VFD noise is one of the most common causes of unstable sensor readings in factories: variable frequency drives switch at high frequency and emit electromagnetic interference (EMI) that couples into sensor and 4–20 mA signal cables, making values jump. Below are proven solutions to eliminate sensor noise from VFDs.
General solutions for sensor noise caused by VFDs
1. Use shielded signal cable
Use shielded twisted-pair cable for sensor signals and earth the shield correctly – at one end only (usually the control panel) to avoid ground loops.
2. Separate power and signal wiring
Never run sensor cables in the same conduit or tray as power cables, especially VFD-to-motor cables. Keep at least 30 cm apart if running in parallel.
3. Fit EMI filters
Install an input EMI filter on the VFD to reduce noise conducted through the supply, and add ferrite cores on signal or motor cables.
4. Use a stable, isolated power supply
Power sensors from a dedicated isolated supply (or linear supply, which is quieter than switching supplies). Use optocouplers or 4-20mA signal isolators for analogue and digital signals.
5. Physically separate VFDs and sensors
Locate drives and sensor equipment apart to reduce electromagnetic field coupling.
6. Use digital / fieldbus communication
Instead of analogue 4–20 mA, use Modbus, Profibus, CAN or EtherCAT, which have better noise immunity.
7. Choose sensors with better noise immunity
Select sensors with built-in EMC protection and filtering.
Specific fixes for 4-20mA signal noise from VFDs
- Proper shielded cable: twisted pair with foil or braid shield (e.g. Belden 8760 type); shield earthed at one end only.
- Ferrite rings or signal filters: wind the 4–20 mA cable 3–5 turns through a ferrite ring to suppress high-frequency noise; add an EMI or analogue filter at the PLC input.
- Separate motor and signal cables: no shared trays, at least 30 cm apart, no close crossings.
- Dedicated isolated 24 VDC supply for sensors, not shared with the VFD; use isolated analogue input modules.
- Software filtering: average over N samples and reject outliers in the PLC or microcontroller.
- Check earthing: panel, VFD and sensors properly earthed, no ground loops, VFD frame earthed per the manual.
Quick diagnosis
- Temporarily stop the VFD and check the 4–20 mA value – if it becomes stable, the VFD is the cause.
- Use a current meter or oscilloscope to see noise on the signal line.
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Where to buy noise filters and signal isolators?
EQUIPMENT INDUSTRIAL SOLUTION CO., LTD (ISE) – Ho Chi Minh City, Vietnam – supplies genuine noise filters and signal isolators with technical consulting, fast quotation and on-site commissioning support across Vietnam and Southeast Asia.
Contact us: Sales +84 909 617 185 – Technical +84 979 117 185 – Email: [email protected]
