DC vs AC Tachometer Generators: Key Differences

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).

DC Tachometer Generator

How it Works

The DC tachometer generator (as shown in Fig. 1) uses a small armature connected to the machine whose speed is being measured. This armature spins within a magnetic field created by a permanent magnet. The voltage (electromotive force or EMF) generated is directly proportional to the product of the magnetic flux and the speed of rotation. Since the flux of the permanent magnet is constant, the generated voltage is directly proportional to the speed of the shaft.

Fig 1: DC tachometer generator

Key Feature

The polarity of the output voltage from a DC tachometer indicates the direction of rotation, making it suitable for applications where this information is important.

AC Tachometer Generator

How it Works

The AC tachometer generator (as depicted in Fig. 2) utilizes a rotating magnet, which can be either a permanent magnet or an electromagnet. The coil is wound around the stator. As the magnet rotates, it induces an EMF in the coil of the stator. The amplitude and frequency of this induced EMF are proportional to the speed of rotation.

Fig 2: AC tachometer generator

Key Features

Unlike the DC tachometer, the AC tachometer is primarily used for measuring speed in one direction.

DC vs AC Tachometer: A Comparison Table

The following table summarizes the key differences between DC and AC tachometer generators: