What is the Core Working Principle for an electric tachometer?

Common Types & How They Generate the Signal:

1.  AC Tachogenerator:

    A small AC generator mounted on the rotating shaft.

    As the shaft rotates, it spins a rotor inside a stator, inducing an AC voltage in the stator windings.

    The frequency (Hz) of this AC voltage is directly proportional to the RPM (`Frequency = (RPM  Number of Poles) / 120`).

    The magnitude (Voltage) is also roughly proportional to speed but can be affected by load. Frequency measurement is preferred for accuracy.

2.  DC Tachogenerator:

    *   A small DC generator mounted on the shaft.

    *   Rotation induces a DC voltage in its windings via commutation.

    *   The magnitude (Voltage) of this DC output is very closely proportional to the RPM.

    *   Provides a simple, direct analog voltage signal. Polarity indicates direction.

3.  Magnetic (Variable Reluctance) Pickup:

    *   A non-contact sensor mounted near the rotating part.

    *   The rotating part has a toothed wheel (reluctor wheel) or protruding ferromagnetic targets.

    *   As each tooth/target passes the sensor's magnetic field, it causes a change in magnetic flux (reluctance).

    *   This change induces a small AC voltage pulse in the sensor coil.

    *   The frequency (Hz) of these pulses is directly proportional to RPM (`Frequency = (RPM Number of Teeth) / 60`).

4.  Hall Effect Sensor:

    *   A non-contact solid-state sensor.

    *   Mounted near a rotating target with magnetic elements (magnets or ferrous material interrupting a magnetic field).

    *   When a magnetic element passes the sensor, it triggers the Hall element, producing a clean digital pulse (on/off).

    *   The frequency (Hz) of these pulses is directly proportional to RPM.

5.  Optical Sensor:

    *   Uses a light source (LED) and a photodetector.

    *   The rotating shaft has a reflective mark, slots, or an encoder disk.

    *   Each pass of a mark/slot interrupts or reflects the light beam, causing the detector to generate a pulse.

    *   The frequency (Hz) of these pulses is proportional to RPM.

Key Components of an Electric Tachometer System:

1.  Sensor/Generator: The part that physically interacts with the rotation to produce the raw electrical signal (voltage, pulses).
2.  Signal Conditioning Circuitry: Processes the raw sensor signal. This may involve:

    *   Amplification: Boosting weak signals (e.g., from magnetic pickups).

    *   Filtering: Removing electrical noise.

    *   Shaping: Converting sine waves into clean digital pulses.

    *   Frequency-to-Voltage Conversion (F/V):Converting pulse frequency into an analog voltage proportional to RPM (common for digital sensors feeding analog displays).

    *   Counting & Timing:For digital systems to measure pulse frequency.

3.  Display/Readout: Presents the RPM value to the user. This can be:

    *   Analog: A moving-coil meter with a needle sweeping across a dial (driven by a DC voltage from a generator or F/V converter).

    *   Digital: An LCD, LED, or vacuum fluorescent display showing numerical digits (driven by a microprocessor counting pulses or reading a digital signal).

4.  Wiring/Cabling: Connects the sensor to the processing/display unit.

Common Applications:

*   Automotive: Dashboard tachometers (measuring engine crankshaft RPM).

*   Aircraft: Engine and propeller RPM monitoring.

*   Industrial Machinery: Monitoring motors, pumps, fans, conveyors, turbines, spindles.

*   Laboratory Equipment: Centrifuges, stirrers.

*   Power Tools: Drills, saws.

*   Marine Engines.

*   Process Control Systems.

In Summary:

Electric tachometers provide a reliable, accurate, and flexible means of measuring rotational speed by transforming mechanical rotation into a usable electrical signal. Their ability to offer remote sensing, compatibility with various display types and data systems, and robustness make them the standard solution for RPM measurement across countless industries