How to handle interference issues with frequency converters!

Jun 09, 2023

How to handle interference issues with frequency converters!

On site, interference problems with frequency converters occur frequently and seriously, even leading to the inability of the control system to be put into use. This has always been a headache. Today, I will talk to you about how to deal with interference problems with frequency converters!

Common phenomena of frequency converter interference

1. As soon as the frequency converter of the heat exchange station is turned on, the pressure transmitter will jump randomly;

2. Using a frequency converter to control the water supply, the voltage change is used as a signal for collecting pressure, and the voltage change is interfered by the frequency converter;

3. When the frequency converter starts the motor, the voltage change signal is unstable and jumps severely;

4. The voltage transformer (4-20mA) jumps randomly after the frequency converter is started, while the nearby integrated thermal resistor (4-20mA) is not affected, and the signal wires are not shielded;

These phenomena are all due to interference from the frequency converter.

Why does the frequency converter cause interference?

Firstly, everyone should know that frequency converters are used to change frequencies. The frequency converter includes a rectifier circuit and an inverter circuit. The input AC power is converted into DC voltage through a rectifier circuit and a flat wave circuit, and then converted into pulse voltage of different widths through the inverter. By using this PWM voltage to drive the motor, the purpose of adjusting the motor torque and speed can be achieved.

This working principle can cause three types of electromagnetic interference:

1. Harmonic interference

The rectifier circuit will generate harmonic currents, which generate voltage drops on the impedance of the power supply system, causing distortion of the voltage waveform. This distorted voltage forms interference with many instruments, and a common voltage distortion is the flattening of the top of the sine wave. When the harmonic current is constant, voltage distortion is more severe in the case of weak power sources. The characteristic of this interference is that it will form interference on equipment using the same power grid, regardless of the distance between the equipment and the frequency converter.

2. Radio frequency conducted emission interference

Due to the pulse like load voltage, the current drawn by the frequency converter from the power grid is also pulse like. This pulse current contains a large amount of high-frequency components, forming radio frequency interference. The characteristic of this interference is that it will cause interference to instruments using the same power grid, regardless of the distance between the instrument and the frequency converter.

3. Radio frequency radiation interference

RF radiation interference comes from the input and output cables of the frequency converter. When there is RF interference current on the input and output cables of the frequency converter, as the cables act as antennas, electromagnetic wave radiation will inevitably occur, resulting in radiation interference. The PWM voltage transmitted on the output cable of the frequency converter also contains rich high-frequency components, which can generate electromagnetic wave radiation and form radiation interference. The characteristic of radiation interference is that when other electronic devices are close to the frequency converter, the interference phenomenon becomes severe.

How to solve the interference problem?

Frequency converter interference processing method

1. The frequency converter should adopt a single point grounding, preferably with short and thick wires for grounding;

2. The signal wire of the sensor adopts a double shielded wire, and the shielding layer is grounded with a cable clamp.

3. Install Line filter, filter magnetic ring or isolator on the power supply of the sensor for isolation.

4. To suppress the harmonics generated by the frequency converter, optional filtering products include: frequency converter input filter, frequency converter output filter, frequency converter input reactor, frequency converter output reactor, etc. Serializing reactors in the input circuit is an effective method to suppress lower harmonic currents.

In addition, to prevent the frequency converter from interfering with signals and control circuits, it is necessary to use separate isolated power supplies for the controller, instrument, and industrial computer.

In fact, the simplest method on site is:

Keep the instrument away from the frequency converter! But not all interference can be eliminated, and the methods still need to be tried one by one.

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