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DC tachometer generator Vs AC tachometer generator | Difference between DC tachometer generator and AC tachometer generator

This page compares DC tachometer generator vs AC tachometer generator and mentions difference between DC tachometer generator and AC tachometer generator.

What is tachometer?: It is the instrument which measures angular velocity of shaft. It performs measurement by registering total number of revolutions during the period of contact or by indicating directly number of revolutions per minute.

DC tachometer generator

DC tachometer generator
Fig:1 DC tachometer generator

DC tachometer consists of small armature which is coupled to machine whose speed needs to be measured. The armature revolves in field generated by permanent magnet.

Emf generated is proportional to the product of flux and speed. As flux in the magnet is constant, generated voltage is proportional to speed. The polarity of voltage output indicates direction of rotation.

AC tachometer generator

AC tachometer generator
Fig:2 AC tachometer generator

AC tachometer has rotating magnet which can be either permanent magnet or electromagnet.

The coil is wounded on stator. The rotation of magnet causes emf to be induced in the coil of stator. Amplitude and frequency of emf is proportional to speed of rotation.

Following table summarizes difference between DC tachometer and AC tachometer.

DC tachometer generator AC tachometer generator
It is used for measurement of speed in both the directions. It is used for measurement of speed in one direction only.
Consists of a horse shoe type of permanent magnet. Consists of arrangement with single stator and single rotor or squirrel cage setup.
Speed is measured with moving coil voltmeter. Speed is measured with moving coil instrument having permanent magnet or electromagnet.


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