Understanding Velocity Transducers

Devices

Portable balancer & Vibration analyzer Balanset-1A

€1,975.00 + VAT (if applicable)

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Vibration sensor

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Optical Sensor (Laser Tachometer)

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Balanset-4

€6,803.00 + VAT (if applicable)

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Magnetic Stand Insize-60-kgf

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Reflective tape

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Dynamic balancer “Balanset-1A” OEM

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A velocity transducer — also called a velometer, seismic sensor, or moving-coil sensor — is a self-generating vibration sensor that produces an output voltage directly proportional to vibration velocity, with no external power and no signal conditioning. It works by electromagnetic induction: a magnet suspended on soft springs moves relative to a coil when the housing vibrates, and that relative motion generates a voltage proportional to velocity. As a member of the seismic transducer family — sensors that use a sprung internal mass as an inertial reference — it measures absolute motion of the surface it is bolted to.

Velocity transducers were the dominant vibration sensor from roughly the 1950s to the 1980s and still serve in permanent monitoring installations and some portable instruments. In new designs, however, they have largely given way to accelerometers, which are smaller, span a wider frequency range, and reach the high frequencies needed for bearing-defect detection.

1. Operating Principle

Electromagnetic induction

The mechanism is a direct application of Faraday’s law:

• A permanent magnet is suspended by springs inside a coil.
• Vibration moves the housing and the coil with it.
• Above the sensor’s resonance, the magnet’s inertia keeps it nearly stationary in space.
• That produces relative motion between coil and magnet.
• The motion induces a voltage in the coil (V ∝ velocity).
• The output voltage is therefore directly proportional to vibration velocity.

Self-generating operation

Because the sensor creates its own signal, it needs no external power — a passive, two-wire transduction that is inherently fail-safe, with no power supply to lose. This is the feature that keeps velocity transducers relevant in specific niches even today.

2. Characteristics

Frequency response

• Low-frequency limit: set by the sensor’s natural frequency, typically 8–15 Hz.
• Usable range: above about 2× the natural frequency, so 16–30 Hz minimum.
• High-frequency limit: typically 1–2 kHz.
• Flat response: a wide, flat region across the usable range.
• Best for: 10–1000 Hz — the band where most general machinery faults appear.

Sensitivity

• Typically 10–500 mV per inch/sec (about 400–20,000 mV per mm/s).
• A common value is 100 mV/in/s (≈ 4000 mV/mm/s).
• Higher sensitivity suits low-vibration applications; lower sensitivity suits high-vibration measurements.

Size and weight

• Relatively large — roughly 50–100 mm long and 25–40 mm in diameter.
• Heavy, often 100–500 g.
• Much bulkier than an accelerometer.
• That mass can mass-load and distort the response of lightweight structures.

3. Advantages

Direct velocity output

The transducer measures velocity directly, with no integration step. That matches the way the machine-vibration standards express limits — ISO 20816 (the successor to ISO 10816) is written in RMS velocity — keeps the signal processing simple, and makes it a natural fit for velocity-based vibration severity assessment.

Self-generating and fail-safe

• No power required.
• Simple two-wire connection.
• Cannot fail from a loss of power.
• Lower system cost, with no power supply to specify.

Good low-frequency response

• Usable down to 10–15 Hz, better than many accelerometers.
• Suits low-speed machinery down to about 600 RPM.
• A natural match for applications that live inside its frequency band.

4. Disadvantages

Limited high-frequency response

• Capped at roughly 1–2 kHz.
• Cannot reach the high-frequency bearing defect energy (5–20 kHz).
• Inadequate for envelope analysis.
• This is the decisive limitation against accelerometers.

Size, weight, and fragility

• Large and heavy, hard to mount on small machines and prone to mass-loading light structures.
• Less portable than an accelerometer.
• The internal springs and moving magnet can be damaged by shock or a drop, so the sensor is sensitive to rough handling and needs more care than a solid-state device.

Temperature limitations

• Magnet strength falls as temperature rises.
• Typically limited to about 120 °C.
• Less high-temperature capability than a charge-mode accelerometer.

5. Where Velocity Transducers Are Still Used

• Legacy permanent installations: older turbomachinery monitoring systems, where replacement-in-kind maintains compatibility with the existing wiring and racks.
• Low-frequency applications: very low-speed equipment (below 300 RPM) and any job where the 10–1000 Hz band is sufficient and high frequencies are not needed.
• Specific requirements: situations that genuinely need a self-generating sensor, intrinsically-safe duties where no powered electronics are allowed, or a preference for a direct velocity output.

6. Mounting

Because the sensor is heavy, mounting integrity is critical — a poorly attached velocity transducer adds its own resonance to the data.

• Methods: stud mounting into a tapped hole (most reliable), bracket mounting with adapter plates, or magnetic mounting where the surface is magnetic and the sensor is not too heavy.
• Considerations: rigid mounting is essential, the sensor must be tightened securely so it does not vibrate independently, the mounting face must be flat and clean, and the cable needs strain relief to prevent pull-out.

7. Modern Alternatives and Field Practice

In most new work the accelerometer has won: it is far smaller and lighter, spans a much wider band (around 0.5 Hz to 50 kHz), is better for bearing-defect detection, is more rugged, and costs less. The standard modern approach is therefore to measure acceleration and integrate to velocity, achieving the velocity reading the standards want while keeping every accelerometer advantage — and modern instruments make that integration completely transparent to the user.

This is precisely how a portable balancing analyser operates. The Balanset-1A uses accelerometers at the bearing housings and integrates to velocity internally, so an engineer gets the direct velocity figure a velocity transducer would provide for an ISO 20816 severity check — together with the high-frequency reach and the 1× amplitude and phase needed for field balancing, none of which a 1–2 kHz velocity transducer could deliver.

8. Calibration and Maintenance

• Calibration: verify sensitivity (mV/in/s or mV/mm/s) and frequency response on a shaker table, with annual calibration typical for critical applications.
• Maintenance: handle carefully to avoid drops and shocks, check the cable condition, verify mounting security, test the output periodically, and replace the sensor if its sensitivity or response drifts.

Velocity transducers, though declining in new installations, remain important in existing permanent monitoring systems and in certain low-frequency, self-powered, or intrinsically-safe duties. Understanding how they work, what they do well, and where they fall short is necessary both for keeping legacy systems running and for making an informed sensor selection when a velocity transducer is still the right choice.

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