divided into:

(1) input filter

There are usually two: 

A, line filter: mainly composed of inductance coil, which increases the line impedance at high frequencies to reduce the frequency of higher harmonic current. 

B, radiation filter: mainly composed of high-frequency capacitors, it will absorb the frequency point is very high, with the harmonic radiation energy components.

(2) the output filter is also composed of inductance 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 higher harmonics in the motor. For the anti-interference measures at the output end of the converter, we must pay attention to the following aspects: 

a, the output end of the converter is not allowed to connect to the capacitor, in order to avoid the power tube on (off) the moment, 

b. when the output filter consists of LC circuit, one side of the filter connected to the capacitor must be connected to the motor side.

7. Use a reactor

In the input current of frequency converter, the proportion of the lower frequency harmonic component (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) is very high, in addition to their potential to interfere with the normal operation of other equipment, because they consume a large amount of reactive power, the power factor of the line greatly reduced. It is an effective method to suppress the lower harmonic current by adding a series reactor in the input circuit. Depending on the location of the wiring, there are two main types:

(1) AC reactor

The series connection is between the power supply and the input side of the inverter. Its main functions are as follows: 

(a) increasing the power factor to (0.75-0.85) by restraining the harmonic current; 

(b) weakening the impact of the inrush current in the input circuit on the frequency converter; 

(c) weakening the influence of the voltage imbalance in the power supply.

(2) DC reactor

Connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple, is to weaken the input current in the high-order harmonic components. However, it is more effective than AC reactor in improving power factor, up to 0.95, and has the advantages of simple structure, small size and so on.

Therefore, the anti-interference measures of frequency converter mainly include adding AC reactor and filter in the input line part of frequency converter, and shielding cable in the input line and output line, the shield layer of all cables is grounded together with the protective ground of reactor, filter, frequency converter and motor, and the grounding point is separated from other grounding points and kept enough distance. At the same time, the signal cable and the power cable inverter should not be arranged in parallel.

In addition, in order to prevent the frequency converter from interfering with the signal and control circuit, it is necessary to supply power to the controller, instrument and industrial control computer with separate isolated power supply.

Source: internet (for study only, intrusion and deletion)