Tachometer Working Principles & Type


         A tachometer is a device that measures the speed of any rotating object, such as a disc or shaft found in a motor or other equipment. However, digital displays are becoming more prevalent. The analogue dial shows the revolutions per minute (RPM).

What the word tachometer meant by?

The words tachos, which means “speed,” and meter, which means “measure,” are both Greek words.

Need of tachometer

  • It is necessary to measure the speed of a rotating shaft or motor in equipment and machinery. E.g A navigating officer on the bridge or an engineer in the ECR (Engine Control Room) of a ship must know the engine’s rotational speed and direction.
  • The tachometer of a car lets the driver choose the best gear and throttle settings based on the road’s circumstances.Consequently, it helps the driver to avoid going faster than the engine’s subsystems, such its spring-retracted valves, can handle.

Working Principle of Tachometer

  • The rotational speed (rpm) of an engine or other rotating machinery is measured using a tachometer. It functions using the electromagnetic induction theory.
  • A sensor or pickup coil, a magnet, and a display mechanism make up the tachometer.
  • Typically, a rotating component, such as an engine flywheel or a machine’s rotating shaft, is located close to the sensor or pickup coil.
  • The component produces a shifting magnetic field as it rotates. These modifications are picked up by the pickup coil, which turns them into electrical pulses.
  • The display mechanism receives the electrical pulses that the pickup coil has generated. The pulses are used to move a needle on a calibrated dial in older analogue tachometers, which shows the rotational speed.
  • The pulses are processed by electronic circuits and shown digitally in contemporary digital tachometers. The rotational speed of the component being measured is correlated with the frequency of the electrical pulses.
  • The tachometer precisely calculates the rpm by counting the number of pulses per unit of time. This data is essential for tracking the speed and efficiency of engines.

Construction of tachometer

         The components of a tachometer include a dial, a needle (for reading indication), and high and low marks for safe and dangerous levels.

Types of tachometers

         Tachometers can be classified based on various parameters:

Based on acquiring data is known as data acquisition

  • Contact
  • Non-Contact

Based on data type

  • Analog
  • Digital

Based on techniques used in Measurement

  • Time Measurement
  • Frequency Measurement

Based on power

  • Mechanical
  • Electrical

Tachometer based on Data acquisition

         Data acquisition is the method of acquiring an electrical or physical parameters such as voltage, current, temperature, pressure, or sound

Contact type tachometers

         Contact type tachometers are those that stay in close proximity to the revolving shaft. It is typically preferred in cases where a machine repair is required.This kind of tachometer uses a magnetic sensor or an optical encoder.

Non-Contact type tachometer

      The rotating shaft does not have to be physically in contact with these tachometers. A laser or optical disc is the primary non-contact type application in mobile devices.

Contact Type vs Non Contact Type Tachometer

Contact TypeNon Contact Type
 The rotating shaft and tachometer must be physically touchingThe rotating shaft does not have to be in direct physical contact with the tachometer.
A magnetic or optical encoder is typically fastened to the tachometer’s shaft. In most cases, a laser or an IR beam is utilised to read an optical disc that is coupled to a revolving shaft.

Tachometer based on Display data Type

Analog Tachometer

  • These tachometers are quite easy to use, making it simple to calculate speed from them. These are made out of a needle that displays the reading at the moment and has markings that indicate a safe and harmful level.
  • They are unable to compute statistics like deviation and average or save the readings. Tachometers, for instance, are analogue devices that display output in numerical form, such as car speed, on bicycles and other two-wheeled vehicles.

Analog Measurement Techniques

         An external frequency to voltage converter is typically used to convert speed to voltage.The tachometer can also function as a generator, generating a voltage corresponding to the shaft’s speed. An analogue voltmeter then shows this voltage.

