Rotor Eccentricity Sensor PR6423/002-030: Identifying EMI and Inspection Priorities

Steam turbine rotor eccentricity monitoring depends entirely on the PR6423/002-030 eddy current sensor. It captures the rotor’s eccentricity value in real time, issuing an alarm if the value exceeds tolerance to prevent shaft rubbing and bearing damage. However, power plant environments are rife with electromagnetic interference (EMI). Nearby motors, high-voltage cables, and variable frequency drives (VFDs) can “bleed” into the sensor signal. A corrupted signal leads to inaccurate monitoring data, resulting in either false alarms that shut down the unit or missed warnings that lead to catastrophic failures.

Here is how to determine if the sensor signal contains EMI and, if interference is present, which parts of the system to inspect first.

How to Determine: Is EMI Mixed into the Signal?

Electromagnetic interference doesn’t “hide”; it reveals itself through signal waveforms and numerical changes. You can make a preliminary judgment without complex instruments by looking at these three points.

1. Check the Signal Waveform: Normal is Smooth, Interference Shows Noise

The normal signal from the PR6423/002-030 eddy current sensor has a smooth waveform. When rotor eccentricity is stable, the waveform is a smooth curve close to a straight line. If the rotor has minor rotational fluctuations, the waveform follows a slow rise and fall, but it should have no sudden spikes or jitters.

If EMI is mixed in, the waveform will “look ugly”:

It may contain many cluttered, small ripples, like a rock was thrown into water.
Sharp peaks may suddenly appear, spiking high and dropping low.
The waveform may display a rapid “tremor” or high-frequency jitter that is completely out of sync with the rotor’s rotation rhythm.

You can easily distinguish this by connecting an oscilloscope to the sensor’s output. If an oscilloscope isn’t available, check the monitoring system’s real-time trend curve—a normal curve is “clean,” while an interfered curve is “messy.” If you are unsure whether your waveform is normal, contact us for comparison images or remote signal curve review assistance.

2. Check the Numerical Fluctuation: Normal is Gradual, Interference Jumps

Rotor eccentricity values do not “jump suddenly.” During normal operation, the value is either stable within a range (e.g., 0.02 – 0.03 mm) or changes slowly with unit load (e.g., slightly increasing as load rises). The fluctuation is small and regular.

When EMI occurs, the value “jumps randomly”: one second it shows 0.03 mm, the next it spikes to 0.08 mm, and then drops back to 0.04 mm, with no discernible pattern. This jumping is unrelated to the actual rotor condition—if the unit load and speed are unchanged, but the value fluctuates repeatedly, it is highly likely due to interference.

Sometimes, the value will “follow the interference source”—for example, the value starts jumping when a nearby feedwater pump starts, and stabilizes when the pump stops. This strongly confirms EMI rather than an issue with the sensor or rotor itself.

3. Check Signal Following: Normal Tracks Rotor, Interference Tracks External Sources

The PR6423/002-030 eddy current sensor‘s signal should only “follow the rotor’s path”—eccentricity increases, signal increases; eccentricity decreases, signal decreases, regardless of other equipment.

If the signal “follows external equipment,” interference is present. For example:

The sensor signal shifts when a high-voltage solenoid valve in the workshop is energized.
The monitoring value changes when a VFD adjusts its frequency.
The signal shows a clear “spike” the moment an adjacent motor starts or stops.

These situations indicate that external electromagnetic signals have “leaked” into the sensor system, affecting the measurement. In such cases, checking the interference source is more reliable than suspecting a rotor fault.

When Interference Occurs: Priority Checks in Four System Areas

Once interference is suspected, do not rush to dismantle the sensor. Checking these four areas will typically uncover the root cause.

Check 1: The Eddy Current Sensor Itself—Installation and Contamination Issues?

A poorly installed sensor can “attract” or amplify interference.

