How does a tachometer measure speed?

Short Answer:

A tachometer is a device used to measure the rotational speed of a shaft or object, usually in revolutions per minute (RPM). It can be mechanical, electrical, or digital depending on the design. It detects how fast an object is rotating and converts that into a readable value.

The tachometer works by counting the number of rotations or pulses over time. In electronic types, a sensor detects each rotation and sends signals to a processor, which calculates and displays the speed. It is commonly used in cars, motors, turbines, and machines.

Detailed Explanation:

Working of a tachometer

A tachometer is an important speed-measuring instrument used in vehicles, electrical motors, turbines, generators, and various industrial machines. It measures how fast a rotating object is spinning. This speed is usually displayed in revolutions per minute (RPM).

There are different types of tachometers, including mechanical, electrical, and digital tachometers. While the designs may vary, the core working principle remains the same: it measures the number of rotations or pulses generated per unit of time and converts that into rotational speed.

1. Mechanical Tachometer:

This is the oldest type and works using mechanical parts.

  • A rotating shaft is connected to the device.
  • As the shaft rotates, centrifugal force causes a spring-loaded pointer or spinning mass to move.
  • The movement is calibrated against speed and shown on a dial.
  • It is simple but can wear out due to friction and moving parts.

2. Electrical Tachometer (AC/DC):

An electrical tachometer uses a tachogenerator that converts mechanical rotation into voltage.

  • A small generator is coupled with the rotating shaft.
  • As the shaft turns, it generates a voltage.
  • The voltage is directly proportional to the speed of rotation.
  • This voltage is then displayed as speed using a voltmeter or digital circuit.

3. Digital or Electronic Tachometer:

This is the most commonly used type today.

  • A sensor, such as an optical, magnetic, or Hall-effect sensor, detects the motion of a rotating part.
  • It produces pulses every time the part completes one revolution.
  • A microcontroller or processor counts the number of pulses in a set time (usually one second).
  • The result is calculated and displayed as RPM on a digital screen.

For example, if the sensor detects 100 pulses in one second, the speed is:
100 pulses/second × 60 seconds = 6000 RPM

Key Components of a Tachometer:

  • Sensing unit: Detects the rotation.
  • Processing unit: Converts signals into speed.
  • Display: Shows the RPM value.
  • Power supply: Required for digital or electrical types.

Applications:

  • Automotive: Shows engine RPM to help in gear shifting and fuel efficiency.
  • Industrial Machines: Monitors motor or spindle speed for safety and control.
  • Generators: Tracks speed to maintain power output.
  • Aerospace and Marine: Used in turbines and engines to prevent over-speeding.

Advantages:

  • Real-time speed monitoring
  • Can be contact or non-contact type
  • Simple to read and use
  • Suitable for both high and low-speed ranges
Conclusion:

A tachometer measures speed by detecting the number of rotations a shaft completes in a given time. Whether mechanical or electronic, it provides a reliable method to monitor RPM in machines and engines. This information helps control operations, ensure safety, and improve performance. Depending on the need, tachometers can be simple analog devices or advanced digital systems with sensors and processors.