A moving-coil velocity sensor generates output voltage through Faraday's law of electromagnetic induction, with the EMF directly proportional to the relative velocity between an internal coil and a permanent magnet [S1][S3]. The device needs no external excitation or IEPE current, and its native mm/s output matches ISO 10816 vibration severity zones for general rotating machinery [S2].
The same induction principle underpins older velocity probes, geophone-style seismic pickups, and most industrial seismic vibration transmitters on a hydraulic turbine, pump, or motor casing [S5][S9]. The mechanical simplicity of one magnet, one coil, and one suspension spring is exactly why the design has survived in condition monitoring for decades.
Faraday's law: the equation that drives the output
The induced voltage in a moving-coil velocity sensor follows e = B·l·v, where B is the radial flux density of the permanent magnet in the working air gap, l is the effective length of the coil turns cutting that flux, and v is the relative velocity between the coil and the magnet [S4][S5]. Because the cut is proportional to v, the output is a true velocity signal, not an acceleration-derived one, which is why Meggitt's low-frequency measurement guidance recommends a velocity transducer with a native velocity output rather than an integrated accelerometer when casing velocity is the parameter of interest [S3].
For a typical industrial geophone, the coil sits on a former wrapped around a cylindrical magnet, and the magnet is suspended on soft helical springs. The working air gap defines the linear motion range over which the turns remain inside the uniform flux zone, and the sensor saturates when the seismic mass leaves that zone [S4]. Sensitivity is usually quoted in mV/mm·s⁻¹ (for example 20–100 mV per mm/s for general-purpose industrial units) and is set by the B·l product and the number of turns.
Inside the housing: magnet, coil, spring, and seismic mass
The transducer body is bolted to the machine casing, so the case accelerates with the structure. A magnet suspended on a soft spring or membrane forms the seismic mass and tries to stay inertial; a coil mounted to the case therefore moves relative to the magnet whenever the casing vibrates [S2][S5]. The relative motion of the coil through the radial field of the magnet is what produces the EMF, exactly as a loudspeaker works in reverse [S6].
Because the spring is tuned to a low natural frequency (commonly 4.5 Hz, 8 Hz, or 10 Hz for general-purpose units), the magnet stays nearly stationary in space over the sensor's flat operating band, and the coil's velocity matches the casing velocity to within a few percent. Above the natural frequency the magnet's inertia drives the relative motion, which is why these units work as seismometers but lose accuracy at very high frequency [S3][S5]. A proper vibration sensor chain therefore specifies the sensor's natural frequency, damping ratio, and useful upper limit, not just its sensitivity.
Self-powered output and why no IEPE current is needed
Piezoelectric accelerometers need an IEPE constant-current source and a charge amplifier because the sensing element is a crystal that generates charge only while stressed. A moving-coil velocity sensor is a true generator: the relative motion of the magnet and coil cuts flux and the output is a real voltage, so the cable carries the signal directly to the data collector, signal conditioner, or 4-20 mA vibration transmitter [S1][S2][S8].
This is also why the unit is intrinsically safe in many hazardous areas, since the housing cannot store enough energy to ignite a gas mixture, and the cable is a passive two-wire circuit. On a bearing housing or gearbox cover the same output can be read by a portable vibration analyzer for route-based data collection or fed into a 4-20 mA vibration transmitter loop for continuous machinery protection.
Operating range, orientation, and accuracy limits
Typical industrial moving-coil velocity sensors cover roughly 2 Hz to 1 kHz, with a flat (within ±10%) velocity response from about 10 Hz to 500 Hz for general-purpose units and from 1 Hz upward for seismic geophones. Sensitivity is set by the B·l product, the working air gap length, and the number of turns in the coil, while the lower frequency limit is set by the suspension stiffness and the seismic mass, and the upper limit is set by the magnet leaving the uniform flux region of the air gap [S4][S5][S9].
Orientation is a hard constraint: a sensor calibrated for vertical mounting gives a different output on a horizontal mount because the suspension spring's gravity preload changes, and a sensor designed for horizontal mounting is inaccurate on a vertical surface [S10]. Damping ratio is the third design lever, and published studies report that sensitivity of an electromagnetic velocity sensor peaks when the damping ratio is around 0.75 of critical, with the lower 3 dB point pulled down to a few Hz when damping is optimised [S4]. At very low frequencies below the natural frequency the output rolls off at 12 dB/octave, which is why a 4.5 Hz geophone and a 10 Hz industrial unit are not interchangeable for slow-speed machinery such as hydro generators, rotary kilns, or agitators [S3].
Where moving-coil velocity wins, and where it loses
For low-speed and mid-speed rotating equipment, hydro turbines, paper machine rolls, cooling-tower fans, and large induction motors, a moving-coil velocity sensor gives the correct native output for ISO 10816-3 severity assessment and survives decades of operation with no active electronics [S1][S3]. On a slow-speed agitator running at 60-300 rpm, the casing vibration sits inside the sensor's flat band, while a small IEPE accelerometer would be integrating noise rather than measuring signal at those frequencies [S3].
For high-frequency bearing and gear-mesh work above 1-2 kHz, an accelerometer is the right tool because the moving-coil magnet has already left its linear flux region, sensitivity is rolling off, and the moving mass limits the mechanical bandwidth. Size is the second constraint: geophone-style units are larger and heavier than MEMS or piezoelectric accelerometers, so they are not used in dense arrays or on small lightweight structures. Comparison of the three main transducer types in industrial monitoring: accelerometer on frequency range and bearing defect detection; moving-coil velocity on ISO 10816 compliance and self-powered simplicity; eddy-current proximity probe on shaft-relative displacement and turbomachinery clearance [S2].
Standards, calibration, and signal conditioning
Industrial moving-coil velocity sensors are commonly specified to API 670 for machinery protection on turbines, generators, and compressors, with ISO 10816-3 used for the vibration severity zones that the mm/s RMS output is read against, and ISO 2954 for the calibration of vibration measurement instruments [S2][S3]. On seismic and structural-health-monitoring work, the same physical sensor is described under the broader category of geophone, and its calibration is reported against a known reference shaker at a defined frequency and amplitude.
Signal conditioning is minimal: a buffered preamp, a high-pass filter above the natural frequency, and integration back to displacement if displacement is the required parameter, since the raw output is already velocity [S3][S5]. For loop-powered monitoring, the output of the velocity sensor feeds a 4-20 mA vibration transmitter that maps the RMS velocity into a process current, which is the standard interface into a PLC or SCADA for continuous machinery protection. Useful internal anchors for further reading: electromagnetic velocity sensor design, industrial velocity transmitter output, and vibration analyzer input chain. The same induction principle shows up in dynamic balancing machine pickup heads, where a velocity-mode transducer resolves both amplitude and phase of the heavy-spot angle on a rotor.
Trackable next signals: monitor the API 670 / ISO 10816 revision cycle in 2026-2027 for any change to the mm/s RMS severity limits, and watch for piezo-velocity hybrids (e.g. Metrix SV6300) that combine the high bandwidth of an accelerometer with the self-powered simplicity of a moving-coil design.