Check the Installation Gap: The PR6423/002-030 has specific installation gap requirements. A gap that is too large or too small not only causes inaccurate measurement but also makes the system more susceptible to external EMI. Use a feeler gauge to measure the gap between the probe and the rotor. If incorrect, adjust it to the standard range (refer to the manual or contact us for specific values).
Check for Probe Contamination: Oil residue or dust accumulated on the probe surface affects signal acquisition and may make it easier for EMI to “cling” to the signal. Clean the probe surface with an alcohol swab, and then check the signal—if the signal stabilizes, it was “pseudo-interference” caused by contamination, not a true electromagnetic issue.
Check Probe Cable Connectors: Loose or oxidized connectors can cause poor signal contact, which easily introduces interference. Disassemble the connector, gently polish the oxidized layer with fine sandpaper, and then plug it back in tightly and secure it to see if the signal recovers.

Check 2: The Cable—Shielding and Routing Vulnerabilities?

The sensor cable is the “signal conduit” and the most common place for interference to “sneak in.” Focus on shielding and routing.

Check the Shielding Layer: The PR6423/002-030 cable has a metal shielding layer to block EMI. This layer will be ineffective if it is broken, ungrounded, or grounded at both ends.
Inspect for Damage: If the outer jacket is broken, the shielding layer might be severed, requiring cable replacement.
Check Grounding: The shielding layer must be “single-point grounded”—either at the sensor end’s ground terminal or the control cabinet’s ground, but not both (double grounding creates a “ground loop,” which introduces interference).
Check Routing: If the cable runs alongside or parallel to strong power lines (e.g., 380 V power lines, high-voltage cables), the strong electromagnetic field will “induce” into the sensor cable.
Inspect Cable Path: Is the cable too close to power lines or VFD cables?
Check Installation Method: Is the cable run in the same cable tray as strong power lines? If so, reroute the sensor cable separately or isolate it with a dedicated metal conduit, keeping it away from strong electrical lines.

Check 3: The Grounding System—Reliability and Ground Loops?

Poor grounding is a “hot zone” for EMI; many interference problems stem from grounding issues.

Check Sensor Grounding: The sensor’s ground terminal must be connected to an “independent protective ground” and not share a ground rod with other equipment (like motors or pumps). Measure the grounding resistance with a ground resistance meter; it should be less than 4 Ω. If the resistance is too high, the grounding is unreliable, and interference can easily enter.
Check Control Cabinet Grounding: The control cabinet receiving the sensor signal must also have separate, reliable grounding. The grounding inside the cabinet must be consistent—signal ground, power ground, and protective ground must be connected separately, not mixed. Mixing grounds causes voltage differences between different grounds, forming a ground loop and corrupting the signal.
Check for “Multi-Point Grounding”: If the sensor, cable shield, and control cabinet are connected to different ground rods, there may be potential differences between these rods, causing current to flow through the system and create interference. In this case, unify all grounds to a single grounding point to eliminate potential differences.

Check 4: Surrounding Interference Sources—Which Equipment is “Broadcasting”?

Identifying the “broadcasting” equipment allows you to either distance the sensor or shield the source, solving the interference problem.

Identify “High-Frequency Interference Sources”: VFDs, high-frequency motors, solenoid valves, and high-voltage switches generate high-frequency electromagnetic fields when energized, making them the most likely sources of sensor interference.
- Test Equipment On/Off: Start and stop these devices individually and observe the sensor signal. If the signal becomes corrupted after a device starts, it is the source of interference.
- Check Distance: The closer the device is to the sensor, the stronger the interference. Try to keep the sensor away from these devices (at least 1 meter distance).
Identify “Strong Electrical Interference Sources”: High-voltage cables, transformers, and high-power motors also affect the sensor with their strong electromagnetic fields.
- Check Cable Path: Does the high-voltage cable pass over or next to the sensor?
- Check Transformer Location: Is a transformer installed close to the sensor mounting point?
If distancing is impossible, shield the interference source (e.g., encase high-voltage cables in metal conduit, add a shielding cover to the motor) to reduce electromagnetic field diffusion.

Electromagnetic interference with the PR6423/002-030 eddy current sensor is a serious matter—inaccurate signals threaten unit safety. If you encounter difficulties in identifying or troubleshooting interference, such as distinguishing between interference and sensor failure, or locating the source, please contact us. We will help ensure a stable sensor signal, accurate rotor eccentricity monitoring, and safe steam turbine operation.

Would you like to schedule a consultation with our engineers regarding the optimal placement or alternative models for high-temperature/high-humidity operation?
E-mail: sales@yoyik.com
Tel: +86-838-2226655
Whatsapp: +86-13618105229

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