Variable frequency drives (VFDs) are a product of advancements in electrical drive technology. With their widespread application, they have increasingly become the largest source of electromagnetic pollution in factory automation. The nonlinear equivalent load of the VFD's DC-AC inverter causes it to not only pollute the factory's power supply system in many system integration projects, but also directly interfere with automation projects, leading to inaccuracies and malfunctions in the measurement and control system, severely damaging the stability of large systems, and even causing the VFD itself to experience "bootstrapping" speed regulation failures due to interference.
In most cases, this is because the frequency converter contaminates the power supply, and then when other electrical devices use the contaminated power supply, they cannot work properly.
This situation can generally be resolved in the following ways:
First, magnetic rings can be added to the input and output lines of the frequency converter for filtering;
Second, if the first method fails, add inverter-specific filters to both the input and output terminals of the inverter.
Third, if the first and second measures are ineffective, inverter-specific reactors can be added to the input and output terminals of the inverter;
Fourth, a power filter and an isolation transformer with an isolation layer can also be added to the single-phase power supply that is required.
The main types of interference in frequency converters are radiated interference and conducted interference.
Radiated interference: For interference signals propagating through radiation, the main methods to weaken them are to install radiation sources and shield the interfered lines.
Conducted interference: Interference signals propagating through lines are mainly dealt with by adding magnetic rings, reactors or filters to the input and output sides of the frequency converter.
Precautions:
1. Signal lines and power lines must cross perpendicularly or be routed in separate channels;
2. Do not use wires of different metals to connect each other;
3. The shielding tube should be reliably grounded, and the connection to grounding should be reliable along its entire length;
4. Twisted-pair shielded cables must be used in signal circuits;
5. The shielding layer grounding should be kept as far away from the frequency converter as possible and separate from the frequency converter grounding point;
6. Magnetic rings can be used on both the input and output power lines of the frequency converter. The specific method is as follows: wind the magnetic ring 4 times in the same direction on the input lines and 3 times in the same direction on the output lines. When winding, ensure the magnetic ring is as close to the frequency converter as possible.
7. Generally, shielding and other interference measures can be taken for equipment and instruments that are subject to interference. Frequency converters are power electronic devices containing electronic components and chips, making them susceptible to external electrical interference. Furthermore, the internal components of frequency converters, such as switching devices, oscillation circuits, and digital circuits, generate continuous interference spectra. This means that when a frequency converter is put into operation, it is necessary to prevent both external interference and interference from the frequency converter itself with external equipment.
