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DC Tachometer Generator

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DC Tachometer Generator

The DC tachometer generator is a speed measuring device. These are used in Machine tools handling & lifting systems, Paper Manufacturing machines , Textile machines, Glass production lines, Rolling mills, railway industry etc.

The word “Tachometer” is derived from the greek word “Tacho” , which means “speed”. Tachometers are of two types, these are : electrical tachometers & mechanical tachometers.

Electrical tachometers are also known as Tachometer generators. Tachometer generator (or, tachogenerator) is a device that is used for measuring the speed of a shaft or rotating body & converting into a voltage so that it can be measured.

Construction :

In its construction, it has

  1.  a permanent magnet,
  2.  armature,
  3.  commutator,
  4.  brushes,
  5.  variable resistor
  6. a moving coil voltmeter

Schematic Diagram :

Figure 1: Schematic diagram of DC Tachogenerator

The armature of the dc tachometer is kept between a permanent magnetic field. The armature is coupled with the machine whose speed is to be measured.

Now, as the shaft of the machine revolves, the armature of the tachometer revolves with it, in the magnetic field, which produces an EMF.

The commutator collects generated current (i.e. EMF) from the armature & converts the AC EMF to DC EMF. The brushes collect current from the commutator & supplies it to the exernal circuitry of the tachometer (i.e voltmeter & resistor). The EMF induced is measured using a moving coil voltmeter. The moving coil voltmeter has an uniform scale, callibrated in speed directly. So, speed of macine can be measured. 

The EMF generated is proportional to the number of the flux & the speed of the revolving armarture (i.e. the speed of the revolving machine). Now, as the electric field of the permanent magnet is fixed, the EMF generated in this case, is directly proportinal to the speed of the machine.

The variable resistor, connected in series, is used to limit the output current flow to short circuit condition. The polarity of the output voltage indicates the direction of rotation, which is a major advantage of using a DC tachometer generator.

Disadvantages of using a DC tachometer generator are that the commutator & brushes require periodic maintenance & the output is nonlinear.

However, there has been a lot of modifications & advancements in the technology of modern day DC tachometer generators.  It is also evident that with the advent of various modern speed measuring technologies, the use of DC tachometer generators has decreased.

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