DC Tachometer Generator vs AC Tachometer Generator

This article explores the differences between DC and AC tachometer generators, both of which are used to measure the angular velocity of a rotating shaft.

What is a Tachometer?

A tachometer is an instrument that measures the rotational speed of a shaft. It can do this by either counting the total revolutions during a specific period or by directly displaying the revolutions per minute (RPM).

What is a 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.

Fig 1: DC tachometer generator

What is a 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.

Fig 2: AC tachometer generator

Difference between DC Tachometer Generator and AC Tachometer Generator