Digital Tachometer

Compare Analog tachometer and Digital Tachometer

Analog TachometerDigital Tachometer
These tachometers are quite straightforward, making it simple to calculate speed.These are comprised of an LCD or LED readout.
They are unable to compute information like deviation and average speed from the readings.They also have storage space in their memory which also employs to provide precise measurement and statistical processes.

Tachometer based on Measurement technique

     Two types of tachometers are distinguished by measurement method:

Based on measurement of time

     The speed is determined by the time difference between the incoming pulses, which is measured by the time measurement device. The duration to take a reading depends on the speed and rises with decreasing speed, making it perfect for low speed readings.

Based on Frequency calculation

     By detecting the frequency of the incoming pulses, the frequency measurement gadget determines speed. It is perfect for measurements at rapid speeds. The speed of the shaft’s rotation has no bearing on how long it takes to take a reading.

Tachometer based on Power

      The tachometer can be categorised into the following groups based on the type of power it uses:

  • Mechanical Tachometer
  • Electrical Tachometer

Mechanical Tachometer

         Mechanical tachometers can compress or stretch  a mechanical spring by focusing on the fact that the centrifugal force on a rotating mass is inversely proportional to rotational speed.These only use mechanical components and motion.

  • Revolution Counters.
  • Centrifugal Force Tachometer


  1. Mechanical tachometers put pressure on the revolving shaft to which they are linked, absorbing power in the process. Not suited to shafts using little power.
  2. More upkeep is necessary.
  3. Defective components produce inaccurate readings.
  4. A counter is typically used to tally the number of revolutions, and a stopwatch is used to measure the passage of time.

Revolution counter

  • Instead of measuring instantaneous rotational speed, revolution counters measure an average of reasonable speed.
  • It is made up of a worm gear, which is often mounted on a spindle. There are two dials on it: an inner dial and an outer dial.
  • The outside dials represent one revolution of the spindle, whereas the inner dials represent one of the outer dials’ revolutions.
  • A stopwatch that serves as a time indicator is attached to the revolution counter on the tachometer.

Advantages of Revolution Counter

  • Efficient to use.
  • The shaft doesn’t need to be attached.

Disadvantages of Revolution Counter

  • Speed is restricted at a modest rate.
  • The occurrence of operational error.

Centrifugal force tachometer

         A shaft has a sliding collar attached on it. The sliding coupling is moved along the shaft against a counterbalancing spring by the hinged arms of the coupling, which carry mass and spread apart when the shaft rotates.

         An arm mechanism transmits the coupling’s location on the shaft, which relies on rotational speed, to an indicator pointer whose dial is calibrated in revolutions per minute. The regulated mechanism may drive the tachometer shaft directly or indirectly through a flexible shaft.

Electrical Tachometer

       Electrical tachometers primarily rely on an electrical signal produced in direct proportion to the shaft’s rotating speed. There are numerous different designs available depending on the type of transducer being utilised.

There are various types of electrical tachometers.

  • The eddy-current or drag type, which produces eddy currents that are proportional to angular speed when a magnet is rotated along with the shaft being measured, is frequently employed in automotive speedometers.
    • Electric generator tachometers produce either an alternating current or a direct current in order to function.

Eddy Current tachometer

  • A permanent magnet serves as the stator, and a solid metallic cylinder with low resistance serves as the rotor, driving the pointer and scale arrangement. The rotor is also attached to a shaft that can make contact with rotating machines whose rpm has to be determined.
  • It is known that when a low resistance metal rotates in a strong magnetic field, eddy current gets induced in the low resistance metal, opposing the cause of action, holding the pointer in equilibrium position and showing some reading that will be calibrated for the corresponding speeds, so it shows the correct rpm in which the machine rotates.

