Technical Foundation
What Is a Magnetic Pickup?
A magnetic pickup (also called a magnetic speed sensor or variable reluctance sensor) is a passive sensor containing a permanent magnet and coil winding that generates an AC voltage pulse each time a ferrous gear tooth passes near its tip, without requiring any external power supply to the sensor itself. The output frequency is directly proportional to rotational speed and the number of gear teeth, making it a standard method for speed sensing, tachometer input and overspeed shutdown on turbines, engines and rotating machinery.
Because the sensor is passive, its signal amplitude depends on gear tooth speed, tooth geometry, and the air gap between the sensor tip and the gear — signal strength drops at low speeds and can fall below a receiving instrument's minimum trigger threshold, which is why magnetic pickups are generally better suited to medium and high speed sensing than very low speed applications where an active (powered) sensor may be preferred instead.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Thread Size | Commonly 3/4-16 UNF, 7/8-14 UNF, or metric | Must match the tapped mounting boss on the gear housing |
| Coil Resistance | Commonly 100-1000+ ohms depending on model | Affects signal amplitude and compatibility with the receiving instrument's input impedance |
| Air Gap Sensitivity | Signal strength drops as gap increases or speed decreases | Gap must be set within the manufacturer's spec range to maintain a reliable signal at minimum operating speed |
| Frequency Range | Output frequency proportional to speed x tooth count | Must fall within the receiving tachometer or controller's rated input frequency range |
| Environmental Rating | Standard, high-temperature, explosion-proof (hazardous location) | Determines suitability for the installation's temperature and hazardous area classification |
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Applications
Where Magnetic Pickups Get Used
Magnetic pickups are specified wherever gear or shaft speed needs to be sensed without external power to the sensor itself.
Turbine Overspeed Protection
Sensing shaft speed for overspeed trip and shutdown protection on steam, gas and hydro turbines.
Diesel & Gas Engine Speed Sensing
Providing tachometer and governor feedback signals on industrial and marine engines.
Compressor & Pump Monitoring
Speed sensing on rotating compressor and pump shafts for control and condition monitoring systems.
Conveyor & Material Handling
Belt and drive shaft speed sensing for conveyor speed monitoring and slip/stall detection.
Wind Turbine Drivetrains
Rotor and generator shaft speed sensing feeding turbine control and overspeed protection systems.
Oil & Gas Rotating Equipment
Explosion-proof rated speed sensors on rotating equipment in hazardous location process areas.
FAQ
Magnetic Pickups Questions, Answered
How does a magnetic pickup generate a signal without external power?
A magnetic pickup contains a permanent magnet with a coil wound around it; as each ferrous gear tooth passes the sensor tip, it disturbs the magnetic field, inducing an AC voltage pulse in the coil purely through electromagnetic induction. This passive operation means no power supply is needed at the sensor, only wiring to carry the generated signal to the receiving instrument.
What air gap should I set between the sensor and the gear?
The correct air gap is specified by the sensor manufacturer, typically as a range (for example 0.010-0.030 inches), and setting it within that range balances signal strength against the risk of the gear tooth contacting the sensor tip. Too large a gap weakens the signal, particularly at low speeds, while too small a gap risks mechanical contact and sensor damage as the gear rotates.
Why is signal strength a bigger concern at low speeds?
Because the sensor generates its signal through the rate of change of magnetic flux as a tooth passes, slower tooth passage produces a weaker induced voltage. At very low shaft speeds, the signal amplitude can fall below the minimum threshold the receiving tachometer or controller needs to reliably detect, which is why magnetic pickups are better suited to medium/high speed applications, with active powered sensors used instead for very low speed sensing.
Do I need an explosion-proof rated magnetic pickup?
An explosion-proof or hazardous-location rated magnetic pickup is required wherever the installation falls within a classified hazardous area, such as around oil and gas process equipment, and the sensor's rating must match the specific area classification (Class/Division or Zone) at the installation site, not just be generically labeled 'explosion proof.'
What determines the coil resistance I should choose?
Coil resistance affects the signal's voltage amplitude and its compatibility with the input impedance of the receiving tachometer, speed switch or controller — the receiving instrument's manufacturer typically specifies a compatible sensor coil resistance range, so the pickup should be matched to that instrument's input specification rather than chosen independently.
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