Abstract: The widespread application of frequency converters in production and process control has brought about a new problem: the harmonics generated by the operation of frequency converters have caused interference problems to systems or instruments close to the frequency converter. For example, some instruments have produced large measurement errors, some instruments cannot even work properly, and some systems may even malfunction.
Frequency converters generate high-power harmonics, and their interference paths are consistent with general electromagnetic interference paths, namely, interference voltage or current generated through circuit coupling, electromagnetic radiation, and inductive coupling. The main interference sources affecting instruments and control systems include: strong electromagnetic fields in the field and fluctuations in the power supply.
Determining whether interference is originating from the frequency converter is easier on-site than determining other types of interference. If stopping the frequency converter when interference occurs and the instruments and system immediately return to normal, it can be confirmed that the interference is caused by the frequency converter, and appropriate measures should be taken.
For power supply disturbances, simply selecting a multi-shielded isolation transformer and properly selecting and regulating the instrument's power supply for filtering and voltage stabilization is sufficient. However, dealing with interference from strong electromagnetic fields is much more complex, as it has a greater impact on instruments and is more difficult to resolve. This is because such interference induces voltage in the circuits of instruments or systems due to electromagnetic induction, thereby affecting the normal operation of instruments and control systems or the normal operation of programs.
The following measures can be taken on-site to overcome the interference generated by the frequency converter:
1. Add a filter circuit to the instrument circuit to block or bypass interference signals.
2. When the instrument has a digital filtering function, a certain time constant can be set to suppress interference.
3. The signal cable is a shielded cable, and the shielding layer is grounded at a single point.
4. Connect a 4.7 μF/100V capacitor in parallel to the transmitter's output signal terminal or connect a capacitor in parallel to ground on the output signal line.
5. The negative terminal of the signal line must be grounded.
6. Use a signal isolator.
