What is an Eddy Current Probe? Non-Contact Displacement Sensor • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is an Eddy Current Probe? Non-Contact Displacement Sensor • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is an Eddy Current Probe? Non-Contact Displacement Sensor

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Understanding Eddy Current Probes

Definition: What is an Eddy Current Probe?

Eddy current probe (also called proximity probe, non-contact displacement sensor, or eddy current transducer) is a sensor that measures the distance (gap) between the probe tip and a conductive target surface without physical contact. In vibration monitoring, eddy current probes are mounted in machine casings pointing at rotating shafts to measure shaft radial position and vibration directly, providing displacement measurements in micrometers or mils with DC to high-frequency response.

Eddy current probes are the standard for permanent vibration monitoring on critical turbomachinery (steam turbines, gas turbines, large compressors, generators) because they measure actual shaft motion rather than bearing housing motion, provide absolute position information for clearance monitoring, and operate reliably in harsh environments (high temperature, contamination) where contact sensors would fail.

Operating Principle

Eddy Current Effect

  1. RF Excitation: Probe coil generates high-frequency RF field (typically 1-2 MHz)
  2. Eddy Current Induction: RF field induces eddy currents in conductive shaft surface
  3. Field Interaction: Eddy currents create opposing magnetic field
  4. Impedance Change: Probe coil impedance changes with gap distance
  5. Signal Conditioning: Electronics convert impedance to DC voltage proportional to gap
  6. Output: Voltage signal representing shaft-to-probe distance

Gap-Voltage Relationship

  • Output voltage inversely proportional to gap
  • Closer shaft → higher voltage
  • Farther shaft → lower voltage
  • Linear range typically 0.5-2.0 mm (20-80 mils)
  • Calibrated in µm/V or mils/V

Key Advantages

Direct Shaft Measurement

  • Measures actual shaft motion, not bearing housing
  • Unaffected by bearing stiffness or mounting structure
  • True rotor vibration vs. transmitted vibration
  • Critical for rotor dynamics analysis

DC to High-Frequency Response

  • Measures from 0 Hz (static position) to 10+ kHz
  • Captures slow roll, transients, resonances
  • No low-frequency limitations like accelerometers
  • Ideal for startup/coastdown analysis

Absolute Position

  • Provides shaft position relative to bearing centerline
  • Monitors clearances to seals, labyrinths
  • Detects rotor shifts or bearing wear
  • Protection function (trip on excessive displacement)

Harsh Environment Capability

  • Non-contact (no wear)
  • High temperature capability (up to 350°C)
  • Not affected by contamination on shaft
  • Reliable in oil mist, steam, dust

Typical Installation

XY Probe Pairs

  • Two probes 90° apart (horizontal and vertical)
  • Measures shaft position in both directions
  • Enables orbit analysis
  • Standard configuration for turbomachinery

Axial Position Probe

  • Mounted facing shaft end
  • Measures axial position and axial vibration
  • Monitors thrust bearing condition
  • Protects against axial rotor shift

Mounting Requirements

  • Rigid mounting in bearing housing or casing
  • Perpendicular to shaft surface
  • Appropriate gap setting (center of linear range)
  • Cable routing and grounding per specifications

Applications

Permanent Monitoring Systems

  • Critical turbomachinery (turbines, compressors > 1000 HP)
  • Continuous monitoring with alarm and trip functions
  • API 670 compliant systems
  • XY probes at each bearing plus axial probe

Rotor Dynamics Testing

Troubleshooting

  • Measure actual shaft vibration vs. bearing housing
  • Determine if problem in rotor or structure
  • Assess bearing condition from shaft motion
  • Clearance verification

Eddy current probes are the gold standard for shaft vibration measurement in critical rotating machinery, providing non-contact displacement measurement with DC response and high reliability. Their ability to measure actual shaft motion, absolute position, and operate in harsh environments makes them indispensable for turbomachinery monitoring, rotor dynamics analysis, and protection systems on high-value rotating equipment.

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