D.C. Tachometer

  • It stands for direct current. This tachometer is essentially a generator of permanent magnets.
  • The output of these instruments ranges from 2 to 10 volts for every 1,000 revolutions per minute (RPM).
  • A voltmeter with a high resistance value is constructed with calibration in RPM for the purpose of Speed indication.
  • The e.m.f produced by a DC generator is dependent on the following two factors
    • Field excitation
    • Speed
  • The armature is linked to the shaft whose speed is being monitored.
  • To measure the generated voltage, a moving coil voltmeter is connected across the brushes. In order to reduce the current flowing through the voltmeter, the variable resistance R is included. The voltmeter can be calibrated in terms of speed (rpm), as voltage is proportional to speed.

A.C. Tachometer

  • The word “AC” means alternating current.
  • This apparatus comprises of a stationary winding and a rotating permanent-magnetic field.
  • The generated frequency and voltage output stay proportional to the speed of rotation.
  • The a.c. tachometer generators are used to get around some of the problems with d.c. tachometers.
  • The spinning magnet used in these tachometer generators could be an electromagnet or a permanent magnet. Because the coil is wound in the described manner, there are no commutator-related issues (as in a d.c. tachometer).
  • The stator coil induces an e.m.f as a result of the magnet’s rotation. This electromagnetic field’s frequency and amplitude are inversely correlated with rotational speed.
  • As a result, the induced voltage’s frequency or amplitude can be utilized to calculate the rotational speed.

Applications of Tachometer

  • Tachometer forms an important part of the machine, used in our daily life.
  • The tachometer in vehicles enables the driver to know whether it is exceeding the speed limit or not and provides safety while driving.

Tachometer in Automobiles

  • Automobile uses Tachometers to measure the rotational speed of mechanical devices, which is typically expressed in RPMs.
  • They are employed to keep an eye on the RPMs in vehicles because running the engine at excessively high RPM rates can significantly reduce engine life.
  • In some instances, a small generator is fastened to the engine’s drive shaft, and the device’s electric current is used to calculate the engine’s revolutions per minute (RPM).
  • Simply measuring the rate at which the ignition system produces sparks to the engine might also be done using this equipment. The number of revolutions per minute an engine produces is shown on a car’s tachometer.

Tachometer in Airplanes

  • Utilise in Flight tachometer is normally required for each engine in an aeroplane, and additional tachometers are required for propeller-powered aircraft.
  • The RPMs at which an airplane’s engines and propellers typically turn are different.
  • The pilot or crew of the aircraft can detect any issues with specific parts by using separate instruments for each type.
  • An aircraft with multiple piston engines uses a different tachometer for each engine.

Laser Instruments

  • Traditional tachometers demand direct physical contact between the measuring objects and the instruments.
  • A laser may be able to collect measurements at a distance in applications where this is not practicable due to technical or safety concerns.
  • A focused beam of light is pulsed against the revolving component to operate laser devices.
  • One reflective spot will be present on the revolving element, and the sensor gauges how quickly the light beam rotates. returns in reflection.
  • They might be fixed components of the system or portable for sporadic spot measurements.

Medical Applications

Even medicine can benefit from using tachometers. A medical expert can use this equipment to determine the rate of blood flow by inserting a small, turbine-like device called a haematachometer into an artery or vein. This might be employed to identify circulation issues like blocked arteries.

Marine Ships

         Naval Ships Marine fleet, to gauge the rotational speed of the ships’ marine diesel engines. This instrument displays the ship’s rotational direction.

Tachometer vs Speedometer

A tachometer measures the rotational speed (rpm) of an engine or other rotating machinery.A speedometer measures the speed of a vehicle in terms of its linear velocity (mph or km/h).
It indicates how fast the engine’s crankshaft is spinning.It indicates how fast the vehicle is moving in relation to the ground.
Tachometers are commonly used in cars, trucks, and boats to monitor the engine’s speed and prevent damage from over-revving.Speedometers are commonly used in cars, trucks, motorcycles, and bicycles to display the vehicle’s speed to the driver.
A tachometer measures the rotational speed of an engine or machine.A speedometer measures the linear velocity of a vehicle
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