The Sensor That Chattered When the VFD Ran
We installed an analog proximity sensor (0–10 V output) near a VFD (variable frequency drive) for a conveyor. The sensor’s reading wandered ±2 V when the VFD ran. The sensor’s output was clean when the VFD was off. The problem: the VFD’s motor cable radiated EMI (electromagnetic interference). The sensor’s analog signal (low voltage, high impedance) picked up the noise. We routed the sensor cable away from the VFD motor cable (separate conduits, 20 cm apart), grounded the cable shield at one end, and used a shielded sensor cable. The wandering stopped. The mistake was running the sensor cable in the same conduit as the VFD motor cable.
Control cabinet grounding and shielding keeps noise out of sensitive signals. This article covers the rules.
Sources of Noise
Factory noise comes from:
- VFDs and servo drives: Fast-switching PWM (pulse-width modulation) on motor cables. Radiates EMI.
- Contactors and relays: Switching inductive loads. Creates voltage spikes.
- Welding equipment: High-frequency noise.
- Long cable runs: Act as antennas.
The sensitive signals (analog sensors, encoder, low-level digital) pick up this noise. Digital signals (24 V) are more tolerant (the noise is small compared to the signal). Analog (0–10 V, 4–20 mA) is the most sensitive.
Step 1: Separate Power and Signal Cables
The #1 rule: keep power cables (motor, VFD, high-current) away from signal cables (sensor, encoder, analog). Separate them physically:
- Separate conduits: Power cables in one conduit, signal cables in another. 20 cm apart minimum.
- Cross at 90°: If they must cross, cross at right angles (not parallel). This minimizes the coupling.
- Don’t share a cable chain: In a drag chain (article 66), keep power and signal cables on separate shelves.
Step 2: Shielded Cables
Use shielded cables for analog and encoder signals. The shield (a metal braid around the wires) blocks EMI. But the shield must be grounded correctly.
- Ground the shield at ONE end: Ground at the cabinet end (the PLC/drive end). Don’t ground both ends (that creates a ground loop, which adds noise).
- Use shielded connectors: M12 connectors with 360° shield termination (not just the pigtail).
- Don’t leave the shield unconnected: A floating shield does nothing.
The grounding rule: Separate power and signal cables (20 cm apart), use shielded signal cables, and ground the shield at one end (cabinet). The sensor that chattered ran parallel to the VFD motor cable. Separate them, and the noise stops.
Step 3: Cabinet Grounding (Earth)
The control cabinet has a ground bus (PE, protective earth). Everything connects to it:
- The cabinet frame.
- The PLC and drives.
- The cable shields (single point).
- The motor frame (via the cable’s ground wire).
The ground bus connects to the building earth (a ground rod). Use thick copper (16 mm² or more). Don’t daisy-chain the ground (each device connects directly to the bus).
Step 4: VFD and Servo Drive Grounding
A VFD is the noisiest device. Its ground must be:
- Direct to the ground bus (short, thick wire).
- The motor cable shield grounded at both ends (the VFD and the motor) — for VFDs, ground both ends (the VFD’s internal filter needs it).
- The motor cable is shielded (not a plain wire).
This is the exception to “ground one end.” For VFD motor cables, ground both ends (the VFD manufacturer specifies this). For signal cables (sensors, encoders), ground one end.
Step 5: Ferrite Beads and Filters
If noise persists, add:
- Ferrite bead: A snap-on ferrite around the sensor cable (chokes high-frequency noise).
- EMC filter: A line filter on the VFD’s input (blocks noise on the power line).
- Isolation: Isolate the analog signal (use an isolated analog module).
| Cable Type | Shield? | Ground Shield |
|---|---|---|
| VFD motor cable | Shielded | Both ends |
| Analog sensor (0-10V, 4-20mA) | Shielded | One end (cabinet) |
| Encoder cable | Shielded (twisted pair) | One end (drive end) |
| Digital I/O (24V) | Not usually needed | N/A |
| Power (mains) | Not shielded (but in conduit) | PE ground |
A Grounding and Shielding Checklist
- Are power and signal cables separated? (20 cm?)
- Do they cross at 90°? (Not parallel?)
- Are analog/encoder cables shielded?
- Is the shield grounded at one end? (For signals?)
- Is the VFD motor cable shielded and grounded both ends?
- Is the cabinet ground bus thick and connected to earth?
- Do all devices ground directly to the bus? (No daisy-chain?)
- Are ferrite beads on noisy cables? (If needed?)
- Is the analog signal isolated? (Isolated module?)
- Is the cabinet IP-rated and grounded? (Article 60?)
The Bottom Line
Control cabinet grounding and shielding separates noise from signals. The sensor that chattered ran parallel to the VFD motor cable. Separate them (20 cm, cross at 90°), use shielded signal cables (ground one end), and ground VFD motor cables at both ends. Use a thick ground bus. The sensor that reads steadily wasn’t the most expensive one — it was wired away from the noise.
