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How Does a Tachometer Work?

written by: Ricky • edited by: Lamar Stonecypher • updated: 2/26/2009

Speed of a rotating shaft or motor needs to be measured as this has several practical uses. For example on a ship a navigating officer at the bridge or an engineer in the ECR needs to know what speed and direction the engine is rotating. How is this measured and how does the instrument work?

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    A tachometer is an instrument which is used to measure the revolution speed of any rotating object such as a shaft or motor. It has it uses in the marine engineering field as well and is used to gauge speed of marine diesel engines on board ships and whether they are rotating in the ahead or astern direction. In this article you will learn about the working principle of a tachometer.

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    Types of Tachometers

    Tachometers may seem similar from outside in that they have a display which shows the speed but internally they are based on various different principles and we have several different types of tachometers such as the following

    • Mechanical Tachometers
    • Magnetic Tachometers
    • A.C. Tachometers
    • D.C. Tachometers

    The basic purpose of the tachometer is same irrespective of its construction and we will learn about two types in this article namely a mechanical tachometer and a D.C. tachometer.

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    Mechanical Tachometer

    Mechanical Tachometer   As you can see in the diagram below this type of tachometer is nothing but a linkage of shafts, gears and rotating weights. When the input shaft which is seen horizontal rotates the vertical shaft it also rotates the weights attached to it which are hinged and free to move inward and outwards. The movement of these flyweights rotates a pointer which is calibrated to give the speed in desired units such as RPM.

    Two main drawbacks of this are that the mechanical weights have inertia and hence not very accurate and secondly it does not give an indication of the direction of rotation.

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    D.C. Tachometer

    The D.C. tachometer can be compared to an Ammeter which is used to measure current in that the pointer in this case is deflected in proportion to the current flowing through a moving coil which gets this current from the D.C. generator. This can be seen in the diagram and the permanent magnets give the necessary magnetic flux for the operation while the spinning motion is provided by the input from the shaft or motor whose speed has to be measured. Apart from telling the magnitude this arrangement also has the advantage of telling the direction of rotation of the shaft i.e. in the ahead or astern direction.

    Hence we see that this instrument can be used on the control panel in the engine control room as well as bridge to tell the duty engineer or duty officer about the speed and direction of rotation of the engine.

    DC Tachometer