How Does a Tacho Generator Work? Inside the Electro-Mechanical Magic
Tacho generators—sometimes called tachogenerators or simply “tachos”—do the deceptively simple job of turning rotary speed into an electrical signal. Yet that voltage output is the heartbeat of countless closed-loop control systems, from steel-mill drives to surgical robots. In this deep dive we’ll peel back the housing, trace the physics, and show why the humble tacho still outperforms many “newer” sensors in harsh, high-power environments.
1. The Core Principle: Faraday’s Law in Action
At its heart a tacho generator is a miniature dynamo:
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Permanent magnet establishes a fixed magnetic field (????).
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Rotor (armature) winding spins within that field at angular speed ω.
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Conductors in the rotor cut magnetic flux lines, inducing an electromotive force (EMF).
The induced voltage is proportionate to speed:
Eout=KT×ωE_\text{out} = K_{\text{T}} \times \omegaEout?=KT?×ω
Where:
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E<sub>out</sub> = output voltage (V)
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K<sub>T</sub> = tachogenerator constant (V · s/rad, often quoted in V/1000 rpm)
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ω = angular velocity (rad/s)
Because the proportionality is nearly linear over the rated speed range, the voltage becomes a direct analogue of shaft speed, perfect for PID controllers and drive feedback loops.
2. Key Electro-Mechanical Components
| Component | Purpose | What Can Go Wrong |
|---|---|---|
| Permanent magnet | Provides constant flux | Demagnetisation at high temps |
| Armature windings | Generate EMF | Worn insulation, open circuits |
| Commutator & brushes (brushed types) | Rectify AC to DC output | Brush wear, noise, carbon dust |
| Bearings | Support rotor | Vibration, grease breakdown |
| Housing & seals | Protect internals | Ingress, corrosion |
Brushless tachos replace the commutator with rotating magnets and stationary coils, eliminating brush maintenance and reducing electrical noise—ideal for servo-grade accuracy.
3. Signal Characteristics That Matter
| Characteristic | Why It’s Important | Typical Values |
|---|---|---|
| Voltage constant (K<sub>T</sub>) | Determines controller scaling | 7–60 V per 1000 rpm |
| Ripple | AC “ripple” rides on DC; too much confuses drives | <1 % on high-quality units |
| Linearity error | Deviation from straight-line V-to-speed plot | ±0.15 % FS to ±1 % FS |
| Output impedance | Affects cable length & loading | 50 Ω to 600 Ω |
| Temperature coefficient | Drift per °C | 0.02 % / °C typical |
4. From Raw Voltage to Closed-Loop Control
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Rotation of the motor shaft spins the tacho.
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Voltage proportional to speed appears across the output terminals.
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Drive amplifier / PLC analogue input reads that voltage.
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Controller compares actual speed (voltage) to set-point, adjusts motor current.
Because the tacho is an analogue transducer, there’s virtually zero latency—critically important for high-bandwidth servo loops or tension control on web-handling machines.
5. Why Still Use a Tacho in 2025?
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Noise immunity: Pure DC output laughs at oily, EMI-heavy factory floors where optical or Hall sensors can falter.
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High power compatibility: No fragile electronics at the rotor tip; only copper, iron, and magnets.
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True speed–proportional voltage: Perfect for vintage drives that cannot ingest pulse trains.
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Cost-effective retrofits: Swap-in replacements keep legacy DC motors alive without rewriting PLC code.
Encoder versus tacho? Encoders win for absolute digital accuracy and multiturn position. Tachos win for robustness, simplicity, and analogue control.
6. Sizing & Selection Checklist
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Speed range: Choose K<sub>T</sub> so max rpm ≈ 90 % of full-scale voltage.
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Mounting style: Foot, flange, or hollow-shaft adapters for easier retrofits.
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Environmental rating: Look for IP65+ sealing and -40 °C to +120 °C ranges if outdoors.
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Brushless vs. brushed: Select brushless for high-precision servo loops or zero-maintenance sites.
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Output polarity: Some drives require positive-only voltage; others accept bidirectional.
7. Maintenance & Troubleshooting Tips
| Symptom | Possible Cause | Quick Check |
|---|---|---|
| Zero-speed voltage present | Residual magnetism or offset | Re-zero controller, inspect wiring screen |
| Erratic signal spikes | Worn brushes, commutator dirt | Visual brush check, clean slots |
| Low output voltage | Partial demagnetisation, open coil | Compare K<sub>T</sub> to spec with handheld tach & DMM |
| High ripple | Bent shaft, bearing wear | Vibration analysis, bearing test |
A 15-minute quarterly inspection—brush length, bearing play, cable strain relief—can extend service life well past 20 years.
Conclusion: The Magic Is in the Marriage of Physics and Practicality
The tacho generator’s astonishing staying power lies in its elegant application of Faraday’s law packaged for real-world, industrial abuse. It turns raw rotation into a clean, scalable voltage that analogue and digital drives alike can understand instantly. Whether you’re retrofitting a decades-old DC motor in a sawmill or engineering a brand-new tension loop for a high-speed slitter, a properly specified tacho generator might still be your simplest, most reliable path to precision speed control.
Need guidance choosing or integrating a tacho generator? Get in touch—our engineers are ready to help translate your rpm requirements into rock-solid feedback solutions