News

On site, the interference from vfd frequency inverters occurs quite frequently and seriously, even leading to the control system being unable to be put into use.

The working principle of a frequency inverter is bound to generate strong electromagnetic interference. A frequency inverter consists of a rectifier circuit and an inverter circuit. The input AC power is converted into DC voltage through the rectifier circuit and the smoothing circuit, and then the DC voltage is transformed into pulse voltages of different widths (referred to as pulse width modulation voltage, PWM) by the vfd inverter.

By driving the motor with this PWM voltage, the purpose of adjusting the motor's torque and speed can be achieved. This working principle leads to the following three types of electromagnetic interference:

1. Harmonic interference

Rectifier circuits generate harmonic currents, which cause voltage drops on the impedance of the power supply system, leading to distortion of the voltage waveform. This distorted voltage interferes with many electronic devices (as most electronic devices can only operate under sinusoidal voltage conditions). A common voltage distortion is the flattening of the top of the sinusoidal wave. When the harmonic current is constant, voltage distortion is more severe in the case of weak power supply. The characteristic of this interference is that it will cause interference to the equipment using the same power grid, regardless of the distance between the equipment and the frequency inverter.

2. Radio frequency conducted emission interference

Since the load voltage is pulse-like, the current drawn by the frequency inverter from the power grid is also pulse-like. This pulse-like current contains a large number of high-frequency components, forming radio frequency interference. 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 frequency inverters.

3. Radio frequency radiation interference
Radio frequency radiation interference comes from the input cable and output cable of the frequency converter. In the above-mentioned situation of radio frequency conducted emission interference, when there is radio frequency interference current on the input and output cables of the frequency converter, since the cables act as antennas, electromagnetic wave radiation will inevitably be generated, resulting in radiated interference.

The PWM voltage transmitted on the output cable of the frequency inverter also contains rich high-frequency components, which can generate electromagnetic wave radiation and form radiation interference. The characteristic of radiated interference is that when other electronic devices approach the frequency inverter, the interference phenomenon becomes severe.

According to the fundamental principles of electromagnetism, the formation of electromagnetic interference must have three elements: an electromagnetic interference source, an electromagnetic interference path, and a system sensitive to electromagnetic interference.To prevent interference, hardware anti-interference and software anti-interference can be adopted.

Among them, hardware anti-interference is the most fundamental and important anti-interference measure. Generally, it starts from the two aspects of anti-interference and suppression to suppress interference. 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 adopt methods such as isolation, filtering, shielding and grounding.

The following are the main steps to solve on-site interference:
1：Adopt software anti-interference measures
Specifically, it is to lower the carrier frequency of the frequency inverter through the human-machine interface of the frequency inverter , and reduce this value to an appropriate range. If this method doesn't work, then the following hardware anti-interference measures can only be taken.

2. Ensure proper grounding
Through the specific on-site investigation, we can see that the grounding situation on site is not very ideal. Correct grounding can not only effectively suppress external interference for 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 problem of frequency inverters.

Specifically, the following points should be achieved:

(1) The main circuit terminal PE (E, G) of the frequency converter must be grounded. This grounding can be shared with the motor driven by the frequency inverter, but not with other equipment. A separate grounding stake must be driven, and this grounding point should be as far away as possible from the grounding points of weak current equipment.
Meanwhile, the cross-sectional area of the grounding wire of the frequency converter should be no less than 4mm ², and the length should be controlled within 20m.

(2) In the grounding wires of other electromechanical equipment, protective grounding and working grounding should be separately set up with grounding electrodes 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 the source of interference
Shielding the source of interference is a very effective way to suppress interference. Usually, the inverter itself is shielded with an iron casing to prevent the leakage of electromagnetic interference. However, the output line of the inverter is best shielded with a steel pipe. Especially when the inverter is controlled by an external signal (4-20mA signal output from the controller), the control signal line should be as short as possible (generally within 20m), and shielded twisted-pair cables must be used. And it is completely separated from the main circuit line (AC380) and the control line (AC220V).
In addition, the circuits of electronic sensitive equipment in the system also require the use of shielded twisted-pair cables, especially for pressure signals.
Moreover, all signal lines in the system must never be placed in the same conduit or trunking as the main circuit lines and control lines.
For the shielding to be effective, the shielding layer must be reliably grounded.

4. Reasonable wiring
The specific methods are as follows: (1) The power lines and signal lines of the equipment should be as far away as possible from the input and output lines of the frequency converter.
(2) The power lines and signal lines of other equipment should avoid being parallel to the input and output lines of the frequency converter.
If the above methods still don't work, then continue with the following methods:

5. Isolation of interference
The so-called isolation of interference refers to separating the interference source from the vulnerable parts of the circuit to prevent them from having electrical contact. Usually, an isolation transformer is adopted on the power line between the power supply and the amplifier circuits such as the controller and transmitter to prevent conducted interference. The power isolation transformer can also apply a noise isolation transformer.

6. Set up filters in the system circuits
The function of the equipment filter is to suppress the interference signals from being conducted from the frequency 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 frequency inverter. To reduce interference with the power supply, an input filter can be set on the input side of the frequency inverter.

If there are sensitive electronic devices such as controllers and transmitters in the circuit, a power noise filter can be installed on the power line of the device to prevent conducted interference.
Filters can be classified according to their different application locations as:

(1) Input filter
There are usually two kinds:
A. Line filter: Mainly composed of inductive coils, it weakens higher-frequency harmonic currents by increasing the impedance of the line at high frequencies.
B. Radiation filter: Mainly composed of high-frequency capacitors, it will absorb harmonic components with radiation energy at very high frequency points.

(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 an anti-interference role, but also can weaken the additional torque caused by the harmonic current generated by high-order harmonics in the motor.

For the anti-interference measures at the output end of the frequency converter, the following aspects must be noted:
Capacitors are not allowed to be connected to the output end of the frequency converter to prevent the generation of a very large peak charging (or discharging) current at the moment when the power transistor is turned on (off), which may damage the power transistor.

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

7. Use reactors
In the input current of a frequency converter, the proportion of low-frequency harmonic components (such as the 5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) is very high. Besides possibly interfering with the normal operation of other equipment, they also consume a large amount of reactive power, significantly reducing the power factor of the line. Inserting reactors in series into 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 input side of the power supply and the frequency converter.
Its main functions are:
A. By suppressing harmonic currents, the power factor is increased to (0.75-0.85);
B. Weaken the impact of inrush current in the input circuit on the frequency converter;
C. Weaken the impact 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, which is to weaken the high-order harmonic components in the input current.
However, it is more effective than AC reactors in improving the power factor, reaching up to 0.95, and has the advantages of simple structure and small volume.

Therefore, the anti-interference measures for frequency inverters mainly include installing AC reactors and filters at the input line of the frequency inverter, using shielded cables for both the input and output lines, and grounding the shielding layers of all cables together with the protective ground of the reactors, filters, frequency inverters and motors, and separating this grounding point from other grounding points to maintain a sufficient distance.

Meanwhile, the signal cables and the power cables of the frequency inverter should not be arranged in parallel.
In addition, to prevent the frequency inverter from interfering with the signal and control loop, it is necessary to supply power to the controller, instrument and industrial control computer with separate isolated power supplies.