Understanding Velocity Transducers
Definition: What is a Velocity Transducer?
Velocity transducer (also called velometer, seismic sensor, or moving-coil sensor) is a self-generating vibration sensor that produces an output voltage directly proportional to vibration velocity without requiring external power or signal conditioning. It operates on electromagnetic induction principles—a magnet suspended on springs moves relative to a coil when vibration occurs, generating voltage proportional to the relative velocity between coil and magnet, which equals the vibration velocity.
Velocity transducers were the dominant vibration sensor from the 1950s-1980s and are still used in permanent monitoring installations and some portable instruments. However, they have been largely replaced by accelerometers in new installations due to accelerometers’ smaller size, wider frequency range, and higher frequency capability needed for bearing defect detection.
Operating Principle
Electromagnetic Induction
- Permanent magnet suspended by springs inside coil
- Vibration moves housing and coil
- Magnet’s inertia keeps it relatively stationary (above resonance)
- Relative motion between coil and magnet
- Motion induces voltage in coil (Faraday’s law: V ∝ velocity)
- Output voltage directly proportional to vibration velocity
Self-Generating
- No external power required
- Passive transduction
- Simple connection (two wires)
- Inherently fail-safe (no power failure issues)
Characteristics
Frequency Response
- Low-Frequency Limit: Natural frequency (typically 8-15 Hz)
- Usable Range: Above 2× natural frequency (16-30 Hz minimum)
- High-Frequency Limit: Typically 1-2 kHz
- Flat Response: Wide flat region in usable range
- Best For: 10-1000 Hz (general machinery frequencies)
Sensitivity
- Typical: 10-500 mV per inch/sec (400-20,000 mV per mm/s)
- Common: 100 mV/in/s or 4000 mV/mm/s
- Higher sensitivity for low-vibration applications
- Lower sensitivity for high-vibration measurements
Size and Weight
- Relatively large (50-100 mm long, 25-40 mm diameter)
- Heavy (100-500 grams typical)
- Much larger than accelerometers
- Mass can affect measurement on lightweight structures
Advantages
Direct Velocity Output
- Measures vibration velocity directly (no integration needed)
- Matches ISO standards specification (RMS velocity)
- Simple signal processing
- Natural for velocity-based analysis
Self-Generating
- No power required
- Simple two-wire connection
- Cannot fail from power loss
- Lower system cost (no power supply needed)
Good Low-Frequency Response
- Usable to 10-15 Hz (better than many accelerometers)
- Suitable for low-speed machinery (down to ~600 RPM)
- Natural for applications matching frequency range
Disadvantages
Limited High-Frequency Response
- Typically limited to 1-2 kHz maximum
- Cannot detect high-frequency bearing defects (5-20 kHz)
- Inadequate for envelope analysis
- Major limitation vs. accelerometers
Size and Weight
- Large, heavy sensors
- Difficult to mount on small machines
- Mass loading affects lightweight structures
- Less portable than accelerometers
Fragility
- Internal springs and moving magnet can be damaged by shock
- Sensitive to handling abuse
- Can be damaged by dropping
- More maintenance than solid-state accelerometers
Temperature Limitations
- Magnet strength decreases with temperature
- Typically limited to 120°C
- Less capability than charge-mode accelerometers
Where Still Used
Legacy Permanent Installations
- Older turbomachinery monitoring systems
- Replacement-in-kind for existing installations
- Maintains compatibility with existing systems
Low-Frequency Applications
- Very low-speed equipment (< 300 RPM)
- Where frequency range 10-1000 Hz adequate
- Simple velocity monitoring without need for high frequencies
Specific Requirements
- Where self-generating advantage needed
- Intrinsically safe requirements (no power)
- Direct velocity output preferred
Mounting
Methods
- Stud mounting to tapped holes (most common)
- Bracket mounting with adapter plates
- Magnetic mounting (if surface magnetic and sensor not too heavy)
Considerations
- Rigid mounting essential (sensor heavy)
- Secure tightly to prevent sensor vibration
- Verify mounting surface flat and clean
- Cable strain relief to prevent pulling
Modern Alternatives
Why Accelerometers Preferred
- Much smaller and lighter
- Wide frequency range (0.5 Hz – 50 kHz)
- Better for bearing defect detection
- More rugged
- Lower cost
- Industry trend toward accelerometers
Integration as Alternative
- Measure acceleration, integrate to velocity
- Achieves velocity measurement with accelerometer advantages
- Modern instruments make integration transparent
Calibration and Maintenance
Calibration
- Shaker table calibration
- Verify sensitivity (mV/in/s or mV/mm/s)
- Check frequency response
- Annual calibration typical for critical applications
Maintenance
- Handle carefully (avoid drops and shocks)
- Check cable condition
- Verify mounting security
- Test output periodically
- Replace if sensitivity or response changes
Velocity transducers, while declining in new installations, remain important sensors in existing permanent monitoring systems and certain low-frequency applications. Understanding their operation, advantages, and limitations is necessary for maintaining legacy systems and making informed sensor selection decisions when velocity transducers might still be the optimal choice for specific low-frequency, self-powered, or compatibility requirements.
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